Effects of Different Hydroxyapatite Binders on Morphology, Ca/P Ratio and Hardness of Nd-YAG Laser Clad Coatings

Hydroxyapatite (HA) is a well known biocompatible coating material used to improve the bonding quality between metallic substrates and the surrounding bone tissue following implantation. In this study, HA is mixed with two different binders, namely water glass (WG) and polyvinyl alcohol (PVA), respectively, and is then clad on Ti-6Al-4V substrates using an Nd:YAG laser beam. The results show that for both binders, the weld surface width, penetration depth and heat affected zone decrease with an increasing travel speed. The PVA binder increases the number and severity of the cracks in the transition layer of the weld bead, whereas the WG binder increases the porosity. In each coating layer, the middle level has the lowest Ca/P ratio, while the lower level has the highest value. Moreover, the Ca/P ratio reduces with an increasing travel speed. The microstructure of the transition layers is mainly composed of CaTiO3, Ca2P2O7, TiP2, Ti, and HA phases. SiO2 and Si2Ti are additional phases found in the specimen used WG as binder material. The WG binder yields a higher hardness of the coating layer and transition layer than the PVA binder. However, for both binders, the hardness of the transition layer is generally far higher than that of the substrate or the coating layer.