NANO METROLOGY OF CYLINDER BORE WEAR

Cylinder bores are multi-process surfaces whose roughness is difficult to characterise for tribological purposes by conventional methods. St atistical approaches may be used to compute asperity densities, summit curvatures and so on, but suffer from the usual disadvantage of tendi ng to infinite values in the absence of a short-wavelength cutoff. A u seful advance in tribological roughness assessment would be to find a means of establishing an appropriate scale of measurement. Using a for m of the plasticity index corrected for anisotropy, a short-wavelength limit hp is derived below which asperities will not take part in long -term tribological interactions. A general relationship is obtained be tween three dimensionless numbers, the short-wavelength limit lambda(p ) normalised by the topothesy Lambda, the fractal dimension D and the material ratio (the ratio of the Hertzian elastic modulus E' to the ha rdness Ii). From this relationship, presented as a carpet plot, the ap propriate scale of roughness measurement for any tribological investig ation of a fractal surface may be determined. With a stylus instrument and an atomic force microscope, a number of cylinder bores were measu red at locations of both high and medium wear before and after running in. By inspection of an ensemble of structure functions, it is shown that cylinder bore surfaces are multifractal, with a transition point (the so-called ''corner frequency'') at about 20 mu m, corresponding t o the average size of a honing grit. Below this length the surfaces ar e selfsimilar fractals down to the limits of AFM resolution. The short wavelength limit using the above formulation appears to be about 40 n m, well below the range of instruments usually employed to measure tri bological surface roughness. (C) 1998 Elsevier Science Ltd.