Effect of crystallization on the bonding strength and failures of plasma-sprayed hydroxyapatite

Hydroxyapatite coatings were synthesized on T1-6Al-4V substrates using the plasma spraying process followed by vacuum and atmospheric post-heat treatments at various elevated temperatures. This study provides an evaluation of the bonding strength and crystallization rate of HACs resulting from the variations in crystallinity and thermal induced cracking. Experimental results provide evidence that the atmospheric heat-treated HACs possessed higher crystallization rate than the vacuum-heated specimens. It implies that the presence of an atmosphere with moisture plays an important role in improving the crystallization of HACs. The bonding strength of all samples was improved with increasing HA crystallization, and the optimal heating condition was found to be about 600°C. However, the crystallization-induced defects result in a serious microstructure and bonding strength degradation when the heating temperatures were higher than 600°C for both post-heat methods. On the basis of the fracture morphologies observation and the bonding strength data fluctuation of both heating conditions, the Weibull distribution function provides a powerful statistical analysis for assessing the failure mechanism and the reliability of plasma-sprayed and post heat-treated HACs.