Influence of a Nanocrystalline surface layer combined with low temperature plasma Nitriding on Microstructure and Tribological Properties

Plasma nitriding has proved to be a useful method for increasing the hardness and wear resistance of austenitic stainless steel without reducing the corrosion resistance of this alloy [1]. However, at low temperature nitriding, the following two problems remain [2]: the thickness of the nitrided layer is relatively low compared to conventional nitriding processes performed at high temperatures, and the hardness exhibits a very steep gradient at the interface between the treated layer and the substrate. Creating a nanocrystalline surface layer prior to plasma nitriding may be one way of improving this [3]. This is because the increased dislocation density and grain boundary fraction near the surface of the nanocrystalline are thought to provide fast diffusion pathways for the nitrogen atoms into the material [4]. In this study, a medical grade AISI 316-ASTM F138 stainless steel was first nanocrystallized by a Surface Mechanical Attrition Treatment (SMAT) followed by plasma nitriding, in order to increase the thickness of the nitrided layer, thereby improving the wear resistance of this stainless steel. Superficial microstructures, nitrogen concentration profiles, phases and micro-hardness of treated specimens were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), glow discharge optical emission spectrometry profiles (GDOES) and micro-Vickers hardness tester. Tribological properties and wear mechanisms were investigated on a pin-on-disc wear tester under Ringer's solution. Also, loss of metal and coefficient of friction values were determined from wear testing.