The Influence of Viscosity of Mineral Oil in the Microstructure and Performances of Lithium-Based Greases

The effects of mineral base oils with different viscosity on the physical and chemical properties, microstructure, tribological properties and rheological properties of lithium grease were studied, and the relationship between different properties was also discussed. Four components from the raw base oil (MVI500) with gradient viscosity were prepared using stirred film evaporator by gradually raising the evaporating temperature. Based on GC-MS analysis, the component of high viscosity was rich in monocylic naphthenes and aromatics but lacked alkanes and polycyclic naphthenes. Then, four corresponding lithium greases were prepared by the same procedure with the soap content of 10%. The grease prepared by high-viscosity component showed good stability at high temperature but poor colloid stability and oxidation stability, while the greases prepared by low-viscosity component were just the opposite. Especially, there was a large gap of oxidation stability between base oil and its corresponding grease, which might be due to the catalytic oxidation effect of metal soap fiber. Scanning electron microscopy analysis showed that the soap fiber structure formed by low-viscosity component was tightly twined and the cavity volume was small, resulting in its high colloid stability. The soap fiber structure formed by high-viscosity base oil was loose and the cavity volume was relatively larger, which was unfavorable for high colloid stability but conducive to the continuous release of the base oil, thus it exhibits good anti-wear and friction-reducing property. Nevertheless, the friction behavior of raw base oil was mainly influenced by the component of low viscosity but not high viscosity. Furthermore, with the increase of viscosity of base oils, the structural strength of grease slightly increased first and then decreased, which was also related to the microstructure of greases. The greases prepared by raw base oil and low-viscosity base oils had similar three-dimensional network structure with high fraction volume of soap fiber, resulting in the relatively higher structural strength compared with greases prepared by high-viscosity base oils. It was worth to note that, unlike the other samples, the grease prepared by the component of 103.80 cSt (40 °C) still maintained relatively high structural strength when the test temperature increased from 40 °C to 80 °C, indicating the intermolecular force of soap fiber formed by this component was relatively stronger than that of other greases. Similarly, the thixotropic area of greases increased first and then decreased with the increase of viscosity of base oils. Combined with the results of structural strength, it indicated that the recoverability of greases with low structural strength was better, and that with high structural strength was worse, which was also closely related to the microstructure of greases. The grease prepared by low-viscosity component showed highly entangled fiber structure with low cavity volume, thus it required more energy to destroy the structure, and the process of recovery was more difficult. Theoretically, it also required larger energy for the structural recovery of high-viscosity samples due to the high-viscosity of base oil, but their loose and weak soap fiber structure made the process of recovery easier. Therefore, compared with the viscosity of base oil, the influence of soap fibrous microstructure was more important to the thixotropy. The research results of this article had a good guiding significance for the selection of base oil in the development of lithium grease products.