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 deposit ion of hydroxyapatite (HA) coatings in recent years. The effect of melting state of HA powders on the coating microstructure and performance is invest igated in this paper. Tensile adhesion lest, X-ray diffraction (XRD) analys is and differential scanning calorimeter (DSC) analysis are used for the ev aluation of the coatings. The coating microstructure is analyzed by scannin g electronic microscopy (SEM). The results show that the tensile bond stren gth of the HVOF sprayed HA coatings can be up to 31 MPa and is largely depe ndent on the spray parameters. XRD result shows that the as-sprayed IIA coa ting using powders with the size around 50 mu m is composed of crystalline HA and very small amount of tricalcium phosphate (alpha-TCP). The coatings deposited using fine powders around 30 mu m contained a lot of amorphous ph ase, crystalline HA and very small amount of alpha-TCP. Result from the DSC indicates that the crystallization temperature of the amorphous phase in t he coatings is around 703 degrees C. Amorphous phases in the as-sprayed coa ting significantly improved bond strength. The structure crystallized. tota lly in heat-treated coatings below 750 degrees C. SEM analyses of the tensi le fractured HA coatings show that fracture occurs within the partially mel ted powders. The interface between the melted and unmelted parts of the pow ders is the weakest zone for crack propagation. This suggests that the full y melted state of the feedstock can result in the formation of amorphous ph ase and simultaneously decrease the bond strength. It also suggests that th e fraction of the powders melted is the most critical factor influencing th e bond strength and phase composition of the coatings. The partial melting state of HA powders is beneficial in terms of bond strength and crystallini ty. (C) 2000 Elsevier Science S.A. All rights reserved.