Effect of nature of nitride phases on sliding wear of plasma nitrided sintered iron

In the present investigation, the effect of the nature of the iron nitrides (γ′-Fe4N or ε-Fe2-3N) on the sliding wear of plasma nitrided unalloyed sintered iron is studied. Plasma nitriding was performed in an industrial scale plasma reactor using two different sets of operative parameters to produce microstructures composed of different dominant iron nitride phases, i.e., γ′-Fe4N and ε-Fe2-3N. Microstructural characterisation was performed using optical microscopy, phase analysis was performed via X-ray diffraction, and topographical analysis was performed using laser interferometry. Dry, ball-on-flat and reciprocating sliding tests were used to perform the tribological characterisation. The wear loss was determined by volumetric loss of the wear scar using laser interferometry. The wear rate of the counter-body was also evaluated. Wear mechanisms were characterised using scanning electron microscopy. Compact, free of porosity compound layers with the same thickness were formed, all of which were topographically similar. The analysis of the worn surfaces showed that several wear mechanisms operate simultaneously during the wear process, with the most significant mechanisms being oxidative and abrasive wear. Based on the wear mechanisms, a model for the wear of the nitrided layers (composite layer and diffusion layer) was proposed. The tribological tests indicate that, for the tribo-system evaluated, the ε-nitride layer exhibited a superior wear performance. The volume loss for the ε-phase was two times lower than that for the γ′-phase.