SINGLE-CYCLE PLASMA NITRIDING AND HARD COATING DEPOSITION IN A CATHODIC ARC EVAPORATION DEVICE

The duplex treatment of alloy steel (nitriding + hard coating) has bee n studied for a few years now. It is an interesting method not only fo r increasing fatigue strength and tempering resistance bur also for im proving corrosion and wear behaviours, particularly under high loads. Such treatments are used for dieing or hot-working applications and ar e often performed with a discontinuous combination of nitriding and ha rd coating. This solution has some disadvantages with regards to inter mediate external steps prior to coating/polishing and grinding. The ai m of this work is to investigate the possibilities and interests of pe rforming duplex treatment during a single cycle within an industrial c athodic are evaporation PVD device. Two types of plasma nitriding were tested: 'conventional' plasma nitriding in N-2 + H-2 mixtures and low -pressure enhanced plasma nitriding assisted by an electron beam produ ced by a new are evaporation source. Hard coatings (TiN or CrN) with g ood adhesion can be deposited on nitrided high-alloy steels if nitridi ng, ion cleaning and coating parameters are adjusted. A smooth surface on the bright nitrided substrate (without compound layer) prior to th e coating stage must be achieved in order to reach the best mechanical properties of the composite. This technology, which does not require additional polishing steps before completion, can result not only in l ower production costs but in high continuity between the two stages of duplex treatment. (C) 1997 Elsevier Science S.A.