Lubricating oil can form a film on the surface of objects to prevent or reduce scratches or damage caused by friction, and the viscosity and thickness of the oil film greatly affect the efficiency of equipment. To achieve better lubrication and wear resistance while keeping the oil film economical, adding suitable friction modifiers is necessary. Friction modifiers generally refer to additives that can enhance the anti-wear performance of lubricating oil, effectively reduce the friction coefficient of contact surfaces, and thus reduce friction and wear. Based on their development history and mechanism, they can be briefly divided into fatty agents, normal anti-wear agents, and extreme pressure anti-wear agents. Fatty agents usually refer to early products like animal and vegetable oils and fatty alcohols, which work by forming an adsorbed film on the metal surface to reduce friction and wear while providing some extreme pressure properties. The distinction between normal anti-wear agents and extreme pressure anti-wear agents in practical use is not very clear, and their mechanisms are basically the same, but the required anti-wear effect and operating environments differ. Extreme pressure anti-wear agents have stricter requirements, such as for heavy-duty vehicles under low-speed, high-torque conditions, which must use high-performance lubricating oils with extreme pressure additives, whereas small vehicles operate more gently and have lower extreme pressure demands. Similarly, in some high-temperature, high-speed conditions, normal friction modifiers cannot keep up with the film formation and reaction rates on the metal surface, easily causing wear or even welding, so specific or special extreme pressure additives are required. All these factors have driven the improvement and development of extreme pressure anti-wear agents.

Up to now, extreme pressure anti-wear additives for lubricating oil and ester products are classified according to their active ingredients into several main categories: Chlorine-containing, with typical products including chlorinated paraffins and chlorinated fatty acids. These were widely used in the early days due to their good extreme pressure performance and low cost, especially in vehicle gear oils and metalworking oils. However, their downsides are obvious, as their extreme pressure and anti-wear performance can be affected by high temperatures, and their chlorine content can cause corrosion and environmental issues. In recent years, their use in various oils has significantly decreased. Sulphur-containing, including mainstream products like sulphurised olefins and sulphurised esters, have better friction and extreme pressure resistance than ordinary fatty agents, with balanced overall performance and relatively strong high-temperature tolerance. They are now commonly used in automotive and industrial gear oils, greases, and cutting fluids. Phosphorus-containing, including phosphate esters, phosphite esters, and ammonium phosphate esters, show differences in film formation speed and film strength compared to sulphur-containing additives. They perform better on rough metal surfaces than other types but generally have lower high-temperature resistance and are usually used in combination with other extreme pressure additives for better results. Organometallic extreme pressure additives, such as zinc dialkyldithiophosphate, combine anti-wear, extreme pressure, and antioxidant properties and are widely used in most oil types. However, due to limited high-temperature tolerance and the potential release of toxic gases during processing, temperature control is crucial. Other organometallic extreme pressure additives, like molybdenum dithiocarbamate, are often combined with other additives for lubricating greases, such as extreme pressure lithium greases and compound lithium greases. These are mature technologies and essential raw materials for lubricating greases.

Traditional extreme pressure anti-wear agents generally contain elements like sulphur, phosphorus, and chlorine, which are harmful to the environment. As environmental awareness grows and laws and regulations improve, the use of traditional extreme pressure anti-wear agents in oils and additive packages formulations is increasingly restricted and may gradually be replaced by environmentally friendly alternatives. According to related information, research and usage of eco-friendly extreme pressure anti-wear agents have been increasing in recent years, including categories like organic esters, organic acids, and their derivatives. Additionally, additives like borate compounds and graphite-containing anti-wear products have already become or are set to become important components of the extreme pressure anti-wear agent market in certain scenarios.

When it comes to practical use, apart from checking whether the extreme pressure anti-wear additives meet the performance requirements of lubricants and comply with environmental laws, there are several other key things users need to think about or test. One is how well they get along with the base oil. Traditional EP anti-wear additives usually work well with Group I base oils, but with more and more use of Group II or even Group III and higher, compatibility can become a bit tricky. Another thing to watch is the possible downsides of the additives in action. For instance, sulfur at high temperatures, especially if there's too much, can corrode copper-containing metals and might negatively affect the base oil's oxidation stability. This means technicians will need to test, tweak, and check formulations repeatedly. On top of that, there's the cost factor. In a commercial setting, a good additive formulation has to balance both affordability and practicality. (The author’s knowledge is limited, so any shortcomings or errors are kindly pointed out by readers.)