The primary purpose of this study is to use a nano-scale optical surface profilometer to assess the feasibility of such instruments in measuring localized friction coefficient on railways, beyond what can be commonly measured by tribometers used by the railroad industry. One of the important aspects of moving freight and passengers on railways is the ability to manage and control the friction between the rails and wheels. Creating a general friction model is a challenging task because friction is influenced by various factors such as surface metrology, properties of materials in contact, surface contamination, flash temperature, normal load, sliding velocity, surface deformation, inter-surface adhesion, etc. With an increase in the number of influencing factors, the complexity of the friction model also increases. Therefore, reliable prediction of the friction, both theoretically and empirically, is sensitive to how the model parameters are measured. In this study, the surface characteristics of four rail sections are measured at 20 microns over a rectangular area using a portable Nanovea JR25 optical surface profilometer and the results were studied using various statistical procedures and Fractal theory. Furthermore, a 2D rectangular area was measured in this study because 1D height profile doesn’t capture all the necessary statistical properties of the surface. For surface roughness characterization, the 3D parameters such as root-mean-square (RMS) height, skewness, kurtosis and other important parameters are obtained according to ISO 25178 standard. To verify the statistical results and fractal analysis, a British Pendulum Skid Resistance Tester is used to measure the average sliding coefficients of friction based on several experiments over a 5 cm contact length for the four rail sections selected for the tests. The results indicate that rail surfaces with lower fractal dimension number have a lower friction. The larger fractal dimension number appears to be directly proportional to larger microtexture features, which potentially increase friction.
A8.1 - Areal optical surface measurement – how to get reliable topography data
