Wear 

Abstract: Although hardness has long been regarded as a primary material property which defines wear resistance, there is strong evidence to suggest that the elastic modulus can also have an important influence on wear behaviour. In particular, the elastic strain to failure, which is related to the ratio of hardness (H) and elastic modulus (E), has been shown by a number of authors to be a more suitable parameter for predicting wear resistance than is hardness alone. There is presently considerable interest in the development of nanostructured and nanolayered coatings, due to the fact that materials with extreme mechanical properties (which are difficult to synthesise by other methods) can be created, particularly when using plasma-assisted vacuum processing techniques. Until now, scientific research has been directed mainly towards the achievement of ultra-high hardness, with associated high elastic modulus, the latter of which, conventional fracture mechanics theory would suggest, is also desirable for wear improvement (by preventing crack propagation). In this study, we discuss the concept of nanocomposite coatings with high hardness and low elastic modulus, which can exhibit improved toughness, and are therefore better suited for optimising the wear resistance of ‘real’ industrial substrate materials (i.e. steels and light alloys, with similarly low moduli). Recent advances in the development of ceramic–ceramic, ceramic–amorphous and ceramic–metal nanocomposite coatings are summarised and discussed in terms of their relevance to practical applications. We also discuss the significance of elastic strain to failure (which is related to H/E) and fracture toughness in determining tribological behaviour and introduce the topic of metallic nanocomposite coatings which, although not necessarily exhibiting extreme hardness, may provide superior wear resistance when deposited on the types of substrate material which industry needs to use.

Abstract: The amount of surface material eroded by solid particles in a fluid stream depends on the conditions of fluid flow and on the mechanism of material removal. The paper first discusses some aspects of the fluid flow conditions which may lead to erosion and then analyses the mechanism of material removal for ductile and brittle materials. For ductile materials it is possible to predict the manner in which material removal varies with the direction and velocity of the eroding particles. The numerical magnitude of the erosion cannot be predicted with accuracy but does correlate with data from metal cutting tests. For brittle materials the conditions leading to initial cracking are deduced and ways of predicting the material removal are discussed. It was not found possible to develop an analysis as detailed as that for ductile materials. In addition, the influence of the properties of the abrasive particle on erosion is briefly considered.

Abstract: In fluid-bed systems, transport lines for solids, etc. heavy erosion may occur. This type of attack has been shown to comprise two types of wear, one caused by repeated deformation during collisions, eventually resulting in breaking loose of a piece of material, the other caused by the cutting action of the free-moving particles. In practice these two types of wear occur simultaneously. Formulae could be derived expressing erosion as a function of mass and velocity of the impinging particles, impingement angle and mechanical and physical properties both of erosive particles and eroded body. In this part of the publication only wear due to repeated deformation is considered; in a second part cutting wear and the combination of these two types will be described. For deformation wear the following equation is found: WD = 12M(Vsinα − K)2e in which WD is erosion in units volume loss, M and V are respectively total mass and velocity of impinging particles, α is impact angle, K is a constant, which can be calculated from mechanical and physical properties and expresses the particle velocity at incipient erosion. e represents the energy needed to remove a unit volume of material from the body surface and describes the plastic-elastic behaviour of the substance. Test results confirm this equation.

Abstract: A simple formulation of “flash temperature” theory is given. This treatment reduces the mathematical complexities and instead emphasises the relevant physical considerations. Simple graphical methods for deducing these temperatures are described. The use of the theory is illustrated by its application to some new experimental results on the breakdown of Perspex and the wear of steels. The extension of the theory to include the influence of surface ( e.g. oxide) and lubricant films is outlined. The temperature in elasto-hydrodynamic films is considered ; it is shown that the temperature difference which exists within such a film can be easily the largest transient temperature in the contact zone and may be several times greater than the surface flash temperature.

Abstract: The third-body approach highlights the many features which are common to different types of materials in different types of rubbing contacts. It suggests that a picture which is globally coherent from a mechanical point of view, in that it obeys as a first step the laws of equilibrium and continuity, can be presented. That picture, which is strongly inspired by lubrication theory, is presented together with its many implications in both fundamentals of tribology and industrial solutions.