How Anti-wear (AW) Agents work?

These additives are typically used to protect machine parts from wear and loss of metal during boundary lubrication conditions. They are polar additives that attach to frictional metal surfaces. 

They react chemically with the metal surfaces when metal-to-metal contact occurs in conditions of mixed and boundary lubrication. 

They are activated by the heat of contact to form a film that minimizes wear.  They also help protect the base oil from oxidation and the metal from damage by corrosive acids. 

These additives become “used up” by performing their function, after which adhesive wear damage will increase.  They are typically phosphorus compounds, with the most common being zinc dialkyldithiophosphate (ZDDP). 

There are different versions of ZDDP — some intended for hydraulic applications and others for the higher temperatures encountered in engine oils.  ZDDP also has some anti-oxidant and corrosion-inhibition properties.  In addition, other types of phosphorous-based chemicals are used for anti-wear protection (e.g., TCP).