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Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.9, pp.765-775, 2011
jmmce.org Printed in the USA. All rights reserved
Characterization and Corrosion Behavior of Plasma Sprayed Pure and
Reinforced HA Coatings in Simulated Body Fluid
Gurbhinder Singh*, Surendra Singh and Satya Prakash
Metallurgical and Materials Engineering Department, Indian Institute of Technology, Roorkee
* Corresponding Author: firstname.lastname@example.org
To evaluate the corrosion behavior in simulated body fluid pure and 10wt% 80Al2O3-20TiO2
reinforced HA coatings were successfully deposited by atmospheric plasma spray technique on
SS-304L substrate alloy. To eliminate non favorable phases on the as sprayed coatings the
coatings were heat treated at 700°C and 800°C for post coating heat treatment technique. Both
pure and reinforced as sprayed and post coating heat treated coatings were characterized by
XRD and SEM/EDAX analysis for surface characterization and further evaluated by
electrochemical test for corrosi on behavior in simulated b ody fluids.
Keywords: Hydroxyapatite, Corrosion, Reinforcement, Post Coating Heat Treatment
Stainless steel (18Cr–8Ni) was the first metallic biomaterial used successfully as an implant in
orthopedic surgery in 1926 [1-2]. In recent years surgical stainless steel grades have been very
commonly used for bio implants due to their comparatively low cost, good corrosion resistance,
easy hot and cold workability and good mechanical properties. These materials when compared
to polymeric and ceramic materials possess more superior tensile strength, fatigue strength, and
fracture toughness, which are the key properties required of structural materials . In recent
years coating of the ceramics material HA on conventionally used metallic implants has attracted
the attention of researchers in the concerned field. By coating HA on metallic parts one can take
advantages of both materials. Where metallic components will impart high mechanical properties
766 Gurbhinder Singh, Surendra Singh and Satya Prakash Vol.10, No.9
to bio implant, the porous ceramics coating will enhance the cell growth on its surface for the
easy fixation of the implant in human body environment. Deposition of HA on metallic
substrates has been applied by a wide range of surface deposition techniques including plasma
spraying, high-velocity oxy-fuel spraying (HVOF), pulsed laser ablation, ion-beam sputtering,
electrophoretic deposition, radio frequency magnetron sputtering, sol–gel and conventional
ceramic processes that involve pressing and sintering [4-8]. Among these techniques thermal
spray techniques offer the most attractive prospect of economy and efficient deposition of HA.
Plasma spray is the only technique which is clinically used and recommended by the Food and
Drug Administration (FDA), USA, due to its excellent coating properties as compared to other
techniques . Although plasma sprayed is the only process which is clinically used for the
coating to be used for biomedical applications but it is reported in the previous literature [10-11]
that during these processes due to rapid cooling of the coated substrate, from very high
temperatures, crystalline HA is converted into tri and tetra calcium phosphates (α-TCP, β-TCP
and TTCP) and oxy hydroxyapatite (o-HA) phases. These phases are generally amorphous in
nature and prone to dissolve in the human blood plasma, which cause instability of bio implant
and ultimately failures of implant after some time . Post heat treatment is reported to be very
beneficial to eliminate these amorphous phases formed during plasma sprayed coating process
[13-16]. It is also reported that with the improvement of structural integrity post heat treatment
plays important role to enhance mechanical properties of HA coating and in the promotion of
cell proliferation and bio integration [17-18].
2. MATERIALS AND METHOD
Commercially available HA powder having high crystallinity and size in range of 100-180 µm
was used for pure HA coating. Volume mean diameter D [4, 3] of the powder was 150.12 µm.
Al2O3- TiO2 (80Alumina-20Titania) having grain size 10-40 µm was used as reinforcement. 10
wt% Al2O3- TiO2 was reinforced in pure HA by weight for reinforced coating as suggested by
the previous literature [11, 17]. Both pure and reinforced HA powders were deposited by
atmospheric plasma spray process. All process parameters of plasma spray process were
explained in the previous work [19, 20, 21]. From the SEM micrograph as shown in Fig.1, it can
be seen that both the HA and reinforced materials have angular crushed shape.
Coating of pure and reinforced HA was carried out by Plasma Spray System at Anod Plasma
Spray Limited, Kanpur, India. Commercially available SS-304L alloy strip (ASTM A 240)
having thickness 5 cm was used as a substrate material. Samples to be coated were grit blasted
using alumina having size 60-80 µm at blasting pressure 0.44 MPa for good coating adhesion.
Average value of surface roughness of the sample after the sand blasting was 7.30 µm.
Parameters of coating are given below in Table I.
Vol.10, No.9 Characterization and Corrosion Behavior 767
Fig .1 SEM micrograph of (a) pure HA and (b) (80Al2O3 –20 TiO2)
Table I. Spraying parameters of pure and reinforced HA coating
S. No Coating Parameter Units
1 Current (A): 750
2 Voltage (V): 50
3 Arc Pressure (PSI): 60
4 Powder Pressure (PSI): 80
5 Hopper RPM: 5.4
6 Hydrogen Pressure (PSI): 10
7 Stand of Distance (mm): 105
8 Powder Rate g/min: 35
9 Plasma Gun diameter (mm): 7
Post heat treatment of as sprayed coating has been carried out at a temperature of 700 ºC and
800ºC for two hours. The coated pieces were placed in the furnace as shown in the Fig. 2 and
then the temperature was raised from room temperature to the desired temperatures i.e. 700 ºC
and 800 ºC at the rate of nearly 5 ºC/min. After attaining the desired temperature samples were
heated for two hours. After two hours furnace power supply was stopped and the coated samples
were left as such for 9-10 hrs for annealing.
X ray diffraction analysis (Bruker- Binary V3) was carried out to analyze the phase structure of
as sprayed and post coating heat treated coated surfaces by taking radiation source CuKα at angle
10º to 60º. There are many methods to determine the crystallinity of HA coatings using X-ray
diffraction like Rutland Method, Relative Intensity Method and Rietveld Method. But in
768 Gurbhinder Singh, Surendra Singh and Satya Prakash Vol.10, No.9
previous literature it is reported that Rutland Method is an accurate method for determining
crystallinity [17, 18]. In this method crystallinity is calculated by comparing the total area under
the diffraction pattern with the area of the amorphous region of the pattern. The % crystallinity is
determined by using equation as:
Crystallinity (%) = x100
∑Ac + ∑Aa
Where ∑Ac is the sum of the areas of all HA crystalline peaks and ∑Aa is the sum of area under
the amorphous peaks.
As sprayed pure and reinforced HA coatings and post coated heat treated coatings were tested for
corrosion in simulated body fluid by electrochemical polarization test. SBF solution was
prepared according to the composition suggested by Kokubo . The ion concentration of SBF
and human blood plasma are compared in Table II. The test was carried out on three electrode
corrosion cell interfaced with a potentiostat of model (PARSTAT, Princeton Applied Research,
USA) which consist of Ag/AgCl as reference electrode and counter electrode. Sample which to
be polarized acted as working electrode. The area of the sample exposed to SBF solution was
1cm2. Samples were immersed in the SBF solution for 2 h before running the test. The corrosion
current density values were calculated by Tafel slope methods which are shown in result section.
Table II. Ion concentrations of SBF and human blood plasma 
S. No. Ion Human Blood
1 Na+ 142.0 142.0
2 K+ 5.0 5.0
3 Mg2+ 1.5 1.5
4 Ca2+ 2.5 2.5
5 Cl- 103.0 147.8
6 HCO3- 27.0 4.2
7 (HPO4)2- 1.0 1.0
8 (SO4)2- 0.5 0.5
3. RESULTS & DISCUSSION
XRD analysis of pure and reinforced HA (as sprayed and post coating heat treated at 700°C and
800°C) coatings are shown in Fig. 2 (a) and (b) respectively. It can be seen from the XRD
Vol.10, No.9 Characterization and Corrosion Behavior 769
analysis that in case of both as sprayed pure and reinforced coatings some amorphous phases are
present which are completely eliminated in case of post coating heat treated coatings at 700°C
and 800°C. Similar phases were reported in previous literature [10, 12]. Further it can be
observed from the graph that HA peaks, which are present in as sprayed coatings in both cases
(i.e. pure and reinforced HA coatings) become more crystalline when the coatings are treated at
700°C and 800°C. Crystallinity and amorphous phases present in these coatings are present in
the Table III. It can bee observed from the Table III that crystallinity of pure HA as sprayed and
sintered coatings have slightly greater than reinforced HA as sprayed and sintered coatings. Non
favorable phases which were present in the as sprayed coatings are completely removed by post
coating heat treatment processes carried out 700°C and 800°C. Similar observation has been
reported by previous research work [13-16]. Non favorable phases, which are amorphous in
nature, present in as sprayed pure and reinforced HA coatings can also be seen in Fig. 2(a) and
2(b) which are not present in pure and reinforced HA coatings sintered at 700°C and 800°C.
Magnifying view of these amorphous phases present in as sprayed pure and reinforced HA
coatings can be seen in Fig. 2(a) and 2(b). Further it can be noticed that amorphous phases
present in reinforced as sprayed HA coatings are slightly grater than pure as sprayed HA
coatings this may be result of reaction of reinforcement contents with hydroxyapatite coating at
higher temperature as suggested by previous literature [13,23].
Table III. Crystallinity and Non favorable phases present in as sprayed and heat treated pure and
reinforced HA coatings
S. No. Coating Crystallinity
1 AS sprayed pure HA
2 As sprayed reinforced
3 Pure HA coating
sintered at 700°C
4 Reinforced HA coating
sintered at 700°C
5 Pure HA coating
sintered at 800°C
6 Reinforced HA coating
sintered at 800°C
770 Gurbhinder Singh, Surendra Singh and Satya Prakash Vol.10, No.9
20 25 30 3540 45 50 55 60
Diffraction angle,2 theeta (degree)
29.5 30.0 30.5 31.0 31.5 32.0
Pure HA coating sinterd at 800oC
Pure HA coating sinterd at 700oC
As sprayed pure HA coating
Relative Intensity (arb.u)
20 25 30 3540 45 50 55 60
29.6 29.8 30.0 30.2 30.4 30.6 30.8
Reinforced HA coating sinterd at 800oC
Reinforced HA coating sinterd at 700oC
As sprayed reinforced HA coating
Relaitive Intensity (arb.u)
Diffraction angle,2 theeta (degree)
Fig. 2 XRD analysis of the (a) pure HA as sprayed and sintered coatings, (b) reinforced HA as
sprayed and sintered coatings
Vol.10, No.9 Characterization and Corrosion Behavior 771
Tafel polarization curves of pure and reinforced as sprayed and post coating heat treated coatings
are presented in Fig. 3(a) and 3(b) respectively. The range of passivation potential for bare SS-
304L substrate, pure HA as sprayed coating, post coating heat treated at 700°C and 800°C are -
660 mV to -160 mV, -650 mV to -150mV, -700 mV to -200 mV and -380mV to 70 mV
respectively, whereas for reinforced as sprayed ,post coating heat treated at 700°C and 800°C
coating it is -700mV to -200mV, -520mV to 50mV and -420 mV to 20mV respectively. Results
of polarization test are reported in Table IV. As it is evident from the Table IV that corrosion
current is maximum in case of bare SS-304L substrate. So all pure and reinforced as sprayed as
well as post coating heat treated coatings are protective in simulated body fluid solution. Pure
HA coating has shown minimum corrosion resistance among the all HA coatings, where as with
incorporation of reinforcement it has improved by 9%. The coatings heat treated at 700°C and
800°C have shown superior corrosion resistance as compared to as sprayed coatings. The results
are compatible with the work presented by the previous researches [24, 25]. Existing literature
[24, 25] suggested the improvement of corrosion resistance may be due to improved crystallinity
after post coating heat treatment process. However it can be also observed from the Table IV
that heat treated coatings at 800°C have shown comparatively poor corrosion resistance as
compared to the coatings heat treated at 700°C.
Table IV Result of Tafel polarization test
Specimen E Corr.
Bare substrate alloy (SS-304L) -408.216 2.743 385.532 296.380
Pure HA as sprayed coating -485.586 1.60 123.769 49.515
Pure HA coating sintered at 700°C -520.504 0.737 74.467 44.570
Pure HA coating sintered at 800°C -199.032 0.979 118.853 66.326
Reinforced HA as sprayed Coating -497.876 1.46 102.266 40.860
Reinforced HA coating sintered at 700°C -250.733 0.468 165.103 93.940
Reinforced HA coating sintered at 800°C -309.812 0.649 169.592 105.626
772 Gurbhinder Singh, Surendra Singh and Satya Prakash Vol.10, No.9
-8.0 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5
HA Coating (as sprayed)
HA Coating heat treated at 700oC
HA Coating heat treated at 800oC
-10.0-9.5 -9.0 -8.5 -8.0 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0
HA Coating (as sprayed)
HA Coating heat treated at 700oC
HA Coating heat treated at 800oC
Fig. 3 Tafel polarization curves for (a) bare ss-304L, pure HA as sprayed and sintered coating at
700°C and 800°C, (b) reinforced as sprayed and sintered coating at 700°C and 800°C
Vol.10, No.9 Characterization and Corrosion Behavior 773
Pure and 10%wt (80Al2O3-20TiO2) reinforced hydroxyapatite coating was successfully
deposited on SS-304L substrate by plasma sprayed process. Both coatings were sintered at
700°C and 800°C. All these coatings were characterized by xrd analysis to evaluate amorphous
phases and crystallinity of the coatings. Theses coatings were also tested for corrosion in
simulated body fluids. Following conclusions can be drawn from the xrd characterization and
corrosion behavior of in SBF.
1. Some non amorphous phases were detected in case of both as sprayed pure and
reinforced HA coatings, which were completely eliminated by post coating heat treatment
of the coatings at 700°C and 800°C.
2. Crystallinity of the both as sprayed pure and reinforced HA coatings was improved by
post coating heat treatment process.
3. Reinforced HA as sprayed coating has shown slightly larger amorphous phase as
compared to pure HA as sprayed coating.
4. All coatings (i.e. as sprayed and heat treated) have shown superior corrosion resistance as
compared to bare SS-304L alloy. However all as sprayed HA coatings have shown poor
corrosion resistance as compared to all HA heat treated HA coatings.
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