Brittle coating/ductile substrate systems (e.g., ceramic coatings on metal alloys) are widely utilized for engineering and biomedical applications such as thermal barrier coatings [1], wear and corrosion resistance [1], dental restorations [2] and medical implants [3]. The hardness of coatings absorbs the impact of external loads and protects the substrate from corrosion, wear and fretting, while the ductility of the substrate retains the structural integrity. The failure of brittle coatings is an important problem of engineering design. Typically the resistance of the coatings to the contact loading is determined by means of indentation [1], which often induces the formation of radial/median cracks in the brittle coatings/ductile substrate systems [4]. Median cracks mostly initiate during the indentation loading half-cycle and will propagate into the coating layer, whereas radial cracks typically emanate from the corners of the indenter during unloading half-cycle [4]. The traditional way of examining the fracture toughness for such cracks is to assume the well-developed crack profile with a half-penny shape and subsequently apply the semi-empirical relationship developed by Lawn et al [5] for brittle monolithic materials. Such approach to analysis of indentation cracks in brittle coating/ductile substrate systems raises practical concerns for the reasons that, firstly, the crack profile may be of an elliptical shape and, secondly, the composite hardness of the coating/substrate system may influence the fracture toughness of the coating as opposed to its monolithic property.
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