Indentation hardness evaluation of cathodic arc deposited thin hard coatings

One trend in the development of wear-resistant vapour deposited coatings is to make them increasingly harder and thinner, by improvement and optimisation of the deposition processes. A complex interdependence exists between the individual properties of a coating and a substrate on the one hand, and those of the ‘composite’ coated system on the other. For example, system stiffness and hardness may vary with indentation depth according to different laws. There is a great need for quantitative modelling methods so that the design of coatings and multi-layered systems can be improved, and the choice of materials optimised. In the present study, various hard coatings produced by filtered cathodic arc deposition were characterised by micro-indentation and macro-indentation methods, and scanning electron microscopy. SEM was used to elucidate the fracture behaviour of these coatings, which exerts an important influence on their hardness performance. Hardness testing results were analysed using a newly proposed modification of the work-of-indentation model, an approach that was recently developed and applied to a range of coated systems. The new development of this model allows more accurate fitting of the empirical data, and yields an estimate for the ultimate coating hardness, and values of dimensionless materials parameters β0 and X. These parameters describe the normalised depth and the degree of abruptness at which the hardness transition from coating to substrate occurs, and are related in a complex way to the ductility and toughness of the thin film and the interface, as well as the substrateʹs yield strength and hardening behaviour. In this study we use the model to interpret micro- and macro-indentation data, and to discuss the results of extrapolating the depth–hardness curve to the important region where the indentation depth lies is between 5 and 10% of the coating thickness, and composite hardness approaches the ultimate film hardness.