Effect of hydrogen on microstructure and mechanical properties of plasma-nitrided pure titanium by cathodic cage plasma nitriding

Surface modification technologies have been applied for the improvement of surface properties, and low temperature nitriding in the mixed N2-H2 atmospheres by cathodic cage plasma nitriding (CCPN) is identified to be an effective way to enhance the mechanical properties of Ti and its alloys. However, little attention was paid on the absorbed H2 in the nitrided specimen under different H2 proportion and the consequent deleterious influence. In this paper, the influence of different H2/(N2 + H2) ratio on microstructure of the nitrided Ti specimens by CCPN were investigated by XRD, EDX, SEM, respectively. And the surface structure of the compound layer is consisted of an outer layer of δ-TiN over the inner layer of ε-Ti2N, formed on the top of diffusion layer consisting of mainly of α-Ti solid solution enriched with nitrogen. It is found that the thickness of compound layer on Ti increases with the introduction of H2. And the N-rich top layer of TiN layer with coarse grain structure is resulted by the combined effect of the introduction of H2 and the plasma action. The surface mechanical properties of nitrided layer were also measured by microhardness tester and tensile test. The results show an average surface hardness of 900 HV0.2–1100 HV0.2 of the nitride surface compared to that of Ti (200 HV0.2 in average). And the surface hardness of nitrided Ti samples increases with the increasing H2 concentration. However, the tensile properties are badly deteriorated independent of hydrogen concentration with the introduction of H2 due to the generation of titanium hydride during nitriding process.