The effect of heat treatment on the structural and performance evolution of plasma electrolytic oxidation coatings

To improve the mechanical durability of biomedical implants, this work examines how heat treatment influences the structure and properties of plasma electrolytic oxidation (PEO) coatings on Ti-6Al-4V alloy. The evolution of the coating was evaluated in terms of phase composition, surface characteristics, and fatigue performance using X-ray diffraction, microhardness tests, and residual stress analysis. It is found that heat treatment markedly improves the crystallinity and hardness of the coating, with the 800 °C treated sample reaching around 670 HV. However, the effect on fatigue life is strongly temperature dependent. A treatment at 400 °C yields the best fatigue resistance, attributed to increased coating densification (porosity reduced to ∼8%) and the preservation of residual compressive stresses, which help inhibit crack initiation. In contrast, treatments above 600 °C promote the anatase-to-rutile phase transition. This transformation introduces residual tensile stresses and encourages microcrack formation, significantly degrading fatigue life. Moreover, surface wettability evolves with phase composition and defect density: the hydrophilic rutile phase formed at 600 °C minimizes the water contact angle, whereas increased microcracking at 800 °C traps air and raises the contact angle. These findings establish heat temperature-dependent structure-property relationships critical for orthopedic coating design.