Lifetime of DLC coatings under cyclic loading

Low friction surface coatings combine low wear rates with low friction coefficients. Both properties enable the protective layers to sustain wide ranges of loading and environmental conditions or even to substitute lubricants. At present, low friction coatings are commonly used on an empirical basis but not as a design element. The reason for the empirical approach is the missing of tools to describe the interaction between the coatings and the substrate and the difficulty to obtain information on the mechanical properties (e.g. strength, toughness, adhesion, residual stresses) of the coatings. A cyclic indentation test is proposed by (1) and (2) to substitute expensive coating tests in cyclic loaded machine elements, e.g. ball bearings. (3) studied the fracture modes of different hard coatings (e.g. TiCN, TiAlN, CrN) under different loading conditions (e.g. in lubricated ball bearings) and found them consistent with the failure modes in cyclic indentation tests under similar str ess conditions. This paper analyses the possibility to predict the lifetime of Diamond like carbon coatings (DLC, a-C:H) under mechanical cyclic loading by the use of the cyclic spherical indentation test. The indentation test is simulated with the help of finite-element-calculations and the results are compared to a ball on plate test. The ball on plate test is used because of the similar stress conditions as in ball bearings