Graphite material has the characteristics of high temperature resistance, corrosion resistance, self-lubrication and so on. As a good solid lubricant and lubrication additive, graphite material is applied to lubrication of mechanical equipment and processing technology in various forms. It plays the role of performance maintenance, energy saving and consumption reduction, and improves production efficiency.
The good lubricity of graphite materials originates from the crystal structure of its own lamellar structure. In the layered graphite crystal structure, carbon atoms form hexagonal graphite layer with SP2 hybrid orbital. The bond energy between carbon and carbon belongs to a resonant σ- bond ( σ- electron covalent bond), which can reach 627KJ/mol. It endows the graphite layer with a strong property, and the interaction between the graphite layer and the layer belongs to a weak Vander WaaIs force, is produced by the interaction of conjugated large H-bonds on both sides of the graphite layer. The bond energy is only 5.4KJ/mol, which is only 1/110 of the covalent σ- bond strength between carbon atoms in the layer. This structural feature determines the load resistance, corrosion resistance, high temperature resistance, radiation resistance of the graphite layer, as well as the good slip between layers, laying the foundation for graphite as a high-performance lubricating material.
Graphite derivatives such as graphite fluoride, metal compound intercalated graphite, expanded graphite and so on, all maintain the layered structure of graphite, and because of the role of intercalated materials, the interlayer spacing is significantly increased, which is very beneficial to the improvement of lubrication performance.
The intrinsic nature of the lubrication of graphite is its lamellar crystal structure. However, the environmental atmosphere and environmental medium of graphite obviously affect its lubrication performance. Under humid atmospheric conditions, the friction coefficient of graphite can be as low as 0.05 (high contact stress) to 0.15 (low contact pressure), while in vacuum, the friction coefficient of graphite rises to 0.5-0.8. When graphite in vacuum is introduced into air, oxygen, water vapor or benzene, ammonia, ethanol, acetone, heptane, etc., the friction coefficient decreases rapidly, but when nitrogen or carbon dioxide is introduced, the friction coefficient does not decrease.
It is generally believed that some gases can be adsorbed along the surface of graphite prism and intruded into the cleavage plane (interlayer), which reduces the surface energy of graphite layer, weakens the bonding strength between layers, and improves the slip of layers. Therefore, the formation of gas molecule adsorption film on and between layers plays a role in reducing friction, thus ensuring the good lubrication performance of graphite. Therefore, graphite intercalations such as graphite fluoride and metal compound intercalated graphite have been used as lubricants. The results show that the existence of interlayer improves the dependence of graphite lubrication on gas medium to a certain extent. In addition, the invasion of lubricating oil molecules to the interlayer of graphite materials is bound to greatly improve its lubricity. Therefore, when combined with lubricating grease, the environmental medium of graphite lubricating material can be improved and its lubrication can be more effectively exerted.
When graphite is used as solid lubricant and lubrication additive, its lubrication performance is also affected by graphite particles. Some research and test results show that graphite powder with average particle size of 4-5 μm can produce good lubrication effect and reduce the friction and wear of the specimen to a minimum, whether natural or artificial graphite. Coarse-grained graphite powder can produce abrasive effect, which is not conducive to antifriction and antiwear; however, it is obvious that fine graphite, such as 1μm particle size, also leads to the decrease of antifriction and antiwear effect, which should not be explained by abrasive effect. Because the fine graphite has obviously lost the friction-reducing mechanism of the slip of the lamellae, accompanied by the adsorption of the fine graphite on the friction surface, the lubrication effect of the solid film would not be satisfactory without the strengthening of the wetting agent or the film-forming agent.
The particle size of solid lubricants such as graphite is generally below 0.5μm in coatings requiring antifriction, wear resistance and decorative effect, and the particle size of solid lubricants such as graphite in suspension oil can be as small as 0.3μm. These are special requirements for the particle size of solid graphite in order to stabilize the dispersion and spread the film.
In the particle factor of graphite, the shape of the particles will also have an effect on its lubrication. The relationship between the specific wear of lubricated materials and the crystal size ratio of graphite powder shows that the specific wear of lubricated materials decreases with the increase of the ratio of graphite particle size. Therefore, thin sheet graphite particles have better lubrication effect.
Author：Zhou Qiang, Xu Ruiqing
