Low-energy plasma spray (LEPS) deposition of hydroxyapatite/poly-e-caprolactone biocomposite coatings
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Low-energy plasma spray (LEPS) deposition of
hydroxyapatite/poly-e-caprolactone biocomposite coatings
Citation for published version (APA):
Garcia - Alonso, D., Parco, M., Stokes, J., & Looney, L. (2012). Low-energy plasma spray (LEPS) deposition of
hydroxyapatite/poly-e-caprolactone biocomposite coatings. Journal of Thermal Spray Technology, 21(1), 132-
134. https://doi.org/10.1007/s11666-011-9695-0
DOI:
10.1007/s11666-011-9695-0
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Published: 01/01/2012
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Download date: 18. Jun. 2022JTTEE5 21:132–143
DOI: 10.1007/s11666-011-9695-0
1059-9630/$19.00 ASM International
Peer Reviewed
Low-Energy Plasma Spray (LEPS)
Deposition of Hydroxyapatite/
Poly-e-Caprolactone Biocomposite Coatings
Diana Garcia-Alonso, Maria Parco, Joseph Stokes, and Lisa Looney
(Submitted May 9, 2011; in revised form August 11, 2011)
Thermal spraying is widely employed to deposit hydroxyapatite (HA) and HA-based biocomposites on
hip and dental implants. For thick HA coatings (>150 lm), problems are generally associated with the
build-up of residual stresses and lack of control of coating crystallinity. HA/polymer composite coatings
are especially interesting to improve the pure HA coatings’ mechanical properties. For instance, the
polymer may help in releasing the residual stresses in the thick HA coatings. In addition, the selection of
a bioresorbable polymer may enhance the coatings’ biological behavior. However, there are major
challenges associated with spraying ceramic and polymeric materials together because of their very
different thermal properties. In this study, pure HA and HA/poly-e-caprolactone (PCL) thick coatings
were deposited without significant thermal degradation by low-energy plasma spraying (LEPS). PCL has
never been processed by thermal spraying, and its processing is a major achievement of this study. The
influence of selected process parameters on microstructure, composition, and mechanical properties of
HA and HA/PCL coatings was studied using statistical design of experiments (DOE). The HA depo-
sition rate was significantly increased by the addition of PCL. The average porosity of biocomposite
coatings was slightly increased, while retaining or even improving in some cases their fracture toughness
and microhardness. Surface roughness of biocomposites was enhanced compared with HA pure coatings.
Cell culture experiments showed that murine osteoblast-like cells attach and proliferate well on HA/PCL
biocomposite deposits.
Keywords biocomposite coatings, cell attachment, hydroxy-
to promote the fixation of host bone cells and enhance
apatite, plasma spraying, poly-e-caprolactone, the bonding strength to the metallic implant. In addition to
statistical design of experiments the intrinsic HA bioceramic properties (bioactivity and
chemical resemblance to mineral bone), process-dependent
coating properties, such as purity, crystallinity, porosity,
thickness, microtopography, and adhesive strength also
play an important role in achieving a homogeneous
1. Introduction bone ingrowth, which guarantees in vivo long-term sta-
bility (Ref 2, 3). Although APS is approved for depo-
Atmospheric plasma spraying (APS) is widely sition of HA coatings on femoral implants by the U.S.
employed to deposit osteoconductive hydroxyapatite Food and Drug Administration (FDA), alternative
(HA) coatings on femoral stems for hip endoprostheses thermal spray technologies, such as high-velocity oxy-
and dental implants (Ref 1, 2). The HA coating function is fuel (HVOF) (Ref 4, 5), vacuum plasma spraying (VPS),
detonation spray (DS) (Ref 6), and micro-plasma spray
(MPS) (Ref 7, 8) have been explored to process HA
coatings with improved structural morphologies and
compositions.
This article is based on an oral presentation at the 2009
International Thermal Spray Conference, Las Vegas, Nevada, The content of crystalline HA in the coating is generally
USA, May 4-7, 2009, and has been expanded from the original attributed to the retention of original unmelted HA parti-
presentation. Proceedings of the ITSC 2009: Basil R. Marple, cles (Ref 1), which negatively affect the coating cohesion
Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. (Ref 9). Controlling the degree of melting of the powder
Lima, and Ghislain Montavon, Ed., ASM International, particles during spraying allows the control of the amor-
Materials Park, OH, 2009. phous/crystalline phases ratio in the coating. It is well
Diana Garcia-Alonso, Joseph Stokes, and Lisa Looney, National known that the high temperature of the plasma jet in con-
Centre for Plasma Science & Technology (NCPST), Dublin City ventional high-energy APS induces undesirable phase
University, Dublin 9, Ireland; and Materials Processing Research changes of the HA feedstock. This results in severe powder
Centre (MPRC), Dublin City University, Dublin 9, Ireland;
Maria Parco, INASMET-Tecnalia, Mikeletegi Pasealekua 2, CP:
decomposition and formation of a mixture of amorphous
20009 San Sebastián, Spain. Contact e-mail: diana.garcia.alonso calcium phosphate (ACP), tricalcium phosphate (TCP),
@gmail.com, garciad2@mail.dcu.ie, and maria.parco@tecnalia. tetracalcium phosphate (TTCP), calcium oxide (CaO), and
com. dehydroxylation products such as oxyhydroxyapatite
132—Volume 21(1) January 2012 Journal of Thermal Spray Technology(OHA) and oxyapatite (OA) (Ref 10-12). Crystallinity of injected at different thermal locations of the plasma jet,
Peer Reviewed
APS HA coatings typically varies in the range of 65-80% very challenging. For this, the optimization of the depo-
(Ref 13, 14). Highly crystalline HA coatings (~92%) have sition process is required to sufficiently melt the HA
been deposited using the MPS process (Ref 15), the lower powder without degrading excessively the PCL powder.
thermal input of which enables the reduction of the over- In this study, novel HA/PCL biocomposites were
heating of powder particles and substrate. However, it must deposited successfully using an experimental low-energy
be said that the optimal phase composition of HA coatings plasma spray (LEPS) system, enabling the thermal deg-
remains open to discussion: crystalline HA is seen as a radation of the polymer to be minimized. Separated
crucial factor for coatings’ in vivo stability (Ref 16, 17), but injection of the two feedstock powders (HA and PCL)
the presence of easily soluble ACP, TCP, or TTCP has been into different zones of the plasma plume was necessary for
found to accelerate the bone attachment rate (Ref 18). that purpose. The aim of this research is to examine the
Customary APS HA coatings are relatively dense effect of the most influential LEPS process parameters on
(porosity 2-10%), and their thicknesses are limited to selected properties of thick HA coatings (>500 lm) and
approximately 150 lm (Ref 10), as thicker coatings to determine the optimum set of process variables to
become too brittle because of the accumulation of residual subsequently produce HA/PCL biocomposite coatings
stresses. Tensile stresses, generally induced in APS HA with improved properties. Finally, a salient feature of this
coatings, should be avoided as they accelerate in vivo research is also to prove the feasibility of processing low-
dissolution and promote multiple cracking of the coatings melting temperature polymers by LEPS without major
(Ref 19). Fracture toughness values for APS HA coatings degradation.
range between 0.39 and 0.55 MPa m1/2, well below the
value of bulk HA (~1 MPa m1/2) (Ref 15, 20). In the case
of the MPS process, the lower thermal input allows for the 2. Materials and Methods
production of 230-lm-thick HA coatings, with a porosity
as high as 20% and without negatively affecting the
2.1 Materials
mechanical properties of the coatings (fracture toughness:
0.6 MPa m1/2) (Ref 15). A commercially available hydroxyapatite powder
In order to improve HA coatings’ performance and to (CAPTAL 60-1, Plasma Biotal Ltd) with an average par-
counteract the well-known brittle behavior of HA, ticle size of 52.1 lm, a purity of 100%, and a crystallinity
extensive research on composite coatings has been con- greater than 99% was used as matrix material. A biore-
ducted (Ref 21-25). The selection of bioresorbable poly- sorbable polymer, poly-e-caprolactone (PCL) from Sigma-
meric second phases can be beneficial for both the Aldrich Co. (Product No. 440744), was employed as a sec-
mechanical and biological properties. First of all, flexible ond phase. PCL was cryogenically milled and sieved to
polymeric second phases may favor the release of residual yield an average particle size of 85.3 lm. Thick deposits
stresses in HA coatings, allowing for thicker coatings with (>500 lm) sprayed onto mild steel substrates
enhanced stability. Second, bioresorbable polymers could (19 9 42 9 2 mm3) were used for process development
eventually be employed to encapsulate drugs and play an studies. Biological testing was carried out on coatings
important role as in situ drug delivery systems (Ref 26). sprayed onto titanium (grade 4) substrates (U = 12 mm;
Among the bioresorbable synthetic polymers, poly-e-cap- 2 mm thick). All substrates were cleaned with alcohol to
rolactone (PCL) is a good candidate as a second phase due remove grease and dust, and subsequently grit blasted with
to its relatively high toughness at body temperature alumina (Al2O3 F24, 600-850 mm) at a blasting pressure of
(~37 C) and because it does not produce an acidic envi- 6 bar to ensure oxides removal and to roughen the surfaces.
ronment during biodegradation (Ref 27). PCL is a biore-
sorbable, anti-inflammatory, and non-mutagenic polymer.
2.2 Low-Energy Plasma Spray (LEPS) System
It has a low melting temperature (Tm = 60 C), but it does
not undergo degradation below Td = 360 C (Ref 28). The The plasma spray system used in this study is an
relatively good thermal stability of PCL may thus allow its experimental low-energy version (max. 15 kW power) of a
deposition by thermal spraying without major degrada- conventional plasma spray system, which was designed by
tion, providing that the process and the injection of the INASMET-Tecnalia (Spain) to deposit osteoconductive
polymer are optimized. coatings on small-sized bone implants. The special parts of
HA/PCL biocomposites have been extensively the system are the power supply (a standard DC power
researched for tissue engineering applications (Ref 29-31), source from a TIG-welding system) with electronic
including drug delivery coatings (Ref 32). However, HA/ autoignition control (Praxair, Triton 400) and the plasma
PCL have never been employed to coat metallic femoral gun that works with just one plasma gas (Argon). The gun
stems or dental implants. Furthermore, the deposition of design allows the system to work in a turbulent plasma gas
HA/PCL, or even PCL, via thermal spraying has never jet flow using a low-energy input without becoming
been reported in the literature. The difference between unstable. A more detailed description of the system has
the thermal characteristics of PCL (Tm = 60 C; Td = been provided elsewhere (Ref 34). The powder injector
360 C) and HA (dehydroxylation temperature: 800- was modified to facilitate the polymer feeding at different
900 C; incongruent melting temperature: 1500 C; thermal locations into the plasma plume. Powder feeding
Ref 33) renders thermal spraying of both materials, rates were chosen so as to supply 0.5-1.3 wt.%
Journal of Thermal Spray Technology Volume 21(1) January 2012—133(1.2-3.7 vol.%) of PCL to the plume. The process deposits with a load of 0.5 N following the procedure
Peer Reviewed
parameters employed to produce the HA and the HA/ reported by Mohammadi et al. (Ref 38). The average
PCL biocomposite series are presented in Table 1. microhardness (HV0.5) was given as an average of 10
indentations. These indentations were also employed to
2.3 Coating Characterization measure the indentation fracture toughness using the
equation reported by Bolelli et al. (Ref 39). The crack and
2.3.1 Thickness and Surface Roughness. The coating half-diagonal lengths were measured using a Reichert Me
thickness was calculated as the average of 10 measure- F2 Universal Camera optical microscope at a magnifica-
ments made with a DualscopeMP0R-FP (Fisher Instru- tion of x500 with at least 10 indentations per sample. The
ments) based on the magnetic induction and eddy current porosity of the components was measured by digital image
methods. The layer thickness (equivalent to the growth analysis (Matlab Image Processing Toolbox) as the
rate in thin film processing) was calculated dividing the average of 10 SEM micrographs (91000) on sample cross
total deposit thickness by the number of sprayed layers sections (Ref 40).
(i.e., the number of gun passes). The coating surfaces and 2.3.4 Biological Characterization. MC3T3 murine cal-
cross sections were studied by scanning electron micros- varial osteoblast-like cells were cultured under standard
copy (SEM EVO LS15, Zeiss). The average roughness tissue culture conditions in a-MEM medium (Gibco Ltd.)
(Ra) was calculated using a Surftest-402 profiler (Mitutoyo) with 10 vol.% foetal bovine serum and 1% penicillin/
as the average of 10 measurements, each one over a line of streptomycin added. The samples were sterilized by
5-mm length. immersion into 70 vol.% ethyl alcohol for 2 h, rinsed with
2.3.2 Crystallinity and Chemical Composition. X-ray sterile phosphate buffer saline (PBS), and stored in culture
diffraction (Bruker AXS D8 Advance, Bragg-Brentano medium inside a CO2 incubator overnight to promote pro-
geometry, Cu Ka, 2h: 20-60, 5 s/step, and step size of 0.2) tein adsorption. The cells were seeded onto the selected
was conducted on HA coatings to determine their crys- coatings at a density of 105 cells/cm2 suspended in 20 lL of
tallinity and phase purity following the methods indicated medium. After an initial attachment period, the medium
elsewhere (Ref 35, 36). Fourier transform infrared spec- was completed to 500 lL. Seeded samples were stained with
troscopy (FTIR Spectrum GX, Perkin-Elmer) was em- DAPI and FITC-labeled phalloidin to assess cell morphol-
ployed to further assess the coatings composition by ogy. Cell proliferation was quantified by an Alamar Blue
qualitative analysis of the HA dehydroxylation and PCL cell proliferation assay (AbBiotech Ltd.) at 7 days and
degradation. The samples were prepared following the 14 days post-seeding as described elsewhere (Ref 41).
procedure reported by Baji et al. (Ref 37). X-ray photo-
electron spectroscopy (XPS Thermo Scientific K-Alpha,
2.4 Statistical Design of Experiments (DOE)
monochromatic Al Ka (hm = 1486.6 eV), x-ray spot:
for Process Development
100 lm) was employed to verify the presence of PCL in
the HA/PCL coatings. Samples were neutralized using a The experimental approach followed in this study con-
flood gun to correct the differential or non-uniform sisted of two steps: 1) examining the effects of the most
charging. XPS high-resolution scans were performed for influential LEPS process parameters on selected properties
the C 1s at pass energy of 50 eV. of thick pure HA coatings to determine the optimum set of
2.3.3 Mechanical Properties and Porosity. The HA deposition parameters to be used for the production of
microhardness and fracture toughness measurements were HA/PCL biocomposite series; 2) assessing the effects of the
carried out using a Vickers indenter (Miniload 2, Leitz). PCL-related process parameters on the quality of HA/PCL
The indentations were made on the cross sections of the coatings. Statistical DOE allows for correlating the variation
Table 1 LEPS parameters for HA and HA/PCL coatings deposition, and DOE-coded factors and levels for statistical
studies of HA (xi) and HA/PCL (xi ) deposition processes (i = 1, 2, 3)
DOE factor code
Parameter HA HA/PCL Factor 21 0 1
Current, A 390 390 … … … …
Power, kW 12 12
Plasma gas flow rate (Ar), slpm 30-42 36 x1 30 36 42
HA feeding rate, g/min 5-9 9 x3 5 7 9
PCL feeding rate, mg/min … 40-120 x1 40 80 120
Carrier gas 1 flow rate (Ar), slpm 4.6 4.6 … … … …
Carrier gas 2 flow rate(N2), slpm … 10-14 x3 10 12 14
Stand-off distance (s.d.), cm 3.5-4.5 3.8 x2 3.5 4.0 4.5
Relative gun velocity, cm/s 20 20 … … … …
Increment y-direction (Dy), mm 3 3 … … … …
Injector 1 position Gun barrel Gun barrel … … … …
Injector 2 position (d), mm … 17-27 x2 17 22 27
Cooling system (compress air), bar None 6 … … … …
134—Volume 21(1) January 2012 Journal of Thermal Spray Technologyof targeted coating properties (called responses) with which represent the quadratic effect of the factor xi2; and e
Peer Reviewed
selected process parameters (called factors), maximizing is the experimental error.
the amount of information that can be obtained from a The accuracy of the statistical models to predict the
given amount of experimental tests. The range of variation experimental results was assessed for a confidence interval
of each factor is described by its lower and upper limits, of 95% considering several key statistical parameters
which are respectively coded as ‘‘1’’ and ‘‘+1’’, while the including lack of fit (p > 0.1), signal-to-noise ratio (ade-
level ‘‘0’’ represents the interval center. Extended infor- quate precision >4), regression coefficient (0.6 < R2
mation about DOE is available in the NIST/SEMATECH < 1), and accuracy ([adjusted R2 predicted R2] < 0.2).
e-Handbook of Statistical Methods (Ref 42).
In this study, six coating properties were selected to
evaluate the deposition processes: average layer thick-
ness deposited per spray pass (L), average surface
3. Results and Discussion
roughness (Ra), crystallinity (C), porosity (P), Vickers
microhardness (H), and fracture toughness (K). The layer 3.1 Hydroxyapatite Coatings
thickness deposited per spray pass, which is related to the The effect of three LEPS process parameters (x1,
deposition rate and the process time efficiency, was stud- plasma gas flow rate; x2, stand-off distance; and x3, HA
ied with the aim of reducing the manufacturing time and feed rate) on the selected properties of thick HA coating
material consumption. The surface roughness, the coating (>500 lm) is reported in this section. The characteriza-
crystallinity, porosity, and mechanical properties were tion of pure HA coatings sets the reference for compari-
included in the analysis because of their critical role for son with the HA/PCL biocomposite coating properties,
the coating in vivo behavior. which are reported in next section. The average layer
A three-factor Box-Behnken design consisting of 17 thickness deposited per spray pass (subsequently referred
experimental runs (12 design points and 5 center points) to as Ôlayer thicknessÕ), average surface roughness, coating
was used for the deposition of pure HA powder. Based on crystallinity, porosity, microhardness, and fracture tough-
screening tests carried out previously (Ref 34), three fac- ness of pure HA coatings are presented in Table 2.
tors were selected (plasma gas flow rate (x1), stand-off The phase purity of the coatings was also analyzed. The
distance (x2), and powder feeding rate (x3)), and their only statistically significant dependencies found were for
levels (-1, 0, +1) were chosen. Table 1 shows the LEPS the layer thickness (L) and coating crystallinity (C). The
parameters employed to produce the pure HA series, the statistical models for these two responses in terms of the
three selected factors, and their respective levels. Once actual factors can be obtained by substituting the coeffi-
the optimum set of process parameters for pure HA cients bij and the statistical power transformation expo-
deposition was determined, the effect of the PCL addition nent k listed in Table 3 in Eq 1:
on the responses was studied.
A full two-level factorial design was selected, using the L ðlmÞ1:59 ¼ 1:23 103 þ 8:6x1 þ 5:19 102 x2
PCL feeding rate (x1 ), PCL injector distance to the gun þ 72x3 2:4x1 x3 68:5x22 þ 3:2x23 (Eq 2)
exit (x2 ), and PCL carrier gas flow rate (x3 ) as the main
factors. The design consisted of 12 experimental runs, 8 of C ð%Þ3 ¼ 7:9 1:07 104 x1 1:80 105 x2 þ 8:92 104 x3
which were design points and 4 center points. The LEPS
parameters employed to produce the HA/PCL composite þ 4:95 103 x1 x2 6:43 103 x23 (Eq 3)
series, the selected factors, and their respective levels are
listed in Table 1. The parameters related to the HA
The regression coefficients and the precision of fit ob-
powder feeding and processing were set to the values
tained from ANOVA are also shown in Table 3. The
previously optimized for pure HA deposition.
statistical models of the four other responses (Ra, P, H,
The statistical evaluation was conducted using Design-
and K) were non-significant; and therefore, the mean va-
Expert 7.0 (Stat-Ease Inc.). The significance of each
lue of the series was adopted for the results discussion.
factor was analyzed with ANOVA (analysis of variance)
3.1.1 Thickness and Surface Roughness. The layer
using a stepwise automatic reduction algorithm. The sta-
thickness (L), which is a measure of the HA deposition
tistical models resulting from the ANOVA study were
rate, varied from 4.8 to 33.0 lm/spray pass for different
expressed by second-order quadratic polynomial equa-
factor combinations (Table 2). The ANOVA shows that L
tions:
X X X depends linearly on the plasma gas flow rate (x1) but
yk ¼ b 0 þ bi xi þ bij xi xj þ bii x2i þ e ðEq 1Þ quadratically on the stand-off distance (x2) and the HA
feed rate (x3). The interaction x1x3 was also found to be
where y is the response; k the statistical power transfor- significant (Table 3). The layer thickness is strongly re-
mation exponent (used in some cases to optimize model lated to the degree of melting of the impinging particles,
fitting); i, j vary from 1 to the number of process factors; because large unmelted particle cores are more likely to
coefficient b0 is the mean of responses of the series, bi bounce off the substrate. Increasing the plasma gas flow
coefficients represent the linear effect of the factor xi; bij, rate (x1) causes a proportional increase of the in-flight HA
are the coefficients of regression representing the xixj particles velocity, which decreases their residence time
interaction effects; bii, are the coefficients of regression inside the plasma plume and thus, reduces their degree of
Journal of Thermal Spray Technology Volume 21(1) January 2012—135Table 2 Properties of LEP-sprayed HA coatings (average (standard deviation))
Peer Reviewed
DOE factor
Layer Roughness Crystallinity, Porosity, Micro Fracture
Sample x1 x2 x3 thickness, lm Ra, lm % % hardness, HV0.5 toughness, MPa m1/2
HA1 1 1 0 24.3 (0.6) 6.47(0.21) 84.2 (3.5) 3.8 (0.5) 334 (50) 0.66 (0.10)
HA2 1 1 0 13.8 (0.5) 5.49 (0.30) 89.1 (1.7) 2.8 (0.4) 279 (43) 0.56 (0.07)
HA3 1 1 0 20.2 (0.7) 6.30 (0.45) 84.5 (1.4) 3.7 (0.3) 329 (47) 0.66 (0.07)
HA4 1 1 0 4.8 (0.2) 6.07 (0.47) 89.7 (1.9) 2.5 (0.4) 244 (24) 0.52 (0.04)
HA5 1 0 1 15.9 (0.9) 6.23 (0.31) 83.9 (0.8) 3.5 (0.7) 332 (48) 0.68 (0.07)
HA6 1 0 1 9.6 (0.3) 5.50 (0.31) 89.2 (2.3) 3.3 (0.3) 275 (47) 0.64 (0.08)
HA7 1 0 1 33.0 (1.8) 6.46 (0.60) 82.8 (0.4) 3.9 (0.7) 351 (38) 0.70 (0.06)
HA8 1 0 1 17.9 (1.1) 5.75 (0.18) 89.1 (2.2) 2.9 (0.4) 292 (46) 0.57 (0.08)
HA9 0 1 1 11.7 (0.8) 5.84 (0.30) 86.9 (2.3) 5.0 (0.7) 260 (42) 0.53 (0.08)
HA10 0 1 1 9.9 (0.3) 5.90 (0.41) 86.0 (1.5) 6.0 (0.5) 271 (34) 0.55 (0.05)
HA11 0 1 1 24.4 (0.8) 5.83 (0.35) 86.6 (1.7) 4.2 (0.6) 307 (66) 0.69 (0.11)
HA12 0 1 1 23.5 (0.8) 5.82 (0.30) 86.8 (2.2) 4.2 (0.4) 337 (41) 0.71 (0.07)
HA13 0 0 0 17.4 (0.9) 6.09 (0.56) 88.0 (1.8) 5.0 (0.2) 332 (52) 0.66 (0.09)
HA14 0 0 0 19.6 (1.0) 5.90 (0.57) 87.5 (1.2) 4.8 (0.4) 331 (54) 0.68 (0.11)
HA15 0 0 0 18.2 (1.0) 6.06 (0.37) 87.8 (1.3) 4.7 (0.6) 339 (49) 0.68 (0.08)
HA16 0 0 0 18.5 (0.8) 5.79 (0.53) 88.0 (0.7) 5.7 (0.4) 278 (40) 0.55 (0.09)
HA17 0 0 0 19.2 (0.8) 6.03 (0.27) 87.1 (0.8) 4.8 (0.8) 322 (42) 0.64 (0.06)
HAopt(a) 36 9 3.8 26.1* 6.00+ 86.5* 4.2+ 307+ 0.63+
(a) HAopt sample was calculated using the DOE optimization tool, and thus the parameters are given in actual factor. The responses are:
*predicted values by statistical models; +HA series mean values
Table 3 Factors coefficients bij, statistical power transformation exponent k, and regression coefficients of polynomial
equations of HA and HA/PCL composite coatings
HA coatings HA/PCL coatings
L, lm C, % L, lm Ra, lm
3 4
b0 1.23 9 10 +7.9 +2.82 9 10 +2.58 9 101
b1 +8.6 1.07 9 104 1.25 9 102 0.7
b2 +5.19 9 102 1.80 9 105 1.02 9 103 1.9
b3 +72.0 +8.92 9 104 … …
b12 … +4.95 9 103 +6.1 3.3 9 103
b13 2.4 … … …5.2 9 103
b23 … … … +0.1
b11 … … … …
b22 68.5 … … …
b33 +3.2 6.43 9 103 … …
k 1.59 3.00 2.09 1.00
R2 0.983 0.946 0.827 0.862
Adj. R2 0.973 0.922 0.807 0.826
Pred. R2 0.951 0.818 0.731 0.689
Precision 37.3 20.6 14.4 12.4
L layer thickness, C crystallinity, Ra average surface roughness
melting. This is why the deposition rate decreased when as a representative average roughness for the series. This
the plasma gas flow rate was too high and the melting of value is in the lower part of the range reported for plasma-
particles insufficient. The deposition rate was also reduced sprayed HA coatings using H2 secondary plasma (Ref 9).
when the stand-off distance (x2) deviated from an opti- 3.1.2 Crystallinity and Chemical Composition. The
mum value found around 4 cm. Finally, as expected, the coating crystallinity (C) varied between 82.8 and 89.7%
layer thickness increased with the HA feeding rate (x3). for different factor combinations, which in any case was
Figure 1 shows the cross section of deposits produced with greater than the minimum of 45% required by the ISO
the factor combinations resulting in the lowest (9.6 ± 13779-2: 2000 standard (Ref 43). These values are also
0.3 lm; sample HA6) and the highest (33.0 ± 1.8 lm; higher than the common 65-80% crystallinity values of
sample HA7) layer thickness values. APS HA coatings for biomedical applications (Ref 13, 14,
The average surface roughness (Ra) ranged from 5.49 to 35), but lower than the values reported for MPS HA
6.47 lm for different factor combinations. Owing to the coatings (Ref 15). The XRD diffraction patterns of the
small range of variation together with the large standard HA coatings with the lowest (HA7) and the highest
deviations (0.50 lm) the series mean (6.00 lm) was selected crystallinity (HA4) are shown in Fig. 2.
136—Volume 21(1) January 2012 Journal of Thermal Spray TechnologyPeer Reviewed
Fig. 1 Cross sections of HA samples LEP-sprayed at different factor combinations: (a) lowest layer thickness HA6 (x1 = 42 slpm,
x2 = 4 cm, x3 = 5 g/min); and (b) highest layer thickness HA7 (x1 = 30 slpm, x2 = 4 cm, x3 = 9 g/min)
Fig. 3 SEM image of Vickers indentation employed to evaluate
Fig. 2 XRD patterns of HA samples LEP-sprayed at different the fracture toughness of sample HA12
factor combinations leading to the lowest crystallinity 82.8%
(HA7: x1 = 30 slpm, x2 = 4 cm, and x3 = 9 g/min); and the highest
crystallinity 89.7% (HA4: x1 = 42 slpm, x2 = 4.5 cm, and x3 = 7 g/ Therefore, it can be said that the purity of LEPS coatings
min), as compared to the HA raw powder is close to 100%, which is above the 95% required by the
ASTM standard ISO 13779-1:2000 (Ref 44). The loss of
The ANOVA shows that the crystallinity depends lin- OH groups was further studied by FTIR. Only the
early on the plasma gas flow rate (x1) and quadratically on samples which received the largest in-flight thermal input
the HA feeding rate (x3). The interaction between x1 and (plasma gas flow rate = 30 slpm) showed a small degree of
x2 was also found to be significant (Table 3). The crys- dehydroxylation. For these samples, a major decrease of
tallinity increased with increasing plasma gas flow rate the OH flexural vibration band intensity at 635 cm1 and
(x1), which confirms that the main source of crystallinity is a minor decrease of the stretching vibration band intensity
the incorporation of unmelted powder particles into the at 3571 cm1 were noticed. An outstanding HA purity was
deposit. As explained above, particles traveling faster observed in samples produced with the highest plasma gas
through the plume (due to greater plasma gas flow rate) flow rate (42 slpm), which led to the shortest particle
melt to a lesser degree, thus maintaining a higher crys- residence time inside the plume.
tallinity. The variation of crystallinity in response to dif- 3.1.3 Mechanical Properties and Porosity. The Vick-
ferent feeding rates (x3) was non-significant compared to ers microhardness (H) and the fracture toughness (K) of
the standard deviations (0.5-2%) when the other factors HA coatings ranged from 244 to 351 HV0.5 and from 0.52
were kept constant. Also, the effect of the stand-off dis- to 0.71 MPa m1/2, respectively (Table 2). A SEM image of
tance (x2) on the crystallinity for a fixed plasma gas flow a sample indentation used for the evaluation of these
rate (x1) was minimal (0.5%), well within the range of the properties is presented in Fig. 3. Both properties have
standard deviation. large standard deviations due to the typical anisotropic
Calcium phosphates other than hydroxyapatite (JCPDS structure and porosity of thermally sprayed coatings. The
9-0432) were not found in any sample of the HA series, ANOVA results suggest that, owing to the large standard
even though detailed XRD scans were performed around deviations of the measurements, the average values
the principal peaks of b-TCP (2h = 31.8 C; JCPDS 9-0169) (308 HV0.5 and 0.63 MPa m1/2) are representative of the
and oxyapatite (OA) (2h = 53.2 C; JCPDS 89-6495). series. The measured values are similar to those reported
Journal of Thermal Spray Technology Volume 21(1) January 2012—137by Mohammadi et al. (Ref 38) for conventional HA crystallinity statistical models (Eq 2, 3) predict values of
Peer Reviewed
plasma-sprayed coatings. 26.1 lm and 86.5%, respectively. The low value of desir-
The average closed porosity (P) ranged between 2.5 ability of this solution (0.56) is related to the opposite
and 6.0% (Table 2). The ANOVA results suggest adopt- trends of crystallinity and layer thickness. It should be
ing the mean value 4.2% as representative of the series. stressed that attainment of optimum deposit properties
However, by looking carefully at the porosity data, it can generally requires parameter tradeoffs (Ref 47); opposite
be observed that the samples sprayed with the highest parameter settings may be called for different deposit
plasma gas flow rate have the lowest porosity. This fact properties. The mean values obtained for the HA series
contradicts the results reported in the literature for con- roughness, porosity, micro hardness, and fracture tough-
ventional plasma spraying (Ref 45, 46). A possible ness were used for reference (HAopt in Table 2).
explanation for this discrepancy is the significant differ-
ence of supplied thermal energy by the different spraying 3.2 HA/PCL Composite Coatings
systems. With a conventional APS system (higher energy),
increasing the plasma gas flow rate results in a reduced The LEPS parameters employed to produce HA/PCL
melting of in-flight HA particles. As a result, the flattening series, the selected factors, and their respective levels are
of impinging particles on the coating is reduced, and listed in Table 1. The parameters related to the HA
therefore, porosity and roughness are increased. Owing to feeding and the process itself were set to the values pre-
the reduced thermal input provided by the LEPS system, viously optimized for pure HA deposition (section 3.1.4).
achieving sufficient melting of in-flight HA particles gen- For this reason, no changes in the HA crystallinity
erally requires a low plasma gas flow rate. As the plasma (86.5%) and HA phase content (~100% pure HA) are
gas flow rate increases, the thermal input becomes insuf- expected in the HA/PCL coatings. The layer thickness,
ficient and particles bounce off the substrate, leading to surface roughness, porosity, microhardness, and fracture
thinner and more compact coatings, with lower porosity toughness of HA/PCL composite coatings are presented in
and surface roughness. The SEM images of the cross Table 4. The degradation of the polymer was also
sections of two samples with different degrees of porosity assessed. Layer thickness and average surface roughness
are shown in Fig. 4. models (Eq 4, 5) were expressed by polynomial equations
3.1.4 Statistical Optimization. The statistical optimi- obtained from the statistical DOE (Eq 1). The coefficients
zation of the selected responses indicates that the best bij, the statistical power transformation exponent k, the
factor combination for deposition of HA coatings with regression coefficients, and the precision of fit obtained
high crystallinity and high layer thickness is: x1 = 36 slpm, from ANOVA are shown in Table 3.
x2 = 3.8 cm, and x3 = 9 g/min. Plasma gas flow rates lower
L ðlmÞ2:09 ¼ 2:82 104 1:25 102 x1 1:02 103 x2
than 36 slpm have been shown to result in slight HA
dehydroxylation, while higher plasma gas flow rates were þ 6:1x1 x2 (Eq 4)
found to decrease the layer thickness and deposition effi-
ciency. The highest powder feed rate (x3) was selected to Ra ðlmÞ ¼ 2:58 101 0:7x1 1:9x2 3:3 103 x1 x2
allow for faster deposition. The stand-off distance (x2) was 5:2 103 x1 x3 þ 0:1x2 x3 (Eq 5)
calculated using the optimization tool included in the
Design-Expert software while fixing the plasma gas flow
rate and powder feed rate to 36 slpm and 9 g/min, 3.2.1 Thickness and Surface Roughness. The layer
respectively. This optimum set of parameters was thickness (L) was found to vary from 34.0 to 84.4 lm/layer
employed to produce the HA/PCL composite coating for different factor combinations (Table 4), which means a
series. Therefore, the responses for this set of parameters significant increase with respect to the HA reference
were calculated to assess the impact of PCL addition on (L(HAopt) = 26.1 lm). The ANOVA showed that the
the coatings (HAopt in Table 2). The layer thickness and layer thickness depends linearly on the PCL feeding rate
Fig. 4 Porosity of HA samples LEP-sprayed at different factor combinations: (a) low porosity 3.7% (HA3: x1 = 30 slpm, x2 = 4.5 cm,
x3 = 7 g/min); and (b) high porosity 6.0% (HA10: x1 = 36 slpm, x2 = 4.5 cm, x3 = 7 g/min)
138—Volume 21(1) January 2012 Journal of Thermal Spray TechnologyTable 4 Properties of LEP-sprayed HA/PCL coatings (average (standard deviation))
Peer Reviewed
DOE factor
Layer Roughness Porosity, Micro Fracture
Sample x1 x2 x3 Thickness, lm Ra, lm % hardness, HV0.5 toughness, MPa m1/2
HP1 1 1 1 84.4 (1.5) 8.28 (0.95) 4.2 (0.6) 269 (65) 0.66 (0.10)
HP2 1 1 1 74.7 (1.9) 13.51 (0.94) 4.9 (0.9) 300 (83) 0.63 (0.14)
HP3 1 1 1 34.0 (0.9) 8.23 (0.90) 7.6 (1.4) 264 (43) 0.55 (0.10)
HP4 1 1 1 71.2 (3.9) 8.28 (0.91) 5.0 (0.6) 331 (33) 0.63 (0.14)
HP5 1 1 1 79.7 (1.2) 7.76 (0.74) 5.8 (0.7) 308 (28) 0.65 (0.18)
HP5 1 1 1 75.8 (2.0) 11.89 (0.89) 5.7 (1.0) 346 (28) 0.72 (0.11)
HP7 1 1 1 45.5 (0.8) 7.93 (0.68) 5.8 (0.5) 261 (19) 0.54 (0.06)
HP8 1 1 1 49.9 (3.3) 12.20 (0.67) 7.5 (1.4) 273 (35) 0.62 (0.07)
HP9(a) 0 0 0 30.8 (2.1) … … … …
HP10 0 0 0 71.2 (2.0) 9.12 (1.08) 5.8 (0.6) 362 (34) 0.78 (0.11)
HP11 0 0 0 78.6 (2.4) 9.74 (0.85) 5.7 (0.8) 302 (65) 0.59 (0.12)
HP12 0 0 0 64.2 (2.3) 9.53 (0.96) 5.6 (0.6) 339 (58) 0.62 (0.19)
(a) Sample HP9 is much thinner than the other center points samples. Possibly the injector nozzle was partially clogged by polymer particles
during the deposition process. As this sample was likely the product of an experimental error, it was discarded
PCL particles on the coating. The ANOVA showed that
Ra is linearly dependent on all factors as well as their
interactions (Table 3). The roughness increased as the
PCL feed rate (x1 ) increased, whatever the value of the
other factors. This trend supports the previous statement,
since a higher amount of polymer splats will leave more
surface defects when impacted by HA particles, leading to
a higher surface roughness. At a fixed PCL carrier gas flow
rate (x3 ), the greater the PCL feeding rate the greater the
influence of the injector distance (x2 ) on the final surface
roughness. The roughness decreased with increasing
Fig. 5 Effect of injector distance on PCL trajectories (s.d. = injection distance, attributed mainly to the turbulence
stand-off distance) close to the substrate as explained previously.
3.2.2 Polymer Degradation. FTIR spectra of HA/PCL
(x1 ), the injector distance (x2 ), and their interaction (x1 x2 ) composite coatings were evaluated taking as a reference
(Table 3). The layer thickness decreased linearly with the FTIR spectra of the PCL and HA feedstock powders,
increasing injector distance. The minimum thickness oc- the characteristic FTIR bands of which can be found
curred for the lower PCL feeding rate (x1 = 40 mg/min) elsewhere (Ref 48-50). The FTIR spectra of all the
when the injector was placed at x2 = 27 mm from the HA/PCL coatings revealed the presence of the carbonyl
nozzle of the plasmatron, i.e., 11 mm from the substrate. If (C=O) group at 1735 cm1 and the -CH2- stretching bands
the injector was too close to the substrate, the turbulences at 2866 and 2940 cm1 characteristic of PCL (with dif-
near the substrate prevented the polymer particles from ferent intensities), regardless of the small quantities of
entering the plume (see Fig. 5 for a schematic represen- PCL (feeding rate of 0.5-1.3 wt.%). The stretching bands
tation of this effect). A shorter injector distance from the corresponding to the PCL C-O and C-C (crystalline at
gun nozzle (x2 = 17 mm; i.e., 21 mm from the substrate) 1293 cm1; amorphous at 1157 cm1), COC (1240 and
allowed for most of the polymer particles entering the 1170 cm1), and OC-O (1190 cm1) overlapped with the
plume, thus increasing the thickness per spray pass. Using mPO43 bands of HA in the HA/PCL composite coatings
this shorter injector distance, the maximum layer thick- which makes the interpretation of the spectra difficult. As
ness was obtained for the minimum PCL feeding rate expected, no significant dehydroxylation of hydroxyapa-
(x1 = 40 mg/min). This counterintuitive effect may have tite occurred, as the characteristic OH band at 3572 cm1
been caused by an insufficient carrier gas flow rate to is present in all spectra. This attests for the low thermal
sustain a higher feeding rate (up to 120 mg/min). Partial energy input during spraying that prevented dehydroxy-
clogging of the feeding line by polymer particles at higher lation of hydroxyapatite toward oxyapatite and decom-
feeding rates may be the reason behind this. position towards tri- and tetracalcium phosphate,
The average surface roughness (Ra) ranged from 7.76 respectively. The spectra of the feedstock powders and
to 13.51 lm for different factor combinations (Table 4), composite coating HP8 are shown in Fig. 6(a). PCL bands
showing a significant increase with respect to the rough- in the range 1490-1200 cm1 can be hardly seen in the
ness of HA samples (Ra(HAopt) = 6.00 lm). This could be coating HP8 (Fig. 6b). Therefore, further assessment was
attributed to the vaporization of polymer or the ejection conducted by XPS to ensure that PCL did not undergo
of polymer (splashing) when HA particles impinging the significant degradation during LEPS deposition.
Journal of Thermal Spray Technology Volume 21(1) January 2012—139Peer Reviewed
Fig. 6 (a) Fourier Transform Infrared (FTIR) spectra of the PCL/HA composite sample HP8 (x1 = 120 mg/min, x2 = 27 mm,
x3 = 14 slpm) and the original PCL and HA powders; and (b) detail of the PCL and HP8 spectra in the range of 1490-1200 cm1
Fig. 7 XPS deconvoluted C 1s spectra of PCL powder, HP1 (x1 = 40 mg/min, x2 = 17 mm, x3 = 10 slpm), HP3 (x1 = 40 mg/min,
x2 = 27 mm, x3 = 10 slpm), and HP8 (x1 = 120 mg/min, x2 = 27 mm, x3 = 14 slpm) samples
XPS confirms the presence of PCL in the HA/PCL shown in the inset of Fig. 7. The assignments of the C 1s
coatings. Moreover, no signal of polymer degradation was components with the following binding energies: C 1sA,
detected. Figure 7 shows the XPS C 1s spectra of the PCL 284.5 eV; C 1sB, 286.1 eV; C 1sC, 288.2 eV; and C 1sD,
powder and three coatings of the HA/PCL series (HP1, 289.8 eV are shown in Fig. 7.
HP3 and HP8). All the spectra were corrected for sample 3.2.3 Mechanical Properties and Porosity. Vickers
charging using as an internal reference the C 1s peak for microhardness (261-362 HV0.5) and fracture toughness
adventitious carbon at a binding energy of 284.5 ± 0.2 eV, (0.54-0.78 MPa m1/2) for HA/PCL series are presented in
C 1sA (Ref 51). The C 1s spectra of the PCL powder and Table 4. ANOVA results for microhardness and fracture
HA/PCL coatings were deconvoluted into four compo- toughness suggest that the use of average values
nents related to the chemical structure of PCL (Ref 52) (305 HV0.5 and 0.66 MPa m1/2) provides a more accurate
140—Volume 21(1) January 2012 Journal of Thermal Spray TechnologyPeer Reviewed
Fig. 8 Cross-sectional micrographs of optimized thick layers produced with the low-energy plasma system at a plasma gas flow rate of
36 slpm: (a) pure hydroxyapatite HA11 (x1 = 36 slpm, x2 = 3.5 cm, x3 = 9 g/min); and (b) HA/PCL composite layer HP1 (x1 = 40 mg/min,
x2 = 17 mm, x3 = 10 slpm)
Fig. 9 Attachment of MC3T3 murine calvarial osteoblast-like cells on rough and highly porous PCL/HA composite coating surfaces
data fit than the respective statistical models. An initial (Ref 54). However, the mechanical properties measured
analysis of these properties can lead to the conclusion that for HA/PCL coatings indicated that the presence of PCL
the addition of PCL did not significantly improve the counteracts this effect. In addition, the overall structure of
mechanical properties of the HA coatings (similar range the remaining deposit seems to be denser with smaller
of variation and mean values of these 3 properties). volume defects than found in HA deposits, when using
However, it must be highlighted that the HA/PCL coat- equivalent processing conditions (Fig. 8).
ings were deposited at a much higher thickness per spray
pass (layer thickness) leading to thicker composite coat-
3.3 Biocompatibility Testing
ings. Deposition of pure HA coatings of similar thickness
(700 lm) at relative high deposition rates (34.0-84.4 lm/ Cell attachment was studied in representative samples
layer) without adding PCL would have led to the building of each series. The HA deposit was sprayed at the opti-
up of residual stresses, resulting in significantly decreased mized parameters described in section 3.1 (x1 = 36 slpm,
coatingÕs mechanical properties. In fact, depositions of x2 = 3.8 cm and x3 = 9 g/min). The HA/PCL deposit eval-
pure HA coatings with layer thicknesses over 30 lm have uated was HP8 with high surface roughness and porosity.
been reported to generate high residual stresses (Ref 53). MC3T3 murine calvarial osteoblast-like cells were able
The porosity ranged between 4.2 and 7.6% (Table 4), to attach well to both HA and HA/PCL composite
therefore the addition of PCL did not significantly deposits. Spread cells on the deposits and their extensions
increase the average porosity of the deposits compared to within pore areas were observed (Fig. 9).
the HA reference. Owing to the large standard deviations After 7 days, the cell numbers were only slightly lower
(up to 1.5%), the porosity statistical model generated did compared to the positive control tissue culture polystyrene
not properly fit the experimental data, showing significant (Fig. 10). Cells continued to proliferate up until 14 days
lack of fit (p = 0.0028), regression coefficient (R2 = 0.58) toward confluence at the deposit surface but at a slower rate
lower than recommended, and an unsatisfactory accuracy compared with the TCPS. The cell number on the HA/PCL
([adjusted R2 predicted R2] = 0.23). The composite deposit is lower than that on the HA deposit, which is
coatings porosity was not homogeneously distributed, and consistent with similar previous studies where HA presence
larger pores were observed. Larger pores in a pure HA improved attachment and proliferation. It is hypothesized
coating would lead to a decrease in mechanical properties that the difference in the concentration of phosphate ions in
Journal of Thermal Spray Technology Volume 21(1) January 2012—141which is particularly important to ensure good coating
Peer Reviewed
biological behavior. Attachment and proliferation of well-
spread MC3T3 murine calvarial osteoblast-like cells was
confirmed on HA/PCL biocomposite coatings, evidencing
their biocompatibility.
Acknowledgments
The authors would like to thank Dr N.E. Vrana
(MPRC, DCU, Ireland) for his help during biological
testing and interpretation of cellular response results,
T. Fernández Landaluce (TU Eindhoven, The Nether-
lands) for her help with the XPS measurements and data
interpretation, and Prof. R. Heimann (TU Bergakademie,
Fig. 10 MC3T3 murine calvarial osteoblast-like cell number Germany) for his advice during the elaboration of this
after 7 and 14 days (average of n = 6 samples) article. This research was supported by a Marie Curie
Early Stage Research Training Fellowship of the
the vicinity of the material could be playing a major role:
European CommunityÕs Sixth Framework Programme
either due to an enhanced dissolution of the HA coating (as
(MEST-CT-2005-020621).
a result of the accumulation of tensile stresses) or the lower
amount of HA available for dissolution on the surface of
the HA/PCL deposit (Ref 55).
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