Periodic crystallization effect in the surface layers of coatings during plasma spraying of hydroxyapatite

A new effect of the amorphous component of periodic crystallization in the surface layers of coatings during hydroxyapatite (HA) plasma spraying is revealed. The effect is caused by significant temperature increases of surface layers due to plasma heating in the process of coating. Under prevalent spraying conditions, the crystallite sizes of the crystallite phase are considerably less than those in dense HA. These non-equilibrium phases crystallize consistently and, as a result, in the surface layers HA crystallites emerge and grow. This leads to oscillating type dependences of crystallite size and crystallinity on coating thickness.