Microstructure and mechanical properties of multilayer TiB2/C and co-sputtered TiB2-C coatings for cutting tools

The feasibility of reducing the friction coe¦cient of TiB2-based coatings by the incorporation of carbon has been investigated for both multilayer and co-sputtered coatings. Characterisation was performed using pin-on-disk tribometry, nanoindentation, glancing angle X-ray di¤ractometry, X-ray photoelectron and Auger electron spectroscopy. The co-sputtered coatings were found to consist of two phases: a hexagonal TiB2-type structure into which carbon is incorporated and a diamond-like carbon (DLC) phase. C is preferentially incorporated into the Ti(B,C)2 phase and the lubricating DLC phase only starts to form once saturation is reached. Consequently, a reduction in the friction is only found at total C concentrations higher than 50 at%. For the multilayers, there was an increase in the overall carbon content required to obtain a friction-reducing e¤ect from about 10Ð50 at% as the TiB2 sublayer thickness was decreased from100 to 1 nm. This was attributed to an increase in the relative proportion of carbon bonded with TiB2 in the interface regions. Coatings with a hardness of about 20Ð30 GPa and friction coe¦cients of(0.2 against a steel ball could be obtained at a suitable composition.