Microstructure and tensile properties of plasma-nitrided TA1 titanium by cathodic cage plasma nitriding in different N2-NH3 gas mixtures

Cathodic cage plasma nitriding (CCPN) is a widely utilized method to improve the surface properties of titanium. Using an NH3-N2 gas mixture has been shown to improve nitriding efficiency, but the effects of varying NH3/ (N2+NH3) ratios on the surface hardness and microstructures of plasma-nitrided titanium remain less explored. Specifically, the question of whether the H2 generated during NH3 decomposition results in hydrogen embrittlement of the substrate is not very concerning. Here, we conduct a series of CCPN experiments on TA1 titanium at different NH3/(N2+NH3) ratios, ranging from 0 % to 70 %, to investigate their impact on the surface hardness, microstructures, and tensile properties. Plasma reactivity and behavior, including electron temperature and density, are analyzed using a single Langmuir probe. Our findings reveal that higher NH3 ratios lead to the formation of a coarse-grained TiN phase and a thicker nitride layer, enhancing surface hardness. Notably, an increase in electron temperature is observed, suggesting improved nitriding efficiency. However, when the NH3 ratio exceeds 30 %, there is a notable decline in tensile properties accompanied by an increase in strength, likely due to the precipitation of multiple titanium hydrides. This study suggests an optimum NH3 ratio of 30 % in the CCPN process to enhance the mechanical properties of titanium and surface integrity and provides insights to the inclination for hydrogen embrittlement due to H2 generated during NH3 decomposition.