Effect of Magnetic Field on Tribological Properties of Lubricating Oils with and without Tricresyl Phosphate

Tribological properties of neat 150 SN mineral base oil and the oils doped with different contents of tricresyl phosphate (TCP) under magnetic field or non-magnetic field were evaluated on a four-ball tribotester, on which an external magnetic field was applied. Furthermore, the morphology and the tribochemical characteristics of the worn surfaces were examined with a scanning electron microscope (SEM) and an X-ray photoelectron spectrograph (XPS). The tribological test results showed that the magnetic field improved anti-wear properties but impaired the friction-reducing properties of neat base oil and the TCP-doped oils. The worn surfaces tested in magnetic field were characterized by a slighter wear than those tested in normal condition, especially for the surfaces lubricated with the TCP-doped oils. Furthermore, the results of XPS analysis indicated that tribochemical films on the surfaces tested with TCP-doped oils were mainly composed of ferric-containing compounds such as Fe2O3, Fe3O4 and FePO4. Under the influence of a magnetic field, the atomic concentrations of oxygen and phosphorous in the frictional sub-surfaces were higher than those without magnetic impact. Thus it can be inferred that the improved anti-wear properties and impaired friction-reducing capabilities of lubricating oils with TCP under a magnetic field were related to the diffusion of phosphorus and oxygen into the substrate induced by magnetism.