New diamond turnable coatings of ternary systems for ultraprecision cutting

Abstract

Moulding tools for optical components need optical surface quality. To reach this high quality surface the machining with monocrystalline diamond tools is most suitable. Steels which are used as materials for moulding tools are not diamond machinable due to heavy chemical wear of the diamond tool. Therefore diamond machinable coatings have to be deposited on the moulding tools before final cutting. Up to now electroless nickel phosphorus coatings are the only diamond turnable coatings, but their hardness and temperature stability is limited. For this reason new diamond machinable coatings with higher hardness and improved temperature stability have to be developed. One promising coating is substoichiometric titanium nitride (TiN x<1 ), which exhibits minor abrasive and chemical wear of diamond. But the crystallite size and hardness of those coatings still has to be lowered in order to be able to compete with the established NiP x coatings. Therefore, in this work copper is added to TiN x<1 coatings to adjust their morphology and hardness. The coatings are deposited by means of reactive DC magnetron sputtering. The titanium to copper ratio and the nitrogen content of the coatings are varied. The chemical composition is investigated by glow discharge optical spectroscopy. To evaluate the reactivity of diamond with the coating surface a contact test is performed in a vacuum chamber, where a monocrystalline diamond is pressed on the coating surface at a pressure of less than 0.1 mPa and a temperature of 800 °C. Scanning electron microscopy and energy dispersive X-ray analysis of the contact areas of the coating and the diamond provide information of the chemical wear of the diamond.