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

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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.