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Tribology and Lubrication Technology April 2016 : Page 12

TECH BEAT Evaluation of a new lubricant additive: Crumpled graphene balls The wear performance of this additive at a low treat rate in PAO is superior to a commercially available 5W-30 engine oil. GRAPHENE IS KNOWN TO EXHIBIT A HIGH DEGREE OF LUBRICITY, and the frictional properties of fluorinated graphene at the nanoscale were discussed in a pre-vious TLT article. 1 The structure of graphene is two-dimensional sheets of carbon organized into hexagonal hon-eycombs. K Y CO KEY CONCEPTS ONC S • Crumpled graphene balls demonstrate greater stability st ility in n dispersions with base stocks such as PAO due to their resistance to aggregation. • Testing for friction and wear shows that crumpled graphene balls exhibit better performance f e over a longer time ti e frame fr ra e than th n other types of graphene disp si . dispersions. • Better ette er results esults for friction fr c tion and a d wear wear r are are found und with with a 0.1% 0. % dispersi di dispersion sp si n of f crumpled crumpl um led graphene balls in PAO compared compar d to a commercially erci lly available a a lable 5W-30 engine 5W-30 i e oil. l. Research has looked at developing materials that can achieve coefficient of friction values below 0.005, which is known as superlubricity. 2 A previous TLT article discussed simulations done by sliding graphene against a diamond-like-surface (DLC). Superlubricity was detected at the nanoscale for the first time when graphene particles wrapped around the DLC surface to form nano-scrolls. The lubricity benefit of graphene has been observed in the material’s use as a solid lubricant, but most lubricant applications involve the use of a fluid. Forming stable dispersions of solid lu-bricants has proven to be difficult. In most cases, solid lubricants will either drop out of solution or—if evaluated at the nanoscale—aggregate into larger particles that do not impart lubricity. Jiaxing Huang, associate professor of materials science and engineering at Northwestern University in Evanston, Ill., says, “We have recognized the limi-tations in dispersing various forms of graphene in liquids. One option that we are evaluating is crumpling gra-phene into balls.” The process for crumpling gra-phene starts with the preparation of its derivative—graphene oxide that is readily dispersible in water. Huang continues, “Graphene oxide sheets are suspended in nebulized, aerosol water droplets produced by a humidifier and then squeezed in all directions through a process called capillary compression. Heat is applied during the evaporation process to reduce graphene oxide to graphene leading to the formation of crumpled graphene balls exhibiting a diameter of a few hundred nanometers.” The process for crumpling graphene starts with the preparation of its derivative—graphene oxide that is readily dispersible in water. Crumpled graphene balls exhibit a rough surface texture with a reduced area of contact when placed on a sur-face. But they become strain-hardened enabling them to maintain the crum-pled shape. Huang says, “The important aspect of crumpled graphene balls is that van der Waals attraction between particles is very weak because the contact area between balls is low, even when they are compressed. In a similar manner to crumpled paper balls, graphene balls of a similar shape do not stack, and as a A. ÎÎÎ 12 The Periodic Table organizes elements according to: alphabetically, increasing atomic number, order of discovery or increasing atomic weight.

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