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Graphite is a traditional dry powder lubricant. The application of graphite in oils has significant economic benefits and has many advantages. With the development of science and technology and the deepening of scientific research, graphite lubricating materials have expanded to appear in the form of graphite derivatives, such as flake nano-graphite, expanded graphite, fluorinated graphite, and compound or straightforward intercalated graphite.The layered structure, and because of the effect of the intercalation substance, makes the layer spacing significantly increased, which is hugely beneficial to the improvement of lubrication performance.
What makes graphite a good lubricant?
Graphite can be used as a lubricant because of its layered crystal structure. In the graphite layered crystal structure, carbon atoms form a hexagonal network of graphite layers with sp2 hybrid orbitals. The carbon-carbon bond energy belongs to a resonance σ-bond (σ-electron covalent bond). It can reach up to 627K J/mol, which gives the graphite layer strong properties; the interaction between the graphite layer and the segment belongs to the weak Van der Waals force, which is caused by the conjugate significant H-bond phase 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 segment. This structural feature determines the load-resistant, corrosion-resistant, high-temperature, and radiation-resistant characteristics of the graphite layer, as well as the excellent sliding properties between the sheets, laying a foundation for graphite as a high-performance lubricant.
Types of solid lubricants (powder graphite lubricant)
1.Graphite and flake nano graphite
Graphite has been developed as a dry powder lubricant for a long time. It has been reported that the use of graphite in vehicle engines has a good effect of saving fuel at about 10%. With the development and popularity of nano engine oil additive, nano-materials have been widely used in solid lubricants and have shown better lubricating properties than traditional materials. The flake nano-graphite studied by Dr. Huang et al. As a lubricant additive. Graphite is more natural to spread into a film on the surface of the friction pair and shows better lubricity than ordinary graphite.
Extruded graphite is a loose and porous worm-like graphite obtained from natural flake graphite by intercalation, washing, drying, and high-temperature expansion. Studies have shown that in lubricating oils, expanded graphite shows excellent compatibility with oil media.
Fluoride graphite is a graphite intercalation compound produced by the direct reaction of fluorine and carbon. Ruffo first reported fluorinated graphite in 1937. The study found that the interlayer bond energy of fluorinated graphite is much lower than that of graphite, and the interval between fluorinated graphite layers is also twice that of graphite layers, which has led scientists to study their lubrication properties.
4.Graphite intercalation compound
Graphite intercalation compounds are formed when graphite absorbs some elements or compounds in the space between the layers. Studies have found that the lubrication effect of graphite intercalation compounds is significantly better than that of pure graphite because compounds or simple substances between graphite layers are easily de-intercalated and show the impact of composite additives. The compounds between the graphite layers are mainly metal chlorides such as FeCl3 and CuCl2.
Dry graphite lubricant uses
Traditional lubricating oils are generally replaced every 5,000 kilometers, and they are not resistant to high temperatures. Many institutions around the world are studying and improving them. In the past, the additives used to enhance the performance of lubricating oils were mostly sulfur and phosphorus substances, which were eventually discharged into the air and caused pollution. Graphite oil lubricant is widely recognized as the ideal solid lubricant material due to its lubricating effect and ultra-stable physical properties. Still, even in the United States, its ultrafine and specific gravity problems have not been solved, that is, the unevenness and precipitation of graphite particles problem.
In a car, the key to energy saving is the engine. It not only affects the operation of the entire vehicle but also affects the amount of fuel. Fine particles in graphite oil lubricant, which are smaller than 1 micron, quickly fill the cylinder wall. Due to scratches, cracks, etc. caused by friction, a flexible protective layer is formed on the surface of the friction pair. At the same time, the conventional metal-to-metal conflict is changed to graphite-to-graphite resistance, which significantly reduces the engine wear and provides the engine with long-term protection.
In summary, although graphite-based lubricant has been used for many years, with the advancement of material technology and the implementation of environmental protection policies, the technique of graphite-based oil is also updated continuously, and graphite will always play an essential role in the lubricant additive industry.