INVESTIGATION OF THICK NITRIDE COATED SYSTEMS FOR HIGH-LOAD APPLICATIONS

Thick (e.g. 10-100 mu m), hard coatings (e.g. TiN and CrN) may be capa ble of improving the tribology and service lifetimes of components whe re the use of thinner coatings is prevented by either substrate surfac e roughness or the scale of in-service contact. In such applications ( e.g. large valves), the acceptable number of cycles to failure may be relatively small, whereas the contact loads can be very large. In this study, the resistance to scratching and high load contact deformation of various thick-coated (15-40 mu m) systems have been investigated. Cast iron, EN8 steel and a duplex stainless steel were used as substra te materials. Prior to ceramic coating, these were treated by either p lasma nitriding or electroless nickel deposition to give an interlayer . CrN or TiN coatings of different thicknesses were then deposited usi ng are evaporation. In order to mimic the scale of in-service contact damage, the samples were tested using high load ball indentation (2 kN ) and a single pass, multiasperity, high load scratch test (33 kN clam ping force). Contact damage was examined using scanning electron micro scopy and scanning electron acoustic microscopy. The latter technique allows subsurface cracking and interfacial debonding to be observed. C onclusions are drawn regarding the roles of the substrate, interlayer and ceramic top layer in determining performance.