On the significance of the H/E ratio in wear control: a nanocomposite coating approach to optimised tribological behaviour
Introduction
Classical theories of wear [1] tend to emphasise hardness as the prime property which defines the wear resistance of a surface. It is often the case that a hard material will also demonstrate a high elastic modulus; nevertheless, it is well-known that some polymeric materials, particularly elastomers, can provide excellent wear resistance in, for example, impact and erosion conditions, yet such materials exhibit an extremely low elastic modulus. Their wear behaviour is typically characterised by a long elastic strain to failure, which can be described in terms of the ratio between the hardness (H) and the elastic modulus (E). Some years ago, it was recognised by several authors that the ranking of materials according to their H/E ratio can provide extremely close agreement to their ranking in terms of wear [2]. Whilst many researchers have, over the years, confirmed the importance of high hardness in mitigating wear [3], the case for a reciprocal relationship between wear resistance and elastic modulus is less well-proven. Indeed, researchers such as Lancaster [4] and Spurr and Newsome [5] have found that wear resistance actually increases with elastic modulus. Such observations are perhaps not surprising, given that materials with a high elastic modulus are generally those that also exhibit high hardness. Despite the lack of conclusive evidence for a need to enhance elasticity (i.e. to reduce E) in order to improve wear resistance, the intuitive logic of this route has remained, with various authors returning to the H/E ratio as a ranking parameter since Oberle’s early study [6], [7], [8]. It is also significant that the ratio between H and E appears in the so-called ‘plasticity index’, which is widely quoted as a valuable measure in determining the limit of elastic behaviour in a surface contact, which is clearly important for the avoidance of wear [6].
Recent developments in surface coating technology, particularly in the production of nanostructured films by physical vapour deposition (PVD) techniques, has revealed exciting possibilities to control H and E with some degree of independence, thereby producing wear-resistant surfaces with combinations of properties which were previously unobtainable. Plasma-assisted PVD allows close control over process parameters, with the resultant possibility to obtain unique, non-equilibrium compositions and structures. This makes it potentially an extremely useful technique in the development of new generations of wear- and corrosion-resistant materials. Current scientific interest in sputter PVD processes to produce so-called ‘nanocomposite’ coatings is directed primarily towards the development of ‘superhard’ (i.e. H≥40 GPa) or ‘ultrahard’ (i.e. H≥70 GPa) wear-resistant materials with associated high elastic moduli. Although impressive performance has been reported for such materials in laboratory tests, they may not be the ideal solution for many commercial applications on ‘real’ components. Furthermore, although the sputter PVD process is an excellent technique for the investigation of different coating structures and compositions, other processes, such as electron-beam (EB) evaporative PVD may be commercially more cost-effective, if appropriate evaporant source materials can be developed. Such factors may be key to the commercial exploitation of these new coatings in, for example, the automotive industry. The following section overviews the recent history of advances in ceramic–ceramic, ceramic–amorphous and ceramic–metal nanostructured coatings development assessing their mechanical and tribiological behaviour in terms of practical applicability. We progress to discuss the significance of elastic strain to failure (i.e. H/E) and fracture toughness in determining coating performance, giving examples of the benefits which nanostructural toughening of coatings can provide and introducing the concept of metal–metal nanocomposite coatings which, although not necessarily ultrahard, may be better suited for use in practical tribological applications.
Section snippets
Background to the development of nanostructured materials and coatings
The current scientific and commercial interest in the development of nanostructured materials, which exhibit enhanced physical properties over their conventional polycrystalline counterparts, is driven by increasing practical demands on the mechanical performance and the wear and corrosion resistance of industrial components. Rapid advances in other applications areas such as semiconductor manufacture, electromagnetic and superconducting materials, bioengineering, etc. are also driving
The significance of H/E in determining coating performance
As suggested above, the imperative for coating tribologists has been, for many years, the development of ever harder (and stiffer) coatings (e.g. thin-film diamond, cubic boron nitride and recently carbon nitride, C3N4); however, it is increasingly recognised that hardness is not necessarily the prime requirement for wear resistance. Coating elasticity and toughness can, particularly in abrasion, impact, and erosive wear, be equally (if not more) important factors. With respect to elastic
Practical examples of coating design to optimise H/E values
As indicated above, the Ti-Al-B-N phase system offers a broad range of possibilities for designing coating structures with optimal properties for different applications. Further to the (by now) obvious benefits of a TiN/TiB2-based nanocomposite structure, the addition of aluminium provides scope for lowering the elastic modulus of the coating (EAl≈70 GPa), as well as generating intermetallic phases or (with sufficient Al-content) the protective high temperature oxidation behaviour of Ti-Al-N.
Conclusions
We have discussed the fact that there is considerable practical merit in using the ratio between H and E as a tool to describe, rank, or (with other parameters known) calculate values for, performance criteria which are important in defining the wear resistance of a material (and particularly of coated materials), such as the elastic strain to failure, the critical yield pressure for plastic deformation and the fracture toughness. It is our opinion that a high H/E ratio is often a reliable
Acknowledgements
Financial support for the Research Centre in Surface Engineering (RCSE) at Hull University by the UK Engineering and Physical Sciences Research Council (EPSRC) and from various industrial collaborators is acknowledged with gratitude, as is the advice and technical support of colleagues in the RCSE. Particular thanks is extended to Dr. C. Rebholz and M.C. Joseph for coating deposition work and provision of test and analysis data. The contribution of Prof. K. Holmberg (VTT, Finland) to
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