Fractal characterisation of inhomogeneous rough surfaces

Load bearing conjunctions are never perfectly flat. They are covered by surface features, which may be either unintentional roughness inherent in the manufacturing process or a combination of such roughness with intentionally introduced surface texture. In either case, only a small proportion of load bearing surfaces are in contact and carry load. This depends on the surface topography, material properties and contacting conditions. Simple surface roughness characterisation parameters such as Ra, Rp etc. although commonly used do not give an adequate description, particularly where surfaces are deliberately textured. Furthermore Imaged surface topographies commonly exhibit features below the resolution of the imaging apparatus. We demonstrate the application of a fractal geometry analysis to honed internal combustion engine cylinder liners, imaged by Atomic Force Microscopy and Confocal Laser Scanning Microscopy. This method enables anisotropic surfaces to be characterised by five parameters which can then be used to generate model surfaces whose characteristics follow very closely those of the measured surface. Analysis of the structure function allows the determination of length scales and roughness features relevant at a given contact length. The reconstruction of anisotropic surfaces is demonstrated. Results from modelled contacts between these surfaces reveal the likely asperity contact geometry to be used in modelling contact and friction in these interfaces.