Microstructural healing with interfacial reaction layers on the adhesive strength enhancement of plasma-sprayed hydroxyapatite coatings

The effect of hydrothermal crystallization and adding reinforced intermediate layers on improving the tensile adhesion of plasma-sprayed HA coatings (HAC) was investigated. The experimental results show that the index of crystallinity (IOC) and phase purity of hydrothermally-treated HAC (HT-HAC) are increased by the low-temperature hydrothermal treatment. The microstructural healing effect with nano-size HA crystallites is significant to diminish the defects and prevent mechanical strength degradation for the HT-HAC. XPS analysis demonstrates that hydrothermal crystallization helps to promote the interfacial TiOH chemical reaction. HA composite coatings with CP-Ti and ZrO₂ intermediate layers provide another strengthening effect compared with the hydrothermal-heating method. The inter-diffusion of Ca results in a chemical bonding at the HA/ZrO₂ interface, which results in the increase of the adhesive strength of composite coatings. The fracture behavior is different between the crystallization-induced HAC and the composite coatings. The HT-HAC remains on the substrate with an evident cohesive failure. The adhesive failure occurred at HA/intermediate layers for composite coatings. Failures with less percentage area of interfacial fracture are indicative of a higher strength of a coating.