Results from the disc machine suggest that it simulates gear contacts reasonably well. Work by the OECD research group on scuffing has also produced some interesting results: they have studied scuffing in 4-ball and spherical pin-on-disc machines. They suggest a two stage approach: breakdown of the elastohydrodynamic film followed by breakdown of boundary lubrication. Dyson, however, has suggested that these transitions can, in fact, overlap and may even occur in reverse order under certain conditions. If the Blok postulate is accepted, an obvious move is to compare the constant total contact temperature at scurf'rag with the temperature for the breakdown of boundary lubrication. Several workers have attempted the comparision, but study of the literature indicates that no strong correlation has yet been found. Hirst and Hollander 6 have provided some interesting data on the relationship between surface topography and boundary lubrication breakdown: the relevance of this work to scuffing, however, is not yet clear. Scuffing is a field where research appears to be following practice. Fundamental work is still required to give empiricists some background on which to work: much of the future experimental work should be oriented to following changes in surface topography during tests.
Delamination theory of wear The delamination theory of wear, introducted in 19737 to explain the nature of sliding wear, departs completely from the classical adhesion theory. Extensive analytical and experimental work s has confirmed the validity of the theory and many of the postulated mechanisms involved. It has been demonstrated that rubbing wear particles appear as thin flakes of metal with highly polished surfaces and not as the hemispherical fragments generally proposed by adhesion theory. The delamination theory of wear is based on the behaviour of dislocations at the surface, sub-surface void and crack formation, and the subsequent joining of cracks by shear deformation of the surface. It predicts that the wear particle shape is likely to be thin flake-like sheets and that the surface layer can undergo considerable plastic deformation. It has been shown that bulk material hardness in itself is not the controlling factor in wear and that the delamination theory and not the adhesion theory satisfies the thermodynamic requirements of the frictional and wear behaviour of metals. The ultimate formation of the wear particle depends on two mechanisms, void formation and crack propagation. For materials such as medium tensile strength steels of good fracture toughness where void nucleation can readily occur, crack propagation may be the controlling mechanism. However, for materials of high tensile strength and low fracture toughness, void nucleation can be difficult but crack propagation can readily take place: void nucleation may then become the controlling mechanism. Surface examination in conjunction with wear particle analysis has led to the hypothesis 9 that the interaction of surfaces in relative motion polishes the surfaces and creates
a shear mix layer of short crystalline order of almost superductile material which spreads over the surface, as first proposed by Beilby I 0. Repeated rubbing causes the shear mix layer to become fatigued and 'Beilby' type particles flake off. The experimental and analytical work to date and the preponderance of plate-like rubbing wear particles produced under lubricated sliding wear of metals and other plastically deforming solids provides evidence of a mechanism such as that postulated in the delamination theory. Further work needs to be done to provide a more complete description of the surface behaviour of materials and the wider application of the delamination theory of wear must await this additional evidence. It may then be possible to predict the wear rate of materials based on first principles and fundamental material properties. Advances in understanding emerge principally through the willingness to question what is generally accepted when logical deductions based on a theory do not coincide with reality. It is to be hoped that the postulation of the delamination theory of wear has fostered the questioning spirit. D. Scott
Wear resistant surface coati=~gs Surface coatings to increase wear resistance andprolong the useful life of components and production plant have been subject to mixed fortune in the last ten years. Materials conservation fervour has produced few tangible results, but anti-pollution legislation has been successfully satisfied by cleaner processes. Some processes such as metalliding have failed to become commercially significant, while others such as chemical vapour deposition are now well established. Thermally-deposited and diffusion coatings, already established ten years ago, continue to be the major engineering surface treatments. The flame spraying of nickel aluminides is a significant development. Plasma and detonation guns have produced higher quality coatings, and the arc gun, bridging the gap between flame spraying and plasma gun, offers fast deposition of different materials to closely controlled compositions. Unless subsequent fusing operations are permissible, porosity remains a problem with thermally deposited coatings where corrosion, as well as wear, resistance is required. The lower temperature (570°C) salt bath treatments have successfully overcome environmentalists' objections to cyanide waste by process modifications which produce more easily disposable cyanates for both nitriding and sulphurising treatments. The trend towards even lower temperatures, with attendant energy savings but more importantly lower distortion, is illustrated by the commercial exploitation of Sulf BT and ion-nitriding. Boriding by salt bath methods has been slow to find applications. Metalliding has almost disappeared due, no doubt, to the inherent safety hazards. An interesting newcomer to the field is manganising, and, despite the handicap of a processing temperature in excess of 1000°C, is creating interest in a wide range of industries. D. Scott, Paisley College o f Technology, High Street, Paisley, PAl 2BE, UK
TRIBOLOGY international February 1978 7