Phosphate esters, thiophosphate esters and metal thiophosphates have been used as lubricant additives for over 50 years. While their use has been extensive, a detailed knowledge of how they work has been a much more recent development. In this paper, the use of phosphate esters and thiophosphate esters as anti-wear or extreme pressure additives is reviewed with an emphasis on their mechanism of action. The review includes the use of alkyl phosphates, triaryl phosphates and metal containing thiophosphate esters. The mechanisms of these materials interacting with a range of iron and steel based bearing material are examined.

One way to improve fuel efficiency in today’s jet aircraft engines is to create an environment for higher operating temperatures and speeds. New and improved lubricants and bearing materials must be developed to remain stable in these elevated operating temperatures. Three lubricants, with varying amounts of tricresyl phosphate added as an anti-wear/extreme pressure additive were tested on two different stainless steels at varying temperatures ranging from 300 °C to 350 °C in vacuum. Significant decomposition of the lubricant base-stocks and the phosphate ester additive did occur in most of the trials resulting in the formation of carboxylic acids and phenols. In these cases a film containing phosphorus was deposited onto the stainless steel substrate

The need for higher performance in jet turbine engines has led to the development of alloys with greater hardness for use in bearings. The increased hardness is a result of carburizing of the surface which leads to a change in the surface chemistry through the formation of metal carbides. The authors have found that metal carbides react differently with the lubricants and additives than the oxides found on the surface of currently used alloys. Of particular concern is the observed reaction of the ester basestock with metal carbides in the presence of phosphate esters. The authors have found that metal carbides can catalyze the decomposition of hindered esters in the presence of aryl phosphates to give the component acids. Products of the reaction include free acids, phenyl esters, and dehydrogenated esters. This reaction is expected to rapidly degrade the performance of the oil due to both acid formation and viscosity breakdown. Examination of the remaining solid material indicates that phosphorus has been incorporated into the structure, presumable as phosphate.