Low temperature diffusion treatments for stainless steel

Austenitic stainless steels are used widely in structural applications and for functional devices where good corrosion resistance is required. The tribological behaviour of such materials is however poor, preventing their use in mechanical devices where (for example) sliding or impact wear may occur.

Attempts to apply conventional diffusion treatments (such as gas nitriding) on such materials to improve wear behaviour show limited success - due both to the surface oxide film present and to the reduction in corrosion resistance which chromium nitride precipitation tends to cause. Recent attempts to apply plasma nitriding processes at treatment temperatures below 450°C have been shown to suppress nitride formation and reveal the development of a so-called ’expanded austenite’ phase (with extreme interstitial nitrogen supersaturation) - which exhibits high hardness and wear resistance. The exact structure and composition of this phase is a matter of some debate; further detailed analytical work is required to understand more about both the nanostructure of this phase and the influence of process parameters on its formation. There is also literature evidence to suggest that carbon-expanded austenite is similarly wear-resistant and can be obtained at higher temperatures (ie. 500°C+) whilst still avoiding the formation of other phases damaging to corrosion resistance - with positive commercial implications for the depth and rapidity of treatment attainable. Low-pressure, high-intensity plasma processing techniques (pioneered in Physical Vapour Deposition of wear-resistant thin films) show excellent promise for the application of such diffusion layers on stainless steels and other candidate materials; further studies of the treatment parameters are however required to achieve process optimisation.