Yttria stabilized zirconia reinforced hydroxyapatite coatings

The low bonding strength between the coating layer and the substrate of plasma sprayed hydroxyapatite (HA) coatings is the weakness in its application as a bio-medical prosthesis. In this study, 30 wt.% of yttria (8 wt.%) stabilized zirconia (YSZ) has been used to reinforce the hydroxyapatite coatings and to improve the bond strength of the HA coatings. Fine YSZ coated HA powders, which can minimize the effects of amorphorization and dissociation of HA by use of the much more thermal resilient YSZ particles to protect HA particles from the high plasma temperature, were prepared by using a ceramic slurry method. These powders are plasma sprayed to obtain the YSZ reinforced hydroxyapatite coatings. The influences of plasma spraying energy on the phase composition, the HA relative phase ratio, microstructure and mechanical properties (bond strength, micro-hardness and fracture toughness) have been studied. The results show that CaO, tri-calcium phosphate (TCP) and tetra-calcium phosphate (TTCP) are reduced significantly and more crystalline HA transforms to amorphous calcium phosphate during plasma spraying with the addition of YSZ. The mechanical properties of the YSZ reinforced HA coatings have thus been improved significantly. The bond strength, micro-hardness and the fracture toughness of the YSZ reinforced HA coatings increase with increasing plasma energy, associated with a greater content of CaO. The results of heat-treated YSZ reinforced HA coatings show that the amorphous calcium phosphate, TCP and TTCP in the as-sprayed coatings transform to the crystalline HA after heating at 800 °C for 2 h.