Statistical Measurement of Real Contact Area on the Basis of Image Intensity Histograms (Part 2) - Application to Paper-Based Wet Friction Materials

Statistical measurement of real contact area was applied to a paper-based wet friction material by using light interferometry images. It was demonstrated that extraction of the real contact area was possible in spite of the difficulty in determining the threshold value for binarization, where a valley did not appear in the lower region of the intensity histogram. The following points were clarified: (1) The intensity histogram, which was obtained through shading correction for non-uniform light illumination, was divided into four domains corresponding to the individual distribution of clearances between a glass plate and a paper-based friction material. (2) The real contact area was estimated by utilizing the "Gauss 1" domain, which was extracted by performing Gaussian distribution fitting to the intensity histogram. The extraction precision was estimated to be 20 nm for a gap conversion range of 3 sigma (standard deviation). (3) By dividing the intensity histogram into real contact area ("Gauss 1") and vicinities of micro-gaps ("Gauss 3", "Area 2", and "Area 4"), it was possible to estimate the statistical ratio of the micro-gap areas, and as a result the dependence of the changes in those areas on the contact pressure was demonstrated. (4) Since the abovementioned knowledge is applicable to general surfaces with rough texture and/or low reflectance as well as to paper-based friction materials, the proposed method is effective as a simple tool for measurement and analysis of the real contact area.