Researchers at Northwestern University’s McCormick School of Engineering have found that crumpled graphene balls are an extremely promising lubricant additive. In a series of tests, a polyalphaolefin base oil with only 0.01–0.1 wt % of crumpled graphene balls outperformed a fully formulated commercial lubricant in terms of friction and wear reduction. A paper on their work is published in the Proceedings of the National Academy of Sciences.
For the average car, 15% of the fuel consumption is spent overcoming friction in the engine and transmission. While oil helps reduce this friction, researchers have long sought additives that enhance oil’s performance. Ultrafine particles are often used as lubricant additives because they are capable of reducing friction and protecting surfaces from wear. They also tend to be more stable than molecular additives under high thermal and mechanical stresses during rubbing. However, they also can aggregate, reducing the effective concentration.
|Crumpled graphene particle. Source: Northwestern.|
While molecular ligands can help the particles to disperse, they tend to degrade under harsh tribological conditions. Therefore, the researchers noted, self-dispersed particles without the need for surfactant are highly desirable.
About five years ago, Northwestern’s Jiaxing Huang, associate professor of materials science and engineering, discovered crumpled graphene balls—novel ultrafine particles that resemble crumpled paper balls. The particles are made by drying tiny water droplets with graphene-based sheets inside.
Capillary force generated by the evaporation of water crumples the sheets into miniaturized paper balls. Just like how we crumple a piece of paper with our hands.—Jiaxing Huang
Shortly after making this discovery, Huang explained it to Northwestern Engineering’s Yip-Wah Chung, professor of materials science and engineering, during a lunch in Hong Kong by crumpling a napkin and juggling it. “When the ball landed on the table, it rolled,” Chung recalled. “It reminded me of ball bearings that roll between surfaces to reduce friction.” That “a-ha!” moment led to a collaboration among the two professors and Q. Jane Wang, professor of mechanical engineering, who was in the middle of editing a new Encyclopedia of Tribology with Chung.
Because of their unique shape, crumpled graphene balls self-disperse without needing surfactants that are attracted to oil. With their pointy surfaces, they are unable to make close contact with the other graphene balls. Even when they are squeezed together, they easily separate again when disturbed.
Borrowing from the analogy that pieces of paper that are crumpled do not readily stick to each other (unlike flat sheets), we expect that ultrafine particles resembling miniaturized crumpled paper balls should self-disperse in oil and could act like nanoscale ball bearings to reduce friction and wear. Here we report the use of crumpled graphene balls as a high-performance additive that can significantly improve the lubrication properties of polyalphaolefin base oil. The tribological performance of crumpled graphene balls is only weakly dependent on their concentration in oil and readily exceeds that of other carbon additives such as graphite, reduced graphene oxide, and carbon black.—Dou et al.
Next, the team plans to explore the additional benefit of using crumpled graphene balls in oil: they can also be used as carriers. Because the ball-like particles have high surface area and open spaces, they are good carriers for materials with other functions, such as corrosion inhibition.
Xuan Dou, Andrew R. Koltonow, Xingliang He, Hee Dong Jang, Qian Wang, Yip-Wah Chung, and Jiaxing Huang (2016) “Self-dispersed crumpled graphene balls in oil for friction and wear reduction” PNAS doi: 10.1073/pnas.1520994113
Jiayan Luo, Hee Dong Jang, Tao Sun, Li Xiao, Zhen He, Alexandros P. Katsoulidis, Mercouri G. Kanatzidis, J. Murray Gibson, and Jiaxing Huang (2011) “Compression and Aggregation-Resistant Particles of Crumpled Soft Sheets” ACS Nano 5 (11), 8943-8949 doi: 10.1021/nn203115u