Pulsed laser modification of plasma-sprayed coatings: Experimental

Surface & Coatings Technology 201 (2006) 2248 – 2255
www.elsevier.com/locate/surfcoat
Pulsed laser modification of plasma-sprayed coatings: Experimental
processing of hydroxyapatite and numerical simulation
S. Dyshlovenko a , L. Pawlowski a,⁎, I. Smurov b , V. Veiko c
a
c
Service of Thermal Spraying at Ecole Nationale Supérieure de Chimie de Lille, BP 90108, F-59652 Villeneuve d'Ascq, France
b
Ecole Nationale d'Ingénieurs de Saint Etienne, 58, rue J. Parot, F-42023 Saint Etienne, France
St.-Petersburg State University of Information Technologies, Mechanics and Optics, 14, Sablinskaya, str., 197101 St. Petersburg, Russia
Received 13 October 2005; accepted in revised form 23 March 2006
Available online 19 May 2006
Abstract
A pulsed CO2 laser was used to treat plasma-sprayed hydroxyapatite coatings. Pulses of 0.74 ms duration and powers equal to 41.6 and 45.3 W
were focused onto a 300 μm spot of the coatings surface. The laser beam was scanned with speeds of 6.4 and 9.6 mm/s. The morphology of lasertreated deposits was observed by scanning electron microscopy (SEM) and the crystal phases identified using X-ray diffraction (XRD). This
technique enabled also the determination of quantitative phase composition. The laser treatment process was modeled using the Fusion-2D,
software and the temperature fields and depth of molten material were predicted. The latter were compared with the experimental ones found in
metallographically prepared cross-sections. A reasonable convergence between the model and experiment was achieved after careful optimisation
of initial material parameters as such coefficient of optical absorption and emissivity.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Hydroxyapatite; Laser treatment of coatings; Numerical simulation; Plasma-sprayed coatings
1. Introduction
Hydroxyapatite (HA, having the chemical formula Ca10
(PO4)6(OH)2) is a bioactive ceramics which owing to chemical
composition and crystal structure similar to those of human
bone can facilitate integration of prostheses into osseous tissue
[1–3]. This ceramics has been used to restore bone tissue and to
decrease the negative consequence of surgical operation for
many years. Because of its limited mechanical strength, HA is
used as coating on surfaces of more resistant metallic prostheses
made of, e.g., Ti-6Al-4V alloy [4]. The environment rich of
calcium and phosphate at the surface of a prosthesis favors
development of bone cells and enhance its adhesion to the bone
[1]. Atmospheric Plasma Spraying (APS) is the most widely
applied method to deposit HA coating onto titanium alloy
prostheses. The method consists of injection of ceramic particles into a high temperature (N 10 000 K) and high velocity
⁎ Corresponding author. Tel./fax: +33 320 33 61 65.
E-mail address: [email protected] (L. Pawlowski).
0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2006.03.034
Table 1
Runs of laser treatment experiments for samples plasma sprayed using 11 or 24
of electric power, with a working gas Ar + H2 of total flow rate of 50 slpm
(volume fraction of Ar was 95% or 97.5%) and carrier gas flow rate of 3 or
3.5 slpm
Plasma spray
samples
abbreviations
Sub11973
Sub11953
Sub24973
Sub24953
Sub119735
Sub119535
Sub249735
Sub249535
Laser power density of
5.9 × 108 W/m2
Laser power density of
6.4 × 108 W/m2
Scan speed
Scan speed
6.4 mm/s
9.6 mm/s
6.4 mm/s
9.6 mm/s
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
An example of abbreviation of a sprayed sample: Sub24973 means that powder
was sprayed onto substrate, using 24 kW of electric power with 97.5 vol.% of Ar
in plasma forming gas and 3 slpm of carrier gas.
S. Dyshlovenko et al. / Surface & Coatings Technology 201 (2006) 2248–2255
Table 2
Laser treatment parameters
Parameter
Value
Pulse duration, s
Pulse rate, Hz
Laser spot dimension, μm
Scanning velocity, mm/s
Average power, W
Pulse power, W
Density of the power, W/m2
Overlap the spots, %
7.4 × 10− 4
47
300
6.4 or 9.6
1.47 or 1.6
41.56 or 45.34
5.9 × 108 or 6.4 × 108
32 or 50
(N800 m/s) plasma jet [5]. The high temperature provokes
dehydration and decomposition of HA. The dehydration produces oxyapatite (OA, Ca10(PO4)6O) and/or oxyhydroxyapatite
(OHA, Ca10(PO4)6(OH)2−xOxVx), in which V means vacancy.
The decomposition, in turn, results in different calcium phosphate phases such as, e.g., calcium oxide (CaO), α-tricalcium
phosphate (α-TCP, α-Ca3(PO4)2), β-tricalcium phosphate (βTCP, β-Ca3(PO4)2), tetracalcium phosphate (TTCP, Ca4P2O9)
and/or the amorphous calcium phosphate (ACP). A spray
particle impacting a surface, at plasma spraying using typical
processing parameters, is composed of a solid core and liquid
shell [6]. Fast cooling of crystal phases in the solid core at impact
results in conservation of high temperature phases. The liquid
shell is most probably transformed at impact into an amorphous
phase. The fractions of HA, OA-OHA, TTCP, α-TCP and ACP
phases in sprayed coatings is the most important factor that
determines their biological behaviour, such as dissolution of the
coating in vivo [7]. Careful control of operational spray
parameters may help in predicting the phase content in the
coating [8]. On the other hand, post-spray treatment of the
coating can modify the phase composition. The treatment can
help in transforming amorphous phase ACP back into crystalline
HA [9]. This renders possible optimization of plasma spray
operational parameters in the way in which most of sprayed
powder particles are molten. Consequently, the mechanical
integrity of the sprayed coating and its adhesion to substrate
would be improved. The physical background of the treatment is
based on the observation that atoms and ions of APC are returned
easily to the positions corresponding to stable crystalline forms
under the action of external factors [10]. Thermal post-spray
treatment and, in particular, laser treatment has been tested to
allow the recrystallization of the amorphous phase [11–14]. In
contrast to annealing, a laser beam does not heat the subjacent
substrate. Moreover, it is an efficient and quick technology that
allows treatment of selected zones with high spatial resolution.
The optimised treatment allows obtaining the desired fraction of
amorphous phase and modification of phase composition [9,15].
The phenomena occuring in ceramics such as HA under action of
laser irradiation can be categorized as a function of the laser
power density as follows [16]:
• During laser treatment with low power density (b108 W/m2)
phase transformations are likely to take place but material
remains solid [9,15]. Treatment under these conditions
hardly modifies the coatings surface. It does produce neither
2249
pores nor fissures at the treated surface. The crystallinity of
the irradiated zone increases. These modifications are similar
to those produced by furnace annealing [17]. The recrystallization of the amorphous phase takes place at temperatures
between 1170 and 1400 K and is accompanied by the
formation of TTCP phase and an increase of HA phase [9].
At a temperature above 1400 K, the fractions of the HA
phase and of the amorphous calcium phosphate both
decrease leading to the formation of TCP and TTCP phases.
The following reactions are then likely to occur:
Ca10 ðPO4 Þ6 ðOHÞ2 →2Ca3 ðPO4 Þ2 þ Ca4 P2 O9 þ H2 O
ð1Þ
Ca10 ðPO4 Þ6 ðOHÞ2−2x Ox Vx →2Ca3 ðPO4 Þ2 þ Ca4 P2 O9
þ ð1−xÞH2 O
ð2Þ
Ca10 ðPO4 Þ6 O→2Ca3 ðPO4 Þ2 þ Ca4 P2 O9
ð3Þ
On the other hand, recrystallization of the amorphous phase
leading to the formation of TCP and TTCP can also occur.
• During laser treatment with the power density higher than
108 W/m2 fusion of the coatings surface occurs. This process
is called often laser glazing [4]. The surface gets smoother
but many cracks will be generated that will weaken the
mechanical integrity of the coatings. The cracking results
from temperature gradients and residual thermal stresses
generated by the laser treatment. Treated ceramics will
become denser [18]. Another characteristic phenomenon of
this type of treatment is a release of entrapped gases escaping
before final solidification and forming of holes in the coating
surface [5]. The thickness of the molten layer depends
strongly on the energy and the spot size of the laser beam.
This type of treatment is realized in the present study by
using a pulsed CO2 laser with different power densities. This
study examines the effect of laser power density on phase
composition and microstructure laser modified coatings. The
numerical code Fusion-2D, described elsewhere [19,20], was
applied to estimate the depth of molten zone in plasma-
Laser
treated
surface
As sprayed surface
100 µm
Fig. 1. SEM (secondary electrons) micrograph of separated laser spots obtained
at laser power density of 6.4 × 108 W/m2.
2250
S. Dyshlovenko et al. / Surface & Coatings Technology 201 (2006) 2248–2255
Table 3
Thermophysical data of HA used in modeling
Properties
Value
HA dense
Melting point
Boiling point
Latent heat of melting
Latent heat of evaporation
Density/molar mass
Heat capacity
Average for solid
Average for liquid
Heat conductivity
Average for solid
Average for liquid
Unit
Reference
HA porous
5%
10%
15%
1843
3500
4.87 × 107
1.44 × 109
2.95 × 1027
–
–
4.63 × 107
1.37 × 109
2.8 × 1027
–
–
4.16 × 107
1.23 × 109
2.52 × 1027
–
–
3.54 × 107
1.05 × 109
2.14 × 1027
K
K
J/m3
J/m3
m− 3
[28]
[6]
[29,30]
[6]
[6]
3.34 × 106
4.25 × 106
3.17 × 106
4.04 × 106
2.85 × 106
3.64 × 106
2.42 × 106
3.09 × 106
J/m3 K
J/m3 K
[6]
1.867
2.259
1.866
2.257
1.865
2.256
1.863
2.254
W/m K
W/m K
[6]
sprayed HA coating. The present paper is a continuation of
the study initiated by Cheang et al. [13] by taking into
account the crystal phases present in the coatings and a
physical understanding of the laser treatment process.
• During laser treatment with laser power density higher than
1010 W/m2 evaporation of the ceramics starts. This type of
treatment can be applied to modify the morphology of the
surface.
by the application of different substrates. The substrates were
blasted prior to processing using alumina grit with size in the
range +125–250 μm. The powder was pure HA prepared by
spray drying and commercialized by Tomita. XRD analysis of
the powder only the HA phase. The powder has a mean
diameter of d50% = 120 μm and the internal porosity of powder
particles is of about 12% [21]. Sprayed coatings had thickness
of about 400 μm.
2. Materials and experimental methods
2.2. Laser treatment of sprayed coatings
2.1. Plasma spraying of HA powder
The plasma-sprayed samples were heat-treated with a CO2
laser. The laser used in this study was a single-mode one,
working with the TEM00 mode at 10.6 μm wavelength. The
principal laser parameters are collected in Table 2. A He–Ne
laser helped in aligning the CO2 laser beam. The ZnSe lens
focused the beam in a spot of about 300 μm size (Fig. 1). Laser
power densities of q = 5.9 × 108 and 6.4 × 108 W/m2 were used in
the experiments. A scanning system allowed the laser spot to
move across the coating surfaces with linear speeds of v = 6.4
and 9.6 mm/s. For each laser power density two different scan
speeds were applied. In total, 32 experiments of laser treatment
were realized (Table 1).
Plasma spraying was carried out using a Praxair installation
including a SG100 torch with anode type P/N 2083-730
equipped with internal powder injector, cathode type 01083A,
manual console type 3710 and powder feeder type 1264. The
internal injection of powder was used throughout the experiments. Principal plasma spray parameters were designed in a
two level experimental plan, described in details elsewhere [8]
and are collected in Table 1 together with laser treatment data.
The spray distance was 10 cm and powder feed rate was about
17 g/min. The as-sprayed sample abbreviations, shown in Table
1, include electric power (in kW), fraction of argon in plasma
gas (in vol.%), and carrier gas flow rate (in slpm). The spraying
was carried out onto aluminum or stainless steel plates with
dimensions 15 × 15 × 3 mm. Although titanium alloy Ti-6Al-4V
is used typically for prostheses, the coating property tested in
the present paper, i.e. phase composition, should not be affected
Table 4
Coefficient of optical absorption of HA used in modeling
Type of material
Value,
10− 4m− 1
Reference
Hydroxyapatite
HA coating
HA coating
HA coating
Enamel composed of HA + 12 wt.%
water + 3 wt.% protein and lipids
8.25
10
1.00 ± 0.25
2.01 ± 0.16
8.02
[31]
[32]
[33]
[34]
[35,36]
200 µm
Fig. 2. Optical micrographs of the sample treated with laser in run 21 (right) and
sketch of laser spots used in the treatment (left). The spots' overlapping was
equal to 36%.
S. Dyshlovenko et al. / Surface & Coatings Technology 201 (2006) 2248–2255
2251
3. Modeling
Modeling was performed using a numerical code Fusion-2D,
described in detail and recently applied to predict the temperature
fields in different samples submitted to a continuous wave and
pulsed CO2 laser [19,20]. Here, the code was adapted to model
pulsed CO2 laser interaction with HA coatings made by APS in
order to simulate the experiments described above.
20 µm
Fig. 3. Optical micrograph of the surface of the sample treated with laser in run
27.
2.3. Methods of powder and coatings characterizations
XRD diagrams were collected with a Guinier Huber 670
imaging camera plate (Ge monochromator, Cu Kα1 radiation of
λ = 0.154056 nm). The stepwise 2Θ angle increase was set to
0.005° and step time to 1 s. This resulted in a scan rate of about
0.3°/min. The samples were prepared following the recommendations of French norms [22], i.e. firstly, coatings were detached
from the substrate and later, crushed prior to the diffraction
experiments. Phase identification was realized by EVA code by
superposing experimental X-ray diagrams with those of the data
base of International Centre for Diffraction Data. Crystalline
phases were identified by the following cards of the data base:
•
•
•
•
α-tricalcium phosphate (α-TCP), JCPDS no. 70-0364;
tetracalcium phosphate (TTCP), JCPDS no. 70-1379;
calcium oxide (CaO), JCPDS no. 75-0264;
hydroxyapatite (HA), JCPDS no. 73-1731.
Quantification of the phases was realized by refining the
diffraction profiles using PowderCell code [23]. The experimental diagrams were compared with the theoretical ones
generated using the data base of Inorganic Crystal Structure
Database (ICSD). The structures of following phases from the
database were helpful in the generation of theoretical profiles:
•
•
•
•
α-TCP, [24];
TTCP, [25];
CaO, [26];
HA, [27].
The shapes of the theoretically generated peaks were modeled using a pseudo-Voigt function. For each diagram,
polynomial function with order 8 was applied to modeling of
the ground noise. Besides the 8 parameters of the ground noise,
scale factor, zero point correction and cell parameters were
refined. The effect of micro stresses was not considered.
Microscopic observations with a scanning electron microscope JEOL type JSM-5300 and optical microscope Nikon
equipped with an electronic camera were made on laser-treated
coatings surfaces and cross-sections. The cross-sections were
prepared metallographically.
3.1. Optical and thermophysical data
The thermophysical data of HA used in numerical simulations
are presented in Table 3 and the coefficient of optical absorption is
shown in Table 4. This coefficient varies in different bibliographical sources from α = 104 to 105 1/m. Because of different spray
conditions resulting in different porosities of HA coatings as well
as diverging optical properties, the modeling of laser treatment
was made by assuming the data in following ranges:
• Porosity from 0% to 15%;
• Coefficient of optical absorption α = 2.1 to 8 × 104 m− 1;
• Emissivity ε = 0.5 to 1.
4. Results
4.1. Experiments
Laser treatment was performed by overlapping of spots
ranging from 36% to 50% (Fig. 2). Cracks formation can be
observed on treated surfaces. Moreover, some pores on the lasertreated surfaces are probably due to the release of entrapped gases
escaping before the melt solidifies [5]. The cracks are likely to be
formed as a result of the large temperature gradients and the
residual thermal stresses that develop after the laser passes over
the surface. A close investigation of melted and solidified surfaces
exhibits dendritically shaped crystals (Fig. 3). The crystals
crystallized from a laser-generated melt composed initially of
amorphous calcium phosphate, HA, and decomposition phases
(TCP, TTCP and CaO). The phases were identified using XRD
Table 5
Results of quantitative phase analysis related to the entire volume of coatings
Plasma spray
samples'
abbreviations
Phase composition
of as-sprayed
samples, vol.%
Phase composition of samples laser
treated with different power density,
vol.%
q = 5.9 × 108 W/m2
Sub11973
Sub11953
Sub119735
Sub119535
Sub24973
Sub24953
Sub249735
Sub249535 a
q = 6.4 × 108 W/m2
HA
αTCP
TTCP
HA
αTCP
TTCP
HA
αTCP
TTCP
73
70
74
73
69
65
63
64
19
12
9
10
11
15
11
8
8
18
17
17
20
20
26
26
60
63
53
56
59
56
60
40
13
19
19
16
17
14
13
14
27
18
28
28
24
30
27
44
56
58
47
52
43
43
59
37
16
9
22
24
20
21
17
16
28
33
31
24
37
36
24
45
a
All samples of Sub249535 as-sprayed and laser treated in different
conditions contain 2% CaO.
2252
S. Dyshlovenko et al. / Surface & Coatings Technology 201 (2006) 2248–2255
Superficial laser melted zone
Underlying as-sprayed layer
20 µm
µm
20
Fig. 4. Optical micrograph of the cross-section of the sample treated with laser in run 25.
analysis. In the analysis should be taken into account that coatings
were detached from the substrate and crushed prior to X-ray
determination. Consequently, the modifications introduced to the
phase composition by laser treatment are related to the entire
volume of the coating and not to the laser-treated zone. The XRD
of as-sprayed coatings for all used processing parameters is
analyzed in detail in previous paper coming from the research
group [8]. Table 5 presents the results of quantitative phase
analysis for the samples sprayed on the substrate using different
spray parameters, and later on, treated by laser with power
densities of q = 5.9 × 108 W/m2 and 6.4 × 108 W/m2. A typical
cross-section of laser-treated sample is shown in Fig. 4. The laser
molten zone can be easily observed. In general, the depth of the
molten layer depends on the laser pulse energy, the laser spot size,
the scan speed and the degree of overlapping of the laser spots. In
the present case, the time interval between laser shots is so long
that the laser induced melt can solidify and cool down before the
next shot arrives. Consequently, in the present processing
conditions, the spots overlapping and scan speed do not influence
depth of the molten zone. The latter depends only on laser pulse
power and laser spot size, i.e., the laser power density. These
depths are shown in Table 6. Their fluctuation for different
samples treated with the same laser power density could have
resulted from variation in porosity of sprayed coatings.
4.2. Modeling
The simulation of laser heating of material with Fusion-2D
code allows determining temperatures and phases (solid, liquid,
and vapor) fields in 2D. The runs of the code were made by using
Table 6
Microscopically determined depths of laser melted zones
Plasma spray
samples'
abbreviations
Laser power density
5.9 × 108 W/m2 (μm)
6.4 × 108 W/m2 (μm)
Sub11973
Sub11953
Sub119735
Sub119535
Sub24973
Sub24953
Sub249735
Sub249535
22
28
29
30
25
23
26
30
36
35
39
30
33
32
37
33
different values of porosity of coatings, optical absorption
coefficients and emissivities. The simulated process parameters
correspond to those shown in Table 1. The results of all
simulations are presented in Fig. 5. The calculated depths of
molten zone were compared with the data found experimentally
at the coatings cross-sections. As the cross-sections did not
reveal any trace of evaporation, all simulations revealing evaporation (Fig. 5c, d, e, and f) which corresponds to absorption
coefficient of α = 5 and 8 × 104 1/m were thought to be unrealistic
due to the overestimated optical absorption. Moreover, the calculated depths of the molten zone were greater than those observed experimentally. Better convergence was reached with
optical absorption coefficient equal to α = 2.1 × 104 1/m. The
zones of convergence of experimentally found and simulated
depths are inside the rectangles of the Fig. 5a and b. These zones
allow also an estimation of coatings emissivity values to be in the
range ε = 0.85–1.
5. Discussion
The powder used for plasma spraying was composed of pure
HA. The plasma-sprayed coatings contained between 63 and
75 vol.% of HA (Table 5). The coatings are relatively porous
(Figs. 1 and 3). An increase of electric power input to the plasma
torch produces more decomposition phases and less HA. It must
be noticed that the XRD analysis does not take into account the
formation of amorphous calcium phosphate. This phase is known
to be present in the coatings and its formation is discussed
elsewhere [8]. The laser treatment associated with melting of the
coatings results in smoothing of the surface (Fig. 2) and in a
further increase of HA decomposition and in the formation of αTCP and TTCP phases with dendritic structure (Figs. 3 and 5).
The decomposition is more pronounced for higher laser power
densities (Table 5). The increase of α-TCP and TTCP fraction in
the coating volume can be explained using the equilibrium phase
diagram. The diagram predicts namely the solidification of αTCP and TTCP from a melt with a composition corresponding to
stoichiometric hydroxyapatite. Another effect that can be
explained with the diagram is an increase of fraction of TTCP
in laser-treated samples (see Table 5). In fact, as P2O5 is an oxide
less refractory than CaO, its selective evaporation from the lasergenerated melt is probable. Consequently, at high temperature of
the melt associated with laser power density would render its
S. Dyshlovenko et al. / Surface & Coatings Technology 201 (2006) 2248–2255
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S. Dyshlovenko et al. / Surface & Coatings Technology 201 (2006) 2248–2255
Fig. 5. Temperature on treated surface (left side) and depth of molten zone (right side) at the end of laser pulse vs. emissivity for different porosities of coatings
calculated for: (a) laser power density q = 5.9 × 108 W/m2 and absorption coefficient α = 2.1 × 104 1/m; (b) laser power density q = 6.4 × 108 W/m2 and absorption
coefficient α = 2.1 × 104 1/m; (c) laser power density q = 5.9 × 108 W/m2 and absorption coefficient α = 5 × 104 1/m; (d) laser power density q = 6.4×108 W/m2 and
absorption coefficient α = 5 × 104 1/m; (e) laser power density q = 5.9 × 108 W/m2 and absorption coefficient α = 8 × 104 1/m; and (f) laser power density q = 6.4 × 108 W/
m2 and absorption coefficient α = 8 × 104 1/m. The zones of convergence of numerical simulations with experiments are inside the rectangles of depths of molten zone
vs. emissivity curves. Symbol “ev” corresponds to evaporation and the figure at the symbol corresponds to the evaporation depth.
composition richer in CaO than that of stoichiometric HA. That is
why the formation of TTCP, corresponding to the rich in CaO
composition of 4CaO·P2O5 on melt solidification is more
probable than that of α-TCP, which corresponds to poorer in
CaO composition of 3CaO·P2O5. This is, in fact, confirmed by the
quantitative XRD analysis shown in Table 5 for laser-treated
samples. The results of this analysis can be interpreted by the
increase of the depth of laser molten zone which, in turn, increases
the quantity of phases that solidify from the melt and increase their
quantity in entire coatings volume. Although P2O5 evaporation
happens on the melt surface, the convective move of the liquid
homogenizes the chemical composition in the entire molten zone.
6. Conclusions
Pulsed laser glazing of plasma-sprayed hydroxyapatite coatings modified the morphology of their surface as well as their
phase composition. The surface becomes smoother. The phase
content was richer in α-TCP and TTCP, being the products of HA
decomposition, than that of as-sprayed deposits. The fraction of
TTCP was greater in all samples treated laser. This can be
interpreted as the result of modification of the chemistry towards
CaO richer compositions. The numerical simulation with the
Fusion-2D code allowed predicting the temperatures fields in the
treated samples and the depth of the laser molten zone. In
particular, the model should help in predicting the laser treatment
condition which results in avoiding of CaO rich phases, which are
not recommended by in vivo experiments. The latter was found
also experimentally, by microscopic observations of sample
cross-sections. The comparison between theoretical and experiment values enabled to estimate the optical coefficients of the
actual deposits. Consequently, the optical absorption coefficient
was found to be equal to 2.1 × 104 1/m and the emissivity to be in
the range of ε = 0.85–1.
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