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Abstract
The in vitro behaviour and characteristics of plasma sprayed hydroxyapatite (HA) coatings
using flame-spheroidized HA feedstock powder on titanium alloy (Ti-6Al-4V) substrates
were investigated in a simulated physiological environment as an attempt to reflect
the actual incubational condition of an implant in a human body system. As-sprayed
and heat-treated HA coatings were immersed in a simulated body fluid with ionic concentrations
comparable to that of human blood plasma for time intervals 2, 4, 6, 8 and 10 weeks.
Rapid dissolution of calcium phosphate was found to occur within the first 4 weeks,
and after the 5th week a retarding rate of 4.1 mM week(-1) was observed where precipitation,
nucleation, and, growth of a carbonate-containing, poorly crystallized or amorphous
calcium phosphate layer on the as-sprayed coatings were noted. The heat-treated coatings
showed minimal or no precipitation on the surface except for the presence of calcite
minerals that is due to carbonation effect. Complete dissolution of other calcium
phosphate phases such as tetracalcium phosphate, tricalcium phosphate and calcium
oxide was also noted after 2 weeks of immersion due to higher ionic solubility relative
to HA. A declining trend in respective microhardness and elastic modulus of the as-sprayed
HA coatings from 207.06 +/- 3.2 H(k300) to 131.8 +/- 5.2 H(k300) and from 31.37 +/-
1.4 to 19.81 +/- 1.6 GPa was observed after 10 weeks of immersion. Tensile bond strength
of both types of coatings showed similar declining trend, with an average dip from
24.5 +/- 2.4 to 7.9 +/- 2.6 MPa. Nevertheless. the heat-treated samples showed rather
reasonable mechanical stability and structural integrity of 26.7 +/-1.4 GPa in elastic
modulus after soaking.