Adhesion of carbon nitride thin films on tool steel

Carbon nitride in thin film form is a good candidate for hard coating but it has poor adhesion on tool steel owing to diffusion of nitrogen or carbon atoms into the substrate at the deposition temperature. The adhesion can be improved when a diffusion barrier is created at the steel surface. TiN or TiCN can be deposited on tool steel prior to carbon nitride deposition, owing to their good adhesion to bulk and to the coating itself. Application of these interlayer is not only expensive but also complex. A process is required to obtain good adhesion of carbon nitride film on tool steel without ally thin him interlayer. In this study AISI H13 and AISI/SAE8620 tool steels are nitrided and carburised respectively in a conventional fluidised bed system and these nitrided and carburised steels are used as the substrates. The films were deposited by a Penning-type opposed target DC magnetron sputtering system on these nitrided tool steel substrates. The adhesion properties of the film was found to be considerably improved on the nitrided samples compared with untreated substrates. As a result of nitriding or carburising, the steel surface is saturated by nitrogen or carbon. Thus no diffusion of nitrogen/carbon atoms occurs to cause breaking of the internal film bonding. X-ray photoelectron spectroscopy was used to observe the diffusion of nitrogen into the steel surface. Raman spectroscopy was used to investigate whether the substrate surface has an effect on film bond structure. The relevant adhesion characteristics along with mechanical properties of the deposited film are discussed. (C) 1999 Elsevier Science S.A. All rights reserved.