Comparative investigation of multilayer TiB2/C and co-sputtered TiB2-C coatings for low-friction applications

Abstract

Composite low-friction coatings have been produced by combining a lubricating phase (C) and a hard wear-resistant phase (TiB2), deposited in the form of either multilayer (TiB2/C) or homogeneous (TiB2-C) films. The multilayer coatings were prepared by sequential magnetron sputtering, and the homogeneous coatings by co-sputtering from TiB2 and C targets. The coatings were characterised by glancing angle X-ray diffractometry and photoelectron spectroscopy. Friction coefficients were measured using a pin-on-disk tribometer, and hardness was determined by nanoindentation. Co-sputtered coatings were found to consist of two phases: diamond-like carbon (DLC) and a hexagonal TiB2-type structure into which carbon is incorporated. In the case of the co-sputtered coatings, overall C concentrations as high as 50 at.% were required to produce a friction-reducing effect. This can be explained by the preferential incorporation of the carbon atoms into the TiB2 lattice, the lubricating DLC phase only starting to form once saturation is reached. In the case of multilayers, it was found that there was an increase in the overall carbon content required to obtain a friction-reducing effect from about 10 to 50 at.% as the TiB2 sublayer thickness was decreased from 100 to 1 nm. This was attributed to an increase in the relative proportion of carbon bonded with TiB2 in the interface regions. It was shown that coatings with a hardness of the order of 20 GPa and displaying friction coefficients against steel as low as 0.1 for temperatures not exceeding 200°C could be obtained by selecting a suitable composition.