Evaluating the effects of plasma diffusion processing and duplex diffusion/PVD-coating on the fatigue performance of Ti–6Al–4V alloy

The effect of triode-plasma enhanced low-pressure oxygen and/or nitrogen diffusion treatments, either as a single process or in conjunction with plasma-assisted physical vapour deposition (PAPVD) on Ti–6Al–4V has been studied under rotating-bending fatigue testing. Following the diffusion treatment, samples exhibit a hardened case, more than 30 μm deep. Semi-logarithmic S–N plots are used to demonstrate and compare the significant changes in fatigue resistance obtained from each process. Fractography and residual stress measurements show that, compared to annealed samples, the fatigue strength of the diffusion-treated samples was superior; although the result changed depending on the processing parameters and microstructure of the substrate material. Also, unsupported and mechanically uncompliant ceramic coatings, such as TiN, promote the initiation of multiple crack sites, which lead to premature failure of the Ti-alloy substrate and a consequent reduction in endurance limit.

► Nitriding and oxynitriding treatments held at 700 oC increase the surface hardness and maintain the high endurance limit of Ti-6Al-4V.
► Triode plasma diffusion processes create a measurable compressive residual stress which improves the fatigue performance.
► PVD coatings reduce the fatigue life when not properly mechanically supported; conversely duplex processes improve the fatigue endurance.