Lubricating oil fails to maintain hydrodynamic lubrication under high temperature and high pressure condition, and seizure and welding are likely to occur in the severe condition. To prevent these catastrophic events, extreme pressure agents (EP agents) that produce tribo-film on the metal surface are indispensable. Chlorine, phosphorus, and sulfur are three major types of EP agents, and the use of these EP agents is decided according to their characteristics. In this paper, we discuss the characteristics of sulfur type EP agents and the behavior of its tribo-film in oil.
