Preferred orientation of plasma sprayed hydroxyapatite coatings

Abstract

The long-term stability of hydroxyapatite (HA) coatings has been under investigation for a long time. The evaluation of crystallinity is important since a relationship exists between the degree of crystallinity and the degradation by in vitro cellular reaction. However, the choice of a representative peak to calculate the crystallinity is the subject of some controversy with the (2 1 1) peak and the (0 0 2) peak being the most commonly analysed peaks. Thus the determination of the degree of crystallinity may be biased due to preferred orientation (texture) in the coatings. Therefore, the dependence of texture on thickness during plasma spraying was investigated by the calculation of a texture coefficient (TC). Experiment results showed that the TC value of the (0 0 2) and (0 0 4) peaks increased with thickness and that the TC value in the (2 1 1) peak decreased. This was caused by a high temperature gradient during spraying and also the growth direction for a hexagonal system. It was observed that appropriately controlled temperature increases during annealing did not bring about notable texture to the recrystallized crystallites. However, if the temperature gradient was high during annealing, notable (0 0 2) texture can exist. The effect of particle size on the texture was also investigated.