Study on Reaction Mechanism of Sulfur and Phosphorus Type Additives Using an Acoustic Emission Technique

Sulfur and phosphorus additives are used in lubricants as extreme-pressure and antiwear agents, which are typically used together to ensure reliability over a wide range of lubrication conditions. However, the working mechanism of the combined additive system has not been clearly defined due to difficulties obtaining information on the material surface where these additives work. This is because this surface is constantly being worn during testing. Therefore, in situ analysis applying an acoustic emission (AE) technique was proposed. AEs are elastic stress waves generated during the deformation and fracture of solids, which can be measured in real-time, providing information with respect to the magnitudes and types of damage. In this paper, an application of the AE helps to clarify how each additive acts on the surface in real-time. The working mechanism to understand improved reliability using both sulfur and phosphorous additives was investigated by the AE technique, along with conventional surface analysis methods. It is concluded that wear reducing properties were improved by the reaction of sulfur additives to remove the protruded parts, followed by the reaction of phosphorous additives to form a protective antiwear film.