Tribological Performance of Lubricating Greases Based on Calcium Carbonate Polymorphs Under the Boundary Lubrication Condition

In this article, we synthesized the calcium sulphonate grease (CSG) based on the calcite using the bright stock (150BS) as the base oil. In order to investigate the tribological performance of lubricating grease containing different calcium carbonate polymorphs under boundary lubrication condition, a calcium sulphonate complex grease (CSCG) based on the vaterite was used as a reference. An oscillating reciprocating friction and wear tester set at a series of applied loads and frequencies was adopted to evaluate the tribological performance under boundary condition. Results showed that the lubricating grease that was composed of crystalline calcite as the partial thickener had excellent friction-reducing and antiwear (AW) properties, regardless of the applied loads and frequencies. The vaterite in CSCG easily experienced a polymorph transformation into calcite or aragonite characterized by Raman spectroscopy. This polymorph transformation was attributed to the highly local friction temperature and activated hydrogen from water or acids oxidated in the rubbing process at high load or frequency. The physical polymorph transformation corresponded to the fluctuations of the friction coefficients, then contributed to the severe wear. XPS analysis indicated that two calcium sulphonate lubricating greases occurred a tribochemical reaction and boundary tribofilms consisted of CaCO₃, CaO, iron oxide and FeSO₄ were formed on the rubbing surfaces. The tribofilm formed by the introduction of the CSG that mainly depended upon the thickeners of calcite structure contributed to an excellent AW protection. The possible boundary friction mechanism for greases based on various calcium carbonate polymorphs was also proposed. Effect of calcium carbonate polymorphs on the tribological performance was discussed.