Blue diode laser-induced nitriding of Ti-6Al-4V ELI and Ti-6Al-7Nb: Microstructure, hardness and wear behavior

Laser-induced nitriding offers an effective route for improving the surface performance of titanium alloys used in biomedical applications. In this study, Ti-6Al-4V ELI and Ti-6Al-7Nb alloys were treated using a low-power blue diode laser (455 ± 5 nm, 22 W) under a controlled nitrogen atmosphere. The influence of laser power on microstructure, phase composition, microhardness, and wear behavior was systematically investigated.Cross-sectional analysis revealed the formation of a multilayer structure consisting of a TiN-rich compound layer, an underlying remelted zone, and a heat-affected zone. The total modified depth increased with increasing laser power, reaching approximately 160-170 μm at the highest processing conditions. X-ray diffraction confirmed the formation of titanium nitride phases, with peak intensities increasing as a function of laser power, indicating enhanced nitrogen incorporation.The development of TiN-based surface layers resulted in a significant increase in hardness, with maximum values of approximately 1758 HV for Ti-6Al-4V ELI and 2233 HV for Ti-6Al-7Nb, compared to substrate hardness values of ~300 HV. The hardness gradually decreased with depth, reflecting the transition from the compound layer to the substrate. Improved wear resistance was observed for all treated samples, particularly at intermediate to high laser powers, where the thicker nitride layer effectively limited plastic deformation and material removal during sliding. The results demonstrate that low-power blue diode laser processing provides an efficient and controllable approach for producing hard, wear-resistant TiN surface layers on biomedical titanium alloys.