# Molecular Science of Lubricant Additives +
## 2 Individual Lubricant Additives and Their Working Mechanism
### Tribo-Improvers
#### 2.1.3 Extreme Pressure Additives (EP)  
Formerly, these were so-called as load carrying agents. When the tribological conditions become severe, at higher load and/or higher temperature, welding of the material may occur. EP can prevent such failure of materials upon rubbing contact. Their functions are unique compared to FMs and AWs. EPs first react with the surface, which is analogous to AWs. However, the product does not protect the surface but mildly removes the atoms on the uppermost surfaces (Figure 7). The reaction of an EP molecule (dialkyl disulfide in this case) with the atom\(-\,\alpha\) on the surface is associated with migration of a bonded-electron from the material, as depicted by a red arrow. It weakens the interaction of the atom\(-\,\alpha\) with other atoms in the material resulting facile migration of atom\(-\,\alpha\) from the material. The wear process releases the mechanical energy at the contact point, otherwise the accumulated energy can induce serious damages to the tribo-materials. The function is so-called \"controlled wear mechanism\". This is closely related to the mechanism of \"corrosive wear\" or \"chemical wear\" where certain chemical substances promote the wear of material. Contrary to the other wear processes such as abrasive and adhesive wear, corrosive wear produces relatively smooth surfaces (less surface roughness). Chemically aggressive substances can induce corrosive wear. Similarly, EPs are relatively reactive with surfaces are compared to substances for FMs and AWs [65].  
It is empirically known that EPs can help the running process in an efficient way. This could be understood by the smooth removing of the initial asperity on the surfaces. In some cases, EPs can provide \"rescue tasks\" when the machine elements are under fatal conditions with severe wear. EPs are expected to lower the contact stress through the corrosive wear. This can make the lubrication regime milder. However, the effect should not last.  
Contrary to the extensive academic researches on FMs and AWs, scientific approaches to EP functions are still fewer, mainly due to the difficulties of analysis. Evaluation of EP such as ASTM D2783 results in welded surfaces with relatively poor repeatability.  
In summary, all tribo-improvers work on the surface under dynamic processes. In other words, the mechanical energy continuously stresses the rubbing contact. It damages the surface more or less, resulting in wear, for example. Here, the importance of recovering the tribo-film has arisen, as pointed out in Figure 8. As discussed supra, the function of all tribo-improvers start with the interaction to the rubbing surface. Then the intermediate material (the transition state) is converted to the tribo-film through adsorption or tribo-chemical reaction, according to the lubrication conditions. When the machine works under high wear rate conditions, the regeneration process should be done quickly. An increase in affinity of the additive to tribo-materials, or an increase in reactivity under the tribological conditions is the usual method of lubricant chemistry. An increase in robustness of the tribo-film is challenging. Multi-functional tribo-improvers are possible by a proper combination of each moiety in FM, AW, and EP molecules (Figure 9).  
Figure 8: The importance of dynamic process for tribo-improvers.  
Figure 7: Working mechanism of EP (controlled wear).