Observation and Analysis of Micro-Behavior Characteristics and Element Contents during Boundary Layer Evolution under Powder Particulate Lubrication

Typical characteristics of the boundary layer under powder lubrication are microscopic reflections of lubrication states and closely related to lubrication properties. Therefore, the current investigation is focused on the changes in typical features of boundary layer with an annular contact, flat-on-flat sliding. A 3D laser profile gauge, scanning electron microscopy, X-ray energy-dispersive spectrometer, and Raman laser confocal spectrometer are utilized to microscopically observe and analyze the boundary layers formed on the brass specimens during the process of graphite-lubricated 0.45 C steel (HRC56) sliding upon brass alloy (H62). The evolution of mixed powder lubrication can be divided into four typical stages: (A) running-in, (B) stable, (C) deterioration, and (D) damage. Accompanied by better lubrication performance, the boundary layers are flat and the surfaces are smooth at early stages. Meanwhile, the surface roughness keeps low level, and the carbon content is consistent. Then, the surface flatness worsens gradually and the surface roughness containing boundary layer increases. Simultaneously, the carbon content on the surface decreases, and the copper content becomes even more. Along with the deterioration of the lubrication, iron is observed on the lower brass specimen, indicating a significantly direct contact between the upper and lower specimens. Moreover, the graphite powder, which used to be an excellent solid lubrication material, transforms to the disordered state in the test. From the Raman spectrometer analysis, the stable phase is the most feasible graphitization stage coupled with optimal lubricity. Friction characteristics are closely associated with the boundary layer states in powder lubrication, and improving the corresponding conditions can delay the damage processes.