Plasma nitriding in a low pressure triode discharge to provide improvements in adhesion and load support for wear resistant coatings

Wear resistant coatings produced by physical vapour deposition (PVD) techniques are gaining increasingly widespread industrial acceptance, particularly as a means of providing improvements in service life and product quality when used on cutting tools, on casting, moulding, and extrusion dies, and in a variety of other materials processing applications. Titanium nitride coatings are now widely used and second generation alloy coatings such as, for example, Ti(C, N), Ti(B, N), (Ti, Al)N, and (Ti, W)C are being considered for specific wear situations. The next stage in extending the applicability of plasma assisted PVD processing is likely to be in the area of third generation multicomponent coatings, which may incorporate multilayers and/or duplex diffusion/coating plasma treatments. These new combined treatments will primarily be designed to improve coating durability (particularly with regard to toughness and corrosion resistance). The use of alternative substrate materials which are cheaper, lighter, and/or more corrosion resistant than traditional tool steels will therefore also become increasingly important. In order to achieve good coating support with these ‘new’ materials, plasma diffusion or implantation treatments are likely to play a major role, especially if adequate coating-substrate adhesion is to be maintained. This paper investigates the duplex low pressure triode plasma diffusion/coating treatment of ASP23 tool steel and demonstrates that plasma nitriding has the potential to provide substantial improvements in PVD coating support, if appropriate treatment conditions are employed. The plasma theory behind the suitability of low pressure triode plasmas for precoating diffusion treatments is examined, with particular regard to surface oxide removal by argon-hydrogen plasma precleaning and white layer control by adjustment of the nitriding discharge parameters. The performance of the nitrided/coated (TiN) system under scratch adhesion testing is also discussed.