THE IMPROVEMENT OF THE ADHESION STRENGTH OF DIAMOND FILMS

One of the expected applications of diamond coatings is in cutting tools for non-ferrous metals and alloys such as Al-Si and non-metals such as hard carbons etc. However, the poor adhesion strength of diamond films must be improved for practical use in cutting. In this study, sintered tungsten carbide (WC) without cobalt metal was used as the substrate and the effect of surface decarburization of the substrate to improve the adhesion strength of diamond films was investigated. Surface decarburization and diamond coating were carried out using a microwave plasma chemical vapour deposition apparatus. Good adhesion was obtained by surface decarburization of the substrate before diamond coating. From the results of observations by scanning electron microscopy and transmission electron microscopy, the improvement in the adhesion strength can be considered to be due to an increase in the contact area between the film and the substrate by generation of fine WC grains on the surface of the substrate with the film embedded in the shape of a wedge at the fine WC grain boundaries. The coated insert demonstrates a very small steady wear without film flaking when milling a hard carbon compared with the large amount of wear exhibited by uncoated cemented carbides.