What is graphene?
Graphite and diamond are the most widely known allotropes of the C element. In 2004, Professor Andre and Researcher Christine of the University of Manchester used scotch tape to paste and tear high-orientation graphite repeatedly. It was the first time to prepare and prove the existence of graphene. The two scholars also won the 2010 Nobel Physics Award Academic awards. The theoretical thickness of a single-layer carbon nanomaterial is only 0.34 nm, which has become another focus of enthusiasm for scientists in many fields after fullerenes and carbon nanotubes.
Anti-friction and anti-wear properties of graphene additives
Graphene has a unique two-dimensional nano-layered structure, high mechanical strength and strong thermal conductivity. Graphene is the basic structural unit of carbonaceous solid lubricating materials, and its nanotribological properties are affected by many factors. Although the number of layers is different in the academic circles, the influence of the number of layers on its surface friction is the most obvious.
Through the research of graphene nanotribological properties, it can be concluded that graphene has a high-quality lubricating effect and high anti-wear properties, so graphene has the potential of high-quality nano-lubricating materials.
(1) Graphene is used as an anti-sticking and anti-friction protective film for micro and nanodevices of various materials. Various graphene-based nano lubricating films reduce the friction coefficient of the substrate surface and extend life.
(2) Graphene is used as an additive for lubricating oil, ionic liquid and water lubrication. It can form a frictional adsorption film and a second transfer film on the friction interface, preventing the direct contact of the second friction surface, thereby improving the bearing capacity of the lubricant and friction. The anti-wear performance of the vice is better.
(3) Graphene is used as a filler for polymers, ceramics and other materials. Graphene makes the mechanical properties of the matrix material better. It uses a continuous transfer film to reduce the friction coefficient of the polymer matrix and dramatically improves the wear resistance of the matrix material. The constant transfer film has self-lubricating and high bonding characteristics.
The structure of graphene is densely layered, which gives it the particular properties required by wear-resistant materials, such as better thermal stability, lower shear strength, and more insufficient surface adhesion. Besides, its ultra-thin sheet structure can quickly enter the contact surface to reduce direct contact between two rough surfaces. Therefore, graphene can be added to lubricants as a friction modifier.
The lattice commensurability between graphene layers determines the nano-friction between the layers. Due to the incommensurability, the sliding friction between the layers formed by the stack is minimal, and even super-lubrication may occur. All these make graphene possess the characteristics of high-performance oil additives. Studies have shown that using an appropriate amount of graphene additives as nano sliding bearings can reduce the friction coefficient and significantly increase the load-bearing and anti-wear properties of lubricants utilizing friction adsorption films. The unique properties of graphene have made it a hotspot for research in many fields, and it has shown a more comprehensive application prospect.
Dispersion stability of graphene additives
Graphene's super-flake layer structure, superior mechanical properties, and self-lubricating properties make it a heat-carrying and anti-wear stuff studied in many fields. However, graphene tends to agglomerate in lubricating oil or water, making its dispersion in solvents unstable. Currently, there are two main methods for solving graphene dispersion. First, add points. Various studies have found that using the right amount of graphene as a lubricating additive can reduce the friction coefficient and significantly increase the dispersant. Utilize the dispersing effect of dispersant to make it evenly and stably dispersed in lubricating oil and water. The disadvantage is that the dispersant may affect the tribological properties of graphene; second, functional modification. The stability of graphene dispersion in lubricating oil or water is enhanced. The key to this method is the choice of active molecules.
The main problems in the development of graphene additives
Due to the advantages of graphene's good thermal stability, low shear strength, and low surface adhesion, its application in tribology has attracted more and more attention from scholars at home and abroad. The application prospects of enhancing heat transfer and improving lubricating friction have significantly been improved—the attention and recognition of experts and scholars in most fields. Many related companies and research institutions have invested a lot of energy in the research and development of graphene lubricants. However, the current research on graphite and functionalized graphene as lubricating materials are still in the early stage, and there are still a lot of problems to be solved:
(1) Many scholars have done a lot of research on graphene agglomeration and formed the leading solution. However, under different graphene specifications, there is a large gap in modification methods and conditions, requiring many experiments to support. And the current research is mostly focused on high-quality graphene, and the price of high-quality graphene is relatively high. It is challenging to apply it to lubricating oil on a large scale under current circumstances. However, there are few studies on the modification of low-quality multilayer graphene. It is challenging to ensure the dispersion stability of graphene in lubricating oil while maintaining the inherent mechanical and tribological properties of graphene. The actual modification of the problem remains to be further studied.
(2) When graphene with different microscopic morphology and other layer specifications is used as a lubricant additive, the tribological properties are also other. When the number of graphene layers is within 30 layers, the anti-friction and anti-wear performance of graphene lubricant additives must be verified by experiments.
(3) Experimental research on the nano-friction and nano-wear mechanisms of various functionalized graphenes remains to be carried out, such as oxidation, nitridation, fluorination, silanization amination of functionalized graphenes exhibiting anti-friction and resistance Grinding performance is more to be explored.
It can be seen from the above that the lubricating oil added with graphene has significantly improved friction and anti-wear effects. However, most of these experiments are carried out on four-ball mills or friction and wear testing machines, and experiments that are carried out on engine benches are rare. In the real engine environment, the lubricating oil is at a higher temperature, pressure, and shear, so it needs to meet the effects of better lubricity, cooling, detergency, and corrosion resistance. Therefore, it is minimal to rely solely on the testing machine to determine the impact of nano-additives in internal combustion engine lubricants.
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