EFFECT OF PARTICLE-SIZE ON MOLTEN STATES OF STARTING POWDER AND DEGRADATION OF THE RELEVANT PLASMA-SPRAYED HYDROXYAPATITE COATINGS

Crystallinity of hydroxyapatite (HA) coatings is an important paramete r to evaluate their stability. Variation of the size distribution of t he starting powder is one way to alter crystallinity of coatings. The fundamental reason might be the variation of molten states of HA powde rs with different particle sizes. In the experiments, HA particles siz ed between 1 and 180 mu m were divided into six groups by sieving. It was observed that the trend of crystallinity of coatings on particle s ize is not linear but fluctuates. The fluctuation of crystallinity was caused by the alteration of molten states of HA powders with differen t size distributions. It is concluded that the molten state of startin g powder also fluctuated with particle size but the trend was differen t from that of crystallinity. Coatings sprayed with different particle sizes were immersed in deionized water for 1 month. After immersion, severe degradation and break-up were observed on the surface of coatin gs with the highest crystallinity, which were sprayed with large sized HA powders. It may be the high porosities in these coatings that caus e the severe degradation. This shows that high crystallinity is not ne cessarily related to high stability of coatings and microstructure is of great importance when stability of coatings is considered. (C) 1996 Elsevier Science Limited.