Effects of yttria-stabilized zirconia on plasma-sprayed hydroxyapatite/yttria-stabilized zirconia composite coatings

The low bonding strength between hydroxyapatite (HA) and the metal substrate interface of plasma-sprayed HA coating has been a point of potential weakness in its application as a biomedical prosthesis. In the present study, yttria-stabilized (8 wt%) zirconia (YSZ) has been used to enhance the mechanical properties of HA coatings. The effects of YSZ additions (in the range 10-50 wt%) on the phase composition, microstructure, bond strength, elastic modulus, and fracture toughness of plasma-sprayed HA/YSZ composite coatings have been studied. The results indicated that decomposition of HA during plasma spraying was reduced significantly with the addition of zirconia. The higher the zirconia content, the lower the amount of calcium oxide, tricalcium phosphate, and tetracalcium phosphate formed in the coatings. In addition, there was a trace of calcium zirconate formed when less than 30 wt% zirconia was present. A solid solution of HA mixed with YSZ formed during plasma spraying; however, the amount of unmelted particles increased as the zirconia increased. The mechanical properties of the HA/YSZ composite coatings, such as bond strength, elastic modulus, and fracture toughness, increased significantly as the contents of zirconia increased.