A typical extreme pressure (EP) additive used in industrial lubricants, including gear oils and greases, is sulfur-phosphorus compounds. These compounds work by forming a protective film on metal surfaces under high-pressure and high-temperature conditions, reducing friction and preventing metal-to-metal contact, which can lead to wear and failure.
Sulfur-phosphorus EP additives typically contain organic sulfur compounds, such as sulfurized olefins or sulfurized esters, along with phosphorus-containing compounds, such as phosphorus esters or phosphorus sulfides. When subjected to extreme pressure, heat, and shear forces, these additives react with metal surfaces to form a chemically bonded layer known as a tribofilm.
The tribofilm acts as a solid lubricant, reducing friction and wear by providing a sacrificial layer that absorbs the stresses and prevents direct metal-to-metal contact. This helps to extend the service life of gears, bearings, and other heavily loaded components in industrial machinery.
It’s important to note that while sulfur-phosphorus EP additives are highly effective in providing protection under extreme conditions, they can also have limitations, such as compatibility issues with certain metals, corrosion concerns, and potential environmental and health considerations. As a result, the formulation and concentration of EP additives must be carefully balanced to ensure optimal performance without compromising other lubricant properties or causing adverse effects.
There are several common extreme pressure (EP) additives, but one of the most typical is molybdenum disulfide (MoS2).
Molybdenum disulfide is a lubricating compound with a layered structure. When used in lubricants, these layers shear easily, reducing friction between metal surfaces. MoS2 is particularly effective under high pressure and low-speed conditions, where the lubricating film between parts may break down.
Here are some other common EP additives:
- Zinc dialkyldithiophosphate (ZDDP): This is another popular EP additive that creates a protective film on metal surfaces under extreme pressure. However, ZDDP has some drawbacks, including environmental concerns and potential corrosiveness to some metals.
- Sulfurized olefins: These are chemically reacted organic compounds that form a protective film on metal surfaces under high pressure.
- Chlorinated paraffins: These were once widely used EP additives, but due to environmental concerns, their use has declined.
The choice of EP additive for a specific lubricant will depend on the application and the desired performance characteristics.