A study of nitrogen-containing, high manganese austenitic stainless steel coatings deposited by sputter PVD

The use of nitrogen as an alloying element in steel is either limited to small quantities (<2 wt% / 7. 5 at%) in bulk materials, where it is used primarily as an austenite stabiliser and mechanical/tribological property enhancer, or introduced in much larger quantities (e.g. 38 at%) in thermochemical surface engineering treatments – where it is used to create a hard, corrosion-resistant diffusion layer of typically 20-30 μm depth, commonly referred to as “Expanded austenite” or “S-phase”. This study examines the effects of nitrogen incorporation in a high-manganese austenitic stainless steel (Staballoy AG 17), at levels that lie between these two extremities with the intention of improving the mechanical and wear properties without compromising the inherent high resistance to corrosion which such alloys possess. Thick, dense and featureless coatings of austenitic-manganese steel containing different levels of interstitial nitrogen were deposited by reactive magnetron sputtering in an argon-nitrogen plasma. The resulting microstructures, characterized by XRD, SEM/EDX, nano-indentation, and fracture analysis were found to exhibit a texture and a small nitrogen gradient across their thickness. With increasing nitrogen content, the hardness was observed to increase and the morphology changed. Moreover, changes in nitrogen content were found to have a more profound effect on the coating properties than any of the process parameters evaluated, the results of which are presented. Excessive nitrogen resulted in the precipitation of Mn4N, apparently avoiding chromium sensitisation. Finally, since the deposited coatings were characterized by high compressive stresses, the stresses involved and the failure modes observed are discussed.