Effect of water on the performance of lubricants and related tribochemistry in boundary lubricated steel/steel contacts

The objective of the overall project is to assess the effect of water on the performance of lubricants and related tribochemistry in boundary lubricated steel/steel contacts. The effect of water on the bulk properties of the lubricant, the tribological performance of lubricants and the mechanism of tribofilm formulation are the focus of this project.
The tribological performance of different lubricants is evaluated by a ball-on-disc test rig as well as a micropitting rig under boundary lubrication and with different levels of humidity. Selected post test samples are analysed to study the reactions occurring during the test and the composition of the tribofilm. The surface analysis techniques used are Fourier Transform Infrared (FTIR), Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS).
The results of this thesis show that water can change the bulk properties (viscosity and TAN) of some lubricants in specific conditions dependent on the water level in the lubricants. The increase of relative humidity always increases wear of the components no matter whether under pure sliding or rolling-sliding conditions, but addition of water in the oil before the tribological test will not always increase wear. It is also observed from XPS results that there is a direct relationship between the oxygen concentration as oxide on the contact surfaces, the layer thickness on the wear scar of post test ball/roller and relative humidity. Also, it is revealed from the results that not only physical parameters but also chemistry must be considered to evaluate the micropitting behaviour, which is found by XPS analysis to be related to the chain length of the phosphate as well as the oxygen concentration as oxide on the wear scar of the surfaces.