Solution Combustion Synthesis of Calcium …
183
of 50–78% and the micropores were in the size range of 0.5–5.0 μm [43]. Efforts were
also made by Liu et al. [44] to fabricate HAp coatings wherein solution combustion
reaction was carried out by placing the Ti substrates in an aqueous redox mixture
containing Ca(NO 3 ) 2 ·4H 2 O, (NH 4 ) 2 HPO 4 , HNO 3 (concentration of 85 vol.%), urea
and NaNO 3 at 500 and 600 °C. This was followed by hydrothermal reaction in 1M
NaOH solution at 80 °C for 10 h to obtain a coating of pure HAp and the addition of
NaNO 3 resulted in improved interfacial bonding strength and coating thickness owing
to the increase in the combustion temperature. Pure HAp coating was obtained when
the Ca/P ratio was 1.75. Fully dense hydroxyapatite ceramic bodies with improved
mechanical strength were synthesized from combustion synthesized nanostructured
powders characterized by a high surface area to facilitate the sintering [45]. The phase
evolution and sintering kinetics of HAp synthesized by solution combustion method
have also been reported [46]. From the literature, it is evident that there are no reports
on the solution combustion synthesis of flowable HAp powders and its utilization
for plasma spray application. The efforts made on the synthesis of flowable HAp
powder by SCS and plasma sprayed HAp coating is discussed in the next section.
3 Fabrication of Plasma-Sprayed HAp Coating Using
Solution Combustion Synthesized HAp Powder
In the present study, the solution combustion method has been explored for the
synthesis of plasma sprayable HAp powders using urea and also urea in combination
with ammonium acetate as the fuels.
3.1 Synthesis of Plasma Sprayable Hydroxyapatite Powder
by the Solution Combustion Process
Calcium carbonate (CaCO 3 ) was converted into calcium nitrate [Ca(NO 3 ) 2 ] using
70% HNO 3 which acted as an oxidizer and ammonium di-hydrogen phosphate
(NH 4 H 2 PO 4 ) was used as the source for phosphate. Mixtures of fuels like urea
and ammonium acetate were used and the ignition temperature was maintained at
400 °C. Varying amounts of urea and ammonium acetate were used to synthesize
the HAp powder like (i) 100% urea, (ii) 100% ammonium acetate, (iii) 75% urea +
25% ammonium acetate, (iv) 50% urea + 50% ammonium acetate, and (v) 25% urea
+ 75% ammonium acetate corresponding to oxidizer to fuel ratio of 1. The redox
mixture was placed in muffle furnace preheated to 400 °C. The solution combustion
synthesized powder was white and was not very fluffy.
183
of 50–78% and the micropores were in the size range of 0.5–5.0 μm [43]. Efforts were
also made by Liu et al. [44] to fabricate HAp coatings wherein solution combustion
reaction was carried out by placing the Ti substrates in an aqueous redox mixture
containing Ca(NO 3 ) 2 ·4H 2 O, (NH 4 ) 2 HPO 4 , HNO 3 (concentration of 85 vol.%), urea
and NaNO 3 at 500 and 600 °C. This was followed by hydrothermal reaction in 1M
NaOH solution at 80 °C for 10 h to obtain a coating of pure HAp and the addition of
NaNO 3 resulted in improved interfacial bonding strength and coating thickness owing
to the increase in the combustion temperature. Pure HAp coating was obtained when
the Ca/P ratio was 1.75. Fully dense hydroxyapatite ceramic bodies with improved
mechanical strength were synthesized from combustion synthesized nanostructured
powders characterized by a high surface area to facilitate the sintering [45]. The phase
evolution and sintering kinetics of HAp synthesized by solution combustion method
have also been reported [46]. From the literature, it is evident that there are no reports
on the solution combustion synthesis of flowable HAp powders and its utilization
for plasma spray application. The efforts made on the synthesis of flowable HAp
powder by SCS and plasma sprayed HAp coating is discussed in the next section.
3 Fabrication of Plasma-Sprayed HAp Coating Using
Solution Combustion Synthesized HAp Powder
In the present study, the solution combustion method has been explored for the
synthesis of plasma sprayable HAp powders using urea and also urea in combination
with ammonium acetate as the fuels.
3.1 Synthesis of Plasma Sprayable Hydroxyapatite Powder
by the Solution Combustion Process
Calcium carbonate (CaCO 3 ) was converted into calcium nitrate [Ca(NO 3 ) 2 ] using
70% HNO 3 which acted as an oxidizer and ammonium di-hydrogen phosphate
(NH 4 H 2 PO 4 ) was used as the source for phosphate. Mixtures of fuels like urea
and ammonium acetate were used and the ignition temperature was maintained at
400 °C. Varying amounts of urea and ammonium acetate were used to synthesize
the HAp powder like (i) 100% urea, (ii) 100% ammonium acetate, (iii) 75% urea +
25% ammonium acetate, (iv) 50% urea + 50% ammonium acetate, and (v) 25% urea
+ 75% ammonium acetate corresponding to oxidizer to fuel ratio of 1. The redox
mixture was placed in muffle furnace preheated to 400 °C. The solution combustion
synthesized powder was white and was not very fluffy.