Cracking caused by cutting of plasma-sprayed hydroxyapatite coatings and its relation to the structural features of coatings deposited at different initial substrate temperatures

The present study estimated the cracking phenomenon in as-plasma-sprayed hydroxyl­apatite coatings (HACs) after they were being subjected to the severe cutting conditions in the direction perpendicular to the coating/substrate interface. In order to evaluate the effects of substrate preheating on the occurrence of micro-cracks, the HACs were depo­sited at different initial substrate temperatures ( T S = 20, 100 and 200 °C). The changes in phase composition and HA splat morphology with T S were observed and were correlated with the cracking occurrence. The results showed that severe cutting conditions intro­duced a localized cracking in the regions of HACs dominantly attributed to the brittle hyd­roxyl-deficient amorphous calcium phosphate (ACP) phase. This effect was particularly observable in the HACs deposited without preheating of substrate. On the other hand, the preheating of substrate reduced the presence of micro-cracks and caused insignificant changes in the average local phase composition. In HACs deposited with preheating of substrate, the HA splats (of which HACs are composed) were thinner and recrystallized HA regions seemed smaller in size and more evenly distributed. These results implied pot­entially important roles of the HA splat formation mechanism on the distribution of ACP and recrystallized HA regions in the as-plasma-sprayed HACs and the cracking resistance of HACs.