Design criteria for wear-resistant nanostructured and glassy-metal coatings

Abstract There is increasing scientific and commercial interest in the development of nanostructured coatings, particularly those based on low-miscibility ‘ceramic–ceramic’ or ‘ceramic–metal’ crystalline/amorphous nanocomposite phase mixtures deposited by plasma-assisted PVD or CVD. In laboratory mechanical testing, extreme values of hardness (which may be in excess of 70 GPa) are often found for such films, similar to those claimed for intrinsically hard materials such as c-BN and diamond. High hardness is, however, often accompanied by an associated high elastic modulus, which although desirable in principle for cutting tool materials and/or coatings, may in practice limit coating durability, on low-strength, low-modulus substrates (e.g. low-alloy steels and the light alloys) and in many wear applications other than metal cutting. In this paper, we discuss the benefits of using the ratio of hardness to elastic modulus ( H / E ) as an indicator of coating durability since this parameter essentially describes the elastic strain to failure capability (and resilience ) of a candidate material. Furthermore, we consider the likely need for tribological coatings to accommodate some degree of substrate deformation; in this respect film toughness , i.e. ‘engineering toughness’ in the sense of an ability to absorb deformation energy (both elastic and plastic) needs to be considered. The concept of predominantly metallic films with a nanograined and/or glassy microstructure (containing little or no high-modulus ceramic constituents) is introduced, through which we point to the importance of retaining ‘sufficient’ coating hardness, whilst reducing coating elastic moduli to more closely match those of candidate substrate materials. With regard to the implications of H / E for practical tribological coating applications, we propose that closer matching of the coating/substrate interfacial elastic properties and thus an improved ability for the coating to accommodate substrate strain, where necessary, is often a more important factor in wear resistance than is extremely high hardness.