الفهرس 
CHAPTER ONE: INTRODUCTIONOnly 14 pages are availabe for public view
 |  | from | 89 |  |  |
| | | from | 89 | | |
Abstract
Solid lubricants are used as additives to improve the performance of grease under condition of boundary lubrication. The present work discusses the effect of solid lubricant additives such as graphite, aluminum powders and molybdenum disulphide (MoS2) on the friction of rolling bearing. 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. %). Besides, the possibility of polymeric powders as solid lubricants was investigated. Six common types of polymeric particles such as polyethylene (PE), polymethylmethacrylate (PMMA), polypropylene (PP), polystyrene (PS), polytetrafluoroethylene (PTFE) and polyvinyl chloride (PVC) of (0 - 50 μm) particle size were used as thickener of the tested grease at concentrations of (5- 10- 15- 20) wt. %.
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 (in order). 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 (in order). The minimum temperature rise was recorded for PMMA followed by PP, PE, PS, MoS2, PTFE, aluminum particles, PVC and graphite for grease contaminated by cement.
The behavior of the solid lubricants used in the present work can be explained on the fact that 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 the ability of polymeric particles 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.