Microwave-Assisted Nitriding of Titanium Alloy Forms a Graded Titanium Nitride Surface with Enhanced Hardness and Preserved Gingival Fibroblast Cytocompatibility

This study investigated the phase composition, surface characteristics, near-surface hardness, and gingival fibroblast response of Ti-6Al-4V modified by microwave-assisted nitriding for potential implant-abutment applications. Mirror-polished Ti-6Al-4V discs were treated by microwave-assisted nitriding at 800 or 850 °C and compared with uncoated Ti-6Al-4V and conventionally ion-plated titanium nitride. Commercially pure titanium was included as a reference for cell assays. Both microwave-treated groups exhibited gold-colored, wave-like surfaces with significantly greater roughness than uncoated Ti-6Al-4V and ion-plated titanium nitride. X-ray diffraction revealed titanium nitride, Ti₂N, and TiNₓ phases, with stronger nitride-related peaks after treatment at 850 °C. Glow discharge optical emission spectroscopy showed a gradual elemental transition extending to approximately 8 μm in the microwave-treated groups, whereas ion-plated titanium nitride showed an abrupt transition at approximately 1 μm. The microwave-treated groups exhibited increased near-surface hardness, with the 850 °C treatment showing the highest hardness. Human gingival fibroblast-1 metabolic activity showed no significant differences among groups after 1 or 6 days of culture. These findings indicate that microwave-assisted nitriding can produce a hard, compositionally graded titanium nitride-containing surface on Ti-6Al-4V while maintaining early gingival fibroblast cytocompatibility in vitro.