Chemical bonding in magnetron sputtered TiNx coatings and its relation to diamond turnability

In ultraprecision turning monocrystalline diamond is up to now the only sui table tool material. However, only a few, relatively soft materials are tur nable, e.g. aluminum or copper. Most alloys of transition metals with highe r wear resistance like steels lead to unacceptable wear of the diamond tool s. Electroless nickel phosphorus coatings (NiPx) are today the hardest, mos t wear resistant material enabling optical surface qualities with diamond t ools. Therefore, steel parts are coated with NiPx, prior to ultraprecision turning. As a possible alternative with higher hardness than Nil, we invest igated TiNx coatings deposited by reactive DC magnetron sputtering. We meas ured the XPS valence band and core level structure of these coatings with n itrogen contents CN = 0-50 at.%. The chemical reactivity of the coatings wi th monocrystalline diamond was checked with a thermal contact test in vacuu m pressing a diamond against the coating at 1000 degreesC. We find that the Ti3d valence band at 2 eV binding energy is visible at all nitrogen conten ts. But as soon as c(N) > 1 at.% a Ti3d-N2p hybrid band at approximately 5 eV emerges and exceeds the intensity of the 3d band in the range c(N) = 5-1 0 at.%. The intensity of the N2s band at approximately 17 eV is correlated with the intensity of the hybrid band. Between CN = 15-30 at.% N the 3d ban d becomes stronger again. At 50 at.% the hybrid band dominates together wit h the N2s-peak whereas the 3d band is very weak. The thermal contact test s hows an adhesion of the diamond up to c(N) = 5 at.%, whereas for c(N) > = 1 0 no interaction was detected. We conclude that the emerging N-Ti covalent bonding indicated by the p-d hybrid band leads to a decreased interaction w ith diamond. This is compared to the Ni-P system, where a strong interactio n between nickel and diamond is observed which decreases by adding phosphor us. In summary, we conclude that adding a certain amount of a main group el ement to a transition metal implies a decreased interaction of the new mate rial with the metastable diamond suppressing carbide formation or graphitiz ation, respectively. This observation is confirmed by diamond turning exper iments of TiNx coatings, where we find a minimum of diamond tool wear for c (N) > 5 at.%. (C) 2001 Elsevier Science B.V. All rights reserved.