Substoichiometric titanium nitride coatings as machinable surfaces in ultraprecision cutting

In ultraprecision cutting, monocrystalline diamond is nowadays the only acceptable tool material. The major problem with diamond tools is that only a few materials are turnable? e.g. aluminium or brass. In contrast, turning ferrous alloys such as steel causes severe diamond tool wear. Therefore, steel parts have to be coated with a diamond turnable material e.g, electroless nickel with phosphorus contents of about 20 at.%. In the search for a new diamond turnable material with better mechanical properties than electroless nickel, we investigated substoichiometric titanium nitride coatings deposited by reactive magnetron sputtering. The coating thickness was about 20 mu m, and the nitrogen content was varied between 0 and 30 at.%. We analyzed the chemical composition of the coatings with GDOES. The coating structure was characterized by SEM and XRD. The mechanical properties were investigated by a hardness measurement and a scratch test. The machinability of the coatings with monocrystalline diamond tools was tested by ultraprecision cutting. We found that coatings consisting of the alpha-Ti phase with solved nitrogen yielded surfaces with optical quality. These coatings are homogeneous with a moderate hardness. Increasing the nitrogen content leads to the formation of the epsilon-Ti2N and delta-TiN phase showing columnar growth and increased hardness, which causes chipping of the diamond tool and decreased surface quality. The adhesion of the coatings was increased by varying the substrate bias and nitrogen content at the coating/substrate interface. With a coating containing 3 at.% nitrogen, a Fresnel structure could be obtained by ultraprecision cutting. (C) 1999 Elsevier Science S.A. All rights reserved.