Diamond-like carbon coatings - a new design element for tribological applications

Diamond-like carbon coatings (DLC) combine high wear resistance with low friction coefficients. Both properties enable the protective layers to sustain wide ranges of loading and environmental conditions. 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 lack of tools for a description of the interaction between the coatings and the substrate. Furthermore it is difficult to obtain information on the fracture properties of the coating substrate system (e.g. fracture toughness, adhesion, residual stresses). A spherical indentation provides a simple technique to measure quantitatively the fracture toughness and the adhesion of brittle coatings on a ductile substrate with standard laboratory equipment. DLC coatings on a 100 Cr 6 steel substrate are indented by silicon nitride balls with different diameters and different loads. Fracture patterns (circular and radial cracks, delamination) are analyzed by finite element calculation and the fracture toughness of the coating itself along with the interface toughness are estimated.