Graphene-Reinforced Lithium Grease for Antifriction and Antiwear

Graphene has attracted tremendous attention as a promising additive in lubricants due to its unique lamellar structure and excellent mechanical strength. Yet, unlike its use in oil and water lubricants, the amount of graphene additive should be considered when it is introduced into the grease which is a twophase colloid. In this work, graphene was added into the lithium grease in different concentrations, and lubrication behaviors were investigated using a four-ball testing method under various operating conditions. Prior to the four-ball friction tests, graphene and grease materials were characterized by scanning (SEM) and transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM), and Raman spectroscopy. Friction test results demonstrate that the graphene concentration in grease varies at different tribological contact conditions to reach the optimum lubrication behavior. On the basis of the results from friction tests and worn scar morphology analysis, a lubrication mechanism was proposed to better understand the interactions among grease elements, e.g. graphene, thickener, and base oil, during the shearing process. It is believed that thickener soap actively participates in the lubrication process at low speeds by releasing enough oil into the friction contacts under high load. Meanwhile, less graphene concentration is required to strengthen the base grease by inhibiting and avoiding severe wear. High rotational speeds negatively affect the "oil-bleed" capability of thickener under lower contact loads due to the churning loss at high centrifugal force. Thus, extra graphene additives are required to retain more oil and separate the contact surfaces. This, in turn, promotes the formation of protective tribofilm on the interface which is the key to the enhancement of antifriction and antiwear performance.