Characterization of hydroxyapatite/nano-zirconia composite coatings deposited by high velocity oxy-fuel (HVOF) spray process

Hydroxyapatite (HA)/nano-zirconia composite coatings were successfully produced through high velocity oxy-fuel (HVOF) spraying of radio frequency (RF) plasma synthesized powders. Microstructure characterization and morphology analysis on the nano-structured coatings were conducted using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), and differential scanning calorimetry (DSC). Elastic modulus of the coatings was determined using nano-indentation on polished cross-sections, which revealed Young's modulus value of ∼130 GPa. Results also showed that the nano-sized zirconia particles (<90 nm) did not coarsen drastically after HVOF deposition, and were uniformly distributed throughout the coating. The crystallite size of tetragonal zirconia in the coating was found to be less than 13 nm. Furthermore, HA phase decomposition and chemical reaction between HA and zirconia was virtually undetected, which is beneficial towards the biological performance of the coatings. XRD analysis together with Rietveld refinement quantitatively revealed the content of the phases present in the coatings, including ∼1.2 wt.% CaZrO₃. The conversion temperature of amorphous calcium phosphate to HA was found to be ∼750 °C and the ancillary influence of the heat treatment on coating structure and properties were revealed.