The mechanism of hydroxyapatite coatings degradation at high substrate temperatures

Abstract

Physical vapor deposition methods used for hydroxyapatite (HA) coatings typically require elevated substrate temperatures and post-deposition annealing to induce crystallization. However, such thermal treatments can degrade both the mechanical integrity and bioactivity of the coating, particularly when substrate temperatures exceed 500 °C. The mechanisms underlying these phenomena remain insufficiently understood. In this study, HA thin films were deposited on silicon and Ti6Al4V substrates using pulsed laser deposition and were systematically characterized to elucidate these mechanisms. XPS and SIMS analyses revealed a temperature-dependent loss of OH− and PO43− groups, an increased Ca/P ratio, and the formation of interfacial oxides, all of which contribute to weakened adhesion. To clarify the temperature-dependent decline in bioactivity, protein adsorption behavior was analyzed using a Kramers-type kinetic framework; the fitted desorption kinetics indicate that coatings deposited near ∼500 °C provide the most stable protein attachment, whereas higher temperatures accelerate desorption due to dehydroxylation and carbonate substitution. Together, these findings provide mechanistic insight into the thermal degradation of HA coatings and offer a framework for optimizing deposition parameters to preserve stoichiometry, adhesion, and bioactivity for long-term biomedical applications.