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A tribological comparison was conducted among micro‐MoS2 (m‐MoS2), platelet‐like nano‐MoS2, and spherical nano‐MoS2 in lithium grease, lithium‐calcium grease, and polyurea grease, respectively. Micro‐MoS2 slightly improved the lubrication of the 3 greases. The 2 nano‐MoS2 samples had obvious superiority over m‐MoS2. The tribofilms produced by nano‐MoS2 contained MoS2 and MoO3. However, the tribofilms with m‐MoS2 did not contain Mo, or Mo only existed in the form of MoO3. The absence of MoS2 in tribofilms was the main reason for the poor lubrication of m‐MoS2 greases. Moreover, the improved lubrication of the spherical nano‐MoS2 greases than the platelet‐like nano‐MoS2 ones was ascribed to the lubricating superiority of the spherical structure to the platelet‐like one.

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Purpose This paper aims to explore the effects of mineral diesel fuel carbon soot (MCS) and biodiesel carbon soot (BCS) on the lubrication of polyalphaolefin (PAO) and diesel fuels. Design/methodology/approach Two styles of carbon soot were prepared from the natural combustion of mineral diesel fuel oil (MDO) and biodiesel oil (BDO). Tribological tests were conducted on a high-frequency reciprocating rig. Friction surfaces were characterized using three-dimensional laser scanning confocal microscopy and Raman spectroscopy. Findings The addition of MCS and BCS to PAO could reduce friction in most cases. MCS had a negligible effect on the wear for contents not exceeding 1.0 per cent. By contrast, BCS exhibited a considerable negative influence on the wear resistance even at low contents. For diesel fuels, MCS reduced both friction and wear, whereas BCS substantially deteriorated the lubrication of BDO. MCS formed a Fe3O4/C composite lubricating film on the friction surface. BCS also entered the contact region, but it did not form an effective Fe3O4/C composite lubricating film. Originality/value This work compared MDO and BDO from a different perspective, i.e. the effects of their combustion carbon soot on the lubrication of lubricating oil and fuel oil. The significant negative effect of BCS on the lubrication of lubricating oil and BDO is a problem that could occur in the industrial application of BDO.

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