High-efficiency low-temperature active screen plasma nitriding by bias voltage optimization of Ti-6Al-4V titanium alloy

Nitriding at low temperatures can prevent the deterioration of mechanical properties by grain growth of metal substrate. However, the formation of a sufficiently thick nitride layer under these conditions remains challenging because of the inherently slow diffusion kinetics. This study presents an efficient active screen plasma nitriding (ASPN) technique applied to a Ti-6Al-4V titanium alloy at low temperatures of 500–600 °C without requiring any supplementary deformation processes. The effects of different bias voltages (400 V and 800 V) on the nitriding behavior within the temperature range of 500–600 °C were comprehensively assessed. A compound layer exceeding 6 μm was formed on the surface of the Ti-6Al-4V titanium alloy after 20 h of nitriding at a low temperature of 600 °C, significantly outperforming conventional plasma nitriding techniques. The optimization of bias voltage greatly enhanced the efficiency of the ASPN process. The study also investigated how the bias voltage influences the ASPN process. Appropriate bias voltages facilitated the migration of nitriding species toward the sample surface and promoted nitride layer growth. However, excessively high bias voltages intensified the ion bombardment, increasing the etching effect on the TiN deposition layer. An optimal bias voltage of 400 V best balanced the deposition and sputter-etching of nitride species during the ASPN treatment of the Ti-6Al-4V titanium alloy.