Researchers at Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences (CAS), have synthesized calcium sulphonate grease (CSG) and investigated its tribological properties using a calcium sulphonate complex grease (CSCG) based on the vaterite as a reference.

Schematic diagram of possible tribological mechanism for vaterite and calcite under boundary lubrication condition

FTIR results elucidated that the crystalline calcium carbonate polymorph in the CSG was calcite, but that in the CSCG was vaterite. Tribological performance of the two lubricating grease under the boundary lubrication condition were investigated and showed that the calcite grease showed excellent friction-reducing and AW properties regardless of the testing applied load and frequency, while the tribological properties of the vaterite-grease were dependent upon the testing conditions. At high load or frequency, the highly local temperature and activated hydrogens resulting from residual water or water in air or acids owing to oxidation became the beneficial requirements for the specialty transformation. As a result, partial vaterites experienced a polymorph transformation into calcite or aragonite characterized by RS. This physical transformation corresponded to the fluctuations of the friction coefficients, then contributed to the severe wear. XPS analysis indicated that boundary tribofilms consisted of CaCO₃, CaO, iron oxide and FeSO₄ were formed on the rubbing surfaces. There is no significant difference in the components of the films, but the tribofilm formed by the introduction of the thickeners of calcite structure contribute to a more excellent protection. Stable calcite is beneficial to improve the tribological performance of grease. The completed conversion of calcium carbonate polymorphs for preparing the CSG in industrial application is the key to improve the tribological performance. Future studies should concentrate on the effect of calcium carbonate polymorphs on the tribological properties.

The work has received support from National Program on Key Basic Research Project of China (973 Program) and the findings have been published in Tribol Lett(Tribol Lett(2012) 47:183–194).