Optimisation of novel bath for plasma electrolytic nitrocarburising of 316L stainless steel and study of tribological properties of treated steel surfaces

A wear resistant nitrocarburised layer was formed on 316L austenitic stainless steel substrate by dc plasma electrolytic nitrocarburising process (PEN/C). A urea based solution was modified such that the temperature of the bath remained around 400°C under the negative bias voltage of 180 V. To get the best tribological properties, the process was performed in three baths containing 5%NH₄Cl and 3, 2 and 1% sodium carbonate. The structural phases, microhardness and wear behaviour of the produced layers were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, a microhardness tester and a wear testing machine. A compound oxide layer, consisting of Fe₂O₃, Fe₃O₄ and Fe(Cr₂O₄)₂, was formed on the surface. The results showed that a sample treated in a bath containing 5%NH₄Cl and 3%Na₂CO₃ exhibited an improvement of tribological behaviour compared to samples which have been PEN/C treated at higher voltages.