Investigation of the diamond machinability of newly developed hard coatings

Presently, coatings of electroless nickel are used for diamond turning molds for injection molding of optical lenses. We have investigated the diamond machinability of substoichiometric hard nitride coatings (TiNx, TiAlNx, and CrNx). These coatings have a superior hardness compared to electroless nickel suggesting an improved wear resistance of molds with optical surface quality. In the case of CrNx and TiAlNx, high tool wear occurred, even after small cutting distances, and the surfaces showed a roughness larger than Ra = 0.5 μm. A considerably higher surface quality was obtained on TiNx coatings. The best results (Ra = 15 nm) were achieved with a nitrogen content of x = 0.03. As a first application, a mold for a diffractive optical element was machined using this newly developed substoichiometric titanium nitride deposit.