Structure of duplex CrN/NbN coatings and their performance against corrosion and wear

In tribological applications the coating-substrate combination can be considered as a system, since both greatly influence the properties of that affect the tribological performance. Further, it is often desirable that both high wear resistance and corrosion resistance can be achieved even when low cost and easily machineable substrate materials are considered. Duplex surface treatment combining pulse plasma nitriding and PVD coating can provide solution for excellent wear and corrosion resistance for low alloy and constructional steels. In this work three different pulse plasma nitriding processes were carried out prior to the CrN/NbN PVD coating to attain high surface hardness and enhanced load bearing behaviour for S154 high strength construction steel. The phase composition of the compound layer, formed in the nitriding process, was found to greatly affect the tribological properties of the duplex system. The compound layer with high amount of epsilon-phase contributed to superior corrosion and wear resistance, whereas the ductile gamma'-phase compound layer provided better impact resistance and enhanced. The best duplex treated S154 samples had wear resistance comparable to that of similarly coated HSS. The corrosion resistance was also improved by duplex process. If anodic current at +500 mV vs. SCE is considered as criteria, the best system has almost 3 orders of magnitude lower corrosion current than with the PVD coating alone. (c) 2007 Elsevier B.V. All rights reserved.