Characteristics of a plasma electrolytic nitrocarburising treatment for stainless steels

In this work, we investigate the feasibility of a plasma electrolytic nitrocarburising (PEN/C) treatment applied to AISI 316 stainless steel using a modified aqueous solution of urea as the treatment electrolyte. The substrate samples were connected cathodically to a high-current DC power supply and biased with a negative voltage in the range 220-260 V. The treatment time was typically in the range of 30-60 s. Investigations of the characteristics of the treated component show that the friction coefficient against a WC-Co ball counterface can be slightly reduced, and that the wear rate decreases by several decades. The microstructure of the treated layers depends strongly on the electrical parameters (e.g. the applied voltage, which controls the treatment temperature) and can be adjusted from a single expanded austenite phase (υ_N) to multi-phase combinations, including mixtures of Fe(Fe,Cr)₂O₄, υ_N, and (Cr,Fe)N_x. The corrosion properties of the treated layers are closely correlated to the microstructure and composition and can be significantly improved, particularly if a thin, but dense, magnetite-based iron-chromium oxide layer is produced at the surface. For improvement of the tribological properties of substrates treated at low temperature, a diamond-like-carbon coating was deposited on the PEN/C pre-treated substrate using a plasma-immersion ion-assisted deposition (PIAD) process. Such duplex treatments show great potential for surface modification of stainless steels for applications in aggressive corrosive-wear environments.