The effects of ZrO2 on the phase compositions of plasma sprayed HA/YSZ composite coatings

The low bonding strength of plasma sprayed hydroxyapatite (HA) coating has been a point of potential weakness in its application as biomedical prosthesis. In the present study, yttria (8wt.%) stabilized zirconia (YSZ) has been used to enhance the mechanical properties of HA coatings. This paper focused on the effects of YSZ additions (in the range 10–50 wt.%) on the phase composition of plasma sprayed HA/YSZ composite coatings. The results indicated that decomposition of HA during plasma spraying reduced significantly with the addition of zirconia. The higher the zirconia content, the lower the amount of calcium oxide (CaO), tricalcium phosphate (TCP), and tetracalcium phosphate (TTCP) that was formed in the coatings. In addition, there was a trace of calcium zirconate (CaZrO3) formed when 30wt.% or less zirconia was added. Enhancing the plasma arc current resulted in an increase in the amount of CaO, TCP, and TTCP. The amorphous calcium phosphates and most of the TCP and TTCP in the as-sprayed coatings transformed into crystalline HA after heat treatment at 800°C for 2 h. The phase of the zirconia changed from tetragonal to cubic upon plasma spraying. This was due to the stabilization of the cubic phase by CaO. The 1000°C annealing showed that only cubic zirconia was present in the coatings.