The sliding friction of bonded solid lubricants
The tribological study of Bonded Solid Lubricants, BSLs is reviewed, this includes a review of the constituent components of a BSL including a table of publications which presents a list of papers containing detailed data about the friction and wear of BSL coatings. The general theories of friction for metals in sliding contact and the friction theory applied to solid film lubrication are reviewed. A section is also included on Herzian contact theory and its application to solid films. A screening test is described for 12 BSLs applied to commercial components. This programme provided information on the friction coefficient and allowed the selection of one of these finishes for a further more in depth study. In general the friction coefficients obtained were higher than those quoted by the material manufacturers, it was presumed that this was due as the consequence of differences in test methods. Three test programmes are described that were conducted on a new pin on disc facility, a major programme to study the performance under various conditions at pressures above the yield pressure of the film. The information was used in the formulation of an empirical equation that can be used to predict the friction coefficient. This programme also demonstrated: that there were three distinct phases of performance, contact life increased with high loads and increased velocity, the friction coefficient in the steady state phase was between 0.18 and 0.22. The second programme examined the evolution of the wear scar on the ball and wear track on the disc. The programme illustrated the development of a transfer film on the ball and the steady increase in friction coefficient to a point where it became stable and the wear scar on the ball changed to a circular flat. The third programme investigated the friction coefficient at very low loads that produced pressures below or close to the yield pressure of the film. The results from this programme demonstrate that as the load increases, the friction coefficient decreases, this can be attributed to the Hertzian contact area being proportional to the load 213 and the friction force is proportional to the area, then the friction coefficient F/W is proportional to the load W-1/3. In the penultimate chapter theoretical predictions for the friction coefficient under various contact conditions are compared with the experimental results obtained, in general the calculated results are higher and some possible causes are given for the discrepancies.