Chapter 22:Solid Lubricants and Self

Transcription

22
Solid Lubricants and
Self-Lubricating Films*
22.1
Introduction
General Characteristics of Solid Lubricants • New Products,
Practices, and Approaches in Solid Lubrication
22.2
Classification of Solid Lubricants
22.3
22.4
Lubrication Mechanisms of Layered Solids
High-Temperature Solid Lubricants
Lamellar Solid Lubricants
Lubricious Oxides, Fluorides, and Sulfates • Composites •
New Approaches to Solid Lubrication at High Temperatures
22.5
Self-Lubricating Composites
Traditional Materials • New Self-lubricating Composite
Coatings and Structures
Ali Erdemir
Argonne National Laboratory
22.6
22.7
22.8
Soft Metals
Polymers
Summary and Future Directions
22.1 Introduction
In most tribological applications, liquid or grease lubricants are used to combat friction and wear; but
when service conditions become very severe (i.e., very high or low temperatures, vacuum, radiation,
extreme contact pressure, etc.), solid lubricants may be the only choice for controlling friction and wear.
Some of the key advantages of solid lubricants in tribological applications over liquid and grease lubricants
are summarized in Table 22.1. A combination of solid and liquid lubrication is also feasible and may
have a beneficial synergistic effect on the friction and wear performance of sliding surfaces. Solid lubricants can be dispersed in water, oils, and greases to achieve improved friction and wear properties under
conditions of extreme pressures and/or temperatures (Barnett, 1977; Broman et al., 1978; Kimura et al.,
1999; Erdemir, 1995).
When present at a sliding interface, solid lubricants function the same way as their liquid counterparts.
Specifically, they shear easily to provide low friction and to prevent wear damage between the sliding
surfaces. Several inorganic materials (e.g., molybdenum disulfide, graphite, hexagonal boron nitride,
*Work supported by U.S. Department of Energy, Office of Transportation Technology, under Contract W-31-109Eng-38.
The submitted manuscript has been created by the University of Chicago as Operator of Argonne National
Laboratory (“Argonne”) under Contract No. W-31-109-ENG-38 with the U.S. Department of Energy. The U.S.
Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license
in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display
publicly, by or on behalf of the Government.
© 2001 by CRC Press LLC
TABLE 22.1
Comparison of Solid and Liquid Lubricants in Tribological Applications
Application Environment
and/or Condition
Solid Lubricants
Liquid and Grease Lubricants
Vacuum
Some solids (i.e., transition-metal dichalcogenides)
lubricate extremely well in high vacuum, have very
low vapor pressure
Pressure
Can endure extreme pressures
Temperature
Relatively insensitive; can function at very low and
high temperatures; low heat generation due to
shear
Electrical conductivity
Radiation
Wear
Some provide excellent electrical conductivity
Relatively insensitive to nuclear radiation
Provide excellent wear performance or durability at
slow speeds and under fretting conditions; lifetime
is determined by lubricant film thickness and wear
rate
Extremely low friction coefficients are feasible
Most liquids evaporate, but
perfluoropolyalkylethers (PFPE) and
polyalfaolefins (PAO) have good
durability
May not support extreme pressures
without additives
May solidify at low temperatures and
decompose or oxidize at high
temperatures; heat generation varies
with viscosity
Mostly insulating
May degrade or decompose over time
Provide marginal performance and
durability at slow speeds and under
fretting conditions; need additives for
boundary lubrication
Depends on viscosity, boundary films,
and temperature
Good
Friction
Thermal conductivity and
heat dissipation
capability
Storage
Hygiene
Compatibility with
tribological surfaces
Resistance to aqueous and
chemically aggressive
environments
Excellent for metallic lubricants; poor for most
inorganic or layered solids
Can be stored for very long times (dichalcogenides
are sensitive to humidity and oxygen)
Better industrial hygiene due to little or no
hazardous emissions; since they are in solid state,
there is no danger of spillage that can contaminate
environment
Compatible with hard-to-lubricate surfaces (i.e., Al,
Ti, stainless steels, and ceramics)
Relatively insensitive to aqueous environments,
chemical solvents, fuels, certain acids and bases
May evaporate, drain, creep, or migrate
during storage
May release hazardous emissions; liquid
lubricants may spill or drip and
contaminate environment; fire hazard
with certain oils and greases
Not suitable for use on non-ferrous or
ceramic surfaces
May be affected or altered by acidic and
other aqueous environments
boric acid) can provide excellent lubrication (Sutor, 1991; Klauss, 1972; Lancaster, 1984; Sliney, 1982;
McMurtrey, 1985; Lansdown, 1999). Most of these solids owe their lubricity to a lamellar or layered
crystal structure. A few others (e.g., soft metals, polytetrafluoroethylene, polyimide, certain oxides and
rare-earth fluorides, diamond and diamond-like carbons, fullerenes) can also provide lubrication
although they do not have a layered crystal structure. In fact, diamond-like carbon films are amorphous,
but provide some of the lowest friction coefficients of all the solid materials (Erdemir et al., 2000). Because
a special chapter (see Chapter 24) in this Handbook is devoted to the friction and wear behavior of
diamond and diamond-like carbon, they will not be covered here.
The solid lubricants with a layered crystal structure are graphite, hexagonal boron nitride, boric acid,
and the transition-metal dichalcogenides MX2 (where M is molybdenum, tungsten, or niobium, and X
is sulfur, selenium, or tellurium). Figure 22.1 shows the layered crystal structures of these solids. Certain
monochalcogenides (e.g., GaSe and GaS) have lattice structures similar to those of dichalcogenides; hence,
they can also provide low friction when present at a sliding interface (Erdemir, 1994).
Major shortcomings of solid lubricants include:
1. Except for soft metals, most solid lubricants are poor thermal conductors and, hence, cannot carry
away heat from sliding interfaces.
2. Depending on test environment and contact conditions, their friction coefficients may be high or
fluctuate significantly.
Carbon
3.35 A°
(a)
(b)
Sulfur
Molybdenum
Oxygen
Boron
Hydrogen
2.96 A°
3.18 A°
(c)
(d)
FIGURE 22.1 Schematic illustration of layered crystal structures of (a) graphite, (b) hexagonal boron nitride, (c)
molybdenum disulfide (representing transition metal dichalcogenides), and (d) boric acid.
3. They have finite wear lives and their replenishment is more difficult than that of liquid lubricants.
4. Oxidation and aging-related degradation may occur over time and present some problems with
transition-metal dichalcogenides.
5. Upon exposure to high temperatures or oxidative environments, they may undergo irreversible
structural-chemistry changes that in turn lead to loss of lubricity and the generation of some
abrasive, nonlubricious by-products.
22.1.1 General Characteristics of Solid Lubricants
Well-known solid lubricants (graphite, HBN, and transition-metal dichalcogenides) owe their lubricity
to a unique layered structure. As illustrated in Figure 22.1, the crystal structures of these solids are such
that while the atoms lying on the same layer are closely packed and strongly bonded to each other, the
layers themselves are relatively far apart, and the forces that bond them (e.g., van der Waals) are weak.
When present between sliding surfaces, these layers can align themselves parallel to the direction of
relative motion and slide over one another with relative ease, thus providing low friction. In addition,
strong interatomic bonding and packing in each layer is thought to help reduce wear damage. While this
mechanism is largely responsible for low friction and is essential for long wear life, a favorable crystal
structure in itself is not sufficient for effective lubrication. The presence or absence of certain chemical
adsorbates is also needed for providing easy shear in most solids. For example, moisture or some other
In contrast, MoS2 and other transition-metal dichalchogenides work best in vacuum or dry running
conditions, but degrade rather quickly in moist and oxidizing environments (Winer, 1967; Farr, 1975;
Kanakia and Peterson, 1987). The friction coefficients of self-lubricating metal dichalcogenides are
typically in the range of 0.002 to 0.05 in vacuum or dry and inert atmospheres, but increase rapidly to
0.2 in humid air. It is generally agreed that no solid can provide very low friction and wear, regardless
of test environment and/or conditions.
Soft metallic lubricants have crystal structures with multiple slip planes and do not work-harden
appreciably during sliding contact. Dislocations and point defects generated during shear deformation
are rapidly nullified by the frictional heat produced during sliding contact. Most high-temperature solid
lubricants rely on thermal softening and/or limited chemical reaction with sliding surfaces that make
them shear with relative ease; whereas self-lubricating polymers consist of long molecular chains with
high chemical inertness and/or very low surface energy, making them non-stick or largely insensitive to
chemical bonding.
Ambient temperature has a strong influence on the lubricity of solid lubricants. Graphite can provide
lubrication up to 400°C, while HBN can withstand temperatures up to 1000°C. Most transition metal
dichalcogenides tend to oxidize at elevated temperatures, and thus lose their lubricity. MoS2 can provide
lubrication up to 400°C, while WS2 endures up to 500°C (Sliney, 1982). In general, those with higher
oxidation resistance or chemical/structural stability perform the best at elevated temperatures. Oxideand fluoride-based solid lubricants (e.g., CaF2, BaF2, PbO, and B2O3) (Sliney, 1993), as well as some soft
metals (e.g., Ag, Au), function quite well at elevated temperatures (Erdemir et al., 1990c; Erdemir and
Erck, 1996; Maillat et al., 1993), but all fail to provide low friction at room or lower ambient temperatures.
The lubricity of these solids at elevated temperatures is largely controlled by their ability to soften and
resist oxidation.
Solid lubricants can be applied to a tribological surface in a variety of forms. The oldest and simplest
method is to sprinkle, rub, or burnish the fine powders of solid lubricants on surfaces to be lubricated.
Fine powders of certain solid lubricants were also used to lubricate sliding bearing surfaces with great
success (Heshmat and Heshmat, 1999; and Higgs et al., 1999). Certain solid lubricants have been blended
in an aerosol carrier and sprayed directly onto the surfaces to be lubricated. Powders of solid lubricants
can be strongly bonded to a surface by appropriate adhesives and epoxy resins to provide longer wear
life (Gresham, 1997). They can also be dispersed or impregnated into a composite structure. Certain
solids (e.g., HBN and boric acid) have been mixed with oils and greases in powder form to achieve
improved lubrication under extreme pressure and temperature conditions (Kimura et al., 1999; Erdemir,
1995). However, in most modern applications, thin films of solid lubricants are preferred over powders
or bonded forms. They are typically deposited on surfaces by advanced vacuum deposition processes
(e.g., sputtering, ion plating, and ion-beam-assisted deposition) to achieve strong bonding, dense microstructure, uniform thickness, and long wear life (Spalvins, 1969, 1971, 1980; Erdemir, 1993). Ion-beam
deposition and mixing can also be used to enhance the durability of solid lubricant coatings (Bhattacharya
et al., 1993; Erck et al., 1992). However, the lifetimes of most solid lubricants are still limited because of
the finite lubricant film thickness. To increase their durability, a self-replenishment or resupply mechanism is needed but very difficult.
22.1.2 New Products, Practices, and Approaches in Solid Lubrication
In recent years, several new lubricants and modern lubrication concepts have been introduced to achieve
better lubricity and longer wear life in demanding tribological applications. Some of the traditional solid
lubricants were prepared in the forms of metal, ceramic, and polymer-matrix composites and used
successfully in a variety of engineering applications (Rohatgi et al., 1992; Prasad and McConnell, 1991;
Gangopadhyay and Jahanmir, 1991; Friedrich, 1995). Carbonaceous films produced by catalytic cracking
of carbon-bearing gases were also shown to provide good lubricity at elevated temperatures (Lauer and
Bunting 1988; Blanchet et al., 1994). Recent developments in PVD and CVD deposition technologies
have led to the synthesis of a new generation of adaptive, self-lubricating coatings with composite or
multilayer architectures (Jayaram et al., 1995; Zabinski et al., 1992, 1995; Voevodin et al., 1999). These
exotic architectures, based on layers of a self-lubricating dichalcogenide (e.g., MoS2, WS2, etc.) and a soft
metallic or hard ceramic layer, were shown to work extremely well under demanding tribological conditions. Multifunctional nanocomposite films (consisting mainly of MoS2 and Ti) have also been produced by magnetron sputtering and are quite hard, moisture insensitive, and self-lubricating, thus raising
the prospect for dry-sliding applications, as well as dry metal-cutting and -forming (Fox et al., 1999).
Duplex/multiplex surface treatments and multilayer coatings with self-lubricating capabilities have also
made their way into the commercial marketplace and have been meeting the ever-increasing performance
demands of more severe applications.
Recently, carbon and WS2 were prepared in the form of hollow nanotubes and demonstrated to provide
high mechanical strength and very low friction coefficients under certain sliding conditions (Tenne, 1992;
Falvo, 1999). Nanostructured ZnO films were also shown to be quite lubricious and relatively insensitive
to variations in ambient pressure, environment, and temperature (Zabinski, 1997). A series of adaptive
lubrication strategies has also been introduced in recent years and shown to be effective over a wide
range of temperatures and pressures (Walck, 1997). Minute oxygen deficiency or sub-stoichiometry in
rutile was shown to lead to the formation of low-shear crystallographic planes and hence high lubricity
(Gardos, 1988, 1990, 1993). A series of plasma-sprayed composite coatings consisting of silver and alkaline
halides (i.e., CaF2, BaF2) as the self-lubricating entities and CrC and/or Cr2O3 as the wear-resisting entities
were also shown to provide excellent lubrication over a wide temperature range (DellaCorte, 1998;
DellaCorte and Fellenstein, 1997). Furthermore, H3BO3 powders, films forming on B2O3, and B4C coatings
were shown to be quite lubricious and highly effective under extreme sliding conditions (Erdemir, 1991;
Erdemir et al., 1990b, 1991c, 1999).
Thiomolybdates and oxythiomolybdates of Cs, Zn (i.e., Cs2MoO2S2, ZnMoO2S2), and a few other alkali
metals were found to be effective in controlling friction and wear at elevated temperatures (King, 1990).
These solids possess a lamellar structure like MoS2 but can endure much higher temperatures than MoS2.
Furthermore, certain complex oxides and oxide-fluorides (i.e., ZnO/SnO/SrF 2 , NiO/BaTiO 3 ,
MgO/ZnO/CaF2, NiO/SrF2) were shown to be rather lubricious at elevated temperatures (Erdemir et al.,
1998). Erdemir (1999) introduced a new crystal-chemical approach to the selection, classification, and
mechanistic understanding of lubricious oxides used to combat friction and wear at elevated temperatures. Based on this approach, one can predict the shear rheology and hence lubricity of an oxide or
oxide mixture at elevated temperatures.
John and Zabinski (1999) investigated the lubrication properties of some sulfate-based (i.e., CaSO4 ,
BaSO4, and SrSO4) coatings at high temperatures as a potential replacement for alkaline halides (i.e.,
CaF2 , BaF2). They found that these sulfates became highly lubricious at 600°C and were able to provide
friction coefficients of ≈0.15 to sliding surfaces. Structural studies revealed the presence of a carbonate
crystal structure, along with a sulfate crystal structure after testing. The carbonate crystal structure
consisted of alternating layers of alkali-earth atoms and carbonate ions. It was proposed that such a
layered structure may have been responsible for the low-friction nature of these sulfates at high temperatures. The new lubricants and lubrication approaches mentioned above are some of the most notable
developments in recent years and certainly have the potential to overcome difficult lubrication problems
that may arise in the future.
In this chapter, solids with self-lubricating capabilities are reviewed first and classified on the basis of
their crystal structures, chemistry, and operational limits. A summary of the recent understanding of the
lubrication mechanisms of both traditional and new solid lubricants is presented next. Then, the present
state-of-the-art in advanced solid lubrication methods and practices is provided. Particular emphasis is
placed on the synthesis and/or applications of solid lubricant films on tribological surfaces by means of
advanced surface engineering processes such as ion-beam-assisted deposition, ion-beam mixing, and
unbalanced magnetron sputtering. Traditional and new applications for self-lubricating composite solid
lubricants are also emphasized. This chapter primarily focuses on developments evolved during the last
decade because several excellent reviews, book chapters, and books cover the earlier developments (Sutor,
1991; Lancaster, 1984; Sliney, 1982, 1993; Singer, 1989, 1992, 1998; McMurtrey, 1985; Klauss, 1972;
Miyoshi, 1996). Also, major emphasis is placed on inorganic solid lubricants with layered crystal structures
and those that provide lubrication at high temperatures. Soft metals and polymers are briefly discussed
because there are several excellent articles providing in-depth information on the properties and applications of these solid lubricants (Sliney, 1986; Dayson, 1971; Sherbiney and Halling, 1977; Wang et al.,
1995; Briscoe, 1990; Zhang, 1997; Bahadur and Gong, 1992; Friedrich et al., 1995).
22.2 Classification of Solid Lubricants
Solid lubricants can be categorized into several subclasses. Table 22.2 provides such a classification based
on the chemistry, crystal structure, and lubricity of the most widely used and recently developed solid
lubricants. As can be realized from Table 22.2, the range of friction coefficients is rather large for a given
solid lubricant. This is mainly because friction is very sensitive to test environment, condition, and/or
configuration. Ambient temperature and the type of counterface materials can also make a big difference
in the frictional property of a given solid lubricant. The specific form or shape of the solid lubricants
(i.e., thin films, powders, bulk, composite, and crystalline or amorphous states) can also play a major
role. For example, the wide range of friction coefficients for MoS2 (i.e., 0.002 to 0.25) stems from several
factors affecting its shear rheology and hence frictional properties. These factors include film microstructure and chemistry, test environment, ambient temperature, contact pressure, film thickness, stoichiometry, and purity. Deposition and/or lubricant application methods can also play a major role in frictional
performance of MoS2 films. Due to their porous, columnar structure, MoS2 films deposited by conventional sputtering methods tend to exhibit higher friction and shorter wear lives than films produced by
more robust ion-beam-assisted deposition and closed-field unbalanced magnetron sputtering techniques.
The MoS2 films deposited by these advanced physical vapor deposition methods can have near-perfect
stoichiometry, purity, and basal plane orientation parallel to the substrate surfaces. These highly optimized films can, in turn, provide friction coefficients as low as 0.002 in ultrahigh vacuum (Martin et al.,
1994; Donnet et al., 1993).
In general, no single lubricant can provide reasonably low and consistent friction coefficients over
broad test conditions, temperatures, and environments. Each lubricant listed in Table 22.2 functions
rather nicely under certain test conditions, but not under all conditions. Researchers have mixed two or
more of these lubricants to broaden the operational range, but in most cases the improvements were
either transitory or short-lived.
22.2.1 Lamellar Solid Lubricants
Lamellar or layered solid lubricants are the class that is most studied by scientists and widely used by
industry. Among the best-known examples are transition-metal dichalcogenides (e.g., MoS2), graphite,
TABLE 22.2
Solid Materials with Self-lubricating Capability
Classification
Lamellar solids
Soft metals
Mixed oxides
Single oxides
Halides and sulfates of alkaline
earth metals
Carbon-based solids
Organic materials/polymers
Bulk or thick-film (>50 µm)
composites
Thin-film (<50 µm) composites
Key Examples
MoS2
WS2
HBN
Graphite
Graphite fluoride
H3BO3
GaSe, GaS, SnSe
Ag
Pb
Au
In
Sn
CuO–Re2O7
CuO–MoO3
PbO–B2O3
CoO–MoO3
Cs2O–MoO3
NiO–MoO3
Cs2O–SiO2
B 2O 3
Re2O7
MoO3
TiO2 (sub-stoichiometric)
ZnO
CaF2, BaF2, SrF2
CaSO4, BaSO4, SrSO4
Diamond
Diamond-like carbon
Glassy carbon
Hollow carbon nanotubes
Fullerenes
Carbon-carbon and carbon-graphite-based composites
Zinc stearite
Waxes
Soaps
PTFE
Metal-, polymer-, and ceramic-matrix composites consisting
of graphite, WS2, MoS2, Ag, CaF2, BaF2, etc.
Electroplated Ni and Cr films consisting of PTFE, graphite,
diamond, B4C, etc., particles as lubricants
Nanocomposite or multilayer coatings consisting of MoS2, Ti,
DLC, etc.
Typical Range of
Friction Coefficienta
0.002–0.25
0.01–0.2
0.150–0.7
0.07–0.5
0.05–0.15
0.02–0.2
0.15–0.25
0.2–0.35
0.15–0.2
0.2–0.3
0.15–0.25
0.2
0.3–0.1
0.35–0.2
0.2–0.1
0.47–0.2
0.18
0.3–0.2
0.1
0.15–0.6
0.2
0.2
0.1
0.1–0.6
0.2–0.4
0.15–0.2
0.02–1
0.003–0.5
0.15
—
0.15
0.05–0.3
0.1–0.2
0.2–0.4
0.15–0.25
0.04–0.15
0.05–0.4
0.1–0.5
0.05–0.15
a Friction values given in this table represent friction measurements made on each solid lubricant over a wide range of
test conditions, environments, and temperatures. The objective here is to show how friction varies depending on test
conditions, as well as from one solid to another.
HBN, and H3BO3 . MoS2, graphite, and boric acid are natural minerals, extracted from deposits around
the world. Other lamellar solids, such as WS2, fluorinated graphite, and transition-metal diselenides and
ditellurides, are synthetic and are used at much smaller scales than graphite, HBN, and MoS2. MoS2 and
WS2 are well-suited for aerospace and cryogenic applications, while HBN is preferred for lubrication at
elevated temperatures. HBN is widely used as a release agent in high-temperature metal-forming operations. Graphite and H3BO3 work extremely well in moist air. The lubricity of graphite persists up to
400°C, while H3BO3 begins to decompose at about 170°C. These two solids do not provide lubrication
in dry or vacuum environments. Graphite fluoride is produced by the fluorination of graphite. This
process increases the spacing between the carbon-carbon layers in graphite from about 0.34 nm to values
as high as 0.8 nm, resulting in easier shear and hence better lubricity, even in dry environments. Among
the lamellar solids, MoS2 and WS2 have the best overall load-carrying capacity as thin films on rigid
substrates.
Most lamellar solids have good wetting capability or chemical affinity for ferrous surfaces. On a rough
or porous sliding surface, they fill in the valleys between asperities and/or pores, thus providing a
smoother surface finish and better support. When applied properly, these solids can also withstand
extreme contact pressures without being squeezed out of the load-bearing surfaces. WS2 is preferred over
MoS2 when applications involve relatively higher temperatures. However, WS2 is a synthetic lubricant
and thus is expensive. Selenides of W, Nb, and Mo can provide even higher temperature capabilities than
their sulfide analogs, but they too are expensive and are used on much smaller scales. Certain selenides
and tellurides (e.g., V, Nb) provide excellent electrical conductivity.
22.2.1.1 Transition-Metal Dichalcogenides
Transition-metal dichalcogenides, MX2 (where M is Mo, W, Nb, Ta, etc., and X is sulfur, selenium, or
tellurium), are among the lowest-friction materials known in dry and vacuum environments (Winer,
1967; Farr, 1975; Kanakia and Peterson, 1987; Singer et al., 1990; Donnet, 1996). They are also well-suited
for cryogenic applications. MoS2 and WS2 are the best-known examples and the most widely used
dichalcogenides. MoS2 is a natural mineral known as molybdenite, whereas WS2 and other dichalcogenides are man-made and therefore expensive. The hardness values of these solids on the Mohs scale
are 1.5 to 2 and their specific gravities lie between 4.7 and 5.5. They are chemically stable and resist attack
by most acids, except aqua regia and hot and highly concentrated HCl, H2SO4, and HNO3. At room
temperature in ultrahigh vacuum, these solid lubricants provide some of the lowest friction coefficients,
but moisture in air has a detrimental effect on their lubricity (Peterson, 1953; Fusaro, 1978). Oxidation
of MoS2 does not begin until the temperature reaches about 375°C. At approximately 500°C, rapid
oxidation begins and MoO3 and SO2 are produced. The thermal and oxidative stability of WS2 is better
than that of MoS2 (Sliney, 1982).
22.2.1.1.1 Preparation and Uses of Dichalcogenides
MoS2 and other dichalcogenides are applied on tribological surfaces as thin, strongly bonded solid films
providing very long wear lives and super-low friction coefficients. Depending on application conditions
(load, speed, temperature, etc.) and the form (crystalline or amorphous), size, purity, stoichiometry, and
film thickness, the friction coefficients of MoS2 and other dichalcogenides vary considerably. In moist
air, the lifetimes of lubricant films are rather short and typical values for friction coefficients are 0.05 to
0.25. Burnished films tend to be short-lived and give higher friction than thin sputtered films (Spalvins,
1971; Fusaro, 1978; Peterson, 1953). Bonded and composite forms of MoS2 last much longer, but their
friction coefficients are generally high (Gresham, 1977).
The advanced physical vapor deposition (PVD) methods used in the deposition of high-quality MoS2
films include magnetron sputtering (Spalvins, 1969, 1971, 1980; Stupp, 1981), ion-beam-assisted deposition (IBAD) (Bolster, 1991; Wahl et al., 1995; Seitzman et al., 1995; Dunn et al., 1998), and ion-beam
mixing (Kobs et al., 1986; Bhattacharya et al., 1993; Rai, 1997). A pulsed laser deposition (PLD) method
can also be used to deposit high-quality MoS2 and other composite films with excellent tribological
performance (Zabinski, 1992; Prasad, 1995). Sputtering has been and is still the most widely used method.
Recently, closed-field unbalanced magnetron sputtering of MoS2 has become very popular and is highly
effective in many tribological applications, including metal-forming and -cutting operations (Fox et al.,
1999). Fil2.2 compares the endurance lives of MoS
2 films prepared by various methods.
MoS2 films may also contain lattice and volume defects (i.e., large voids or porosities) in their microstructures (Spalvins, 1980; Lince and Fleischauer, 1987; Hilton and Fleischauer, 1991). Furthermore,
some contain significant amounts of oxygen and carbon impurities that may have been present in the
deposition chamber or introduced during film deposition. They may also come from the source or target
material used during deposition. Depending on the level of contaminants, resultant MoS2 films may show
Maximum Thrust Bearing Endurance
of *1 µm* MoS2 Coatings
burnished
dc sputtered
rf sputtered
NRL S-modulated
OSMC pure
OSMC AuPd
NRL Pb-alloyed
0
2
4
6
8
10
12
Revolutions to Failure
(Millions)
FIGURE 22.2 Endurance lives of MoS2 films produced by various methods. (From Singer, I.L., Bolster, R.N.,
Seitzman, L.E., Wahl, K.J., and Mowery, R.L. (1994), Advanced Solid Lubricant Films by Ion-Beam Assisted Desposition., Naval Research Laboratory, NRL/MR/6170-94-7633.)
significant differences in their tribological properties (Suzuki, 1998). Crystalline films with porous columnar structures tend to wear out rather quickly. Tilting or bending of columnar grains with poor cohesion
during sliding results in fracture of the top portions of each column; the remaining lower part is smeared
on the surface and the basal planes of the MoS2 crystal are eventually oriented parallel to the sliding
surfaces (Spalvins, 1980; Hilton, 1991). The rate and degree of reorientation appear to depend on the
initial microstructure. Recent studies have indicated that films with dense morphology, preferred basal
orientation, and high purity provided the best overall performance. In fact, the lowest friction coefficients
(0.002 to 0.01) were reported on a phase-pure (oxygen-free) and stoichiometric MoS2 film (Donnet,
1993; Martin et al., 1993). These super-low friction coefficients are obtained in ultrahigh vacuum and
are attributed to a combination of perfect basal orientation of the MoS2 layers and to the absence of any
adsorbed species or contaminants on sliding surfaces.
22.2.1.1.2 Modern Practices
Increasing demand for higher performance, longer wear life, and better efficiency in advanced mechanical
systems that depend primarily on solid lubrication for safe operation has intensified interest in new and
exotic lubrication practices in recent years. One of the major reasons for this interest was that conventional
lubrication practices could no longer meet the performance and durability needs of advanced mechanical
systems. Most studies have concentrated on MoS2 and have resulted in a better understanding of the
friction and wear mechanisms of this solid lubricant. Such mechanistic understanding is, in turn, used
to develop the new and better lubrication practices that are in wide use by industry today.
With the advanced PVD methods mentioned earlier, MoS2 films can be grown at subzero, room, or
elevated temperatures. At lower deposition temperatures or under high-energy ion bombardment, one
can obtain amorphous and sub-stoichiometric films with relatively poor tribological properties. During
sliding or upon annealing, the crystallinity, and hence the lubricity, of MoS2 may be restored (Zabinski
et al., 1994). Ion-beam mixing of sputtered MoS2 or WS2 films (50 to 70 nm thick) with sapphire, Si3N4,
and ZrO2 substrates can also result in an amorphous microstructure with a sub-stoichiometry of MoS1.8.
In these studies, 2-MeV Ag+ ions at 5 × 1015 cm–2 dose were used. During tribological tests in dry N2,
FIGURE 22.3
substrate.
Friction performance and durability of sputtered and Ag+ ion-beam mixed MoS2 films on sapphire
friction coefficients of 0.03 to 0.04 were measured in both the as-deposited and ion-irradiated films.
However, the sliding lives of Ag+ ion-irradiated films were found to increase 10- to 1000-fold over those
of as-sputtered films on all ceramic surfaces studied. The improvements in wear lives were correlated
with a significant improvement in film/substrate adhesion (Bhattacharya et al., 1993). Figure 22.3 shows
the friction performance and durability of sputtered and Ag+ ion-beam mixed MoS2 films on sapphire
substrates. Similar improvements in wear lives of WS2 film were found after ion-beam mixing (Rai et al.,
1997).
Recently, researchers have developed novel means to dope MoS2 films with certain metals (e.g., Au,
Ni, Ti, Pb, C, etc.) and compounds (TiN, PbO, Sb2O3, etc.) (Zabinski et al., 1992, 1995; Spalvins, 1984;
Stupp, 1981; Hilton et al., 1992, 1998; Wahl et al., 1995; Lince et al., 1995). Tribological studies have
demonstrated that when doped properly and in the correct proportions, these dopants can substantially
improve the mechanical and tribological properties of MoS2 films. For example, Au-doped MoS2 films
were shown to have more stable frictional traces and lower friction than undoped sputtered MoS2 films,
as shown in Figure 22.4. Figure 22.5 compares the friction and wear performance of conventional MoS2
with that of Ti-doped MoS2 in increasingly humid air. Furthermore, doping of MoS2 with Pb, Ti, Ni, Fe,
Au, and Sb2O3 resulted in film amorphization or densification and in reduction of the crystallite size,
which in turn reduced the mean and variance of the friction coefficients and substantially increased their
wear lives (Zabinski et al., 1992, 1995; Wahl et al., 1995, 1999). The exact mechanisms responsible for
lifetime improvements in doped MoS2 films are not yet fully understood. However, researchers have
noticed that doping generally results in preferential alignments of basal planes parallel to the substrate
surface and thus lower susceptibility of MoS2 to oxidation or moisture-induced degradation. It is speculated that such favorable alignment, together with increased resistance to oxidation, may have been
responsible for increased wear life and lubricity.
Films with duplex and/or alternating layers of MoS2 and metals or hard nitrides have also been
produced in recent years and used in a variety of applications (Hilton et al., 1992; Jayaram, 1995; Seitzman
et al., 1992). For example, MoS2 films prepared by RF magnetron sputtering on AISI 440C and 52100
steels, and multilayer coatings of MoS2 with either nickel or Au-(20%)Pd metal interlayers (with layer
COEFFICIENT OF FRICTION, µ
.05
MoS
2
.04
Au-MoS
2
.03
.02
.01
0
10000
20000
30000
40000
50000
SLIDING DURATION, cycles
FIGURE 22.4 Effect of Au doping on friction behavior of sputtered MoS2 films. (From Spalvins, T. (1984), Frictional
and morphological properties of Au-MoS2 films sputtered from a compact target, Thin Solid Films, 118, 374-384.
With permission.)
0.30
Pin-On-Disc
Substrate: WC
Counterpart: 6 mm dia. WC Ball
Track Radius: 3.5 mm
Speed: 500 rpm
Load: 10 N
Friction Coefficient (µ)
0.25
0.20
0.15
0.10
0.05
MoS2
MoST™
0.00
0
20
40
60
% Relative Humidity
80
100
FIGURE 22.5 Friction performance of conventional and Ti-doped MoST films at different humidity levels. (Courtesy of Multi-arc, Inc.)
thicknesses ranging from 0.2 to 1.0 nm) on silicon substrates, had very dense microstructures that in
some cases exhibited significant orientation of MoS2 basal planes parallel to the substrate. Some of the
optimized films exhibited excellent endurance and friction coefficients of 0.05 to 0.08 in UHV (Hilton,
1992).
Overall, these novel coating practices have led to favorable changes in crystallite size and film density
and reduced edge orientation in growing films, which in turn resulted in increased coating endurance.
Some dopants (Pb, Ti, PbO) resulted in an amorphous microstructure but with no detrimental effect
on the low-friction and wear behaviors of the films. In fact, despite the formation of an amorphous
microstructure, significant increases in wear lives are attained with Pb- and Ti-doped films (Fox et al.,
1999; Wahl et al., 1995, 1999). However, the mechanism(s) responsible for such remarkable performance
has not yet been resolved.
In recent years, researchers have demonstrated that low-friction surface films can be formed in situ
on the surfaces of Mo, W, and Mo- or W-containing metallic alloys by adding sulfur-bearing gases such
as H2S and SO2 into the test chamber (Singer et al., 1996a,b; Sawyer and Blanchet, 1999). In a model
experiment run in high vacuum, where a small amount of H2S was admitted to maintain an S partial
pressure of 13 Pa, Singer et al. (1996) recorded a friction coefficient of 0.01 on the resulting films on Mo
substrates.
Recently, WS2 was prepared as nanoparticles having structures similar to those of nested carbon
fullerenes and nanotubes. Preliminary test results showed that these nanoparticles are highly effective in
reducing friction and wear and do outperform the solid and thin film forms of WS2 and MoS2 when
tested under the same test conditions (Rapoport et al., 1997). For the excellent durability and performance
of these nanoparticles, high chemical inertness and a hollow cage structure were proposed. Apparently,
hollow structures are chemically very stable and do not interact with oxygen or water molecules in the
environment. Because of their high rigidity, they impart high elasticity, which allows these particles to
roll rather than slide.
22.2.1.2 Monochalcogenides
Sulfides and selenides of gallium and tin (i.e., GaS, GaSe, SnSe) have crystal structures that resemble
those of transition-metal dichalcogenides (i.e., MoS2, WS2, WSe2) which are well-known solid lubricants.
Figure 22.6 shows the layered structure of GaSe. These solids are known as sandwich semiconductors in
FIGURE 22.6 (a) Crystal structure of GaSe, and (b) SEM photomicrograph of fractured GaSe pellet. (From Erdemir,
A. (1994), Crystal chemistry and self-lubricating properties of monochalcogenides gallium selenide and tin selenide,
Tribol. Trans., 37, 471-476.)
solid-state physics and have been studied extensively for their electrical and optical properties (Phillips,
1969). Tin selenide represents a group of layered compounds that also comprise SnS and the sulfides and
selenides of germanium, whereas GaSe belongs to a class that also includes the layered gallium sulfide
and the sulfides and selenides of indium.
Using a pin-on-disk machine, Erdemir (1994) performed friction tests on large crystalline pieces and
compacts of GaSe and SnSe monochalcogenides against sapphire and 440C steel balls to assess their
lubricity. For the specific test conditions explored, friction coefficients of the sapphire/GaSe and sapphire/SnSe pairs were ≈0.23 and ≈0.35, respectively. The friction coefficients of 440C pin/440C disk test
pairs with GaSe and SnSe powders were ≈0.22 and ≈0.38, respectively. The friction data, together with
the crystal-chemical knowledge and electron microscopy evidence, supported the conclusion that the
lubricity and self-lubricating mechanisms of these solids are closely related to their crystal chemistry and
the nature of their interlayer bonding.
In a series of earlier studies, Boes and Chamberlain (1968) and Gardos (1984) explored the tribological
and thermal oxidation properties of composite lubricants consisting of indium/gallium and WSe2. The
principal goal of these studies was to achieve better oxidation resistance on WSe2 by alloying it with lowmelting-point indium and gallium. Upon curing the composite mixture at high temperatures, the investigators found that both indium and gallium underwent chemical reaction with WSe2 to form the selenides
of these metals. Further studies by Gardos (1984) demonstrated that, compared to the parent WSe2, the
new composite lubricant exhibited superior oxidation resistance over a wide range of ambient temperature. Also, the lubricating capability of this InSe/WSe2 composite was much superior to that of WSe2
alone, especially at elevated temperatures. Apparently, a protective film resulting mainly from the preferential oxidation of sub-stoichiometric indium selenides was primarily responsible for the superior
oxidation resistance of this new lubricant. The protective film was thought to effectively shield the
lubricating entities against oxidation.
As discussed later, chalcogenides owe their low-friction nature to their lamellar structures in which
strongly bonded atoms form extensive rigid sheets (see Figures 22.1 and 22.6). In the cases of dichalcogenides such as MoS2 or MoSe2, the crystal structure is composed of a monolayer of Mo ions sandwiched
between layers of S or Se ions. However, in the case of monochalcogenides such as GaS or GaSe, the
crystal structure is composed of double layers of Ga sandwiched between Se ions (Figure 22.6).
22.2.1.3 Graphite
Graphite is another classic example of lamellar solids that provides low friction and high wear resistance
to sliding surfaces. Because of its good lubricity, abundance, and low cost, it is used in many industrial
applications. Like diamond, graphite is a polymorph of carbon. Both occur naturally and are recovered
from deposits around the world; both can also be produced by synthetic means. Synthetic graphite is
primarily produced by heating petroleum coke to about 2700°C. Chemically, both graphite and diamond
are the same, but differ totally in their structures and properties. For example, graphite is perhaps one
of the softest materials, while diamond is the hardest of all natural materials. Diamond has the highest
thermal conductivity, whereas graphite is a relatively poor thermal conductor. However, graphite is a
good electrical conductor, but diamond is an excellent electrical insulator. Graphite has a sheet-like crystal
structure (see Figure 22.1) in which all of the carbon atoms lie in a plane and are bonded only weakly
to the graphite sheets above and below. Each carbon atom in the plane joins to three neighboring carbon
atoms at a 120° angle and at a distance of 0.1415 nm. The distance between atomic layers is 0.335 nm
at room temperature, and the layers are held together by van der Waals forces.
In moist air, the friction coefficient of graphite varies from 0.07 to 0.15, depending on test conditions,
sliding contact configuration, form of graphite used (powder, bulk, thin film, purity, crystallite orientation), and test machine. The lowest friction coefficient of 0.01 was observed during a nanotribology
experiment in which a W tip was slid against the cleaved graphite flakes (Mate, 1987). The dense and
highly oriented pyrolitic graphite (HOPG) performs extremely well in humid air, giving friction coefficients of about 0.1. In dry air, inert atmospheres, or vacuum, graphite’s lubricity degrades rapidly, the
friction coefficient increases to as high as 0.5, and it wears out quickly. Experimental studies carried out
FIGURE 22.7 Effect of water vapor pressure on wear rate of graphite. (From Savage, R.H. (1948), Graphite
lubrication, J. Appl. Phys., 19, 1-10. With permission.)
by research groups have confirmed that the lubricity of graphite is not due to its layered crystal structure
alone, but depends strongly on the presence or absence of certain condensable vapors, water vapor being
one. Research has shown that only a small amount of condensable vapor is needed to improve the lubricity
of graphite (Rowe, 1960). Certain vapors appear to be more effective than others. For example, a test
run by Savage (1948) showed that n-heptane and isopropanol are much more effective than water vapor
in terms of increasing the lubricity of graphite. The beneficial effect of condensable vapors on the lubricity
of graphite has been attributed to the saturation in its lattice of π-electrons, which otherwise make atomic
layers slide with difficulty. Figure 22.7 shows the relationship between wear rate and water vapor pressure
for graphite.
Graphite can provide lubrication up to about 500°C in open air, although friction tends to increase
as the temperature rises. At higher ambient temperatures, it begins to oxidize and lose its lubricity. In
vacuum, the friction coefficient is initially high (i.e., 0.4), but decreases to about 0.2 at 1300°C. In most
sliding experiments, thin transfer films are formed on the surfaces of sliding counterfaces. These transfer
layers are thought to be important for achieving longer wear life and possibly even lower friction. When
small amounts of sodium thiosulfate (Na2S2O3) or sodium molybdate (Na2MoO4) were added to graphite
to improve the transfer film forming behavior, researchers observed longer wear life and lower friction
against sliding steel counterfaces (Langlade et al., 1994). During these tests, transformations of the
graphite structure to a turbostatic phase was observed as a thin layer by means of electron microscopy
and X-ray diffraction.
Graphite is inexpensive and readily available in various forms. It is resistant to both acids and bases.
In practice, graphite is used in powder, colloidal dispersion, solid, and composite forms to combat friction
and wear. It is a key ingredient of electrical brushes used in many motors. It can be dispersed in water,
solvents, oils, and greases to achieve better lubricity under extreme application conditions, such as
lubrication of molds and dies in metal-forming, as well as flange faces of rails and railcar wheels. Graphite
is also used as a self-lubricating filler in various metal-, ceramic-, and polymer-matrix composites
(Rohatgi et al., 1992; Prasad and McConnell, 1991; Gangopadhyay and Jahanmir, 1991). Carbon-graphite
composites are rather common and widely used in various engine, aircraft, and seal applications.
Graphite fluoride is prepared by fluorinating graphite at stoichiometries from X = 0.3 to 1.1 in CFx .
It is prepared by direct reaction of graphite with fluorine gas at controlled temperatures and pressures
and can be regarded as an intercalation compound of graphite. Fluorination increases the distance
between atomic planes from about 0.34 nm to as high as 0.8 nm and, hence, results in easier shear and
better lubricity (Fusaro and Sliney, 1970). It also causes basal planes of graphite to distort and lose their
planar configuration. CFx is electrically insulating and nonwettable with water, but decomposes at about
450°C.
Fluorination of graphite was shown by Fusaro and Sliney (1970) to substantially improve the lubricity
and durability of this solid and make it less sensitive to variations in ambient humidity. Earlier studies
indicated that burnished CFx was capable of providing friction coefficients of 0.1 or less up to about
480°C in open air. Compared to those of MoS2 and even HOPG under the same test conditions, such
friction values were considerably lower. It is possible to prepare composite structures and resin-bonded
films of CFx in order to achieve longer life; however, due to its high cost, CFx is rarely used by industry.
In a recent study, CFx was used as an additive to WS2 thin films to reduce their sensitivity to moisture
(Zabinski et al., 1995). These films were produced on AISI 440C steel substrates by a pulsed laser
deposition method. Substrate temperature and CFx concentration were varied to control film microstructure and chemistry. Tribological tests were conducted over a wide range of relative humidities (i.e.,
<1 to 85% RH). Coatings with a low concentration of CFx exhibited ultra-low friction in dry air (friction
coefficients <0.01), but the coefficients increased with increasing relative humidity. Films grown at
elevated temperatures (300°C) or with higher concentrations of CFx showed insensitivity to humidity,
but the friction coefficients were relatively high (0.04 in dry air).
22.2.1.3.1 Modern Practices
Graphitic lubricious precursors can also result from catalytic cracking of certain carbon-bearing gases
and can be used to lubricate surfaces, especially at high temperatures (Ashley, 1992; Lauer and Bunting,
1988; Blanchet et al., 1994). This is done by injecting a stream of hydrocarbon-bearing gases into the test
chamber where hot ceramic or metal surfaces are maintained and slid against one another. The hydrocarbons in the gas turn into a thin coating of graphite-like carbon that is responsible for lubrication.
In recent years, a few attempts were made to lubricate sliding surfaces by other carbon forms, such as
bucky-balls (C60) (Bhushan et al., 1993) and hollow nanotubes of carbon (Falvo, 1999). In addition to
the powder form, sublimation or thermal evaporation methods were used to deposit C60 as strongly
bonded and dense films on metallic and ceramic substrates. Depending on the form, density, and adhesion
of these films to their substrates, friction coefficients of 0.15 to 0.5 were obtained. Ion irradiation of such
films with 2 MeV Ag+ and B+ ions at various doses resulted in partly crystalline to amorphous films that
were able to provide friction coefficients of <0.1 (Bhattacharya et al., 1996).
Recently, a series of new boron-doped and partially graphitized carbon composites were developed to
achieve better lubricity at elevated temperatures. Long-duration friction and wear tests were run as a
function of both increasing and decreasing temperatures to assess the durability and the friction and
wear performance of the composites. As shown in Figure 22.8, the friction coefficients of the borondoped carbon composite against a ceramic counterface were in the range of 0.05 to 0.1 at temperatures
up to 500°C. Based on analytical studies, it was concluded that the boron doping was essential for
achieving higher oxidation resistance on these graphitic materials.
Vitreous or glassy carbon materials are made by pyrolysis of thermosetting polymers. Structurally,
they are different from graphite, but the interatomic bonding and local arrangement at nanoscale are
more graphitic than diamond. They are extremely hard and, hence, more wear resistant than graphite,
which is very soft. Just like graphite, the friction coefficient of glassy carbon shows high sensitivity to
relative humidity of the test environment. Glassy carbon materials have very low fracture toughness, but
can be reinforced with metallic/nonmetallic fibers to achieve improved toughness. Copper-containing
glassy carbon composites have high electrical conductivity and can be used for electrical contacts or
brushes (Burton and Burton, 1989).
FIGURE 22.8
Friction performance of boron-doped carbon composite at temperatures to 525°C.
Quite recently, researchers have produced highly disordered graphitic carbon layers on silicon carbide
by reaction with chlorine and chlorine/hydrogen gas mixtures at 1000°C. The thickness of the graphitic
layer can vary from a few to 100 µm, depending on process time. When such a graphitized surface is
subjected to sliding friction tests, very low (0.1 to 0.15) friction coefficients are achieved (Gogotsi et al.,
1997). It is possible that such graphitic layers on rigid SiC substrates can be used to control friction and
wear of microelectromechanical systems, sliding bearings (e.g., mechanical seals), electrical contacts, and
biomedical implants.
22.2.1.4 Hexagonal Boron Nitride (HBN)
HBN is a synthetic solid lubricant with high refractory and lubricity qualities at elevated temperatures.
Below 1000°C, oxidation is negligible. It is chemically inert and resists attack by molten metals, oxides,
glasses, slags, and fused salts. Its crystal structure is similar to that of graphite as shown in Figure 22.1.
The atomic planes are made of two-dimensional arrays of boron and nitrogen atoms, configured in a
honeycomb pattern (Rowe, 1960). As in graphite, the bonding between the atoms of HBN in each layer
is covalent and very strong, while bonding between the layers is of the weak van der Waals type.
HBN is typically produced by reacting B2O3 with urea or ammonia gases at high temperatures. Unlike
graphite, HBN has a white color. Its cubic analog (i.e., cubic boron nitride) is like diamond and is
extremely hard and resistant to wear. HBN is generally produced in powder form. Depending on manufacturing conditions, different grades (i.e., turbostatic, quasi-turbostatic, meso-graphitic, and graphitic)
of HBN are obtained. In terms of lubrication performance, the graphitic grade provides the best results.
Purity and powder size of final products can also affect lubrication performance. The presence of boron
oxide in the structure or as a binder makes a significant difference in the tribological performance of HBN.
HBN can be compacted into dense, solid pieces or parts by hot-pressing and can also be prepared as
a composite structure. It can be plasma-sprayed with other ceramics to obtain a self-lubricating coating.
Recently, very fine particles of HBN were incorporated into electroplated Ni coatings to provide superior
friction and wear properties under unlubricated and high-load, high-temperature sliding conditions
(Funatani and Kurosawa, 1994; Pushpavanam and Natarajan, 1995). HBN can also be used as a selflubricating phase in ceramic composites. Such materials will be very attractive for mechanical face seal
applications. In a recent study, Westergard et al. (1998) investigated the tribological performance of
Si3N4 /SiC composites containing 0 to 8 wt% HBN. All specimens were produced by hot isostatic pressing.
The results indicated that the presence of HBN in the composite body lowered the friction coefficients
of test pairs from a range of 0.4 to 0.9 to a range of 0.02 to 0.1. Analytical studies revealed that sliding
surfaces were covered by a thin, well-adhering tribofilm, which may have been responsible for the
improved tribological performance.
HBN has also been used as an additive in oils and greases. Recent tests by Kimura et al. (1999) showed
that addition of HBN in concentrations as little as 1 wt% results in an order of magnitude reduction in
TABLE 22.3 Effect of Various Gases at Various Pressures on Friction
Behavior of HBN Sliding Against Itself
Environment
UHV, 10–8 Pa
CO, C3H8, H2O, air (50% RH); 10–3 Pa
CO, N2, O2; 10 Pa
Air (50% RH); 10 Pa
C3H8; 10 Pa
Air (50% RH); 105 Pa
Air (50% RH), atmospheric pressure
Steady-state Friction Coefficient
0.6–0.7
0.6–0.7
0.6–0.7
0.4
0.4
0.2
0.1
Data from Martin, J.M., LeMogne, T., Chassagnette, C., and Gardos, M.N.
(1992), Friction of hexagonal boron nitride in various environments, Tribol.
Trans., 35, 463-472.
wear of bearing steels sliding against each other in line contacts. At higher concentrations, the reduction
in wear is even greater. Similar improvements in antiwear and antifriction properties were reported for
HBN-containing greases by Denton and Fang (1995).
HBN has high thermal and chemical stability and does not appreciably oxidize up to about 1000°C.
Typical friction coefficients of HBN in air are 0.2 to 0.3 up to about 700°C. It has been used as a release
agent in metalworking operations involving high temperatures (Golubus, 1970). In high vacuum, HBN
loses its lubricity. Buckley (1978) reported a friction coefficient of 1 for HBN sliding against itself in
ultrahigh vacuum. Much earlier tests by Deacon and Goodman (1958), Rowe (1960), and Haltner (1966)
gave friction coefficients of 0.4 to 0.7 in high vacuum in the outgassed states. Admission of certain organic
vapors into the test chamber reduced friction coefficients to the 0.2 level. Recent fundamental studies by
Martin et al. (1992) in ultrahigh vacuum (10–8 Pa) and under partial pressures of CO, C3H8 , H2O, air
with 50% humidity, N2, and O2 resulted in friction coefficients of 0.1 to 0.7; Table 22.3 summarizes their
experimental results. These experiments further reinforced the initial assertion that HBN is not only
similar to graphite in its crystal structure, but also in its lubrication behavior (Rowe, 1960; Rabinowicz,
1964). Thus, one can understand why HBN is often referred to as “white graphite.”
22.2.1.5 Boric Acid
Boric acid is a lamellar solid lubricant (Figure 22.9) with a crystal structure similar to those of graphite
and HBN (Erdemir, 1991). It has a triclinic unit cell in which boron, oxygen, and hydrogen atoms are
arrayed to form extensive atomic layers parallel to the basal plane of the crystal (see Figure 22.1). Because
of the triclinic crystal structure, the c-axis is inclined to the basal plane at an angle of 101° (see Figure 22.1).
This inclination causes shifting of alternate layers along the c-axis. Bonding between the atoms lying on
FIGURE 22.9
SEM photomicrograph of lamellar structure of boric acid.
0.09
0.07
µ
0.05
0.03
0.01
1N
SAPPHIRE BALL
2N
5N
ALUMINA BALL
10N
STEEL BALL
FIGURE 22.10 Variation of friction coefficient of boric acid films under different loads during sliding against steel
and ceramic balls. (Adapted from Erdemir, A. (1991), Tribological properties of boric acid and boric-acid-forming
surfaces. I. Crystal chemistry and mechanism of self-lubrication of boric acid, Lubr. Eng., 47, 168-172.)
the same plane is of the covalent/ionic and hydrogen type; the layers are 0.318 nm apart and held together
only by weak van der Waals forces.
Boric acid exists in two major crystalline forms: metaboric acid (H2O·B2O3 or HBO2) and orthoboric
acid (3H2O·B2O3 or H3BO3). Furthermore, metaboric acid has been reported to crystallize in three
different forms: orthorhombic or α-metaboric acid, monoclinic or β-metaboric acid, and cubic or
Γ-metaboric acid. Among these, orthoboric and orthorhombic metaboric acids exhibit layered-crystal
structures and thus can provide low friction. Orthoboric acid exists as a natural mineral known in
mineralogy books as sassolite. It is stable up to about 170°C.
Due to its layered-crystal structure, H3BO3 is a self-lubricating solid. To demonstrate its lubricity,
Erdemir (1991) performed extensive friction tests with solid compacts of H3BO3 on a pin-on-disk
machine. Cylindrical rods with a nominal diameter of 1.27 cm were compacted from 99.8 wt% H3BO3
powders by cold-pressing at about 35 MPa. To establish point contact during friction tests, one end of
the rod-shaped compacts was finished with a hemispherical cap of 5-cm radius. Subsequently, the boric
acid pin was attached to the pin holder of a pin-on-disk machine and rubbed against a 50-cm-diameter
AISI 52100 steel disk.
The friction coefficient of the pin/disk pair described above was measured as a function of sliding
distance. The initial friction coefficient of this tribosystem was approximately 0.2; it then decreased
steadily with distance and eventually reached a steady-state value of 0.1 after sliding about 20 m.
H3BO3 can spontaneously form on the surfaces of boron and B2O3 films. Erdemir et al. (1990b)
investigated the formation and tribological characteristics of such boric acid films formed on the surfaces
of vacuum-evaporated B2O3 layers. They found that H3BO3, which formed spontaneously on the surfaces
of B2O3 coatings, is remarkably lubricious. For a sliding pair of sapphire ball/B2O3-coated Al2O3 disk,
they reported friction coefficients ranging from 0.02 to 0.05 in open air with 50% relative humidity,
depending on applied force. Figure 22.10 presents the friction coefficients of various balls sliding against
a B2O3-coated Al2O3 disk under different contact loads. The use of a harder, more rigid ball (e.g., sapphire)
results in a lower friction coefficient because the true contact area between a hard, rigid ball will be
smaller than that between a soft, less-rigid ball. Friction force, which is a product of the true contact
area multiplied by the shear strength of the contact interface, will be much lower when hard, rigid balls
are used in sliding contact.
Based on surface and structure analytical studies, it was concluded that low friction is a direct consequence of the layered crystal structure of H3BO3 films forming on the exposed surfaces of the B2O3 coating
by the spontaneous chemical reaction:
(
)
(
)
1 2 B2O 3 coating + 3 2 H2O moisture → H 3BO3
∆H298 = −45.1 kJ mol
The above reaction occurs naturally, and a thin layer of H3BO3 forms everywhere on the exposed surface
of B2O3 coatings. Formation of such self-lubricating and self-replenishing films was later demonstrated
on VB2, B4C, and borided steel surfaces, affording very low friction coefficients to sliding metal and
ceramic surfaces (Erdemir et al., 1991, 1996b,d, 1998, 1999; Bindal and Erdemir, 1996).
As described previously, H3BO3 crystallizes in a triclinic crystal structure essentially made of atomic
layers parallel to the basal plane. The atoms lying on each layer are closely packed and strongly bonded
to each other. The bonds between the boron and oxygen atoms are mostly covalent, with some ionic
character. Hydrogen bonds strongly hold the planar boron/oxygen groups together. The atomic layers
are widely spaced (e.g., 0.318 nm apart) and held together by weak forces (e.g., van der Waals). Because
of the ionic character of interatomic bonds, boric acid can dissolve in water and some other solvents.
With its layered crystal structure, H3BO3 resembles other layered solids well-known for their good
lubrication capabilities (e.g., MoS2, graphite, and HBN).
Erdemir (1990) proposed that under shear stresses, plate-like crystallites of H3BO3 can align themselves
parallel to the direction of relative motion. Once so aligned, they can slide over one another with relative
ease and thus impart the low friction coefficients shown in Figure 22.10.
Boric acid films were shown to bond strongly to the surface of aluminum and its alloys and provide
excellent lubricity when used as a metal-forming lubricant (Erdemir and Fenske, 1998). Used as a filler
in polymers, boric acid and boron oxide can substantially lower friction and increase the wear resistance
of base polymers (Figure 22.11) (Erdemir, 1995). It was demonstrated that sub-micrometer size powders
of boric acid can be dispersed in oils and greases to impart better lubricity and extreme pressure capability
(Erdemir, 1995, 2000b).
For applications at elevated temperatures, the use of H3BO3 is not recommended for several reasons.
First, above about 170°C, H3BO3 tends to decompose and eventually turn into B2O3, thus losing its layered
crystal structure and hence its lubricity. Second, at temperatures greater than about 450°C, B2O3 becomes
liquid-like and tends to react with underlying substrates. For metals, the chemical reaction is negligible,
FIGURE 22.11
Friction performance of polyimide and boron oxide-filled polyimide.
L
INTRAFILM FLOW
V
L
INTERFACE SLIDING
V
L
INTERFILM SLIDING
V
FIGURE 22.12 Schematic representation of three ways by which sliding can be accommodated between an uncoated
and a coated surface. (From Singer, I.L. (1992), Solid Lubrication Processes, in Fundamentals of Friction: Macroscopic
and Microscopic Processes, Singer, I.L. and Pollock, H.M. (Eds.), NATO-ASI Series, Vol. 220, Kluwer Academic, London,
237-261. With permission.)
and low friction can be reinstated by viscous-flow lubrication. However, for ceramics — especially for
the oxides — the situation is different. Liquid B2O3 can react with these ceramics and lead to high
corrosive wear. Friction can also be very high because of the highly viscous nature of the reaction products.
22.3 Lubrication Mechanisms of Layered Solids
In general, the lubricity and durability of a solid lubricant are controlled by a mechanism that involves
interfilm sliding, intrafilm flow, and film/substrate or interface slip, as illustrated in Figure 22.12. It has
been found that the lamellar solid lubricants discussed above provide lubrication by an interlayer shear
mechanism, mainly because the crystal structures of these solids are such that while the atoms lying on
the same layer are closely packed and strongly bonded to each other, the layers themselves are relatively
far apart and the forces that bond them (e.g., van der Waals) are weak (see Figure 22.1). Strong interatomic
bonding and packing in each layer give these solids the very high in-plane strength that is essential for
longer wear life or reduced wear damage during sliding. When present on a sliding surface, crystalline
layers of these solids align themselves parallel to the direction of relative motion and slide over one
another with relative ease to provide lubrication. Furthermore, the formation of a smooth transfer film
on the sliding surfaces of counterface materials is also important for long wear life and the accommodation
of sliding velocity, as well as for dissipation of frictional energy.
Recent electron microscopy studies of H3BO3-lubricated rubbing surfaces of steel test pairs clearly
revealed some plate-like crystallites exhibiting a preferred alignment parallel to the sliding direction
(Figure 22.13). Similar observations were made by TEM on sputtered MoS2 films after sliding tests. Several
other studies used X-ray diffraction to further verify that indeed some crystalline orientation occurs on
most lamellar solids during sliding tests (Wahl et al., 1995, 1999; Martin et al., 1994; Moser and Levy,
1993). Crystalline layers can be made of single or several atomic planes. For example, in graphite, H3BO3 ,
and HBN, the layers are made of a single atomic plane; while in MoS2 and other transition-metal
dichalcogenides, the layers consist of three atomic planes (see Figure 22.1); in monochalcogenides, there
are four atomic planes in each layer (see Figure 22.6).
FIGURE 22.13
cated surface.
Physical evidence for preferred crystalline orientation and intercrystallite slip on boric acid-lubri-
The electronic states of atoms in each layer play a major role in the lubricity of each solid. Graphite
and HBN are similar in electronic states. The only major difference between the bonding configuration
of these two solid lubricants is that the π-bonding and π-antibonding bands overlap weakly at the
Brillouin-zone boundary in graphite, which is responsible for the fairly good electrical conductivity of
graphite; whereas these bands are separated by an energy gap of several electron-volts in stoichiometric
BN, causing it to be an insulator. In both cases, if residual π attractions between atomic layers are not
eliminated or reduced, high friction and wear may result. In graphite, π-bond interactions are generally
reduced or eliminated by intercalation. Both donor (e.g., alkali metals) and acceptor (metal chlorides)
type intercalants can be used for this purpose (Levy, 1979; Dresselhaus, 1996). As a result, the interlayer
shear properties of graphite are markedly improved. However, attempts to find effective intercalation
species for HBN were mostly unsuccessful.
Within the layered-crystal structure of transition-metal dichalcogenides, metal atoms are sandwiched
between the chalcogen atoms in a planar array of S-Mo-S, while the layers in monochalcogenides consist
of four atomic layers. For example, each layer of GaSe consists of strongly bonded Ga and Se atoms in
the sequence Se-Ga-Ga-Se (see Figure 22.6). The Ga atoms are paired to form the two atomic planes
inside, while the chalcogen atoms form the top and bottom planes. Note that the removal of one layer
of Ga atoms in the GaSe crystal structure would have produced the exact crystal structure of MoS2, as
illustrated in Figure 22.1(c); this suggests that mono- and dichalcogenides are indeed closely related. The
interatomic bonding within the layers of mono- and dichalcogenides is strong and mainly of the covalent
type, whereas the bonding between adjacent layers is weak and of the van der Waals type. Within the
rigid two-dimensional layers of MoS2 crystals, the S atoms have a trigonal prismatic coordination around
the Mo atoms, while the Ga atoms in the GaSe structure are tetrahedrally coordinated.
Previous studies have clearly demonstrated that the lubricity of a solid lubricant is controlled by a
number of intrinsic and extrinsic parameters. For example, intrinsically, both graphite and MoS2 have
layered crystal structures, but the extent of their lubricity and durability is largely controlled by extrinsic
factors such as the presence or absence of vapors or gaseous species in the test environment. Graphite
functions best in humid air, while MoS2 lubricates best in dry and vacuum environments. As mentioned
earlier, the lubrication behavior of HBN and H3BO3 is similar to that of graphite. This contrasting
behavior of self-lubricating solids has been — and still is — the subject of numerous fundamental studies.
Thus far, suggestions have been made that the enhanced lubricity of graphite and HBN in a humid
environment may be related to the weakening effect of water molecules on the residual π-bonds between
the layers of their crystals. As for the poor lubricity of MoS2 in a humid environment, it has been suggested
that water molecules react directly or indirectly with MoS2 and thus alter the interatomic array and
bonding, which in turn increase friction. When tested in open air, MoS2 and other dichalcogenides were
found to react with oxygen and form MoO3 or other types of complex oxides. It has been speculated
that the chemical reactions leading to MoO3 formation occur predominantly at the prismatic edges where
reactive dangling bonds exist.
Another intrinsic parameter that can affect the lubricity of a layered solid is the interlayer-to-intralayer
bond-length ratio. It has been reported that this ratio is a crude but revealing indicator of the lubricity
of a lamellar solid (Jamison, 1972, 1978). In general, it was found that the greater the interlayer-tointralayer bond-length ratio, the weaker the interlayer bonding with respect to the intralayer bonds, and
thus the higher the lubricity. For MoS2 and GaSe crystals, the interlayer-to-intralayer bond-length ratios
are 1.5 and 1.6, respectively (Zallen and Slade, 1974).
The presence or absence of electrostatic attractions between the layers of mono- and dichalcogenides
constitutes yet another intrinsic parameter that can affect the lubricity of these solids. For example, of
the numerous metal dichalcogenides, only a few can impart low friction to sliding tribological interfaces.
Previous research has shown that despite their layered crystal structures, NbS2, TiS2, VS2, TaS2, etc., are
not as lubricious as MoS2 or WS2 (Clauss, 1972; Jamison, 1978). Based on the molecular orbital and
valence bond theories, Jamison (1972, 1978) proposed the following explanation for the poor lubricating
performance of NbS2, TiS2, VS2, TaS2, etc. In the layered crystal structure of these solids, there is a region
of negative electrical charge that not only concentrates above the chalcogen atoms of a given layer, but
also extends well into the pockets between the chalcogen atoms of neighboring layers. Because the bottoms
of the pockets are positively charged (due to the exposed ion cores of the surrounding atoms), an
electrostatic attraction exists between the layers, making the layers of these solids shear with difficulty.
As for the excellent solid-lubricating capacities of MoS2 and WS2, the region of negative electrical
charge is contained within the layers. Thus, the surfaces of the chalcogen atoms are positively charged,
creating an electrostatic repulsion between the layers and making interlayer slippage exceedingly easy
(Jamison, 1978). Relatively greater interlayer separation in MoS2 and WS2 crystals is thought to result
from the same electrostatic repulsion between successive layers.
In general, previous studies have clearly demonstrated that the friction and wear performance of solid
lubricants are strongly affected by both the intrinsic (crystal-specific) and extrinsic (operating-environment-specific) factors. Therefore, no single solid lubricant can provide low friction and wear in all
environments. Furthermore, not all layered solids are good solid lubricants. The type and magnitude of
interlayer bonds are also important.
22.4 High-Temperature Solid Lubricants
For applications in open air and at temperatures above 500°C, most of the lamellar solids mentioned
above lose their lubricity and become useless. Furthermore, most sliding interfaces (including metals
and non-oxide ceramics) become oxidized (Quinn and Winer, 1985). Thin oxide films that form on the
sliding surfaces may, in turn, dominate the friction and wear behavior of these interfaces. In particular,
wear debris particles trapped at sliding interfaces could be very abrasive and cause high wear. If the sliding
bodies differ chemically or if there is a third or fourth body at the sliding interface, two or more oxides
may form on the sliding surface and control friction and wear. In past years, significant research has been
carried out to study the shear rheology of such oxides and to formulate alloys or composite structures
that can lead to the formation of oxides with very low shear strength (Peterson et al., 1994). These are
often referred to as lubricious oxides.
22.4.1 Lubricious Oxides, Fluorides, and Sulfates
Certain oxides (e.g., Re2O7, MoO3, PbO, B2O3, NiO, etc.), fluorides (e.g., CaF2, BaF2, SrF2, LiF, and MgF2),
and sulfates (e.g., CaSO4, BaSO4, and SrSO4) become soft and highly shearable at elevated temperatures
and hence can be used as lubricants (Sliney et al., 1965; Sliney, 1969; John and Zabinski, 1999). When
applied as thin or thick coatings (by means of PVD, plasma spraying, fusion bonding, etc.), these solids
0.6
Friction Coefficient
25 C
0.5
300 C
0.4
0.3
0.2
600 C
0.1
1.3 kg
0
0
1000
2000
3000
Sliding Cycle
FIGURE 22.14 Effect of test temperature on friction coefficient of an ion-beam-deposited Cu-Mo film; lower
friction coefficients at high temperatures are attributed to formation of CuO-MoO3 films. (From Wahl, K.J., Seitzman,
L.E., Bolster, R.N., Singer I.L., and Peterson, M.B. (1997), Ion-beam deposited Cu-Mo coatings as high temperature
solid lubricants, Surf. Coat. Technol., 89, 245-251. With permission.)
can provide acceptable levels of friction coefficients and long wear life. They can also be mixed with other
solid lubricants to obtain lubrication over much wider temperature ranges. Major drawbacks of oxidebased lubricants are that they are inherently brittle and thus may fracture easily and wear out quickly.
Furthermore, most oxide-based lubricants do not provide lubrication down to room temperature. Potential applications for lubricious oxides include high-temperature seals, bearings, and gears, valves and
valve seats, variable stator vanes, and foil bearings.
Recent systematic studies have demonstrated that the oxides of Re, Ti, Ni, W, Mo, Zn, V, B, etc., become
highly lubricious and can provide fairly low friction at elevated temperatures (Kanakia et al., 1984;
Kanakia and Peterson, 1987; Peterson et al., 1960, 1982, 1994). Mixed oxides (e.g., CuO-Re2O7, CuOMoO3, PbO-B2O3, PbO-MoO3, CoO-MoO3, Cs2O-MoO3, NiO-MoO3) can also provide wider operational
ranges and can be prepared as alloys or composite structures to provide longer durability. The lubricious
layers that form by oxidation of alloy surfaces are very desirable and exceptionally advantageous when
compared with the solid lubricant coatings with finite lifetimes. At high temperatures, as the oxide layer
is depleted from the surface by wear, the alloying ingredients diffuse toward the surface where the oxygen
potential is higher; they oxidize again to replenish the consumed lubricious layers that have low shear
strength and/or surface energy to decrease friction (Peterson et al., 1982, 1994). Figure 22.14 shows the
frictional performance of CuO-MoO3 at different temperatures (Wahl et al., 1997).
In a series of fundamental studies, Gardos (1988) demonstrated that at a very narrow range of anion
vacancies and at high temperatures, crystalline TiO2 (rutile) and rutile-forming surfaces can provide very
low friction coefficients to sliding tribological interfaces. Further work by Gardos (1993) and Woydt et al.
(1999) demonstrated the formation of Magneli phases on sliding surfaces containing titanium-based
alloys and compounds. Their findings suggested that Magneli phases are principally the result of tribooxidation and that once formed, they can dominate the tribological behavior of sliding ceramic interfaces,
mainly because of their unique shear properties. However, TiOx-based solid lubricants have not yet found
wide use, mainly because of the difficulty in achieving and maintaining the very narrow range of oxide
stoichiometry needed for good lubricity.
A new breed of lubricious zinc oxide films was recently synthesized by pulsed-laser deposition, and
their tribological properties were explored over a wide range of test conditions (Zabinski et al., 1997).
The stoichiometry and microstructure of these films were found to have profound effects on lubricity
and were controlled by adjusting substrate temperature and oxygen partial pressure during deposition.
Zinc oxide films with oxygen deficiency and nanoscale structure were found to provide low friction
coefficients and long wear lives at room temperature. However, as the chemical stoichiometry and crystal
structure approached those of the bulk zinc oxide, the tribological properties and load/speed sensitivity
Cycles
1.00
PLD ZnO Film
0.80
One-Million-Cycle Test
0.60
µ
0.40
µ=0.2
0.20
Cycles
1x106
FIGURE 22.15 Variation of friction coefficient of pulsed-laser deposited ZnO film with number of sliding cycles.
(From Zabinski, J.S., Saunders, J.H., Nainaparampil, J., and Prasad, S.V. (2000), Lubrication using a microstructurally
engineered oxide: performance and mechanisms, Tribol. Lett., 103-116. With permission.)
of the films degraded. Figure 22.15 shows the variation of friction coefficient of a pulsed-laser-deposited
ZnO film during sliding against an AISI 440C steel ball.
22.4.2 Composites
Plasma-sprayed self-lubricating composites and adaptive lubricants were recently engineered to combat
friction and wear problems at high temperatures. The composite coatings consist of silver and alkaline
halides (i.e., CaF2, BaF2) as the self-lubricating entities and chrome carbide and/or oxide as the wearresisting entities (DellaCorte and Sliney, 1987, 1990; Sliney, 1993; DellaCorte and Fellenstein, 1997).
Thick plasma sprayed coatings (0.1 to 0.2 mm) and bulk powder metallurgy composite forms of these
solid lubricants provide friction coefficients ranging from 0.2 to 0.5, depending on ambient temperature,
load, and speed. Over the years, these solid lubricants have been highly optimized and carefully formulated
and the latest formulations are capable of providing lubrication over much broader temperature ranges
than their earlier versions. Figure 22.16 shows the friction performance of PS-304 (consisting of 20 wt%
Cr2O3, 10 wt% Ag, 10 wt% BaF2/CaF2 eutectic composition, and NiCr as the binder) against an alumina
ball at temperatures up to 870°C. Recent studies have also demonstrated that these lubricants are very
FIGURE 22.16
Friction performance of PS-304 self-lubricating composite coating at temperatures to 870°C.
suitable for high-speed sliding bearing surfaces and provide excellent durability and frictional performance, especially when used in foil bearing applications (DellaCorte, 1998).
To achieve low friction from room temperature to very high temperature, a series of adaptive solid
lubricants has recently been developed. A good adaptive lubricant is made of several ingredients that
provide low friction at low temperatures, and as the temperature increases, these lubricious ingredients
react with each other and/or oxygen in air to form a high-temperature solid lubricant phase providing
low friction. The lubricating entities in this case were selected from those metals that can react with the
environment to form the kind of lubricious layers needed (Wlack et al., 1997; Zabinski et al., 1992). One
problem is that the oxidation is not reversible, so when the temperature returns to low values, the friction
may increase. To solve this problem, researchers have used very thin diffusion-barrier layers to limit the
extent of oxidation to the very top surface rather than to the bulk or over a thick layer. Another approach
was to use capsules of high-temperature adaptive lubricants in a low-temperature matrix. While the lowtemperature matrix provides lubricity at lower temperatures, the capsules with a protective shell on the
surface react with oxygen and become lubricious, thus providing the needed level of lubricity.
22.4.3 New Approaches to Solid Lubrication at High Temperatures
Recently, a crystal-chemical approach was introduced by Erdemir (2000a) to classify lubricious oxides
on the basis of lubrication performance and operational limits. This approach was proposed to serve as
a guide for determining the kind(s) of lubricious oxides needed on a sliding surface at high temperatures.
Apparently, the crystal chemistry of certain oxides that form on sliding surfaces relates strongly to their
shear rheology and hence their lubricity at high temperatures.
The principle of the crystal-chemical approach is based essentially on the ionic potential of an oxide
and is defined as γ = Z/r, where Z is the cationic charge and r is the radius of the cation. Erdemir (1999)
proposed that using this principle, one can establish model relationship(s) between the quantum-chemical characteristics and the lubricity of oxides at high temperatures. Specifically, it is possible to establish
a correlation between the ionic potential or the cationic field strength of an oxide and its shear rheology,
and hence its lubricity.
Apparently, ionic potential controls several key physical and chemical phenomena in oxides. In general,
the higher the ionic potential, the greater the extent of screening of a cation in an oxide by surrounding
anions such as B2O3 or Re2O7. Oxides with highly screened cations are generally soft and their melting
points are low. Their cations are well-separated and completely screened by anions; hence, they have
little or no chemical interaction with other cations in the system. Most of their bonding is with surrounding anions. Conversely, oxides with lower cationic field strengths or ionic potentials (e.g., Al2O3,
ZrO2, MgO, and ThO2) are very strong, stiff, and difficult to shear, even at high temperatures, because
their cations interact with each other and form strong bonds.
The crystal-chemical approach can be used to predict the extent of adhesive interactions between two
or more oxides at a sliding interface; hence, it can be used to predict frictional performance. Extensive
research by previous investigators has already identified several lubricious oxides that afford fairly low
(≈0.2) friction coefficients at elevated temperatures (Kanakia et al., 1984; Kanakia and Peterson, 1987;
Peterson et al., 1960, 1982, 1994). Some of these oxides and their friction coefficients at high temperatures
are shown in Figure 22.17. As can be deduced from this figure, the higher the ionic potential, the lower
the friction coefficient. This means that oxides with higher ionic potentials appear to shear more easily
and thus exhibit lower friction at high temperatures. As mentioned earlier, the higher the ionic potential,
the greater the screening of a cation in an oxide by surrounding anions. The highly screened cations in
an oxide will interact very little with other cations in their surroundings, and this will allow them to
shear more easily at elevated temperatures.
In most tribological situations, two or more dissimilar solid bodies may be rubbing against each other,
and often the sliding surfaces are covered by more than one kind of oxide. The crystal-chemical approach
introduced in this chapter can also be used to predict the lubricity of such complex binary oxide systems.
Specifically, crystal chemistry can be used to estimate the solubility, chemical reactivity, number of
FIGURE 22.17 Relationship between ionic potentials and friction coefficients of single oxides. (Adapted from
Erdemir, A. (2000a), A crystal chemical approach to lubrication by solid oxides, Tribol. Lett., 8, 97-102.)
compounds formed, and eutectic temperature or lowering of the melting point of an oxide when a second
oxide is present. For example, the eutectic temperature and compound-forming tendencies of two oxides
are closely related to the cationic field strengths or ionic potentials of the involved elemental species. The
ability of an oxide to dissolve in or react with other oxides or to form complex oxides is estimated from
the difference in relative ionic potentials of the oxides in the system. In general, the greater the difference
in ionic potential, the lower the eutectic temperature and the greater the tendency to form complex oxides.
Figure 22.18 shows several cases in which two oxides (CuO-Re2O7, CuO-MoO3, PbO-B2O3, PbO-MoO3,
CoO-MoO3, and NiO-MoO3, etc.) were either present at or purposely introduced to the sliding interfaces
to achieve low friction at high temperatures. Most of these data were extracted from papers and progress
reports authored by Peterson et al. (Peterson, 1987; Peterson et al., 1960, 1982, 1994), who have had
extensive experience with lubricious oxides. Nickel-based superalloys, because of their relevance to hightemperature applications, were used as substrates in most of their studies. The specially formulated nickel
alloys contained Ti, Ta, W, Re, B, and Mo as potential lubricious oxide formers. Note that the scatter in
the friction values shown in Figures 22.17 and 22.18 is large; this is not unusual in the field of tribology
because test machines, conditions, or parameters vary greatly from study to study.
Recently, Cs-based oxides were reported to be very promising for lubricating Si-based ceramic components at high temperatures. At 600°C, 0.02 to 0.1 friction coefficients have been reported for Cs2Olubricated Si3N4 ceramics (Strong and Zabinski, 1999). During sliding at high temperature, a mixed oxide
layer consisting of Cs2O and SiO2 was found and believed to be responsible for low friction. As can be
seen from Figure 22.18, such a combination would result in a large difference in the ionic potentials of
these two oxides.
From Figure 22.18, it can be seen that as the difference in ionic potential increases, the lubricity of the
oxide species also increases. There are two fundamental reasons for this phenomenon. One is that as the
difference in ionic potential increases, the ability of oxides to form a low-melting-point or readily
shearable compound improves; hence, oxides tend to exhibit lower hardness and shear strength at elevated
temperatures because the anions are able to better shield or screen the cations and thus make them less
likely to interact with neighboring cations. The second reason for the phenomenon is that the ability or
affinity of ionic species to form highly stable compounds (that exert very little chemical or electrostatic
FIGURE 22.18 Relationship between friction coefficient and difference in ionic potentials of double oxides. (From
Erdemir, A. (1999), A crystal chemical approach to lubrication by solid oxides, Tribol. Lett., 8, 97-102.)
attraction) improves as the difference in ionic potential increases. Lower attraction between sliding
surfaces means lower adhesive forces across the sliding contact interfaces, and hence lower friction
(Erdemir, 2000a).
22.5 Self-Lubricating Composites
22.5.1 Traditional Materials
Self-lubricating composites have been available for a long time and are used rather extensively by industry
to combat friction and wear in a variety of sliding, rolling, and rotating bearing applications. They are
generally prepared by dispersing appropriate amounts of a self-lubricating solid (as fillers, preferably in
powder form) with a polymer, metal, or ceramic matrix. With powder metallurgy techniques, fillers and
matrix materials can be thoroughly mixed, compacted, and then sintered (if necessary) to obtain the
desired shape. They can also be extruded, rolled, or hot/cold-pressed into useful shapes. Recently,
compositionally and functionally gradient self-lubricating composite structures were also manufactured
and offered for industrial use. While the core is made of nearly pure matrix material to provide high
strength, hardness, and toughness, the near-surface regions where sliding will occur are enriched in selflubricating powders to achieve lubricity. Composite structures prepared in this fashion are used for a
wide range of tribological applications, such as bushings, bearings, and a variety of gears and traction
devices. For example, copper-graphite and silver-graphite composites are used in electrical brushes and
contact strips, while aluminum-graphite composites are well-suited for bearings, pistons, and cylinder
liners in engines and a host of other mechanical systems (Kumar and Sudarshan, 1996).
Recent tribological studies have demonstrated that when mixed at correct concentrations with optimal
particle sizes, self-lubricating filler materials can have a substantial beneficial impact on the mechanical
and tribological properties of matrix materials. For example, it was shown that graphite, MoS2, and boric
acid fillers tend to increase the wear resistance of nylon and polytetrafluoroethylene (PTFE)-type polymers (Blanchet and Kennedy, 1992; Fusaro, 1990). Aluminum-graphite composites exhibit excellent
lubricity, durability, and resistance to galling under both dry and lubricated conditions (Rohatgi et al.,
1992). When the graphite content in aluminum-matrix composites exceeds ≈20 vol%, the friction coefficient approaches that of pure graphite and becomes highly independent of the matrix alloy. AluminumWS2 composites were also found to be very effective in reducing galling and in providing excellent lubricity
and durability, especially in high-vacuum environments (Prasad and Mecklenburg, 1994). The presence
of WS2 particles in the matrix results in significantly increased resistance to seizure and enables the
composite body to operate under very high loads without galling.
Recent studies concluded that improved tribological behavior was mainly due to the formation of a
thin transfer layer on the sliding surfaces of counterface materials. In the case of polymers, a significant
increase in mechanical strength was also observed and thought to be responsible for high wear resistance.
It was found that, initially, transfer films were not present but formed as a result of surface wear and
subsurface deformation. They are continuously replenished by embedded graphite particles dispersed in
the matrix (Rohatgi et al., 1992).
In addition to metal-matrix composites, a series of self-lubricating polymer and ceramic matrix
composites have also been developed, tested, and offered for industrial use in recent years (Gangopadhyay
and Jahanmir, 1991; Prasad and Mecklenburg, 1994; Fredrich et al., 1995). These composites are emerging
as an important class of tribological materials, offering new means to combat friction, wear, and galling
under extreme conditions. In a recent study, tribological properties of fine-grain alumina (20%)-graphite
composites were explored as potential candidates for advance sealing applications. Pin-on-disk wear tests
showed that friction coefficients can be reduced from 0.5 for alumina-on-alumina to ≈0.25 for aluminagraphite composite (Yu and Kellett, 1996).
In another study, ceramic-matrix composites were fabricated by drilling a series of small holes in
alumina and silicon nitride ceramics and then filling the holes with NiCl2-intercalated graphite under
high pressure. Addition of graphite to silicon nitride considerably reduced the friction coefficient, but
the alumina-graphite composites exhibited only a marginal reduction in friction coefficient compared
to that of the alumina. The reduction in friction coefficient for silicon nitride-graphite composite can
be explained by the formation of transfer films consisting of a mixture of materials from both contacting
surfaces. However, for the alumina-graphite composites, the graphite regions were completely covered
with steel wear particles, inhibiting the formation of graphite-containing transfer films (Gangopadhyay
and Jahanmir, 1991).
Mixing of Sb2O3 with MoS2 was shown to act synergistically to improve the friction and wear behavior
of MoS2. Specifically, the tribological behavior improves because only the thin layers of MoS2 residing
on top are exposed to the environment, while the MoS2 at the bottom is protected against thermal or
environmental degradation by Sb2O3, which also acts as a beneficial support for MoS2. The proposed
mechanism suggests that composite structures containing Sb2O3 were also found to be more resistant to
tribo-oxidation than was pure MoS2 alone (Zabinski et al., 1993).
22.5.2 New Self-lubricating Composite Coatings and Structures
Recent advances in PVD and CVD technologies have led to the development of a new generation of selflubricating nanocomposite films and multilayer coatings. One such film is based on the MoS2 and Ti
system and is produced by closed-field unbalanced magnetron sputtering. This film is much harder and
more wear-resistant than conventional MoS2 coatings, yet it still has the low friction characteristics of
conventional MoS2 films. Its friction coefficient against a steel ball is ≈0.02 in humid air and <0.01 in
dry N2. The Vickers hardness value could be >1500 HV (Teer et al., 1997; Fox et al., 1999). Furthermore,
this coating is not greatly affected by moisture in the test environment. It is proposed for use in a variety
of dry sliding and machining applications (e.g., milling, drilling, tapping, cold-forming dies and punches,
stamping, bearings, and gears for aerospace and vacuum applications).
New coating architectures based on layers of a self-lubricating solid (e.g., MoS2, WS2, etc.) and a metal,
ceramic, or hard metal nitride or carbide (i.e, Ti, TiN, TiC, Pb, PbO, ZnO, Sb2O3) were also produced
in recent years and were shown to work extremely well under demanding tribological conditions. These
coatings can be prepared by co-sputtering of MoS2 and TiN or TiB2 targets, or a single target composed
of TiN and MoS2. The resultant coatings may consist of distinct TiN and MoSx phases in the form of a
nanodispersive system. The hardness of these coatings could be as high as 20 GPa, while their friction
coefficients are generally low (i.e., ≈0.1), even in open-air environments. Because of high hardness and
low friction, they can be used in both sliding and machining applications (Gilmore et al., 1998a,b).
Recently, researchers have also produced multilayers of MoSX/Pb and MoSX/Ti (with individual layer
thickness in the 4 to 100 nm range) by magnetron sputtering at room temperature. Sliding wear tests in
50% relative humidity showed great improvements in wear life over that of pure MoSx coatings (Simmonds et al., 1998). The three-dimensional design of adaptive coatings based on a multicomponent
MoS2/TiC/DLC coating architecture resulted in improved tribological properties over broad ranges of
environmental humidity and other test parameters (Voevodin et al., 1998). While the hard coatings of
TiC and DLC provided high strength and resistance to wear, solid lubricants DLC and MoS2 provided
low friction at the sliding surface. The coating had friction coefficients of 0.15 in humid air and 0.02 in
dry nitrogen, thus increasing the prospects for use in space mechanisms.
PbO/MoS2 and ZnO/WS2 nanocomposite films were also produced in recent years and tested for their
lubricity and durability in a variety of environments. The volume fraction of WS2 decreased with
increasing depth from the surface (Wlack et al., 1994). Composite films perform significantly better
during tribotesting than films composed entirely of MoS2 or PbO and ZnO. In addition, the composite
films demonstrate the properties of “adaptive” lubricants. MoS2 provides lubrication at room temperature; however, when the films are exposed to oxidizing environments at elevated temperatures, they adapt
by forming PbMoO4. This compound has been noted to display lubricant properties at high temperature.
Thus, there is significant potential for tailoring film compositions so that the components react to produce
lubricious wear debris and lubrication over extended temperature ranges (Wlack et al., 1997; Zabinski
et al., 1992).
Electroless nickel, chromium, nickel-phosphorus coatings containing small amounts of graphite, MoS2,
PTFE, and diamond particles were also developed in recent years and used to achieve relatively thick
films with self-lubricating properties. The deposition of MoS2 containing Ni-P composite coatings (containing ≈3 wt% MoS2) resulted in significant improvements in wear resistance and reduced the friction
coefficient of the base Ni coatings (Moonir-Vaghefi et al., 1997).
22.6 Soft Metals
Mainly because of their low shear strengths and rapid recovery as well as recrystallization, certain pure
metals (e.g., In, Sn, Pb, Ag, Au, Pt, Sn, etc.) can provide low friction on sliding surfaces (Wells and De
Wet, 1988; Sherbiney and Halling, 1977). They are used chiefly as solid lubricants because the attractive
properties they combine are unavailable in other solid lubricants. For example, in addition to its soft
nature, silver has excellent electrical and thermal conductivity, oxidation resistance, good transfer-filmforming tendency, and a relatively high melting point; thus, it has been commercially used to lubricate
the high-speed ball bearings of rotating anode X-ray tubes for many years. The Mohs hardness values of
soft metals are generally between 1 and 3. Reported friction coefficients of soft metals range from 0.1 to
0.4, depending on the metal and test conditions. Pb, In, and Sn provide better lubricity at room temperature
than Ag, Au, and Pt. At elevated temperatures, Pb, Sn, and In melt and undergo oxidation. Ag, Au, and
Pt have fairly high melting points, do not oxidize appreciably, and hence are preferred for high-temperature
lubrication purposes (Erdemir and Erck, 1996; Maillat et al., 1993; Seki et al., 1995). Au remains in metallic
form regardless of the temperature, while Ag2O decomposes as the temperature increases and Pt oxidizes
only slightly. Bronze and babbitts prepared by alloying some of these soft metals with Al, Zn, Cu, have
been used as bushings, bearings, and other tribological applications for a number of years.
Soft metals are generally produced as thin films on surfaces to be lubricated. Simple electroplating
and vacuum evaporation can be used to deposit most of these metals as self-lubricating films, but dense
and highly adherent films are produced by ion plating, sputtering, or ion-beam-assisted deposition
techniques (Erdemir et al., 1990a). Film-to-substrate adhesion is extremely critical for achieving long
FIGURE 22.19 Variation of friction coefficient of indium films as a function of film thickness. (From Sherbiney,
M.A. and Halling, J. (1977), Friction and wear of ion-plated soft metallic films, Wear, 45, 211-220. With permission.)
wear life or durability, especially on the surfaces of ceramic tribomaterials (Spalvins and Sliney, 1994;
Spalvins, 1998). The thickness of the soft metallic films also plays a major role in both friction and wear.
The lowest friction coefficients and wear rates are usually obtained with thinner films (i.e., 0.5 to 1 µm
thick) and under higher contact pressures (Dayson, 1971; El-Sherbiny and Salem, 1986). Figure 22.19
shows the variation of friction coefficient of an indium film with thickness (Sherbiney and Halling, 1977).
However, too thin a film tends to wear out quickly. Also, the friction coefficients of most soft metals tend
to decrease as the ambient temperature increases, mainly because of additional softening and rapid
recovery from strain hardening. Thick films result in large contact areas and hence high friction.
The combination of very high thermal conductivity with low shear strength and chemical inertness
makes silver and gold coatings ideal for applications involving high frictional or ambient heating, such
as sliding ceramic interfaces. As can be seen in Figure 22.20, thin Ag films can lower wear rates of zirconia
balls and disks by factors of 2 to 3 orders of magnitude. Reduction in wear is more dramatic at higher
sliding speeds. This is mainly because of the fact that zirconia has a very poor thermal conductivity, and
thus suffers severe thermomechanical wear at high sliding velocities. However, when a highly thermally
FIGURE 22.20 Wear performance of uncoated and silver-coated zirconia (calcia-partially stabilized) test pairs at sliding
velocities up to 2 m/s. (Adapted from Erdemir, A., Busch, D.E., Erck, R.A., Fenske, G.R., and Lee, R.H. (1991b), Ionbeam-assisted deposition of silver films on zirconia ceramics for improved tribological behavior, Lubr. Eng., 47, 863-867.)
FIGURE 22.21
zirconia ball.
Physical condition of a wear track formed on silver-coated zirconia disk during sliding against
conductive film like silver is present at the sliding interface, the wear rate decreases dramatically, mainly
because frictional heat is dissipated rapidly from the sliding interface. The low friction coefficient of silver
also helps in reduced frictional heating. Figure 22.21 shows the condition of a wear track formed on a
silver-coated zirconia disk. Overall, the film is still intact; only the tips of substrate asperities are exposed,
but the base zirconia is well-protected against wear. Figure 22.21 also reveals some physical evidence for
shear deformation experienced by soft silver film during contact sliding.
Silver is used as a lubricant in X-ray tubes, certain satellite parts, ball bearings, bolts, and other sliding
parts in nuclear reactors. When applied as a dense and adherent coating on the surfaces of these
components, it can effectively dissipate frictional heat that can otherwise cause thermomechanical and
tribochemical wear. Used on ceramic surfaces, it shears easily, thereby reducing the friction and microfracture-induced wear of the sliding ceramic surfaces (Erdemir et al., 1990a, 1991). Silver and other soft
metallic coatings can also protect the sliding surfaces against environmental and/or tribochemical degradation under dry and oil-lubricated sliding contact conditions (Ajayi et al., 1993; Erdemir et al., 1992;
DellaCorte et al., 1988). Under lubricated sliding conditions, thin silver films were extremely effective in
reducing friction and wear at temperatures up to 300°C (Ajayi et al., 1993, 1994; Erdemir et al., 1992,
1996a). One of the major shortcomings of metallic solid lubricants is that most of them react with sulfur
and chlorine (if present in the operating environment) and may undergo rapid corrosive wear.
22.7 Polymers
Polymers in various forms are widely used in tribology. They are lightweight, relatively inexpensive, and
easy to fabricate. They can easily be blended with other solids to make self-lubricating composite structures. Certain polymers (polytetrafluoroethylene [PTFE], polyimide, nylon, ultra-high-molecular-weight
polyethylene [UHMWPE], etc.) are self-lubricating when used in both the bulk and thin-film forms, or
as binders for other solid lubricants (Lancaster, 1984; Fusaro, 1988, 1990; Gresham, 1994; Jamison, 1994).
Coatings can be produced on a tribological surface by first spraying or sprinkling the powders, then
consolidating and curing them at high temperatures. The most common polymer-based solid lubricant
is PTFE, which is widely known as Teflon (an E.I. DuPont de Nemours Co. trade name). It is a “nonstick”
surfacing agent used in cookware, seals, and gaskets to facilitate release. It is also used in various other
forms (powder, composite, colloidal dispersion in oils and greases) to achieve low friction. Its friction
coefficient ranges from 0.04 to 0.2, depending on test conditions. PTFE can be used at temperatures up
to about 250°C. Polyimide and its coatings can also provide low friction. It can also be composited with
a self-lubricating inorganic filler to enhance its mechanical and tribological properties, especially at
elevated temperatures (Fusaro and Sliney, 1973; Blanchet and Kennedy, 1992). Fine PTFE powders have
also been used as additives in various oils and as thickeners in greases (Willson, 1992).
UHMWPE is another popular polymer used widely in total joint replacements (Kurtz et al., 1999).
Because of the very long molecules and highly entangled molecular chains, it provides better wear
resistance than PTFE. However, wear of this polymer still poses a major obstacle for the longevity of the
total joint replacements. Recent efforts to solve these problems have increased interest in the structure,
morphology, and mechanical properties of the UHMWPE and in various surface and structural treatment
processes (such as crosslinking, carbon-fiber reinforcing, recrystallization). It was reported that
crosslinked UHMWPE has much better wear properties and thus is a promising alternative for total joint
replacements. There are several excellent review articles and book chapters devoted to the tribological
uses of UHMWPE and other polymers in various applications. It is impossible to cover all of them here,
but readers can refer to the references provided here for further information (Wang et al., 1995; Briscoe,
1990; Zhang, 1997; Bahadur and Gong, 1992; Friedrich et al., 1995).
22.8 Summary and Future Directions
This chapter further demonstrates that solid lubricants have much to offer for demanding tribological
applications. Their use in advanced tribosystems is expected to increase in the near future, mainly because
the operating conditions of future tribosystems are becoming more and more demanding. One major
problem is that there exists no such lubricant that is capable of providing reasonably low and consistent
friction coefficients over broad test conditions, temperatures, and environments. The results of previous
studies demonstrate that the performance of layered solid lubricants are very much dependent on
tribological and environmental conditions. For example, the lubricity of transition-metal dichalcogenides
is adversely affected by moisture, while graphite depends on moisture for good lubricity. Layered solid
lubricants can be doped or intercalated with a number of metals and compounds to achieve lesser
sensitivity to ambient humidity and temperature.
Nowadays, most solid lubricants are produced as thin solid films on sliding surfaces. They are also
used as fillers in self-lubricating metallic, ceramic, and polymeric composites. In most cases, a transfer
film is found on the sliding surfaces. In general, formation of such a film at sliding interfaces seems to
be key to achieving low friction and long wear lives in most solid-lubricated surfaces. For solid lubricant
films, strong adhesion is key for long service life. Modern sputtering techniques and ion-beam processes
are quite capable of imparting strong adhesion between solid lubricant films and their substrates. Ionbeam mixing of conventional solid lubricants, such as MoS2, with ceramics is also feasible and appears
promising for severe tribological applications.
For materials with poor thermal conductivity, Ag and Au films combining high thermal conductivity
with low shear strength and good chemical inertness should be considered. Silver is primarily used as a
lubricant in ball bearings of rotating anode X-ray tubes. A unique solid lubricant, boric acid, which forms
naturally on the surfaces of boric oxide- and boron-containing ceramics, has recently been discovered.
It was shown that this lubricant can impart remarkably low friction coefficients (e.g., 0.02) to sliding
interfaces in moist environments where MoS2 is known to be ineffective.
For applications involving high temperatures, most layered solid lubricants appear ineffective. A
combination of solid and liquid lubrication may provide short-term solutions to this problem; but for
a long-term solution, the development of effective lubricious oxides, fluorides, and other compounds is
essential. Recently, a crystal-chemical approach was introduced to classify lubricious oxides on the basis
of their lubrication performance and operational limits at high temperatures. This approach may serve
as a basis for determining the kind(s) of lubricious oxides needed on a sliding surface at high temperatures.
Lubrication from vapor phases and by catalytic cracking of carbonaceous gases also appears promising.
Recently, sulfates of Ca, Ba, and Sr were shown to provide quite a low friction coefficient at high
temperatures. A series of adaptive lubrication strategies was also introduced in recent years and shown
to be effective in achieving lubrication at broader temperature ranges.
Certain polymers are also used as solid lubricants because the attractive properties they combine are
unavailable in other solid lubricants. Polymers are particularly favored for applications where cost, weight,
corrosion, and biocompatibility are the major considerations. In short, solid lubricants have been around
for a long time and they have been meeting some very important and critical tribological needs. They
are expected to be in high demand for many more years to come.
References
Ahsley, S. (1992), Lubricating ceramic engines with exhaust, Mechanical Eng., 114, 72-73.
Ajayi, O.O., Erdemir, A., Hsieh, J.H., Erck, R.A., Fenske, G.R., and Nichols, F.A. (1993), Boundary film
for structural ceramic materials, Wear, 164, 1150-1155.
Ajayi, O.O., Erdemir, A., Fenske, G.R., Erck, R.A., Hsieh, J.H., and Nichols F.A. (1994), Effect of metalliccoating properties on the tribology of coated and oil-lubricated ceramics, Tribol. Trans., 37, 656-661.
Barnett, R.S. (1977), Molybdenum disulfide as an additive for lubricating greases, Lubr. Eng., 33, 308-313.
Bahadur, S. and Gong, D. (1992), The action of fillers in the modification of the tribological behavior
of polymers, Wear, 158, 41-59.
Bhattacharya, R.S., Rai, A.K., McCormick, A.W., and Erdemir A. (1993), High-energy (MeV) ion-beam
modifications of sputtered MoS2 coatings on ceramics, Tribol. Trans., 36, 621-626.
Bhattacharya, R.S., Rai, A.K., Zabinski, J.S., McDevitt, N.T., and Neil, T. (1994), Ion beam modification
of fullerene films and their frictional behavior, J. Mat. Res., 9, 1615-1618.
Bhushan, B., Gupta, B.K., Van Cleef, G.W., Capp, C., and Coe, J.V. (1993), Sublimed C60 films for tribology,
Appl. Phys. Lett., 62, 3253-3255.
Bindal C. and Erdemir, A. (1996), Ultralow friction behavior of borided steel surfaces after flash-annealing, Appl. Phys. Lett., 68, 923-926.
Blanchet, T.A. and Kennedy, F.E. (1992), Sliding wear mechanism of polytetrafluoroethylene (PTFE) and
PTFE composites, Wear, 153, 229-243.
Blanchet, T.A., Lauer, J.L., Liew, Y.-F., Rhee, S.J., and Sawyer, W.G. (1994), Solid lubrication by decomposition of carbon monoxide and other gases, Surf. Coat. Technol., 68-69, 446-452.
Boes, D.J. and Chamberlain, B. (1968), Chemical interactions involved in the formation of oxidationresistant solid lubricant composites, ASLE Trans., 11, 131-139.
Bolster, R.N., Singer, I.L., Wegand, J.C., Fayeulle, S., and Gossett, C.R. (1991), Preparation by ion-beamassisted deposition, analysis and tribological behavior of MoS2 films, Surf. Coat. Technol., 46, 207-216.
Briscoe, B.J. (1990), Materials aspects of polymer wear, Scripta Met. Material., 24, 839-844.
Broman, V.E., DeJovine, J., DeVries, D.L., and Keller, G.H. (1978), Testing of Friction Modified Crankcase
Oils for Improved Fuel Economy, SAE Reprint No. 780597.
Buckley, D. (1978), Friction and transfer behavior of pyrolytic boron nitride in contact with various
metals, ASLE Trans., 21, 118-124.
Burton, R.A. and Burton, R.G. (1989), Friction and wear experiments on glassy carbon based materials,
Proc. 35th Meet. IEEE Holm Conf. on Electrical Contacts, IEEE, New York, 31-34.
Clauss, F.J. (1972), Solid Lubricants and Self-lubricating Solids, Academic Press, New York.
Dayson, C. (1971), The friction of very thin solid film lubricants on surfaces of finite roughness, ASLE
Trans., 14, 105-115.
DellaCorte C. and Fellenstein, J.A. (1997), The effect of compositional tailoring on the thermal expansion
and tribological properties of PS300: a solid lubricant composite coating, Tribol. Trans., 40, 639-645.
DellaCorte, C., Sliney, H.E., and Deadmore, D.L. (1988), Sputtered silver films to improve chromium
carbide based solid lubricant coatings for use to 900°C., STLE Trans., 31, 329-334.
DellaCorte, C. and Sliney, H.E. (1987), Composition optimization of self-lubricating chromium-carbide
based composite coatings for use to 760°C, ASLE Trans., 30, 77- 83.
DellaCorte, C. (1998), The Evaluation of a Modified Chrome Oxide Based High Temperature Solid
Lubricant Coating for Foil Bearings, NASA/TM-1998-208660.
DellaCorte, C. and Sliney, H.E. (1990), Tribological Properties of PM 212: A High-temperature SelfLubricating Powder Metallurgy Composite, NASA TM-102355.
Denton, R.M. and Fang, Z. (1995), Rock Bit Grease Composition, U.S. Pat. No. 5589443.
Donnet C. (1996), Advanced solid lubricant coatings for high vacuum environments, Surf. Coat Technol.,
80, 151-156.
Donnet, C., Le Mogne, Th., and Martin, J.M. (1993), Superflow friction of oxygen-free MoS2 coatings
in ultrahigh vacuum, Surf. Coat. Technol., 62, 406-411.
Dresselhaus, M.S. (1996), Intercalation in Layered Materials, NATO ASI Series, Vol. 148, Plenum Press,
New York.
Dunn, D.N., Seitzman, L.E., and Singer, I.L. (1998), MoS2 deposited by ion beam assisted deposition: 2H
or random layer structure?, J. Mat. Res., 13, 3001-3007.
El-Sherbiny, M. and Salem, F. (1986), Tribological properties of PVD silver films, STLE Trans., 29, 223-228.
Erck, R.A., Erdemir, A., Fenske, G.R., and Hsieh, J.-H. (1992), Ion-beam assisted surface modifications
for friction and wear reduction, Lubr. Eng., 48, 307-312.
Erdemir, A., Fenske, G.R., Erck R.A., and Cheng, C.C. (1990a), Ion-assisted deposition of silver films on
ceramics for friction and wear control, Lubr. Eng., 46, 23-30.
Erdemir, A., Fenske, G.R., and Erck, R.A. (1990b), A study of the formation and self-lubricating mechanisms of boric acid films on boric oxide coatings, Surf. Coat. Technol., 43/44, 588-532.
Erdemir, A., Fenske, G.R., Nichols F.A., and Erck, R.A. (1990c), Solid lubrication of ceramic surfaces by
IAD-silver coatings for heat engine applications, Tribol. Trans., 33, 511-516.
Erdemir, A. (1991), Tribological properties of boric acid and boric-acid-forming surfaces. I. Crystal
chemistry and mechanism of self-lubrication of boric acid, Lubr. Eng., 47, 168-172.
Erdemir, A., Erck, R.A., and Robles, J. (1991a), The relation of Hertzian contact pressure to friction
behavior of self-lubricating boric acid films, Surf. Coat. Technol., 49, 435-438.
Erdemir, A., Busch, D.E., Erck, R.A., Fenske, G.R., and Lee, R.H. (1991b), Ion-beam-assisted deposition
of silver films on zirconia ceramics for improved tribological behavior, Lubr. Eng., 47, 863-867.
Erdemir, A., Fenske, G.R., Erck, R.A., Nichols F.A., and Busch, D.E. (1991c), Tribological properties of
boric acid and boric-acid-forming surfaces. II. Mechanisms of formation and self-lubrication of
boric acid films on boron- and boric oxide-containing surfaces, Lubr. Eng., 47, 179-185.
Erdemir, A., Erck, R.A., Fenske, G.R., and Hong, H. (1996a), Solid/liquid lubrication of ceramics at
elevated temperature, Wear, 203/204, 588-594.
Erdemir, A., Bindal, C., Zuiker, C., and Savrun, E. (1996b), Tribology of naturally-occurring boric acid
films on boron carbide, Surf. Coat. Technol., 86/87, 507-511.
Erdemir, A., Fenske, G.R., and Munson, J.H. (1996c), Proc. 29th Int. Symp. on Automotive Technology and
Automation, June 3-6, 1996, Florence, Italy, 766.
Erdemir, A., Halter, M., and Fenske, G.R. (1996d), Preparation of ultralow-friction surface films on
vanadium diboride, Wear, 205, 236-240.
Erdemir, A. and Erck, R.A. (1996), Effect of niobium interlayer on high-temperature sliding friction and
wear of silver films on alumina, Tribol. Lett., 2, 23-28.
Erdemir, A., Ajayi, O.O., Fenske, G.R., Erck, R.A., and Hsieh, J.H. (1992), Synergistic effects of solid and
liquid lubrication on the tribological behavior of transformation-toughened ZrO2 ceramics, Tribol.
Trans., 35, 287-292.
Erdemir, A. (1993), Friction and Wear of Ceramics, S. Jahanmir (Ed.), Marcel Dekker, New York, 119-162.
Erdemir, A., Eryilmaz, O.L., Nilufer, I.B., and Fenske, G.R. (2000), Effect of source gas chemistry on
tribological performance of diamondlike carbon films, Diamond Rel. Mat., 9, 632-637.
Erdemir, A., Eryilmaz, O.L., and Fenske, G.R. (1999), Self-replenishing solid lubricant films on boron
carbide, Surf. Eng., 15, 291-295.
Erdemir, A. (2000a), A crystal chemical approach to lubrication by solid oxides, Tribol. Lett., 8, 97-102.
Erdemir, A. (1994), Crystal chemistry and self-lubricating properties of monochalcogenides gallium
selenide and tin selenide, Tribol. Trans., 37, 471-476.
Erdemir, A., Bindal, C., and Fenske, G.R. (1998), Lubricated Boride Surfaces, U.S. Pat. No. 5, 840, 132.
Erdemir A. and Fenske G.R. (1998), Clean and Cost-Effective Dry Boundary Lubricants for Aluminum
Forming, SAE Paper No. 980453.
Erdemir, A. (1995), Lubrication from Mixture of Boric Acid with Oils and Greases, U.S. Patent No. 5,
431, 830.
Erdemir, A. (2000b), Lubrication with Boric Acid Additives, U.S. Patent No. 6,025,306.
Falvo, M.R., Taylor, R.M., Helser, A., Chi, V., Brooks, F.P., Washburn, S., and Superfine, R. (1999),
Nanometre-scale rolling and sliding of carbon nanotubes, Nature, 397, 236-238
Farr, L.P.G. (1975), Molybdenum disulfide in lubrication. A review, Wear, 35, 1-22.
Fox, V., Hampshire J., and Teer, D. (1999), MoS2/metal composite coatings deposited by closed-field
unbalanced magnetron sputtering: tribological properties and industrial uses, Surf. Coat. Technol.,
112, 118-122.
Friedrich, K., Lu, Z., and Hager, A.M. (1995), Recent advances in polymer composites’ tribology, Wear,
190, 139-144.
Funatani, K. and Kurosawa, K. (1994), Composite coatings improve engines, Adv. Mat. Proc., 146, 27-29.
Fusaro, R.L. (1988), Evaluation of several polymer materials for use as solid lubricants in space, Tribol.
Trans., 31, 174-181.
Fusaro, R.L. and Sliney, H.E. (1970), Graphite fluoride (CFx)n — A new solid lubricant, ASLE Trans., 13, 56-65.
Fusaro, R.L. and Sliney, H.E. (1973), Lubricating characteristics of polyimide bonded graphite fluoride
and polyimide thin films, ASLE Trans., 16, 189-196.
Fusaro, R.L. (1990), Self-lubricating polymer composites and polymer transfer film lubrication for space
applications, Tribol. Int., 23, 105-108.
Fusaro, R.L. (1978), Lubrication and Failure Mechanisms of Molybdenum Disulfide Films. Part I. Effect
of Atmosphere, NASA TP-1343.
Gangopadhyay, A. and Jahanmir, S. (1991), Friction and wear characteristics of silicon nitride-graphite
and alumina-graphite composites, Tribol. Trans., 34, 257-265.
Gardos, M.N., Hong H.-S., and Winer, W.O. (1990), The effect of anion vacancies on the tribological
properties of rutile (TiO2-x). II. Experimental evidence, Tribol. Trans., 32, 209-220.
Gardos, M.N. (1993), The effect of Magnéli phases on the tribological properties of polycrystalline rutile
(TiO2-x), Proc. 6th Int. Congr. on Tribology, Eurotrib 93, M. Kozma (Ed.), Vol. 3, Aug. 30-Sept. 2,
201-206.
Gardos, M.N. (1984), An Analysis of the Ga/In/WSe2 Lubricant Compact, ASLE Preprint # 84-AM-6C-1,
Gardos, M.N. (1988), The effect of anion vacancies on the tribological properties of rutile (TiO2-x), Tribol.
Trans., 31, 427-436.
Gilmore, R., Baker, M.A., Gibson, P.N., Gissler, W., Stoiber, M., Losbichler, P., and Mitterer, C. (1998a),
Low-friction TiN-MoS2 coatings produced by DC magnetron co-deposition, Surf. Coat. Technol.,
108-109, 345-351.
Gilmore, R., Baker, M.A., Gibson, P.N., and Gissler, W. (1998b), Preparation and characterization of lowfriction TiB2-based coatings by incorporation of C or MoS2, Surf. Coat. Technol., 105, 45-50.
Gogotsi, Y.G., Jeon, I.D., and McNallan, M.J. (1997), Carbon coatings on silicon carbide by reaction with
chlorine containing gases, J. Mater. Chem., 7, 1841-1848.
Gresham, R.M. (1997), Bonded solid lubricants, CRC Handbook of Lubrication: Theory and Practice of
Tribology, Vol. III, E. R. Booser (Ed.), CRC Press, Boca Raton, FL, 167-181.
Heshmat, H. and Heshmat, C.A. (1999), The effect of slider geometry on the performance of a powder
lubricated bearing, Tribol. Trans., 42, 640-646.
Higgs, C.F., Heshmat, C.A., and Heshmat, H.S. (1999), Comparative evaluation of MoS2 and WS2 as
powder lubricants in high speed, multi-pad journal bearings, J. Tribol., 121, 625-630.
Hilton M.R., Bauer, R., Didziulis, S.V., Dugger, M.T., Keem, J., and Scholhamer, J. (1992), Structural and
tribological studies of MoS2 solid lubricant films having tailored metal-multilayer nanostructures,
Surf. Coat. Technol., 53, 13-23.
Hilton, M.R., Jayaram, G., and Marks, L.D. (1998), Microstructure of co-sputter-deposited metal- and
oxide-MoS2 solid lubricant thin films, J. Mat. Res., 13, 1022-1032.
Hilton, M.R. and Fleischauer, P.D. (1991), Structural, chemical, and tribological studies of sputterdeposited MoS2 solid lubricants films, Advances in Engineering Tribology, Chung, Y.W. and Cheng,
H.S., (Eds.), STLE SP-31, 31-36.
Jamison, W.E. (1978), Electronic effects on the lubricating properties of molybdenum disulfide and related
materials, ASLE Proc. 2nd Int. Conf. Solid Lubrication, ASLE Sp-6, STLE, Park Ridge, IL, 1-8.
Jamison, W.E. (1972), Structure and bonding effects on the lubrication properties of crystalline solids,
ASLE Trans., 15, 296-305.
Jamison, W.E. (1994), Plastics and plastic matrix composites, in CRC Handbook of Tribology and Lubrication, Vol. 3, CRC Press, Boca Raton, FL, 121-147.
Jayaram, G., Marks, L.D., and Hilton, M.R. (1995), Nanostructure of Au-20%Pd layers in MoS2 multilayer
solid lubricant films, Surf. Coat. Technol., 77, 393-39.
John, P.J. and Zabinski, J.S. (1999), Sulfate based coatings for use as high temperature lubricants, Tribol.
Lett., 7, 31-37.
Kanakia, M.D. and Peterson, M.B. (1987), Literature Review of the Solid Lubrication Mechanisms,
Southwest Research Institute, Interim Report, BFLRF #213, San Antonio, TX, 6-18.
Kanakia, M., Owens, M.E., and Ling, F.F. (1984), in Proc. Workshop on Fundamentals of High Temperature
Friction and Wear with Emphasis on Solid Lubrication for Heat Engines, Ling, F.F. (Ed.), Industrial
Tribology Institute, Troy, NY, pp. 19-38.
Kimura, Y., Wakabayashi, T., Okada, K., Wada, T., and Nishikawa, H. (1999), Boron nitride as a lubricant
additive, Wear, 232, 199-206.
King, J.P. and Forster, N.H. (1990), Synthesis and Evaluation of Novel High Temperature Solid Lubricants,
AIAA Paper # 90-2044.
Kobs, K., Dimigen, H., Huebsch, H., Tolle, H.J., Leutenecker, R., and Ryssel, H. (1986), Enhanced
endurance life of sputtered MoSx films on steel by ion beam mixing, Mat. Sci. Eng., 90,281-286.
Kumar, R. and Sudarshan, T.S. (1996), Self-lubricating composites: graphite-copper, Mat. Technol., 11,
191-194.
Kurtz, S.M., Muratoglu, O.K., Evans, M.E., and Avram A. (1999), Advances in the processing, sterilization,
and crosslinking of ultra-high molecular weight polyethylene for total joint arthroplasty, Biomaterials, 20, 1659-1688.
Lancaster, J.K. (1984), Solid lubricants, in CRC Handbook of Lubrication: Theory and Practice of Tribology,
Vol. II, Theory and Design, Booser, E.R. (Ed.), CRC Press, Boca Raton, FL, 269-290.
Langlade, C., Fayeulle, S., and Olier, R. (1994), Role of additives in the physico-chemistry of graphitebased transfer films, Thin Solid Films, 237 38-47.
Lansdown, A.R. (1999), Molybdenum Disulphide Lubrication, Tribology Series, 35, Dowson, D. (Ed.),
Elsevier, Amsterdam.
Lauer, J.L. and Bunting, B.G. (1988), High temperature solid lubrication by catalytically generated carbon,
Tribol. Trans., 31, 339-350.
Levy, F. (1979), Intercalated Layered Materials, D. Reidel, Boston.
Lince, J.R. and Fleischauer, P.D. (1987), Crystallinity of RF-sputtered MoS2 films, J. Mater. Res., 2, 827-838.
Maillat, M., Chattopadhya, A.K., and Hintermann, H.E. (1993), Preparation of silver coatings to obtain
low friction in alternating sliding at 570°C, Surf. Coat. Technol., 61, 25-29.
Martin, J.M., LeMogne, T., Chassagnette, C., and Gardos, M.N. (1992), Friction of hexagonal boron
nitride in various environments, Tribol. Trans., 35, 463-472.
Martin, J.M., Pascal, H., Donnet, C., LeMogne, T., Loubet, J.L., and Epicier, T. (1994), Superlubricity of
MoS2: crystal orientation mechanisms, Surf. Coat. Technol., 68-69, 427-432.
Martin, J.M., Donnet, C., LeMogne, T., and Epicier, T. (1993), Superlubricity of molybdenum disulfphide,
Phys. Rev., B48, 10583-10586.
Mate, M.C., McClelland, G.M., Erlandsson, R., and Chiang, S. (1987), Atomic-scale friction of a tungsten
tip on a graphite surface, Phys. Rev. Lett., 59, 1942-1945.
McMurtrey, E.L. (1985), Lubrication Handbook for the Space Industry. Part A. Solid Lubricants, 1985,
NASA TM-86556
Miyoshi, K. (1996), Solid Lubrication Fundamentals and Applications, NASA TM-107249.
Moonir-Vaghefi, S.M., Saatchi, A., and Hedjazi, J. (1997), Tribological behaviour of electroless Ni-P-MoS2
composite coatings, Z. fur Metallkunde, 88, 498-501.
Moser, J. and Levy, F. (1993), MoS2-x lubricating films: structure and wear mechanisms investigated by
cross-sectional transmission electron microscopy, Thin Solid Films, 228, 257-260.
Peterson, M.B., Murray, S.F., and Florek, J.J. (1960), Consideration of lubricants for temperatures above
1000°F, Trans. ASLE, 2, 225-233.
Peterson, M.B., Calabrese S.B., and Stupp, B. (1982), Lubrication with Naturally Occurring Double Oxide
Films, Office of Naval Research, Final Report, Contract No: N00014-82-C-0247.
Peterson, M.B., Li, S.Z., and Murray, S.F. (1994), Wear Resisting Oxide Films for 900°C, Final Report,
Argonne National Laboratory, ANL/OTM/CR-5.
Peterson, M.B. (1953), Friction and Wear Investigation of Molybdenum Disulfide. I. Effect of Moisture,
NACA TN-3055.
Peterson, M.B., Calabrese, S.J., Li, S.Z., and Jiang, X.X. (1994), Friction of alloys at high temperature,
J. Mater. Sci. Technol., 10, 313-318.
Phillips, J.C. (1969), Excitonic instabilities and bond theory of III-VI sandwich semiconductors, Phys.
Rev., 188, 1225-1228.
Prasad, S.V., Zabinski, J.S., and McDevitt, N.T. (1995), Friction behavior of pulsed laser deposited
tungsten disulfide films, Tribol. Trans., 38, 57-62.
Prasad, S.V. and Mecklenburg, K.R. (1994), Self-lubricating aluminum metal-matrix composites containing tungsten disulfide and silicon carbide, Lubr. Eng., 50, 511-518.
Prasad, S.V. and McConnell, B.D. (1991), Tribology of aluminum metal-matrix composites, Lubrication
by graphite, Wear, 149, 241-253.
Pushpavanam, M. and Natarajan, S.R. (1995), Nickel-boron nitride electrocomposites, Metal Finish., 93, 97-99.
Quinn, T.F.J. and Winer, W.O. (1985), Thermal aspects of oxidational wear, Wear, 102, 67-76.
Rabinowicz, E. and Imai, M. (1964), Frictional properties of pyrolytic boron nitride and graphite, Wear,
7, 298-300.
Rai, A.K., Bhattacharya, R.S., Zabinski, J.S., and Miyoshi, K. (1997), Comparison of the wear life of asdeposited and ion-irradiated WS2 coatings, Surf. Coat. Technol., 92, 120-128.
Rapoport, L., Bilik, Yu., Feldman, Y., Homyonfer, M., Cohen, S.R., and Tenne, R. (1997), Hollow nanoparticles of WS2 as potential solid-state lubricants, Nature, 387, 791-793.
Rohatgi, P.K., Ray, S., and Liu, Y. (1992), Tribological properties of metal matrix-graphite particle
composites, Int. Mat. Rev., 37, 129-149.
Rowe, G.W. (1960), Some observation on the frictional behavior of boron nitride and graphite, Wear, 3,
274-285
Savage, R.H. (1948), Graphite lubrication, J. Appl. Phys., 19, 1-10.
Sawyer, W.G. and Blanchet, T.A. (1999), Lubrication of Mo, W, and their alloys with H2S gas admixtures
to room temperature air, Wear, 225, 581-586.
Seitzman, L.E., Singer, I.L., Bolster, R.N., and Gossett, C.R. (1992), Effect of titanium nitride interlayer
on the endurance of composition of a molybdenum disulfide coating prepared by ion-beam assisted
deposition, Surf. Coat. Technol., 51, 232-236.
Seitzman, L.E., Bolster, R.N., and Singer, I.L. (1995), Effects of temperature and ion-to-atom ratio on
the orientation of IBAD MoS2 coatings, Thin Solid Films, 260, 143-147.
Seki, K., Suzuki, M., Nishimura, M., Hasegawa, M., and Moriyama, M. (1995), Performance of ball
bearings operated at temperatures up to 500°C in vacuum, Lubr. Eng., 51, 753-763.
Sherbiney, M.A. and Halling, J. (1977), Friction and wear of ion-plated soft metallic films, Wear, 45, 211-220.
Simmonds, M.C., Savan, A., Van Swygenhoven, H., Pfluger, E., and Mikhailov, S. (1998), Structural,
morphological, chemical and tribological investigations of sputter deposited MoSx/metal multilayer
coatings, Surf. Coat. Technol., 108-109, 340-344.
Singer, I.L. (1992), Solid Lubrication Processes, in Fundamentals of Friction: Macroscopic and Microscopic
Processes, Singer, I.L. and Pollock, H.M. (Eds.), NATO-ASI Series, Vol. 220, Kluwer Academic,
London, 237-261.
Singer, I.L., Mogne, T., Donnet, C., and Martin, J.M. (1996a), Friction behavior and wear analysis of SiC
sliding against Mo in SO2, O2 and H2S at gas pressures between 4 and 40 Pa, Tribol. Trans., 39,
950-956.
Singer, I.L. (1989), Solid lubricating films for extreme environments, in New Materials Approaches to
Tribology, Pope, L.E., Fehrenhacher, L., and Winer, W.O. (Eds.), MRS Proc., 140, 215-226.
Singer, I.L., LeMogne, T., Donnet, C., and Martin, J.M. (1996b), In situ analysis of the tribochemical
films formed by SiC sliding against Mo in partial pressures of SO2, O2, and H2S gases, J. Vac. Sci.
Technol. A, 14, 38-45.
Singer, I.L., Bolster, R.N., Seitzman, L.E., Wahl, K.J., and Mowery, R.L. (1994), Advanced Solid Lubricant
Films by Ion-Beam Assisted Deposition., Naval Research Laboratory, NRL/MR/6170-94-7633.
Singer, I.L. (1998), How third-body processes affect friction and wear, MRS Bull., 23 (6), 37-40
Singer, I.L., Bolster, R.N., Wegand, J., and Fayeulle, S. (1990), Hertzian stress contribution to low friction
behavior of thin MoS2 coatings, Appl. Phys. Lett., 57, 995-997.
Sliney, H.E. (1993), Solid lubricants, Metals Handbook, Vol. 18. Friction, Lubrication, and Wear Technology, ASM-International, Metals Park, OH, 115-122.
Sliney, H.E. (1982), Solid lubricant materials for high temperatures. A review, Tribol. Int., 15, 293-302.
Sliney, H.E., Storm, T.N., and Allen, G.P. (1965), Fluoride solid lubricants for extreme temperatures and
corrosive environments, ASLE Trans., 8, 307-322.
Sliney, H.E. (1969), Rare Earth Fluorides and Oxides — An Exploratory Study of Their Use as Solid
Lubricants at Temperatures to 1800°F, NASA TN D-5301.
Sliney, H.E. (1986), The use of silver in self-lubricating coatings for extreme temperatures, Trans. ASLE,
29, 370-375.
Spalvins, T. (1980), Tribological properties of sputtered MoS2 films in relation to film morphology, Thin
Solid Films, 73, 291-297.
Spalvins, T. (1984), Frictional and morphological properties of Au-MoS2 films sputtered from a compact
target, Thin Solid Films, 118, 374-384.
Spalvins, T. (1969), Deposition of MoS2 films by physical sputtering and their lubrication properties in
vacuum, ASLE Trans., 12, 36-43.
Spalvins, T. (1971), Lubrication with sputtered MoS2 films, ASLE Trans., 14, 267-274.
Spalvins, T. (1992), Lubrication with sputtered MoS2 films: principles, operation and limitations, J. Mat.
Eng. Perform., 1, 347-352.
Spalvins, T. (1998), Improvement of Ion Plated Ag and Au Film Adherence to Si3N4 and SiC Surfaces for
Increased Tribological Performance, NASA TM-207415.
Spalvins, T. and Sliney, H.E. (1994), Frictional Behavior and Adhesion of Ag and Au Films Applied to
Aluminum Oxide by Oxygen-ion Assisted Screen Cage Ion Plating (SCIP), NASA TM-106522.
Stupp, B.C. (1981), Synergistic effects of metals co-sputtered with MoS2, Thin Solid Films, 84, 257-266.
Sutor, P. (1991), Solid lubricants: overview and recent developments, MRS Bull., 16, 24-30.
Suzuki, M. (1998), Comparison of tribological characteristics of sputtered MoS2 films coated with different apparatus, Wear, 218, 110-118.
Teer, D.G., Hampshire, J., Fox, V., and Bellido-Gonzalez, V. (1997), The tribological properties of
MoS2/metal composite coatings deposited by closed field magnetron sputtering, Surf. Coat. Technol., 94-95, 572-578.
Tenne, R., Margulis, L., Genut, M., and Hodes, G. (1992), Polyhedral and cylindrical structures of tungsten
disulphide, Nature, 360, 444-445.
Voevodin, A.A., Bultman, J., and Zabinski, J.S. (1998), Investigation into three-dimensional laser processing of tribological coatings, Surf. Coat. Technol., 107, 12-19.
Voevodin, A.A., O’Neill, J.P., and Zabinski, J.S. (1999), Nanocomposite tribological coatings for aerospace
applications, Surf. Coat. Technol., 119, 36-45.
Wahl, K.J., Seitzman, L.E., Bolster, R.N., Singer I.L., and Peterson, M.B. (1997), Ion-beam deposited
Cu-Mo coatings as high temperature solid lubricants, Surf. Coat. Technol., 89, 245-251.
Wahl, K.J., Dunn, D.N., Singer, I.L. (1999), Wear behavior of Pb-Mo-S solid lubricating coatings, Wear,
230, 175-183.
Wahl, K.J. Seitzman, L.E., Bolster, R.N., and Singer, I.L. (1995), Low-friction, high-endurance, ion-beamdeposited Pb-Mo-S coatings, Surf. Coat. Technol., 73, 152-159.
Walck, S.D., Zabinski, J.S., McDevitt, N.T., and Bultman, J.E. (1997), Characterization of air-annealed,
pulsed laser deposited ZnO-WS2 solid film lubricants by transmission electron microscopy, Thin
Solid Films, 305, 130-143.
Walck, S.D., Donley, M.S., Zabinski, J.S., and Dyhouse, V.J. (1994), Characterization of pulsed laser
deposited PbO/MoS2 by transmission electron microscopy, J. Mat. Res., 9, 236-245.
Wang, A., Sun, D.C., Stark, C., and Dumbleton, J.H. (1995), Wear mechanisms of UHMWPE in total
joint replacements, Wear, 181/183, 241-249.
Wells, A. and De Wet, D.J. (1988), The use of platinum in thin tribological coatings, Wear, 127, 269-281.
Westergard, R., Ahlin, A., Axen, N., and Hogmark, S. (1998), Sliding wear and friction of Si3N4-SiCbased ceramic composites containing hexagonal boron nitride, J. Eng. Tribol., 212, 381-387.
Willson, B. (1992), PTFE as a friction modifier in engine oil, Ind. Lubr. Tribol., 44, 3-5.
Winer, W.O. (1967), Molybdenum disulfide as a lubricant: a review of the fundamental knowledge, Wear,
10, 422-452.
Woydt, M., Skopp, A., Dorfel, I., and Witke, K. (1999), Tribol. Trans., 42, 21-31.
Yu, C.-Y. and Kellett, B.J. (1996), Tribology of alumina-graphite composites, Proc. Ceramic Engineering
and Science, 20th Annual Conference on Composites, Advanced Ceramics, Materials, and Structures —
American Ceramic Soc., Westerville, OH, 17, 220-227.
Zabinski, J.S., Donley, M.S., Prasad, S.V., and McDevitt, N.T. (1994), Synthesis and characterization of
tungsten disulphide films grown by pulsed-laser deposition, J. Mater. Sci., 29, 4834-4839.
Zabinski, J.S., Donley, M.S., and McDevitt, N.T. (1993), Mechanistic study of the synergism between
Sb2O3 and MoS2 lubricant systems using Raman spectroscopy, Wear, 165, 103-108.
Zabinski, J.S., Corneille, J., Prasad, S.V., McDevitt, N.T., and Bultman, J.B. (1997), Lubricious zinc oxide
films: synthesis, characterization and tribological behaviour, J. Mat. Sci., 32, 5313-5319.
Zabinski, J.S., Donley, M.S., Walck, S.D., Schneider, T.R., and McDevitt, N.T. (1995), Effects of dopants
on the chemistry and tribology of sputter-deposited MoS2 films, Tribol. Trans., 38, 894-904.
Zabinski, J.S., Donley, M.S., Dyhouse, V.J., and McDevitt, N.T. (1992), Chemical and tribological characterization of PbO-MoS2 films grown by pulsed laser deposition, Thin Solid Films, 214, 156-163.
Zabinski, J.S., Florkey, J.E., Walck, S.D., Bultman, J.E., and McDevitt, N.T. (1995), Friction properties of
WS2 /graphite fluoride thin films grown by pulsed laser deposition Surf. Coat. Technol., 76-77, 400-406.
Zabinski, J.S., Saunders, J.H., Nainaparampil, J., and Prasad, S.V. (2000), Lubrication using a microstructurally engineered oxide: performance and mechanisms, Tribol. Lett., 103-116.
Zallen, R. and Slade, M. (1974), Rigid-layer modes in chalcogenide crystals, Phys. Rev. B., 9, 1627-1637.
Zhang, S.W. (1998), State-of-the-art of polymer tribology, Tribol. Int., 31, 49-60.

Chapter 22:Solid Lubricants and Self

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