CHARACTERIZATION OF TIN COATINGS DEPOSITED ON PLASMA NITRIDED TOOL STEEL SURFACES

Wear-resistant TiN coatings deposited on tool steels are used frequently in industry. There is a trend towards further optimizing these coatings, e.g. by plasma nitriding the tool surface prior to TiN deposition. In this work the influence of the nitriding conditions on the surface properties of AISI 304 and ASP 23 tool steels was investigated. The plasma nitriding was carried out in a triode ion plating configuration normally used to deposit TiN coatings. At the surface of AISI 304 stainless steel, only a thin compound layer (Fe4N, Fe3N) was found, probably as a consequence of the rather slow nitrogen diffusion in the austenite matrix. For ASP 23 high speed steel, the different nitriding behaviour of the martensitic matrix causes the formation of a diffusion layer which results in an increase in hardness at the surface. On an analogous set of specimens the TiN deposition was started immediately after the plasma nitriding. To optimize this combined treatment, the influence of the nitrogen content of the nitriding medium and that of the thickness of an intermediate titanium layer were examined. By means of X-ray diffraction only an influence on the lattice parameter of TiN and the nitriding layers was found. as compared with the lattice parameter for the separate treatments. Bad adhesion of TiN coatings on plasma nitrided AISI 304 was observed and was explained in terms of the high internal stresses in the compound layer on the nitrided surface. For the ASP 23 substrate a better coating adhesion was observed, probably due to a different structure of the nitriding layer and to the enhanced substrate hardness. At this stage of the research, none of the above-mentioned nitriding treatments results in a sufficient adhesion of the TiN layer, although some useful facts could be established for further research.