Tailoring of diamond machinable coating materials

Microturning with diamond tools is a promising technique to produce surfaces with highest precision. But only some workpiece materials are suitable for this technique, e.g., copper, aluminum, germanium, PMMA, some compounds, and organic materials. Diamond machining of most transition metal alloys like steel leads to a rapid tool wear which is believed to originate from chemical reactions in the contact zone between tool and workpiece. NiPx>0.2 coatings are widely used as diamond machinable coating material used for optical applications. In contrast to diamond machining of pure nickel, these coatings do not wear diamond tools rapidly. The chemical reactivity of the coatings with single crystal diamond was investigated with a thermal contact test. It is shown that a sufficient amount of phosphorus suppresses reaction with diamond. Quantum mechanical cluster calculations reveal that this is mainly explained by a strong hybridization of electronic valence orbitals, i.e., covalent bonding. We also investigated magnetron sputtered TiNx coatings. The reactivity test shows that an amount of approximately 10 at.% nitrogen can reduce the reactivity with diamond significantly. Cluster calculations reveal that this is due to covalent bonding between titanium and interstitial nitrogen atoms. Calculations and reactivity tests for NiSix and NiTix coatings were also performed. The latter case shows that also alloying of two transition metals may provide enough covalent bonding to inhibit chemical reaction with diamond. We conclude that a sufficient amount of covalent bonding in transition metal containing materials is necessary for machinability with diamond tools. On the basis of these investigations a generalized requirement profile for diamond machinable coating materials is derived.