Extreme-Pressure and Anti-Wear Additives

When two metallic surfaces are lubricated in a hydrodynamic regime, the oil film is stable and problems of wear are not very important. In severe service, the film can be destroyed from then on the metallic parts rubbing on each other can cause first metal loss and then even the seizing of the parts by welding. 

The role of anti-wear and extreme-pressure additives is to create a solid lubricant at the interface of the metal by chemical reaction. 

These additives must thus be capable of decomposing under heat action by liberating the species that react with the moving metal or metals by creating an interphase more fusible than the metal itself. 

Examples of dithiophosphoric derivatives used as anti-wear and extremel pressure additives. 

For example, olefin sulfuri2ation products (Lubrizol, 1980), dithiophosphomolybdates (Mobil, 1980), or more simply the dithiophosphates of alcohol (Shell, 1980) whose anti-oxidant properties have been announced, are used in oil formulations for their anti-wear properties. 

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Table 4.6 Extreme pressure and anti-wear additives (14).

The use of nanoparticles as extreme pressure and anti-wear additives for liquid lubricants assumes significance. WS2 particles (average diameter 100 nm) have been shown to reduce the wear of self-mated steel shding couples by a factor of approximately two for loads below 400 N and by a factor of ten above this... 

Nanoparticles have received widespread acceptance as extreme pressure and anti-wear additives for liquid lubricants. If the nanoparticle deposits can act as a third body to help reduce asperity interactions and increase the load-bearing capacity of rubbing pairs (Rapoport et al, 2003) then action of such a third-body layer on tribocorrosion behavior will be of much interest. In addition, the possibility of using aqueous suspensions of nanoparticles as an alternative to oil-in-water emulsion is worth exploring. 

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