Résumé
Plasma nitriding is generally used to harden surfaces and thus to improve their wear resistance. However, when applied to austenitic steel it often deteriorates its corrosion properties. After the discovery of expanded austenite [1], it was observed that low temperature nitriding produces a thin and very hard layer with an abrupt layer/core interface. Furthermore, a high dislocation density together with a small grain size enhances the nitrogen diffusion [2-3]. These can be obtained by surface mechanical attrition treatment (SMAT), but unfortunately an oxide layer is also formed on the surface, which significantly diminishes the nitrogen flux into the bulk material [4]. The aim of this investigation is to study the effect of SMAT followed by low temperature plasma nitriding. This duplex treatment is supposed to facilitate the diffusion of nitrogen into austenitic stainless steel. In this study, plasma surface alloying treatments were carried out on SMATed medical grade ASTM F138 in the temperature range 350°C-425°C. Superficial microstructures of the SMATed specimens prior to plasma nitriding were characterized using a micro-Vickers hardness tester, optical microscope, scanning electron microscope and X-ray diffraction. The characterizations show that SMAT prior to nitriding can either enhance or diminish the nitrogen diffusion, because the latter depends on the surface features before and after SMA treatment. In this context, two processes were studied. In the first approach, the samples are SMATed and then nitrided. These samples exhibit a negative SMAT effect, in terms of hardness, nitrided phase formed and homogeneity of nitride layer. Other authors reported similar results in a previous study [4]. In the second approach, the samples are treated in the same manner, but they undergo another treatment between SMATing and nitriding. This increases the thickness of nitride layer and enhances several mechanical properties such as micro-hardness and wear resistance.