Influence of Substrate Temperature and Hydrothermal Treatment on the Phase Composition of Plasma-Sprayed Phosphate Coatings

Phosphate coatings have been produced by plasma-spraying hydroxyapatite (HA) and tricalcium phosphate (TCP) powders onto Ti substrates at initial temperatures of 20, 300, and 550°C, followed by hydrothermal treatment (HTT) at 650°C, and the variation in the phase composition of the coatings has been examined in relation to the phase composition of the plasma-sprayed powders: 100 wt % HA, 100 wt % α‑TCP, and 100 wt % β-TCP. The as-prepared coatings produced by plasma-spraying the HA powder consisted of 87–91 wt % HA and 9–13 wt % CaO, and after HTT their phase composition was 89–93 wt % HA and 7–11 wt % CaO. The coatings produced by plasma-spraying the α-TCP powder at initial substrate temperatures from 20 to 550°C consisted entirely of a crystalline α-TCP phase. The coatings produced by plasma-spraying the β-TCP powder consisted of both β-TCP and α-TCP, and the content of the latter phase decreased from 100 to 80% as the substrate temperature was raised from 20 to 550°C. After HTT, the coatings contained 26–28% HA, independent of the phase composition of the starting TCP powders.