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

Abstract

a b s t r a c tThe 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 Ti6Al-4V has been studied under rotating-bending fatigue testing. Following the diffusion treatment, samples exhibit a hardened case, more than 30 lm 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.

Evaluating the effects of PIRAC nitrogen-diffusion treatments on the mechanical performance of Ti–6Al–4V alloy

“…8. The results for untreated Ti-6Al-4V presented in this plot are consistent with those previously reported in [5]. The fatigue limit appears to decrease following PIRAC nitrogen-diffusion.…”

“…The lower temperature treated samples were clearly compromised by the much longer diffusion treatment time of 48 h. Finally, Ti-6Al-4V samples exhibiting the largest decline in fatigue limit (approximately 25%) were those treated at 900 1C for 2 h, when compared to untreated Ti-6Al-4V. The improved performance in fatigue life for the treatments performed at 800 1C (as opposed to treatments at 700 1C and 900 1C) is attributed to the increased build-up of compressive residual surface stresses at 800 1C which inhibit crack propagation by resisting cyclic slip [5,43]. Moreover, longer plasma nitriding durations have been associated with recrystallisation and grain growth -which may lead to premature failure, decreasing the overall fatigue life [5].…”

“…It has been reported that the rate of nitrogen diffusion in the titanium α-phase is 30 times higher than for nitride compounds such as TiN and Ti 2 N; the formation of compound layers of these titanium nitride phases thus effectively impedes further nitrogen diffusion in the bulk [5], which in turn may dictate longer (or higher temperature) treatments for improved efficacy and thicker compound layer formation. Indeed, according to the Arrheniustype relationship governing nitrogen diffusion, and Fick's second law for non-steady-state diffusion, the treatment temperature is a dominant parameter.…”

“…The improved performance in fatigue life for the treatments performed at 800 1C (as opposed to treatments at 700 1C and 900 1C) is attributed to the increased build-up of compressive residual surface stresses at 800 1C which inhibit crack propagation by resisting cyclic slip [5,43]. Moreover, longer plasma nitriding durations have been associated with recrystallisation and grain growth -which may lead to premature failure, decreasing the overall fatigue life [5]. Indeed, the treatment at 800 1C for 4 h was characterised by the second least increase in grain size out of all the PIRAC treatments studied in this work.…”

“…A number of nitrogen diffusion techniques are known to produce TiN on titanium alloys to improve hardness and wear resistance while also reducing friction. These include chemical vapour deposition (CVD) and reactive physical vapour deposition (PVD) [4][5][6][7]. However these processes deposit a coating on the substrate which forms weak metallurgical bonds upon interacting with a substrate, thus producing a coating which may be susceptible to delamination and premature failure [7].…”