The present work discusses the effect of solid lubricant additives such as graphite, aluminum powders, molybdenum disulphide (MoS2), polytetrafluoroethylene (PTFE), polyethylene (PE), polymethylmethacrylate (PMMA), polypropylene (PP), polystyrene (PS) and polyvinyl chloride (PVC) on the friction of rolling bearings. The tested solid lubricants are dispersed in lubricating greases. Performance tests were carried out using tribometer testing machine. A comparison performance was conducted using cement particles as contaminant in the tested grease at 10 wt. % content. Based on the experimental observations, it was found that grease dispersed by solid lubricants and free of cement showed a decreasing trend of friction coefficient, where the lowest values were displayed by MoS2 followed by aluminum particles then PTFE, PE, PP, PVC, graphite, PMMA and PS. While the lowest friction values of grease contaminated by cement were displayed by PMMA followed by MoS2, PP, PVC, PE, PS, aluminum particles, graphite and PTFE. The behavior of solid lubricants used in the present work can be explained on the ability of polymeric particles to adhere into the sliding surfaces by the help of the electric static charge generated on their surfaces as a result of the friction with the steel surface. Adhesion force depends on the deformed surface area of the polymeric particles which increases for soft polymers such as PE, PP, PTFE and PVC. As for hard polymers such as PMMA and PS, their particles roll on the sliding surfaces protecting them from extra friction. In addition, the ranking of polymeric materials in reducing wear caused by cement particles predicted that their ability depends on their adherence to the surfaces of solid contaminants and contact area. Knowing that the force of adhesion depends on the amount of electric charge generated during friction adherence, it can be recommended to select the polymeric materials according to their triboelectrification properties.
Marzouk et al. (Wed,) studied this question.