Nanoscale multilayer WC/C coatings developed for nanopositioning, part II: Friction and wear

Tribological performance of tungsten carbide/carbon (WC/C) multilayers of nanometers in period was investigated by a microtribometer. Coatings with various layer thicknesses and two different top layers, WC or C, were prepared by physical vapour deposition. Continuous friction measurement and quasi-continuous wear measurement were cross-examined with respect to the evolution of the morphology and the composition of wear scars by a scanning electron microscope and a profilometer. When a steel sphere slides on the WC top layer coatings, adhesive wear occurs and high friction coefficients and high wear rates are observed. When the sphere slides on the carbon top layer coatings, a thin transfer film is observed, and low friction coefficients of 0.1 and wear rates of 3 nm/h are obtained. Thick carbon layers in the coating are beneficial for reducing friction coefficients, but result in high wear rates. A linear relationship between the friction force and the normal load is observed in the carbon top layer coatings. Potential applications of the coatings for nanopositioning are discussed.