TaN/CrN/ZrN functionally graded thin-film coatings on Ti-6Al-4V for load-bearing biomedical applications: Microstructure, wettability, and nanomechanical properties.

Surface modification of Ti-6Al-4V alloys is essential to improve their mechanical performance in demanding applications. In this work, stepwise functionally graded TaN/CrN/ZrN coatings were synthesized on Ti-6Al-4V substrates using a combined DC/RF magnetron sputtering technique. The influence of substrate temperature on coating microstructure, surface characteristics, wettability, and nanomechanical behavior was investigated by varying the substrate temperature from 250 °C to 550 °C while maintaining identical sputtering conditions. Cross-sectional and surface FE-SEM analyses revealed continuous, dense coatings with a well-defined graded structure and gradual changes in grain morphology and surface features with increasing substrate temperature. Grazing-incidence X-ray diffraction and Raman spectroscopy indicated enhanced crystallinity and structural ordering at elevated temperatures without the formation of secondary oxide phases. Surface characterization showed temperature-dependent changes in roughness and wettability, with a gradual transition from hydrophilic to hydrophobic behavior. Nanoindentation measurements revealed a substantial increase in hardness and elastic modulus with increasing substrate temperature, reaching maximum values of 43.52 GPa and 295.85 GPa, respectively, for coatings deposited at 550 °C. Scratch adhesion testing demonstrated improved coating adhesion and scratch resistance with increasing substrate temperature up to 450 °C, while the coating deposited at 550 °C exhibited comparatively higher brittle fracture tendency under progressive loading. These results confirm that substrate temperature is a key parameter for tailoring the structural, mechanical, wettability, and adhesion behavior of stepwise functionally graded TaN/CrN/ZrN coatings on Ti-6Al-4V substrates.