ICMCTF2003 Session B8-2: Hard and Multifunctional Nano-structured Coatings

There is increasing scientific and commercial interest in the development of nanocomposite coatings, based particularly on coherent or semi-coherent mixtures of ceramic/amorphous or ceramic/metal phases deposited by PVD or CVD. In laboratory mechanical testing, extreme values of hardness (in some cases 70GPa, or higher) are often found for such nanocomposite coatings, 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 (eg. 600GPa+ for c-BN and 850GPa+ for diamond) which, although desirable (in principle) for a cutting tool material and/or coating - where dimensional stability under load is obviously important - may in practice limit coating durability on low-modulus substrates and/or in wear applications other than machining. In reality, acceptable wear resistance can (depending on the contact conditions) often be achieved in a material whose hardness is little more than 30% higher than that of the counterface material with which it is in contact; thus high hardness (beyond a certain minimum requirement) is unlikely to provide further significant wear reduction. Conversely, differences in modulus between coating and substrate, if large, have the potential to generate substantial interfacial stresses under load, which may compromise coating adhesion and durability. Since the elastic moduli of many ceramic coating materials are in excess of 400GPa, whereas most engineering metals have moduli of ~200GPa (or substantially less), such circumstances are not uncommon. As well as elastic deflections under load, in some applications (eg. metal forming, or heavily-loaded gears) there may even be a risk of asperity - or near-surface - plastic deformation, which a coating may also have to accommodate.

In this paper therefore, we discuss the concept of H/E as an indicator of durability - since it provides a method to describe the 'elastic strain-to-failure' of a coating. However, rather than prescribing the H/E parameter as an arbitrary term, of universal applicability, we point to the importance of retaining sufficient coating 'H' (within the scope of trying to reduce coating 'E', to match that of the substrate). Furthermore, we consider the need (particularly in heavily-loaded contacts) for a coating to accommodate extensive substrate deformation; in this respect, film toughness (ie. 'engineering toughness' - in the sense of an ability to absorb deformation energy, whether elastic or plastic) needs to be considered. It may be that ceramic/metal and (particularly) metal/metal nanocomposites - the latter of which we are currently investigating - are able themselves (like ceramic/metal multilayers) to undergo some measure of plastic deformation in the 'soft' phase component, to accommodate large substrate strains, whilst continuing to provide wear protection. We discuss hardness, elastic modulus and wear test measurements (eg. pin-on-disc, reciprocating-sliding, impact or abrasion) on such nanocomposite coatings, with regard to the implications of the ratio of H to E for practical tribological coating applications, where close matching of the coating/substrate interfacial elastic properties (and/or an ability to withstand plastic strain, if necessary) may be of more importance than extreme hardness.