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Lubricant additives refer to additives that reduce the coefficient of friction of lubricants under boundary lubrication conditions. In the fluid lubrication state, the surface of the friction pair is entirely separated by the lubricating oil film. The friction force is proportional to the viscosity of the lubricating oil. When the viscosity increases, the viscous resistance increases. However, if the thickness of the lubricating oil is too low, or the load on the friction pair is high, the lubricant film between the friction pairs is fragile, and the fluid lubrication state cannot be maintained. The friction force has nothing to do with the viscosity of the lubricating oil but only depends on the chemical properties of the lubricating oil, that is, the surface-active substances present in the lubricating oil.
Since most of the original surface-active materials in the lubricating oil are refined and removed during processing, the best friction modifier needs to be added to improve the lubricating performance of the refined oil. The lubricant friction additive is based on this principle. Through the chemical action of Lubricant additives, the micro convex portions of the friction surface are softened and smoothed, or the additives are adsorbed on the surface of the friction pair, and the micro concave portions are filled to improve the oil film state on the surface of the friction pair.
To reduce the friction under severe boundary lubrication conditions, the purpose of increasing power, reducing fuel consumption, and reducing wear is achieved. Filling the depression with friction additive can also improve the sealing of the combustion chamber of the engine and maintain the proper technical condition of the engine.
Dry lubricant additives
The robust suspension type was developed and applied earlier, and it is dispersed in the lubricant in the form of particles. The most famous solid suspension additives are molybdenum disulfide, graphite, and Teflon oil additive. Molybdenum disulfide and graphite are multilayer lamellar crystals. They have a strong adsorption force on the metal surface. Still, the interlayer shear resistance is feeble, so they have an excellent anti-friction and anti-friction effect in the boundary lubrication state. Resistant to high temperatures. teflon oil additive is generally considered to be unable to form an adsorption film on the metal surface. Still, its particles can accelerate the running-in of the friction pair surface under boundary lubrication conditions, improve the smoothness, and reduce friction.
The disadvantage of dry lubricant additives is that the additive particles may be aggregated and delaminated during storage, and its anti-friction effect will be reduced. At the same time, the lubricant will become cloudy, the color will be darkened, and the appearance will be damaged. Most critically, there is also the possibility of particles clogging the filter or being separated from the lubricant by a centrifugal filter.
Due to the inherent disadvantages of Dry lubricant additives, such additives have been gradually phased out in recent years. Vigorously develop oil-soluble additives that are entirely compatible with lubricating oils. Most of these additives are polar or active oil-soluble macromolecular compounds. The stronger the polarity of Oil additives, the longer the hydrocarbon chain and the lubricating ability.
The adsorption force of molecules and metals decreases with increasing temperature. When the oil temperature reaches 150 to 200 ° C, the polar molecules fall off, and the adsorption film is destroyed and fails.
In recent years, oil additives have used compounds such as organic molybdenum, organic tungsten, natural boron, and ashless organic esters as friction modifiers, which not only have anti-friction properties at ordinary temperatures but also can react chemically and improve at high temperatures. The condition of the friction surface makes the friction surface smoother, and the friction coefficient is about 0.05.
It often has anti-wear effects, but it is different from anti-wear and extreme pressure anti-wear additives. Anti-friction additives are used to form anti-friction adsorption film or friction improvement film under boundary lubrication conditions. The role of anti-friction additives or extreme pressure agents is that in the case of a lubricating film rupture, the active chemical elements (sulfur, phosphorus, etc.) contained in it will chemically react with the metal to form an anti-wear protective film, preventing the friction surface from sticking, seizing or sintering. The anti-wear film can support higher loads than the boundary lubrication film, but the friction coefficient is much larger than the boundary lubrication film.
For automobiles, the boundary lubrication mainly occurs in the piston ring of the engine and its corresponding cylinder liner at the top dead center, and the surface of the main reducer gear. Besides, the crankshaft bearings and connecting rod bearings will experience boundary lubrication when the engine starts, stops, and accelerates under heavy load or suddenly loads at a certain speed; camshafts and lifters, rockers and valves of high-compression engine valve mechanisms When the tappet is running under heavy-duty, the friction surface is also in the state of boundary lubrication. Therefore, adding appropriate anti-friction additives to automobile lubricants is an effective measure to reduce friction and save fuel. The fuel-saving rate is generally between 1% and 4%. Extreme pressure anti-wear agent refers to an additive that can form a high melting point chemical reaction film with the metal surface under high temperature and high-pressure boundary lubrication conditions to prevent melting, seizure, and scratching. Its role is to react its decomposition products with metals at high friction temperatures to produce compounds with lower shear stresses and melting points than pure metals, thereby preventing contact surface seizure and welding and effectively protecting metal surfaces. Extreme pressure anti-film agent is mainly used in industrial gear oil, hydraulic oil, guide rail oil, cutting oil and other oils with excessive pressure requirements to improve the extreme pressure and wear resistance of oil products.