The influences of plasma spraying parameters on the characteristics of hydroxyapatite coatings: a quantitative study

Abstract

All laboratory-made plasma-sprayed hydroxylapatite coatings (HACs) were found to undergo, to different degrees, changes in phase composition, crystallinity, morphology and roughness dependent on plasma spraying parameters (PSPs). The PSPs, which were systematically varied, included the plasma atmosphere, the spraying current and the stand-off distance. Through the determinations of the concentration of impurity phase (CIP) and the index of crystallinity (IOC), the extent of phase purity and the degree of crystallinity of HACs were quantitatively assessed, respectively. Coatings consisting of at least 50% (IOC>50%) of the original crystalline structure and almostly 95% (CIP<5%) apatite with barely detectable extra phases were obtained. The microstructure of HACs exhibited great deviations both in morphology from molten to partial molten state and in roughness from coating of high irregularity (R a=14.48μm) to a smoother (R a=4.46 μm) one, dominantly influenced by the spraying atmosphere. As the terms of CIP and IOC are defined and established, the biological responses related to phase purity and crystallinity of HACs can be further evaluated in vitro and in vivo.