الفهرس 
CHAPTER ONE: INTRODUCTIONOnly 14 pages are availabe for public view
 |  | from | 117 |  |  |
| | | from | 117 | | |
Abstract
Solid lubricant films have received considerable research attention in the last decades owing to their remarkable improved tribological characteristics. The aim of the present work is to study the effect of thermoplastic polymers (PP and PTFE), as a solid lubricant additives on coating performance under lubricated conditions with lithium grease, to reduce the effect of main components of white cement such as sand, kaolin and limestone contaminating lithium grease on friction coefficient and wear of steel test specimens using a cross pin tester. The effect of amount of contaminants on the tribological performance of these films was systemically studied. The tests were carried out at a sliding velocity of 0.5m/s and load of 10N. The rotating specimens were greased before the test and further greasing was carried out every 30 sec during the test. The test period was 5 min. The wear scar diameter was measured for the upper stationary pin using optical microscope within an accuracy of ±1 µm. Experiments were carried out at room temperature using lithium based grease, the solid additives with range of grain size was from 0 to 300 µm and the range of grain size of the contaminants was from 0 to 150 µm.
Experimental results show that, the addition of 15 wt. % Polytetrafluoroethylene to lithium grease gives excellent anti-friction and wear resistance performances under components of white cement compared to lithium grease as received, due to its good lubricating properties. It was found that lithium grease should be dispersed by 20 wt. % oil to balance the contaminants and get the lowest friction and wear values. As the oil content, added to the grease, increased friction coefficient and wear slightly decreased, the significant differences of surface roughness, hardness, microstructure and intrinsic lubricating property directly lead to the different tribological performances and worn morphology. The formed composite transfer layer plays a vital role in reducing friction and wear due to its anti-friction and shielding action of the film surface from the hard metal asperities. Two main abrasive wear mechanisms (three-body rolling wear and two-body grooving wear) occur simultaneously in the tribological process under sand-dust environments. A transfer layer-hardening composite wear modeling was established to further explain the anti-wear mechanisms and friction-reducing capacity of these solid lubricant films under dusty environments.