38 n BEARINGS AND BELTS March 2024 www.drivesncontrols.com Fighting friction with graphene The most basic de nition of friction is the force that prevents smooth and easy movement of two moving surfaces in contact with each other. If not reduced or controlled e ectively, high levels of friction often lead to high wear and, ultimately, to poor reliability and equipment failures. Friction is a complicated problem for engineers. Bearing friction, for example, is not constant and is addressed using tribological phenomena that occur in lubricant lms between rolling elements, raceways and cages. Whether a design engineer opts for a dry, silicon or mineral lubricant, an oil or grease, the aim is to match the lubricant to the application to reduce friction between the internal surfaces of the bearing’s components by reducing or preventing metal-to-metal contact between rolling elements and raceways. While this is a good way of reducing wear and preventing corrosion, there are new materials that can reduce friction dramatically compared to other lubrication and surface treatment options. On the macro scale, friction results from microscopic imperfections in surfaces. At the atomic scale, however, friction involves attractive forces between individual atoms. This opens up the phenomenon of “superlubricity”, where atomic-scale structural mismatches make it impossible for multiple atoms in one surface to get close to atoms in the other, leading to extraordinarily low friction. Since super-lubricity was rst proposed in 1990, several groups have observed the e ect, but it has been dicult to scale because of the inconsistency between macroscopic surfaces. Until now, that is. Grease additive The potential of graphene as a lubricant remains relatively unexplored. Several studies have investigated the sliding properties of graphene-based dry lubricants, however few have explored its use as an additive to oil or grease in rolling contacts. A 2014 study that investigated using multi-layer graphene as a lubricating additive in bentone grease concluded that the solid additive was well dispersed, thus achieving a lower coecient of friction and less wear than graphite and ionic liquids. Graphene, being a two-dimensional material, o ers unique friction and wear properties. In addition to its thermal, electrical, optical and mechanical properties, graphene can serve as a liquid or colloidal lubricant for bearings. Graphene is ultrathin even in multiple layers. It can be applied to systems with oscillating, rotating and sliding contacts to reduce friction and wear, as well as protecting bearings from corrosion when exposed to water – a process known as tribo-corrosion. This is due to graphene exhibiting a slippery texture, potentially making it an excellent lubricant. For applications involving heavy loads, grease is the lubricant of choice. It is simply a mixture of oils with a thickener. Although greases have more lubricant drag than oils or dry lubricants, a reduced ll can result in lowtorque bearings, while also being appropriate for high-speed applications. However, grease lubricants have certain disadvantages that must be overcome to meet the demands of operating conditions in modern equipment. Grease with a graphene additive is being explored to reduce frictional torque further, potentially making it suitable for applications where it wouldn’t have been considered previously. A 2019 study by Leibniz University investigated the frictional torque for sliding contacts and rolling bearing applications using graphene as a grease additive. Two test rigs were used to monitor pivoting movements for more than one million test cycles, comparing frictional torque levels for a standard grease lubricant compared to one containing graphene in various thicknesses. The frictional torque for bearings lubricated using pure grease averaged A study has found that using low-friction graphene as a lubricant can deliver substantial energy savings. Chris Johnson, managing director of SMB Bearings, explains this so-far little-explored possibility could result in improved, or even limitless, bearing wear. Graphene is a form of carbon consisting of a single layer of atoms arranged in a hexagonal lattice structure.
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