Rheological Behavior of Carbon Nanotubes as an Additive on Lithium Grease

The rheological behaviors of carbon nanotubes (CNTs) as an additive on lithium grease at different concentrations were examined under various settings of shear rate, shear stress, and apparent viscosity. The results indicated that the optimum content of the CNTs was 2%. These experimental investigations were evaluated with a Brookfield Programmable Rheometer DV-III ULTRA. The results indicated that the shear, stress and apparent viscosity increase with the increase of CNTs concentration. The microstructure of CNTs and lithium grease was examined by high resolution transmission electron microscope (HRTEM) and scanning electron microscope (SEM). The results indicated that the microscopic structure of the lithium grease presents a more regular and homogeneous network structure, with long fibers, which confirms the rheological stability. 1. Introduction Grease is a solid or semifluid which would normally have been employed together with a thickener, additive, and anti-oxidant agent. The fluid lubricant that performs the actual lubrication can be petroleum (mineral oil), synthetic oil, or vegetable oil. The thickener gives grease its characteristic consistency and is sometimes believed as a “sponge” that holds the oil in place [1]. The majority of greases on the market are composed of mineral oil blended with a soap thickener. Additives enhance the performance and protect the grease and lubricate surfaces. The influence of the rheological properties of CNTs additives is very important for all the grease lubricating bearings. To characterize a lubricant comprehensively, the rheological properties at all working conditions, pressures, and temperatures have to be known [2]. Grease is widely used as a lubricant in the wheel assembly, journal bearings, and rolling element bearings. Grease is also used in other areas that need occasional service like the brake or stopper assembly to help keep these fittings rust-free and make removal of dirt and grime easier. Grease is applied to machines that can be lubricated infrequently and where lubricating oil would not stay in position. It also acts as a barrier to prevent entering of water and the incompressible materials. CNTs used as a performance enhancing additive in gear lubricants for extended lifetimes, lower operating costs, and improved power efficiency. Numerous laboratory investigations and industrial experience indicate that using of CNTs has significant advantages compared to conventional solid lubricants in both mild and extreme pressure conditions. Lubricating grease consistency has been evaluated for years