Hydroxyapatite-zirconia composite coatings via the plasma spraying process

Hydroxyapatite (HA) coatings have been deposited on artificial hip prosthesis as a bioactive surface to induce a direct chemical bond with natural bones and hence achieve a biological fixation onto implants. However, its intrinsic weak mechanical properties have always been the main obstacle to its applications, especially in the area of orthopedic applications, where implants have to withstand medium-high loading. In this study, HA and yttria-stabilized zirconia (YSZ) powders were ball-milled and then sprayed onto Ti-6Al-4V alloy substrates. The effect of plasma arc current on the phase composition was investigated. The amount of decomposition of HA increases with the increase of plasma arc current from 600 to 1000 A. The impurities, as a result of the decomposition, were α-TCP (alpha-tricalcium phosphate) and CaO, although a small amount of TTCP (tetra-calcium phosphate) was also detected. The influence of ZrO₂ on the decomposition of HA was also studied. The results show that ZrO₂ reacts with CaO in HA to form CaZrO₃. This reaction caused an increase in the decomposition of the CaO-poor HA.