Photoelastic visualization of contact phenomena between real tribological surfaces, with and without sliding

Abstract

The design, construction and operation of a machine for visualizing real contact interfaces and testing sliding wear is described. The bodies that form the interface are both opaque—the slider is a carbon graphite sample (National Electrical Carbon Company brush grade 634) and the rotor surface is copper. The visualization technique uses contact indentation stresses and photoelasticity to generate optical fringes (shadows) directly beneath the contact interface. The resultant fringes indicate the size and location of the islands of real contact that exist within the apparent contact region. Resolution is sufficient to distinguish two neighbouring millimetre-sized contacts. For calibration and comparison purposes, photoelastic pictures of sharp graphite pencil points and hard plastic spheres of various diameter indenting the copper slip ring surface are presented. Photographs of photoelastic images of contact interfaces between an opaque carbon sample sliding against an opaque copper slip ring are also presented. Photographs are presented that show the formation of concentrated contact (and possible evolution to thermal mounding) on the carbon slider surface. Collective data from other photographs measures the relative occurrence of concentrated contact and suggests operating and surface conditions that can discourage concentrated contact and formation of thermal mounds.