Surface Deformation Behaviour of Tic and Tin Coated Steels: 1 Indentation Response

Indentation hardness testing using loads of 25–1000 gf on a Vickers profile diamond indenter has been used to study the deformation and fracture behaviour of CVD TiC and CVD and PVD TiN coatings on a range of steel substrates. The coatings were prepared by proprietary processes and were 2–5 /lm thick. The variation of hardness with indentation size and depth has been compared with a number of models for the composite hardness behaviour of coated systems. Good fits were found both to a law of mixtures model incorporating indentation size effect behaviour, and to a simple empirical polynomial. In both cases, parameters can be calculated from which the hardness of the coating itself can be assessed for purposes of quality control etc. For the first time, the variation of hardness with temperature for coated systems has been investigated. For the coating/substrate composites studied here, hardness fell rapidly with temperature, and ‘displayed marked hysteresis of both hardness andfracture behaviour. This reduction in hardness can be so marked that the composite hardness can decrease to below that of the substrate itself above ∼ 300°C. Also, stress relaxation at elevated temperatures leads to copious cracking of the coating during indentation. The observations are discussed and the implications assessed for the tribological applications of coatings. A simple model relating the hysteresis in hardness with residual stress is also presented.