Development of Functionally Graded HA-Al₂O₃-TiO₂ Composite Coating on Ti Alloy: Microstructural Characterization, Nano-mechanical Properties, and Wear Behavior

Functionally graded HA-Al₂O₃-TiO₂ composite coating and HA coating have been developed on Ti-6Al-4V alloy by using plasma spraying. Subsequently, post-heat treatment of the coatings has done at 750 °C for 2 h. The effect of graded composition and post-heat treatment on microstructure, phase, nano-mechanical properties, and wear behavior has been investigated in detail. The microstructure analysis shows that there is a reduction in the area fraction of porosities and an improvement in surface roughness in the functionally graded coating. After heat treatment of the coating, the area fraction of porosities and surface roughness reduces, which is due to the sintering effect during post-heat treatment. There is a reduction in the magnitude of residual stress in the functionally graded coating in comparison with HA coating. However, after post-heat treatment, the residual stress magnitude reduces, and nature changes from compressive (as-sprayed condition) to tensile in nature (post-heat-treated condition), which is due to the annealing effect. There is improvement in nano-hardness and Young’s modulus after post-heat treatment of the coating, which is due to the heat treatment leading to densification of microstructure and amorphous to crystalline phase transformation. The wear rate is marginally higher in HA coating (4 × 10⁻⁷ g/s) as compared to the functionally graded HA-Al₂O₃-TiO₂ composite coating (1.1 × 10⁻⁷ g/s– 1.6 × 10⁻⁷ g/s), whereas the improvement is more in the heat-treated functionally graded HA-Al₂O₃-TiO₂ coating (1.1 × 10⁻⁷ g/s).