Role and effect of mechanical polishing on the enhancement of the duplex mechanical attrition/plasma nitriding treatment of AISI 316L steel

Abstract : This paper analyses a recently developed duplex process combining a Surface Mechanical Attrition Treatment (SMAT) with low-temperature plasma nitriding. The thickness of the nitrided layer significantly increased by adding a polishing step between SMAT and plasma nitriding at 425 °C for AISI 316 austenitic steel. Contrarily it was also observed that without this polishing step the hardness decreased and the thickness and the homogeneity of the nitrided layer diminished with respect to the standard duplex process. The reason for this phenomenon is not well understood. Therefore, the aim of this investigation is to characterize and to discuss the effect of SMAT on the nitrogen diffusion into an austenitic stainless steel AISI 316 subjected to the duplex surface treatment. A second objective is to study the effects of the intermediate polishing step on the friction coefficient and hardness. In order to better understand the link between the generated microstructures and the diffusion kinetics and tribological responses, samples have been characterised by many different techniques such as Optical and Scanning Electron Microscopy (OM and SEM), X-Ray diffraction, hardness and ball-on-disc tests. Special attention was paid to the formation of oxide layers. The superficial and in-depth chemical composition of the SMATed samples were characterised by Energy Dispersive X-ray spectrometry (EDS) and Glow Discharge Spectrometry Profiles (GDOES), respectively.