evaluation of wear and friction mechanisms of different piston ring

EVALUATION OF WEAR AND FRICTION MECHANISMS
OF DIFFERENT PISTON RING AND CYLINDER LINER
MATERIALS
A. KOMPA, A. KRATZSCH, H.-J. FÜßER
DaimlerChrysler AG, Dept. FT4/ST, Wilhelm-Runge Strasse 11, D-89081 Ulm, GERMANY;
e-mail: [email protected]
A. HEUBERGER
DaimlerChrysler AG, Dept. EP/MVO, D-70546 Stuttgart, GERMANY
K.-H. ZUM GAHR
University of Karlsruhe, Dept. of Mechanical Engineering, WKII, Kaiserstrasse 12, D-76128 Karlsruhe, GERMANY
SUMMARY
In the field of engine development one has to meet constantly growing requirements due to fuel consumption
restrictions, weight reduction and emission standards, often combined with supercharged motors. These requirements
lead to higher stresses of material components. Especially contact stresses of cylinder liner / piston ring surfaces are
increased. Particularly, dead center areas are highly stressed, since hydrodynamic friction changes into boundary
friction. This paper deals with modelling boundary lubrication using a reciprocating test rig. The influence of different
lubrication modes on friction and wear of selected cylinder liner and piston ring materials is examined.
Keywords: tribological properties, reciprocating sliding conditions, boundary lubrication, gray cast iron, aluminiumsilicon alloys
1
INTRODUCTION
Regarding cylinder liners, classical gray cast iron has
shown great performances. A couple of years ago
different aluminium based liner concepts have been
introduced for different reasons like weight reduction
and some thermal characteristics [1, 2]. Since
aluminium-based crankcases are often part of these
solutions, aluminium based liners would be
advantageous considering the better match of material
properties. The aluminium-silicon alloys applied to
liners should proof sufficient friction and wear
properties. Different authors consider eutectic alloy
systems in general as promising [3, 4]. For future
developments of these concepts more detailed
information about friction and wear properties and basic
tribological mechanisms is necessary. The aim of our
work is to develop adequate test routines to evaluate the
material properties with respect to the tribological stress
at the dead centres under engine-like conditions.
2
2.1
MATERIALS AND EXPERIMENTAL
METHODS
Methods
For all tests, a reciprocating test machine was used
(Optimol, SRVIII) that has been modified and advanced
for our specific requirements. The general experimental
set-up is shown in Fig. 1. All test samples have been
taken from engine components from series production.
normal force FN
driving linkage
oscillation f
opposing
body
active
heating system
bath lubrication
basic body
force sensor
Figure 1: Schematic tribometer set-up for bath
lubrication
The adapted specimens are mounted in devices where
liner segments and piston ring segments serve as basic
body and oscillating opposing body, respectively
(Fig. 2). Tribometer tests were carried out under varying loads (normal forces FN up to 200 N), whereas
temperatures, stroke (∆x = 3 mm or 4 mm), frequency
(f = 30 Hz) and test duration (t = 3 h) were kept
constant. The temperature of the heating system is
controlled and set to 180°C, which corresponds with an
oil bath temperature of 165°C. Different modes of
lubrication have been applied: bath lubrication (Fig. 1)
and a flowing-through lubrication (lubricating oil flow
rate about 3 ml/min). For the latter an inclined set-up
(inclination angle 25°) was arranged (Fig. 3). During
experiments friction values were recorded continuously.
The final linear wear was measured using a stylus
profilometer. Scanning electron microscopy (SEM) and
me-tallographic sections were applied to evaluate
tribologi-cally induced topographical and morphological
changes in the near surface region. In the bath lubricated
system the oil volume of approximately 1 ml has been
analysed afterwards by means of atomic emission
spectroscopy (AES).
piston ring
segment
b
a
oscillating
direction
10 µm
20 µm
d
c
liner segment
Figure 2: Arrangement of system liner/ring in bath
lubrication
f
T
3
T
FR
flowing-through
lubrication
Figure 3: Inclined system with flowing-through
lubrication
2.2
Materials
In this work two different tribological systems have been
investigated: liner made of gray cast iron versus chromium-ceramic coated piston rings and liner made of an
hypereutectic aluminium-silicon alloy versus nitrided
steel piston rings.
b
4
50 µm
50 µm
c
RESULTS
Considering kinematics, reciprocating test methods are
well suited to model the engine’s piston stroke in the
region of top and bottom dead centres, where relative
velocities of piston ring versus cylinder liner are small,
become zero and finally change direction The two dead
centre areas therefore are highly stressed since hydrodynamic friction changes into boundary friction. Whereas loads on top and bottom dead centres caused by gas
forces and/or ring tensions differ in real engine systems,
in reciprocating test rigs usually two symmetric dead
centre areas are generated. Since relative velocities are
small, boundary lubrication is predominant which results
in intensified test conditions. Nevertheless the same
wear appearance can be produced. Fig. 6 compares the
profiles of a real engine’s top dead centre area and a
wear trace after an oscillating tribometer test.
linear wear wlin [µm]
a
20 µm
Figure 5: SEM micrographs of surface topographies: a)
aluminium-silicon alloy, b) nitrided steel, c) gray cast
iron, d) chromium-ceramic coated cast iron
FN
oil supply
system
200 µm
0
0
-2
-4
-4
-6
-6
-8
0
1
2
3
4
5
trace [mm]
two overlapped dead centers
2
-2
engine test bench
d
4
top dead center
first compression ring
2
6
7
-8
0
1
2
3
4
5
trace [mm]
6
7
tribometer test
Figure 6: Comparison of top dead centre’s wear trace:
engine test bench versus reciprocating test rig
50 µm
50 µm
Figure 4: Metallographic sections of used materials: a)
aluminium-silicon alloy (liner), b) nitrided steel (ring),
c) gray cast iron (liner), d) chromium-ceramic coated
cast iron piston ring
A lubricant of the viscosity class SAE 10 W-40 has been
used for all experiments. Metallogra-phic sections and
SEM micrographs of the original test samples can be
found in Figure 4 and Figure 5, respectively.
Because of the small stroke, the two dead centres overlap with increasing wear. The final wear is determined
as the maximum difference of the original and worn surface. The linear wear number wlin is formed by averaging the heights of three different measurements at
each sample.
When using bath lubrication linear wear of the liner
segments is increased with higher loads as expected.
Consequently, the oil volume is enriched with worn
particles. Fig 7 shows the linear wear of the liner
segments as well as the aluminium and iron content in
the oil volume detected by AES.
8
7
6
5
90
80
70
60
50
4
40
3
30
2
20
1
10
0
0
25
50
75
100
125
150
175
200
0
normal force FN [N]
Figure 7: Linear wear of liner segments and element
content of oil volume versus load (∆x=3mm, f=30Hz,
T=180°C, t=3h)
Fig. 8 shows coefficients of friction of an aluminiumsilicon alloy in dependence of the mode of lubrication.
The bath lubricated systems always result in higher
coefficients of friction over a sliding distance of about
2500 m. This also holds for the system gray cast iron
versus chromium-ceramic coated piston ring. During
oscillating motion wear particles are produced and
partly carried with by the piston ring over one or several
amplitudes. If placed just into the tribological contact
between ring/liner they can also be pressed into the
upper surface region again.
coefficient of friction µ [-]
In order to check the reproducibility of the wear
numbers, measurements of ten different samples at
FN = 100 N, ∆x = 4 mm, f = 30 Hz, T = 180°C, t = 3 h
were done, which resulted in a scatter range of 20%
(also shown in Fig. 9). Within this scatter range all
different applied test parameters show similar results.
10
bath lubrication
Al-Si alloy, stroke
"
gray cast iron
"
flow
Al-Si alloy
gray cast iron
9
8
7
6
5
4
3
∆x= 4 mm
∆x= 3 mm
∆x= 4 mm
∆x= 3 mm
∆x= 4 mm
∆x= 4 mm
2
0,30
Al-Si alloy vs nitrided steel
1
bath
0,25
0
0
25
50
75
100
125
150
175
200
normal force FN [N]
0,20
Figure 9: Linear wear of liner segments as a function of
normal force: bath vs. flow lubricated system (f=30Hz,
T=180°C, t=3h)
0,15
0,10
flow
0,05
0,00
Fig. 9 shows linear wear numbers of liner segments of
different test runs as a function of load. The aluminiumsilicon alloy always shows higher wear rates than the
gray cast iron system. Whereas wear rates of gray cast
iron seems to be linear over the whole tested range, the
Al-Si alloy shows a different and increased dependence
on the applied load. In the region of lower loads (up to
about 75 N) wear rates differ slightly in comparison to
gray cast iron. In regions of loads higher than 75 N wear
rates show a strong increase with load. This is consistent
with Torabian et al. [5] who found three different
regions with different but constant slopes of the wear
curve (i.e. wear rate over load).
linear wear wlin [µm]
linear wear of liner segments
Al-Si alloy, stroke ∆x= 3 mm
gray cast iron,
∆x= 3 mm
element concentration in oil volume
Al
Fe
9
element content [mg/kg]
linear wear of liner segment wlin [µm]
10
0
500
1000
1500
sliding distance [m]
2000
2500
Figure 8: Coefficients of friction of Al-Si alloy, bath vs.
flow lubrication (FN=100N, ∆x=4mm, f=30Hz,
T=180°C, t=3h)
Using the bath lubricated system, the amount of oil
available for lubrication is highest, but it is not possible
to remove wear particles produced during the test.
Furthermore, since the arrangement of the bath
lubrication facility is an open system, evaporation losses
lead to a reduced oil volume resulting in an even higher
concentration of wear debris. Altogether the phenomena
mentioned above result in continuously changing
lubrication conditions. These disadvantages can be
avoided or lessened by using flowing-through
lubrication. Because of the limited sensitivity of
detection, AES cannot be applied here for element
detection any more. Independent of the mode of
lubrication, friction values recorded during experiments
scatter in range of about ±5%.
Fig. 10 shows SEM micrographs of the surfaces of the
liner segments (Al-Si alloy and gray cast iron) after
tribometer tests with the different lubrication modes.
Considering the Al-Si alloy there are concise
differences. In bath lubrication wear debris is produced
but maybe not removed out of the contact area.
Therefore the probability of being worked into the
surface and/or to fill up pores is high which leads to a
more `closed´ appearance of the surface. In contrast to
this, the worn particles are being moved out of the
tribological contact by the lubricating oil flow and the
original surface shows a more porous appearance, which
is similar to the original surface in Fig. 5a).
Gray cast iron shows little wear in general in the applied
range of test parameters. After the experiment often a
polishing effect can be observed only by visual
inspection under certain angles of incidence of light.
This is due to a smoothing effect of surface roughness
on the honing plateaus.
We tried to examine the wear mechanism of the Al-Si
alloy in more detail by applying a high load
(FN = 200 N) under the servere condition of starved
lubrication in the flow arrangement.
a
b
20 µm
c
20 µm
d
200 µm
200 µm
Figure 10: SEM micrographs of surfaces of liner segments: a) Al-Si alloy, bath, worn surface; b) Al-Si alloy, flow,
worn surface; c) gray cast iron, bath; d) gray cast iron, flow (FN=100N, ∆x=4mm, f=30Hz, T=180°C, t=3h).
The arrows show the sliding direction; dashed lines on the gray cast iron liner segments show the transition to the
original surface.
The metallographic section of this sample in Fig. 11
clearly shows the enrichment of smaller Si particles in
the near surface region which seems to verify the
hypothesis, that silicon crystals are being fractured and
pressed into the surface again.
20 µm
Figure 12: SEM micrograph of top dead center area of
a cylinder liner of a run engine
20 µm
Figure 11: Metallographical section of Al-Si alloy
under high load (FN=200N) and starved lubrication)
Fig. 12 shows an aluminium-silicon alloy cylinder liner
taken from a run engine. Wear appearance seems to be a
transition between the tribometer tests with bath and
flow lubrication shown in Fig. 10.
4
SUMMARY
In this work a test method has been developed to model
the tribological contact piston ring versus cylinder liner.
Since wear appearances in the wear traces are similar to
that of run engines, the test seems to model the real
conditions at engine’s dead centres. More work has to
be done to compare tribometer tests and engine test runs
with respect to possible friction and wear mechanisms.
5
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(1997) 9. 502-508.
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