The addition of hydrogen to nitrogen facilitates the formation of nitride phases in the plasma nitriding processes of stainless steels, though it also induces the deterioration of their mechanical properties. This study presents a hydrogen-free plasma nitriding process for fabricating a nitrogen-expanded austenite phase (γN) on an AISI 316 stainless steel surface. The steel substrate was nitrided in N2-Ar plasma with various gas compositions discharged by radio frequency (RF) and direct current (DC) modes. The process using the RF mode enabled the fabrication of a layer composed of a γN phase with a thickness of approximately 3 μm on the steel surface regardless of the gas composition, thereby enhancing its surface hardness. In contrast, such a layer was not observed in the DC mode, and the steel's hardness was similar to that of the untreated surface. This difference in layer formation was attributed to the mitigation of surface etching by the Ar active species using the RF mode because of the lower bias voltage compared with that of the DC mode. This phenomenon suppresses the removal of the nitride phase formed during the process, which is a key factor contributing to nitrogen penetration. In conclusion, an N2-Ar plasma nitriding process using the RF mode is demonstrated to be a hydrogen-free process for fabricating a layer of a γN phase.
Keywords: N2-Ar gas mixture; austenitic stainless steel; etching mitigation; hydrogen-free plasma nitriding; nitrogen-expanded austenitic phase; radio frequency discharge mode.