Characteristics of a plasma electrolytic nitrocarburising treatment for stainless steels

In this work, we investigate the feasibility of a plasma electrolytic nitro carburising (PEN/C) treatment applied to AISI 316 stainless steel using a m odified aqueous solution of urea as the treatment electrolyte. The substrat e 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 characteristic s of the treated component show that the friction coefficient against a WC- Co bail counterface can be slightly reduced, and that the wear rate decreas es by several decades. The microstructure of the treated layers depends str ongly on the electrical parameters (e.g. the applied voltage, which control s the treatment temperature) and can be adjusted from a single expanded aus tenite phase (gamma (N)) to multi-phase combinations, including mixtures of Fe(Fe,Cr)(2)O-4, gamma (N) and (Cr,Fe)N-x. The corrosion properties of the treated layers are closely correlated to the microstructure and compositio n and can be significantly improved, particularly if a thin, but dense, mag netite-based iron-chromium oxide layer is produced at the surface. For impr ovement of the tribological properties of substrates treated at low tempera ture, 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 sta inless steels for applications in aggressive corrosive-wear environments. ( C) 2001 Published by Elsevier Science B.V. All rights reserved.