Formation of S-phase layer on plasma sprayed AISI 316L stainless steel coating by plasma nitriding at low temperature

Low-temperature nitriding of austenitic stainless steels can produce expanded austenite, known as S-phase, leading to improved surface hardness while maintaining corrosion resistance. Sprayed AISI 316L coatings include oxide layers and defects (pores and cracks), and their structure is considerably different from AISI 316L steel plate structures. In this paper, plasma sprayed AISI 316L coating was treated to produce S-phase by low-temperature plasma nitriding. The effects of nitriding temperatures and spraying conditions (input electrical power and particle size of the spray powder) on the S-phase layer were investigated, and also the nitrogen diffusion process was discussed. Under optimized spraying conditions, the sprayed coatings formed thicker nitride layers than those formed on AISI 316L steel plates; this was the result of a small amount of trapping of nitrogen by dissolved Cr.