Indentation Fracture, Acoustic Emission and Modelling of the Mechanical Properties of Thin Ceramic Coatings

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Abstract

Fracture is an almost inevitable consequence of highly loaded contacts in ceramiccoated systems. For relatively thick coatings fracture is often similar to that observed in bulk samples of the coating but as the coating thickness is reduced the substrate plays an increasing role in influencing or even controlling fracture behaviour. Both through-thickness and interfacial fracture may be observed depending on the relative toughness of the substrate, coating and interface. Through-thickness, fracture is exacerbated by plastic deformation in the substrate and therefore the load support from the substrate is critically important in determining the type and extent of fracture observed. In this paper, nanoindentation testing and post facto atomic force and scanning electron microscopy is used to characterise the types of fracture which occur for hard coatings on softer substrates and multilayer coatings on glass. The effect of fracture on the nanoindentation load-displacement curves and the hardness and Young’s Modulus values obtained from them is discussed and a simple model to account for the observed behaviour is introduced. The use of acoustic emission to monitor plasticity and fracture during the indentation cycle is also discussed.