Abstract
Metal-matrix self-lubricating composites can exhibit excellent tribological properties owing to the release of solid lubricant from the matrix and the formation of a lubricating film on the tribosurface. The coverage of the lubricating film on a worn surface significantly influences the sliding process. However, it is difficult to quantify the film coverage owing to the thin and discontinuous character of the lubricating film and the high roughness of the worn surface. A quantitative characterization of the lubricating film coverage based on X-ray photoelectron spectroscopy (XPS) analysis was developed in this study. The friction tests of Cu-MoS2 composites with a MoS2 content of 0–40 vol% were conducted, and the worn surfaces of the composites were observed and analyzed. Further, the influence of the MoS2 volume content on the coverage of the lubricating film on the worn surface was investigated. The relationships among the volume fraction of the lubricant, coverage of the lubricating film, and the friction coefficient were established. The friction model for the metal matrix self-lubricating composites was developed and verified to facilitate the composition design and friction coefficient prediction of self-lubricating composites.
Highlights
Metal-matrix self-lubricating composites (MMSCs) are widely used in tribological applications wherein oil or grease lubrication cannot be implemented [1−3]
The contact surface of the MMSC is partially covered by a solid lubricant of area fraction of αf, which is equal to the volume fraction of the solid lubricant (V) in the composite
Based on the results reported in the literatures, the counter surfaces of MMSCs are usually covered with continuous transfer layers, which have the same chemical composition as MMSCs; the lubricating film on the transfer layer can be assumed to have an area fraction equivalent to that of the worn surface of the composite [13, 28]
Summary
Metal-matrix self-lubricating composites (MMSCs) are widely used in tribological applications wherein oil or grease lubrication cannot be implemented [1−3]. MMSCs are fabricated using two constituent components: a metal matrix and reinforced solid lubricant particles. Molybdenum disulfide, tungsten disulfide, and black phosphorus are widely used solid lubricants. They have a lamellar crystal structure with stacked layers weakly bonded with each other, while the bonding within the layers is quite strong. MMSC is self-lubricating because of the transfer of the solid lubricant to the tribosurface and its formation into a thin film, which can prevent direct contact between the two mating surfaces. As an MMSC can maintain a continuous supply of solid lubricant to replenish the lubricating film formed at the tribosurface, it could provide effective lubrication throughout a long-term frictional operation [7−9]
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