Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.2, pp.173-184, 2011
jmmce.org Printed in the USA. All rights reserved
173
Post Heat Treatment of Plasma Sprayed Pure and Alumina-Titania
Reinforced Hydroxyapatite Coating on SS 304 Steel
Gurbhinder Singh*, Surendra Singh, Satya Prakash
Metallurgical and Materials Engineering Department, Indian Institute of Technology, Roorkee
247667, India
* Corresponding author: [e-mail], Tele. [phone], fax [phone],
ABSTRACT
It has been reported by previous research work that in plasma spray process hydroxyapatite
decomposes into some amorphous phases due to rapid cooling of the coated substrate during the
coating process, which causes instability of coated implant. Post coating heat treatment has been
expected to minimize these phases and also enhance the long term reliability of hydroxyapatite
coatings on metal substrates. In this paper post coating heat treatment is carried out at different
temperatures suggested by the literature to study its effect on the microstructural and
mechanical properties of coating. Coated substrates were characterized by X-ray diffraction
(XRD) and SEM/EDAX analysis.
Keywords: Hydroxyapatite, Post Heat Treatment, Amorphous Phases, Plasma Spray
1. INTRODUCTION
In recent years Hydroxyapatite (HA) coated metallic implants are recognized as one of the
promising implant materials for load bearing conditions like hip and knee prosthesis [1]. HA
coated metallic implants have favorable osto-conductivity due to ceramic HA coating and
superior mechanical properties due to metallic substrate. For the bio medical applications the
crystallinity of Hydroxyapatite (HA) coating should be of high degree to ensure low dissolution
rates in vitro [2-4]. Plasma spray technique is the only process which is clinically recommended
by Food and Drug Administration (FDA), USA due to unmatchable properties of coating
obtained by this process among other coating techniques [5]. But there are some problems which
are associated with this process. First is that during coating process rapid cooling of coated

174 Gurbhinder Singh*, Surendra Singh, Satya Prakash Vol.10, No.2
substrates leads to formation of some amorphous phases like try-calcium phosphate (α and β
TCP) and tetra- calcium phosphates (TTCP) [6]. These phases are rapidly soluble in human
blood plasma and can cause implant instability after some time of implantation [7-9]. These
phases must be eliminated or minimized to prolong applications of bio implant. It has been
observed by previous research work that pure HA coating has poor mechanical properties like
Young’s modulus, micro hardness and adhesion to substrate [10-11]. Another controversy
regarding HA coated implant is its long term effectiveness in human body environment [7-9].
Lot of research work has been carried out to increase the effectiveness of the coating by
optimizing coating parameters, reinforcing some secondary materials and by post coating heat
treatment process [12-16]. It is reported in the previous literature that post coating heat treatment
eliminates all non favorable phases formed during coating operation [17-18]. Khor and Cheang
[19] suggested that post heat treatment also plays important role to enhance mechanical
properties of HA coating. Furthermore Lue et al [20-21] observed some ultrafine particles after
post coating heat treatment which may be the cause of improvement in mechanical properties of
coating.
2. MATERIALS AND METHOD
Commercially available surgical stainless steel grade (SS-304L) having thickness 6 cm was used
as substrate material. The composition of substrate material was checked by Optical Emission
Spectrometer (Thermo Jarrell F., UK). Compositions provided by the industry and calculated in
laboratory are shown in Table I.
Table I: Composition of Substrate Material
Element C Si Mn Cr Ni P S Cu Mo Fe
%age
(Provided)
0.02-
0.04
0.25-
0.75
1.0-
2.0 18-20 8-12 0.03- 0.05
0.01-
0.03 --- --- Balance
%age
(Calculated) 0.0260 0.03505 2.20 19.46 7.082 --- --- 0.4780 0.2048Balance
For pure HA coating commercially available HA powder having size in range of 100-180 µm
was used. For reinforcement of coating 10% (80 alumina-20 titania) having size in the range of
10-40 µm was added by weight. SEM micrograph of pure HA, reinforce materials and reinforced
HA are shown in the Fig.1(a), 1(b), 2(a), 2(b) and 3(a), 3(b) at different magnifications. It can be
seen from the SEM micrographs 1(a) and 2(b) that both HA and reinforced materials have
angular shape. At higher magnification it is revealed from the micrograph 1(b) that HA grain is
made of very small particles.

Vol.10, No.2 Post Heat Treatment of Plasma Sprayed Pure and Alumina-Titania 175
1(a) Pure HA at Magnification 100X 1(b) Pure HA at Magnification 2000 X
Fig.1 SEM Micrograph of Pure HA
2 (a) Reinforce Material at Mag. 100X 2(b) Reinforce Material at Mag. 2000X
Fig. 2 SEM Micrograph Reinforcing Material (80Alumina-20Titania)
HA Reinforcement HA Reinforcement
3 (a) Reinforced HA at Mag. 100X 3(b) Reinforced HA at Mag. 2000X
Fig.3 SEM Micrograph of Reinforced HA

176 Gurbhinder Singh*, Surendra Singh, Satya Prakash Vol.10, No.2
Both coatings i.e. pure and reinforced HA were successfully deposited by Jet Air Plasma Spray
Technique at Anod Plasma Spray Limited, Kanpur (India). The substrate material were cut into
desired shape i.e. 20x15x5cm and 10x15x5cm then washed by acetone and dried in ordinary
oven. Samples were grit blasted by alumina having size 350-450 µm followed by air blasting to
remove the alumina from the substrate surface just before coating. Coating parameters are given
in Table II.
Table II 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
As sprayed pure and reinforced HA coated samples were heat treated at 700 ºC and 800 ºC to
evaluate the role of heat treatment process on coating properties. Samples were placed in the
furnace as shown in the Fig.4 and were heated to desired temperatures i.e. 700 ºC and 800 ºC at
the rate of 5 ºC /min. respectively. After attaining the desired temperature samples were heated
for 2 hours. After two hours the furnace was switched off and samples were allowed to cool in
the furnace for 8-10 hours for annealing.

Vol.10, No.2 Post Heat Treatment of Plasma Sprayed Pure and Alumina-Titania 177
Coated sample inside the furnace
Fig. 4 Experimental set up of the furnace for post coating heat treatment process
X-Ray diffraction analysis was carried out to characterize as sprayed and sintered samples to
reveal the effect of post coating heat treatment. Cu target was used for analysis at speed 2º/min in
the range of 10-90º at 2θ angle. SEM/EDAX (FEI Quanta 200F, Made in Czech Republic)
analysis was carried out to check the morphology of as sprayed and sintered coated samples.
Surface roughness (Ra) values of the coated samples were carried out by surface roughness tester
(Wyko NT 1100, USA). Each reported value of surface roughness (Ra) is the mean of five
observations taken at different locations. Each value of Ra is the average roughness calculated
over the entire measurement array given as Ra= ૚
ࡹ ෌ሼ
ࡹ
ࡶୀ૚ Zj} Where Zj is the height of each pixel
after the zero is removed.
To check the micro hardness of coatings, the coated samples were cut through cross-sectional
and mounted in epoxy and polished on various grade emery papers like 1/0, 2/0, 3/0, 4/0, 5/0 and
then cloth polishing with alumina paste grade II. Micro Hardness Tester (Leitz, Germany) fitted
with a Vickers pyramidal diamond indenter was used for this test. 10 gm load was applied to
check the hardness. Hardness value was calculated from the relation Hv = 1854.4 * F/d2. Where F
is for load in grams and d is for the diameter of the indenter in micrometer. Each reported value
of the micro hardness is the average value of five measurements
Zeiss Axiovert 200 MAT inverted optical microscope, fitted with imaging software Zeiss
Axiovision Release 4.1, (Germany) was used to determine the porosity of the as sprayed and post
coating heat treated samples, which is compatible with ASTM B276 standard. Porosity was
measured by colour contrast method. About 20 readings were taken to avoid overlap between
two locations.

178 Gurbhinder Singh*, Surendra Singh, Satya Prakash Vol.10, No.2
3. RESULTS
SEM micrographs of as sprayed pure HA coating, post heat treated coating at 700ºC and 800ºC
are shown in Fig 5(a), 5(b) and 5(c) respectively. It can be seen from the SEM micrograph 5(a)
that coating is uniform however few unmelted grains may be observed in as spray coating which
are not present in post coating heat treated coatings at 700ºC and 800ºC as shown in 5(b) and 5
(c). This may be due to recrytallization of coating during heat treatment. Similar observations
were reported by previous research work [16, 21]. Coating tends to become denser in case of
post heat treated coatings.
5(a) 5(b)
5(c)
Fig. 5 SEM micrograph of as sprayed pure HA coating 5(a), post coating heat treated coating at
700ºC 5(b) and 800ºC 5(c)
Similarly in case of reinforced HA coatings, as sprayed, post heat treated coatings at 700ºC and
800ºC are shown in Fig. 6(a) ,6(b) and 6(c) respectively. Here also continuous coating with few
un melted grains may be seen from Fig 6(a).Fig.6 (b) shows almost all grains fully melted. In
Fig. 6(c) it can be seen that coating becomes comparatively denser. This may be due to
recrystallization of coating during heat treatment process.

Vol.10, No.2 Post Heat Treatment of Plasma Sprayed Pure and Alumina-Titania 179
6(a) 6(b)
6(c)
Fig. 6 SEM micrograph of as sprayed reinforced HA coating 6(a), post coating heat treated
coating at 700ºC 6(b) and 800ºC 6(c)
X-Ray diffraction analysis of pure and reinforced HA powders, as sprayed coatings, post heat
treated coatings at 700 ºC and 800 ºC are shown in Fig. 7(a) and 7(b). Analysis shows that non
amorphous phases like tri calcium phosphates (α-TCP. Β-TCP) and tetra calcium phosphate
(TTCP) are observed between 31º to 33º are present in both as sprayed pure and reinforced
coatings which were not present in their original powders before coatings. Similar phases were
reported by previous research work [11, 12, and 22]. These phases were successfully eliminated
by post heat treatment process at 700 ºC and 800 ºC.

180 Gurbhinder Singh*, Surendra Singh, Satya Prakash Vol.10, No.2
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Pure HA Powder
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o-Hydroxyapatite, a-αTCP, b-βTCP, t-TTCP, c-CaO
Fig 7 (a) XRD analysis of pure HA powder, as sprayed coating and heat treated coatings at
700ºC and 800ºC
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Fig 7 (b) XRD analysis of reinforced HA powder, as sprayed coating and heat treated coatings at
700ºC and 800º C

Vol.10, No.2 Post Heat Treatment of Plasma Sprayed Pure and Alumina-Titania 181
Surface roughness of the coating is very important for bio implant applications. Coating must be
sufficiently rough to facilitate the cell growth in the body environments. Surface roughness of as
sprayed pure HA and reinforced HA, post heat treated pure and reinforced HA coatings at 700ºC
and 800ºC temperatures is shown in the Fig.8. Surface roughness of the reinforced as sprayed
HA coating is slightly higher than that of pure HA coating. That may be due to the role of
reinforcement. Post heat treatment process results in reduction in surface roughness in case of
both pure and reinforced HA coatings. This may be due to refinement of grains due to sintering
process.
0
1
2
3
4
5
6
Reinforced HA
Pure HA
Heated at 8000C
Heated at 8000C Heated at 700
0
C
Heated at 700
0
C
As Sprayed
As Sprayed
3.52
4.45
6.32
3.40
4.20
5.36
Surface Roughness (µm)
Fig.8 Surface roughness of pure and reinforced as sprayed and post heat treated HA coating at
700ºC and 800ºC temperature
Micro hardness is an important property for a bio implant material. For trouble free long service
of coated bio implant in body it must possess sufficient hardness so that it may withstand the
high stresses in high load bearing conditions like hip and knee prosthesis. Micro hardness of as
sprayed pure and reinforced HA coatings and post treated pure and reinforced coatings at 700ºC
and 800ºC temperatures are shown in Fig.9.It can be observed that with reinforcement hardness
is slightly increased. Post coating heat treatment also has a positive impact on improvement of
microhardness. This improvement in hardness may be attributed to recrystallization of the
coatings due to heat treatment.

182 Gurbhinder Singh*, Surendra Singh, Satya Prakash Vol.10, No.2
0
100
200
300
400
500
600
700
800
Reinforced HA
Pure HA
Heated at 8000C
Heated at 8000C
Heated at 7000C
Heated at 7000C
As Spr ay ed As Sprayed
643
645
593
562
496
458
Micro Hardness (Hv)
Fig.9 Micro hardness of pure and reinforced as sprayed and post heat treated HA coating at
700ºC and 800ºC temperature
Porosity of pure as sprayed HA coating is in the most promising range i.e. less than 2%.
Reinforcement has reduced the porosity slightly. Whereas post coating heat treatment led
reduction in porosity less than 1% .Which may attribute refinement of grains.
4. CONCLUSIONS
Pure and 10% wt (80 alumina-20 titaina) reinforced HA coatings were successfully deposited on
SS-304 steel substrate by plasma spray process. Post coating heat treatment was carried out at
700ºC and 800ºC temperature. Following conclusions may be drawn from the study:-
1. Improvement in microhardness and surface roughness was observed with the
reinforcement with slight decrease in porosity.
2. Some non favorable amorphous phases were observed in both pure and reinforced as
sprayed coatings which were not present in their original powders.
3. These phases were successfully eliminated by post coating heat treatment at 700ºC and
800ºC temperatures.

Vol.10, No.2 Post Heat Treatment of Plasma Sprayed Pure and Alumina-Titania 183
4. With elimination of amorphous phases the post heat treatment process had also improved
the mechanical and microstructural properties like micro hardness and porosity of both
pure and reinforced HA coatings with slight decrease in surface roughness.
5. Few un melted grains of HA was observed in both as sprayed pure and reinforced HA
coatings which were eliminated by recrystallization during post coating heat treatment
process.
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