IR spectroscopic analysis of grease lubricant films in rolling contacts

In this paper, the composition of grease lubricant films formed in rolling contacts has been studied using IR reflection spectroscopy. A simple ball-on-flat test arrangement has been used. Greases are two-phase lubricants composed, primarily, of a thickener dispersed in a basestock. The lubricant film in the contact region however, does not necessarily reflect the bulk composition, but will depend critically on the mechanism of lubricant supply to the contact. If flow of bulk grease into the contact occurs then both thickener and base oil will be present. The grease structure however will be heavily degraded by-the very high local shear stresses as it passes through the contact. If the contact is starved of bulk now then film generation is due to local base oil flow from the grease reservoir. In this case, the separating film is primarily base oil. Thus the composition of the film depends on the lubricant supply and is likely to change with operating condition and time. In this work, the composition of grease films under both fully flooded and starved conditions has been studied using IR reflectance spectroscopy. This technique provides information on the concentration and distribution of the grease components both within the track and in the reservoir at the sides; Under fully flooded conditions both thickener and base oil are present in the inlet, although the thickener concentration varies depending on speed and proximity to the Hertzian zone. Under starved conditions there is an initial deposition of bulk grease in the track as fresh grease is overrolled at the start of the experiment. As the test continues however, the grease structure is broken down and base oil is expelled from the track. There is also evidence of shear degradation and oxidation of the thickener within the track. The condition of the grease dose to the track also changes as the test proceeds and there are local thickener concentration changes in this region. These findings are discussed in the light of current theories of grease lubrication.