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

Abstract

In ultraprecision turning monocrystalline diamond is up to now the only suitable tool material. However, only a few, relatively soft materials are turnable, e.g. aluminum or copper. Most alloys of transition metals with higher wear resistance like steels lead to unacceptable wear of the diamond tools. Electroless nickel phosphorus coatings (NiP x ) are today the hardest, most wear resistant material enabling optical surface qualities with diamond tools. Therefore, steel parts are coated with NiP x prior to ultraprecision turning. As a possible alternative with higher hardness than NiP x we investigated TiN x coatings deposited by reactive DC magnetron sputtering. We measured the XPS valence band and core level structure of these coatings with nitrogen contents c N=0–50 at.%. The chemical reactivity of the coatings with monocrystalline diamond was checked with a thermal contact test in vacuum pressing a diamond against the coating at 1000°C. We find that the Ti3d valence band at 2 eV binding energy is visible at all nitrogen contents. 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=5–10 at.%. The intensity of the N2s band at approximately 17 eV is correlated with the intensity of the hybrid band. Between c N=15–30 at.% N the 3d band becomes stronger again. At 50 at.% the hybrid band dominates together with the N2s-peak whereas the 3d band is very weak. The thermal contact test shows an adhesion of the diamond up to c N=5 at.%, whereas for c N>=10 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 with diamond. This is compared to the Ni–P system, where a strong interaction between nickel and diamond is observed which decreases by adding phosphorus. In summary, we conclude that adding a certain amount of a main group element to a transition metal implies a decreased interaction of the new material with the metastable diamond suppressing carbide formation or graphitization, respectively. This observation is confirmed by diamond turning experiments of TiN x coatings, where we find a minimum of diamond tool wear for c N>5 at.%.