Effect of the powders' melting state on the properties of HVOF sprayed hydroxyapatite coatings

High velocity oxy-fuel (HVOF) spray technique is being used for the deposition of hydroxyapatite (HA) coatings in recent years. The effect of melting state of HA powders on the coating microstructure and performance is investigated in this paper. Tensile adhesion test, X-ray diffraction (XRD) analysis and differential scanning calorimeter (DSC) analysis are used for the evaluation of the coatings. The coating microstructure is analyzed by scanning electronic microscopy (SEM). The results show that the tensile bond strength of the HVOF sprayed HA coatings can be up to 31 MPa and is largely dependent on the spray parameters. XRD result shows that the as-sprayed HA coating using powders with the size around 50 μm is composed of crystalline HA and very small amount of tricalcium phosphate (α-TCP). The coatings deposited using fine powders around 30 μm contained a lot of amorphous phase, crystalline HA and very small amount of α-TCP. Result from the DSC indicates that the crystallization temperature of the amorphous phase in the coatings is around 703°C. Amorphous phases in the as-sprayed coating significantly improved bond strength. The structure crystallized totally in heat-treated coatings below 750°C. SEM analyses of the tensile fractured HA coatings show that fracture occurs within the partially melted powders. The interface between the melted and unmelted parts of the powders is the weakest zone for crack propagation. This suggests that the fully melted state of the feedstock can result in the formation of amorphous phase and simultaneously decrease the bond strength. It also suggests that the fraction of the powders melted is the most critical factor influencing the bond strength and phase composition of the coatings. The partial melting state of HA powders is beneficial in terms of bond strength and crystallinity. (C) 2000 Elsevier Science S.A. All rights reserved.