The objective of this study is to evaluate the boundary lubrication performance of ionic liquids under high vacuum and low temperature by taking the cosmic space environment into consideration, as a screening stage prior to evaluating lubrication performance in actual space mechanisms. The boundary lubrication performance of ionic liquids was evaluated at room temperature with a reciprocating linear motion tribometer, and at low temperature (from −80°C to room temperature) with a unidirectional rotation tribometer. Low-temperature rheometry was also carried out. Ionic liquids showed a supercooling state and crystallization. This crystallization was prevented by mixing different ionic liquids together. The equimolar mixture of 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) amide (EMI-TFSA), 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) amide (BMI-TFSA), and 1-hexyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) amide (HMI-TFSA) showed no crystallization in our experiment. The antiwear performance of this sample oil mixture was similar to that of MAC and much better than that of PFPE at low temperatures. This mixture prevented metal contact at −80°C, most likely due to its high viscosity and high adsorption of molecules.
Measurements of both thermal and lubrication properties of urea-based grease composites were conducted with and without additives of vapor grown carbon nanofiber (VGCNF) and solid lubricant molybdenum disulfide (MoS 2 ). As a result, a synergetic effect was found to greatly improve both the wear resistance and thermal properties of the urea grease composite with VGCNF and MoS 2 .
Comprehensive interpretations of grease properties are presented. Dropping point, penetration and rheological properties are examined for a wide variety of greases involving both lithium soap greases and diurea greases. Types and viscosity of the base oils, types and content of the thickeners and manufacturing conditions are extensively varied in the greases examined. Grease exhibits the rubbery region over a wide range of frequency, indicating the network structure formed by thickener fibers. The magnitude of storage modulus, G', in the region is related to the network density of the thickener fiber in analogous to polymer rheology. Two regions are found in the dropping point of grease. In the Region A, dropping point decreases as the network density decreases. In the Region B, the dropping point is almost independent of the network density, but dominated by network collapse. Moreover, the loss modulus, G'', at non-linear regime, representing viscous dissipation of energy, is found to be a controlling factor for the penetration, and it allows for excellent correlation with the extensive body of penetration data.
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