The present work discusses the effect of sand particles, contaminated in the lubricating oil, as well as sand particles embedded in the sliding surfaces, on the friction coefficient displayed by polyamides sliding against steel. Stribeck curve is used to compare the friction performance in boundary, mixed and hydrodynamic regimes. Experiments were carried out to investigate the friction coefficient of the tested materials when the oil was contaminated by sand particles of different particle sizes and contents. In the presence of sand particles, the friction increase was attributed to the abrasion of the sliding surfaces by sand particles. In the mixed lubrication regime, the combined effects between boundary and hydrodynamic lubrication was controlling friction coefficient for clean oil. The minimum friction coefficient appears in the mixed lubrication regime. Sand particles in oil disturbed the hydrodynamic film and altered the values of friction coefficient. It was observed that friction coefficient with sand particles was higher than that displayed by clean oil. The presence of sand particles in the oil and as embedded particles in the sliding surfaces strongly affected the friction coefficient. Only when the sand particle size was higher than the film thickness and the particles were embedded into one surface and cut into the other one abrasion occurred and friction coefficient increased.
A. S. Al-Osaimy (Wed,) studied this question.