An approach to evaluate the resistance of hard coatings to shock loading

Nowadays hard coatings are widely used to prolong lifetime of most tools and machinery elements and to improve their working efficiency. The main benefits of hard coating are based on the combination of high hardness and toughness: properties that are especially important in conditions of dynamic impact wear, when high cyclic local loading is the main cause of coating degradation. Cavitation test seems to be a useful tool for analyses of deformation, crack initiation and their propagation in hard coatings. The TiN and Cr–N coatings were deposited on steel substrate by means of the cathodic arc evaporation method at various temperatures and bias voltage, and at various thicknesses. The coatings were tested in a cavitation tunnel with a system of barricades to determine the parameters describing coating properties related to the dynamic loading resistance and to explain the failure mechanism of hard coatings under dynamic loading conditions. The analysis of degradation mechanism of thin hard coatings under repeated impact loading has resulted in deriving a new empirical parameter suitable for description of the hard coating cavitation resistance. The derived parameter is proportional to the plasticity index, H/E, (defined as ratio of hardness, H, and Youngʹs modulus, E), adhesion force, LC2, ratio of thermal conductivity of the coating to that of the substrate, and inversely proportional to the number of phases in the coating phase composition, the ratio of the thermal expansion of the coatings to that of the substrate and the coating thickness. The cumulative mass loss determined in the tests shows very good correlation with the new parameter.