Journal of Minerals & Materials Characterization & Engineering, Vol. 9, No.12, pp.1059-1069, 2010
jmmce.org Printed in the USA. All rights reserved
1059
Role of Post Heat Treatment of Plasma Sprayed Pure and Al2O3-TiO2
Reinforced Hydroxyapatite Coating on the Microstructure
and Mechanical Properties
Gurbhinder Singh*, Surendra Singh and Satya Prakash
Metallurgical and Materials Engineering Department, Indian Institute of Technology, Roorkee
247667, India
* Corresponding Author:
ABSTRACT
Pure and 10 wt% 80Al2O3-20TiO2 reinforced hydroxyapatite coatings were successfully
deposited on Ti6Al4V substrate by plasma spray technique for bio medical applications. Post
coating heat treatment was carried out to study the effect of heat treatment on improvement in
micro structure and mechanical properties of coatings. Coating was characterized by
SEM/EDAX and XRD analysis. Positive impact of post coating heat treatment was observed in
microstructure, surface roughness and micro hardness of both coatings.
Keywords: Hydroxyapatite, Pla sma Spray, Heat treatment, Microhardness
1. INTRODUCTION
Hydroxyapatite (Ca10 (PO4)6 (OH)2,(HA) coating on metallic substrates is now becoming popular
in clinically used applications due to excellent mechanical properties of substrate and good
biological response of HA coating [1]. It is reported by the previous research that thermal spray
techniques can develop thick, adherent and crystalline HA coatings as compared to other
processes [2-4]. Hence due to superior coating characteristics of plasma spray process among the
others, it is the only process which is recommended by Food and Drug Administration (FDA),
USA, for clinical use [5]. It is advantageous to use plasma sprayed HA coated metallic implant in
body, however it is concluded by the previous research work that in plasma spray process due to
rapid cooling from very high temperature decomposition of HA takes place to form tetra calcium
phosphate (TTCP), tri calcium phosphate (α-TCP, β-TCP) phases. These phases are amorphous

1060 Gurbhinder Singh, Surendra Singh and Satya Prakash Vol.9, No.12
in nature and prone to dissolve in body fluids, which may cause instability of implant after some
time [3]. There is another concern and controversy as to long term effectiveness of coated
implant in body environment [6-8]. To improve the effectiveness many studies have used some
secondary materials like silica, alumina, bio active glass and carbon nano tubes (CNTs) etc. as
reinforcement [9-13]. Post heat treatment is reported to be very beneficial to eliminate the
amorphous phases formed during plasma sprayed coating process [14-17]. It is also reported that
with the improvement of structural integrity post heat treatment plays important role to enhance
mechanical properties of HA coating [6, 18] and in the promotion of cell proliferation and bio
integration [19]. Further more heat treatment results in formation of ultrafine particles as
reported by Lu et al [15, 20] which may be the cause of improvement of mechanical properties.
In this paper the study of post heat treatment of pure HA and 10wt % Al2O3- TiO2 reinforced
coatings on Ti6AL4V was carried out to assess its effects.
2. MATERIALS AND METHOD
Commercially available HA powder having grain size in the range of 100-180 µm was used in
the pure HA coating where as 10wt % Al2O3-TiO2 (80% Alumina and 20% Titania having purity
99.85 %) manufactured by Industriekeramik Hochrhein GmbH (Germany), having grain size 10-
40 µm was used as reinforcement in HA for reinforced coating. It can be seen from the SEM
micrograph that both HA and reinforcement have angular grains.
TiO2 Al2O3
(a) (b)
Fig .1 SEM micrograph of (a) pure HA and (b) (80Al2O3 –20 TiO2)
Standard grade Ti6AL4V alloy strip (ASTM F1472) having thickness 3 cm was used as
substrate. Strip was cut into pieces of 20 x 15 and 15x10 cm, prior to spraying the pieces were
washed with acetone. Then grit blasting of the substrate material was carried out with the

Vol.9, No.12 Role of Post Heat Treatment 1061
alumina having size 350-450 µm. After air blasting, the substrates were coated with pure and
reinforced HA by air jet plasma spray technique at Anod Plasma Spray Limited Kanpur,
(India).Coating parameters are given in the Table I.
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.
Coated Sample Hot Zone inside the furnace
Fig. 2 Experimental set up of the furnace for post coating heat treatment process

1062 Gurbhinder Singh, Surendra Singh and Satya Prakash Vol.9, No.12
For the morphology of as sprayed and heat treated coating samples SEM/EDAX (FEI Quanta
200F, Made in Czech Republic) analysis was carried out. X ray diffraction analysis of as spray
coated and heat treated samples was carried out with scan speed of 2º/min between 10-90º at 2θ
angle to check the effect of post coating heat treatment. Cu target was used as X-ray source for
analysis.
Surface roughness (Ra) values of the plasma sprayed coated specimens 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.
Porosity of the as sprayed and heat treated coating samples were measured by the Zeiss Axiovert
200 MAT inverted optical microscope, fitted with imaging software Zeiss Axiovision Release
4.1, (Germany), which measures the porosity on the basis of colour contrast. About twenty (20)
separate locations were selected to avoid the overlap between two locations and determine the
area percent porosity.
Micro Hardness Tester (Leitz, Germany) fitted with a Vickers pyramidal diamond indenter was
used to check the micro hardness of the as sprayed and head treated coating on samples.10 gm
load was applied to check the hardness. Coated samples were cut through cross section and
mounted in epoxy and polished on 1/0, 2/0, 3/0, 4/0, 5/0 emery papers followed by cloth
polishing with alumina paste grade II for this test.
3. RESULT
From the SEM micrographs of pure HA as sprayed coating, treated at 700ºC and 800ºC as shown
in Fig. 3 (a), (b) and (c). Large particles may be seen in as sprayed coating 3(a) are converted
into small particles in heat treated coatings 3(b, c) due to recrystallization. Similar observation
was reported by previous literature [15, 20]. Presence of few un melted HA particles are being
revealed in as sprayed coating where as post heat treated coatings do not show such grains.
Similarly in case of reinforced HA coating as shown in Fig. 4 (a), (b) and (c) molten particles in
heat treated coatings may be observed and post heat treated coating seems to be denser than as
sprayed coating.

Vol.9, No.12 Role of Post Heat Treatment 1063
Un Melted particles
(a) (b)
(c)
Fig. 3 SEM micrograph of pure HA coating (a) as sprayed coating, (b) coating heat treated at
700 ºC, (c) coating heat treated at 800 ºC

1064 Gurbhinder Singh, Surendra Singh and Satya Prakash Vol.9, No.12
(a) (b)
(c)
Fig. 4 SEM micrograph of reinforced HA coating (a) as sprayed coating,(b)heat treated coating
at 700 ºC,(c) heat treated coating at 800 ºC
From XRD analysis of pure and reinforcing coatings as shown in Figs. 5 and 6, the presence of
amorphous phases like tri calcium phosphates (αTCP. βTCP) and tetra calcium phosphate
(TTCP) are observed between 31º to 33º, these phases were not present in original powders
before coating process. Similar phases were reported in previous literature [1, 12 and 21].
However in both coatings these phase are completely removed by post coating heat treatment at
700ºC and 800ºC temperature. Improvement of crystallinity is also observed by post coating heat
treatment process.

Vol.9, No.12 Role of Post Heat Treatment 1065
20 25 3035 40 4550 55 6065 70 7580 85 90
a
bc
Heat treated at 800ºC
Heat treated at 700ºC
As sprayed coating
Powder
Reletive intensity (arb.u)
Diffraction angle,2 theeta (degree)
a-αTCP, b-βTCP, c-TTCP
Fig. 5 XRD analysis of pure HA powder, as sprayed coating and heat treated coatings at 700ºC
and 800ºC temperature
20 25 3035 4045 50 55 60 65 70 7580 85 90
c
b
a
t
Heat treated at 800ºC
Heat treated at 700ºC
As sprayed coating
Powder
Reletive intensity (arb.u)
Diffraction angle,2 theeta (degree)
a-αTCP, b-βTCP, c-TTCP
Fig. 6 XRD analysis of reinforced HA powder, as sprayed coating and heat treated coatings at
700ºC and 800 º C temperature

1066 Gurbhinder Singh, Surendra Singh and Satya Prakash Vol.9, No.12
Surface roughness of reinforced HA coating is slightly higher as shown in the Fig.7. This may be
due to presence of reinforcement. Further effect of heat treatment on surface roughness of the
coatings may be observed which shows slight decrease in surface roughness in case of both
coatings. This may be due to the refinement of grain size by heat treatment.
0
1
2
3
4
5
6
C
3.61
B
4.52
A
6.06
C
3.54
B
4.40
A
5.86
Reinforced HA Coating
Pure HA Coating
Surface Roughness (µm)
A-as sprayed coating, B-post heat treated coating at 700 ºC and C- post heat treated coating at
800 ºC temperature
Fig.7 Surface roughness of pure and reinforced as sprayed and post heat treated HA coating at
700ºC and 800ºC temperature
Porosity of coating plays an important role in case of bio implants. It is observed by the previous
literature that metallic parts dissolve toxic metallic ion in body fluid environments [22, 23].So
the coating must be continuous and free from through porosity. Porosity of pure and reinforced
plasma sprayed coatings is in the most promising range i.e. less than 2%.Post coating heat
treatment led reduction in porosity less than 1%, which may attributed refinement of grains.
Micro hardness is the important property for an implant to be used in body environment. It must
possess sufficient hardness to bear the stresses involve in the high load conditions like hip and
knee prosthesis. Reinforcement has improved the microhardness of the coating. Post coating heat
treatment has further enhanced this property as shown in the Fig. 8. It can be observed from the
figure that micro hardness gets improved with post coating heat treatment in case of both
coatings and as the temperature is increased from 700ºC to 800ºC the further improvement in
micro hardness is observed.

Vol.9, No.12 Role of Post Heat Treatment 1067
0
100
200
300
400
500
600
700
800
C
754
B
618
A
507
C
694
B
598
A
486
Reinforced HA Coating
Pure HA Coating
Micro Hardness (Hv)
A-as sprayed coating, B-post heat treated coating at 700 ºC and C- post heat treated coating at
800 ºC temperature
Fig.8 Micro hardness of pure and reinforced as sprayed and post heat treated HA coating at
700ºC and 800ºC temperature
4. CONCLUSIONS
Pure and 10 wt% 80Al2O3-20TiO2 reinforced HA coatings were deposited by plasma spray
process. Post coating heat treatment was carried out at 700ºC and 800ºC to study its effect on
micro structure and mechanical properties. Following conclusions may be drawn from the
study:-
1. Recrystallization of the coating is observed by post coating heat treatment, by which
some un melted particles got melted and converted into fine particles.
2. Some non favorable amorphous phases like tri calcium phosphates were observed in as
sprayed pure and reinforced coatings. These phases were successfully eliminated by the
heat treatment process at 700ºC and 800ºC.
3. Improvement in micro hardness was observed by reinforcement which was further
improved by post coating heat treatment process in case of both coatings.
4. Slight increase in surface roughness was observed by reinforcement, which was slightly
decreased by the post coating heat treatment process.
5. Porosity of both pure and reinforced HA coatings has been observed in acceptable range
i.e. less than 2%.Reinforcement has reduced the porosity slightly, but post heat treatment
led substantial decrease in porosity i.e. it is less than 1%.

1068 Gurbhinder Singh, Surendra Singh and Satya Prakash Vol.9, No.12
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