Phenomena in microwear experiments on metal-free and metal-containing diamond-like carbon coatings: friction, wear, fatigue and plastic deformation

Atomic force microscopy based microwear experiments allow deeper understanding of mechanisms of wear on the microscopic scale due to a well defined single asperity contact, elimination of roughness effects, and the high lateral and vertical resolution in the micro- and nanometer range. In this paper, results of linear oscillating microwear experiments on metal containing diamond like carbon (DLC) coatings, thin pure DLC-coatings, Si- and Si:O-doped DLC-coatings will be presented. The experiments have been performed using either a diamond tipped cantilever-based system or an electrostatic transducer system (Hysitron Inc.) in conjunction with a standard AFM. Diamond tip radii of less than or equal to 1 μm and loads in the range of some millinewtons lead to contact areas of only 0.1–0.2 μm2 and contact pressures in the range 2–20 GPa. Under these conditions, for metal-DLC coatings (e.g. W-DLC) material fatigue on a nanometer scale can directly be observed and identified as an important wear mechanism. Furthermore, the columnar growth structure of the film and percolation of the metallic nano-particles inside the film, i.e. metal content and metal type, strongly influence the fatigue and wear resistance of the coatings. Oscillating microwear experiments on thin DLC, Si-DLC and Si:O-DLC coatings on glass substrates are analysed with regard to friction, wear and plastic deformation. The friction coefficient μ can be understood in terms of a combination of Hertzian elastic contact and an additional ploughing term μ=aL−1/3+bLn (L=load) where n strongly changes during the first wear cycles. Comparison of residual wear depth and residual indentation depth shows that the wear volume may completely be due to plastic deformation and not to a real material loss. In other cases, material loss and plastic deformation both contribute to the observed wear volume. Therefore, evaluating only residual wear marks may lead to a misinterpretation of nanowear results.