Microstructural self-healing effect of hydrothermal crystallization on bonding strength and failure mechanism of hydroxyapatite coatings

Hydroxyapatite coatings (HACs) are synthesized on Ti–6Al–4V substrates using the plasma spraying process followed by autoclaving hydrothermal treatment at 125 and 150 °C. The quantitative analysis of X-ray diffraction indicates that the hydrothermal treatment is available to promote HA crystallization and further eliminate the amorphous and impurity phases of the HACs. The microstructural self-healing effect of hydrothermally treated HA coatings (HT-HACs) can be recognized as a nucleation and grain growth of nano-crystalline HA which tends to diminish the spraying defects through the hydrothermal crystallization. The significant hydroxy ion (OH−) peak in XPS spectra detected from HT-HAC specimens represents that the hydroxyl-deficient state of plasma-sprayed HACs is significantly improved with the abundant replenished OH− groups during the hydrothermal treatment. XPS analysis also demonstrates that the hydrothermal crystallization helps to promote the interfacial Ti–OH chemical reaction. The bonding strength of HA coatings is significantly improved from 32.4 MPa for the as-sprayed HACs to 38.9 MPa after 150 °C hydrothermal treatment. Through the statistical analysis of Weibull distribution function, the strengthening HT-HACs are generally reliable materials with a wear-out failure model. The failure morphologies of HT-HACs represent homogeneity with a larger area fraction of cohesive failure and a decreased adhesive failure area. The phenomena were resulted from the microstructural self-healing effect and the enhanced interfacial adhesion of HT-HACs to Ti–6Al–4V substrate.