The effect of surface mechanical attrition treatment on low temperature plasma nitriding of an austenitic stainless steel

The combined effect of superficial nanocrystallisation by SMAT (Surface Mechanical Attrition Treatment) followed by plasma nitriding on the mechanical properties of a medical grade austenitic stainless steel was studied. SMAT conditions were optimised to enhance nitrogen diffusion. Experimental observations (energy dispersive X-ray spectroscopy profiles, cross-sectional optical micrographs, phase analysis by X-ray diffraction and micro-hardness profiles) show that polishing away a very thin layer after SMAT and before nitriding significantly improves nitrogen diffusion into the substrate, yielding a 50% thicker nitrided layer. Possible causes for this improvement are discussed.

► The formation of a nanocrystalline surface layer improves the nitrogen diffusivity. ► It yields a 50% thicker nitrided layer. ► This layer is 5 times harder than the 316L substrate. ► This result is obtained by adding a polishing step between SMAT and nitriding.