Alternative methods for residual stress and failure strain determination of amorphous hard coatings
Hardness, residual stress and adhesion strength to the substrate are decisive properties of thin hard and wear resistant amorphous carbon coatings for forming tools, bearings or gear wheels, because the coatings for such applications are subjected to high dynamical loads. Especially the residual stress due to the layer growth process is a limiting factor for the adhesion strength. For amorphous carbon coatings a direct determination of the residual stress is not possible by X-ray diffraction (XRD) or by the borehole method after focussed ion beam (FIB) preparation.
Different tungsten doped hydrogenated amorphous carbon coating systems (a-C:H:W) were applied on one side of thin metallic substrate foils by reactive magnetron sputtering. The residual stress in the films was calculated from the resulting substrate bending according to the calculation model of Brenner et al. and other approaches. The foils were subsequently examined by unidirectional tensile tests in order to determine the critical failure strain of the coatings. In the context of these experiments, an event and spectrography based acoustic emission (AE) technique was used in order to detect and analyse the crack formation in the films. This method is regarded as the basis for determining the film toughness and represents a first step towards investigating the fatigue properties of highly dynamically stressed thin films.