The real contact area between transparent non-flat surfaces and a rubber plate under steady contact was analyzed on the basis of visualization images generated by using light-induced fluorescence. We examined the fundamental characteristics of a plano-convex lens, and we developed a technique for extracting the real contact area for a rough surface model by Gaussian distribution fitting. An aqueous rhodamine-B solution was used for fluorescence measurements, where the results confirmed the linearity of the relation between intensity and film thickness for thicknesses up to 0.1 mm. In the case of a plano-convex lens specimen, we determined the starting point of contact, that is, the loading where a peak appeared on the lowest intensity side of the intensity histogram. A Gaussian distribution was fit to the histogram upon obtaining the difference from the histogram at zero loading, in which the extracted distribution domain equivalent to the real contact area agreed with the theoretical Hertzian contact area. In the rough surface model specimen, a peak emerged in the part of the histogram with lowest intensity, and the real contact area was measured by using a fitting method based on a Gaussian distribution.
Diamond-Like Carbon (DLC) coating provides low friction properties without lubricants and, with lubricants such as organic acid, should provide ultra low friction. In this study, the friction property of two types of DLC coating, which were hydrogenated amorphous carbon (a-C:H) by the Plasma-Enhanced Chemical Vapor Deposition (PECVD) process and tetrahedral amorphous carbon (ta-C) by the T-Shaped Filtered Arc Deposition (T-FAD) process, were evaluated with oleic acid lubrication. It was found that friction coefficient of ta-C with deuterated oleic acid lubrication provided much lower than that of a-C:H. The tribo-film on wear track was analyzed by time-of-flight secondary ion mass spectrometer and X-ray photoelectron spectroscopy. As a result, it was found that larger amount of C-O and C=O bond derived from oleic acid were detected in the upper layer of tribo-film formed on the sliding surface of ta-C disc. It was suggested that difference of friction property between a-C:H and ta-C seemed to be caused by different composition of tribo-film generated during sliding.