E xtreme Pressure (EP) additives contain compounds that enter into chemical reaction with coactivating friction pair materials.In this way, new compounds are formed to create a strongly adhering lubricating film resistant to tearing and breaking.
This film increases the efficiency of the lubricating material and blocks the direct metal- metal contact between friction surfaces, and prevents adhering and sticking. For lubrication, it must be known that decreasing the coefficient of friction is not the only purpose. Lubricity (lubrication) additives form boundary films, which have higher load bearing capacity and lower friction coefficients, on friction pair surfaces by physical adsorption and chemical adsorption (chemisorption) with more polar molecules than unreinforced lubricants. Extreme pressure additives are generally effective at high surface temperatures, like the additives which prevent the surfaces from adhering.
The sudden increase in load in case of lubricating film breakage causes instantaneous extreme temperature flashes on very small surfaces. This leads to the formation of very hot local surfaces. Under these conditions, the additive is disintegrated and initiates a reaction, mostly forming an inorganic layer (particularly chloride, sulfide, phosphate compound, phosphide or eutectic with low melting point). This layer is resistant to micro-welding resistance and abrasion. As the temperature flashes die away simultaneously, the temperature drops below the decomposition temperature of additives and the formation of the layer stops. If the layer breaks, the process is repeated. This self-regulating mechanism becomes operational again only when the local temperature rises above the decomposition temperature of the additive. Lubricity additives, which are mostly organic compounds, are only effective at adsorption temperatures. These temperatures are lower in the case of physical adsorption, but higher in the case of chemical adsorption. In general, they do not exceed 150 °C. If the chemical adsorption turns into a chemical reaction with the temperature increase, the transition between the two types of additives may become continuous. This prevents the surfaces from sticking together (adhering) completely with the anti-wear effect. Both groups of additives also contribute to the reduction of adhesion and abrasion, which in turn results in a reduction in wear of the friction surfaces, particularly in sliding and rolling movements such as mixed friction, shear and high pressure. The figure shows the change in the coefficient of friction with temperature. In the figure; The curve I shows the base oil. The weak bond between the non-polar oil and the surface gets even weaker as the temperature and the friction coefficient increases. The curve II represents a material produced with an additive, for example a soap produced with a fatty acid. The friction coefficient merely changes until it reaches the softening temperature (Tm), but increases after reaching this temperature. The curve III shows that the Extreme Pressure additive greatly reduces the friction coefficient after reaching the reaction temperature (TR). At this point, the extreme pressure additive starts to react with the surface, and after the softening temperature it remains almost constant at the lowest friction coefficient value. The curve IV theoretically indicates the combination of lubricity additive and extreme pressure additives.
