Influence of Ionic Liquid as Additive on the Conductivity and Tribological Properties of Greases

Rolling bearings, essential components of high-end equipment, require effective grease lubrication to function optimally. However, in industrial areas, such as new energy vehicles and wind power generation equipment, motor bearings are often exposed to electrical environments. When the electric potential difference between the two lubrication-film separated surfaces is generated and its threshold value is exceeded, the lubrication film will be broken, resulting in electric damage on the contact surfaces of bearings, which is known as "bearing electrical pitting" and has become an increasing concern in the field of tribology. Addressing the issue of bearing electrical pitting, a cost-effective industrial approach involves using conductive greases. This type of greases can reduce the potential difference across the oil film, thereby alleviating the electrical pitting. Ionic liquids (ILs), which are organic salts with low melting points below 100 ℃, having exceptional electrical conductivity and tribological properties. Formulating conductive greases with the incorporation of IL additives provides a practical solution to mitigate bearing electrical pitting. In this paper, the double trifluoro-sulfonate IL, which was designed by some of the authors, was used as a conductive additive to formulate lithium-based greases, and conductive greases with 10% IL (denoted as IL-1) and 20% IL (denoted as IL-2) were prepared. This IL could harmonize both the polarity and conductivity to ensure conductivity and oil solubility. Tests showed that the ionic liquid could remain stably dispersed in the grease over an extended period. The electrical conductivity of the greases was measured using a self-designed resistance measuring device. The results showed that with the IL, the resistivity of the grease was significantly reduced. The resistivity of grease IL-1 was lower than that of grease IL-2, indicating that as the mass fraction of ionic liquid increased, the resistivity of greases was reduced. In addition, the resistivity of grease with nano-graphite was slightly greater than that of IL-1 and IL-2 greases. At the same time, as the electrode distance increased, the resistivity values of IL-1 and IL-2 did not change significantly, while the resistivity of nano-graphite added grease increased. Ionic liquids were liquid salts composed of anions and cations. Due to the presence of ions, ILs created a good conductive path in the grease and the corresponding resistivity showed less dependence on the electrode distance. Moreover, with the increase of the applied voltage the resistivity of IL-1 and IL-2 decreased because the increased electric field activated the ions more. Using a UMT-3 tribometer, the tribological properties of the greases IL-1 and IL-2 were tested under steel-steel contact. It showed that IL-1 and IL-2 presented lower friction and wear than the base grease. And with a heavier load, IL-2 demonstrated better tribological behavior than IL-1. In addition, the IL grease was treated by the different voltages for some time, and the related friction and wear test were carried out to study the influence of the electric field pretreatment on the lubrication of the greases. It was interesting to find that with a load of 30 N and a frequency of 4 Hz, about 10% decrease of friction could be achieved for applied voltage of 100 V and pretreatment time of 60 min. Moreover, it was found that with increasing applied voltage, more decrease in friction and wear could be attained. Unfortunately, no sound explanation could be given at present, and more research was needed to clarify this electric pretreatment.