Triboemission-lubricant interactions
Creators: name: Puhan, Debashis
Publisher: name: Imperial College London
Year: 2017

description: There is need to increase the efficiency of machines in order to limit damage to the environment. The key to doing this is through the implementation of advanced materials, superior designs and better lubricants. For better formulation of lubricants, a complete knowledge of lubricant behaviour under full film and boundary lubrication conditions is a prerequisite. However, several aspects of the latter remain a mystery. For example, the pathways, by which certain anti-wear film form, are still not so clear. One possibly important factor, which limits understanding in this area, is a poor knowledge of the phenomena known as Triboemission. The aim of this PhD project is therefore to elucidate the tribological role of sub atomic particles and plasma emitted during the sliding process. To achieve this, two approaches are taken. Firstly, test equipment is developed to monitor the effects of emitted particles from a sliding contact under vacuum, ambient and submerged conditions. This staged approach is required due to the difficulty in detecting triboemission effects under submerged conditions. The second approach is to artificially introduce electrons into a sliding lubricated contact in order follow their effects. Particular focus is on non-conductive materials since these are known to produce intense emission behaviour when rubbed. The study first presents spatially resolved measurements of electron emission from Polytetrafluoropolyethylene (PTFE) when scratch tests are carried out in a vacuum tribometer, in which microchannel plates coupled to a phosphor screen are used to image electron emission. The results show that electron emission occurs at specific locations on the worn surface and these sites remain active exhibiting after emission events which decay with a time constant of up to several seconds. To study triboemission events under ambient and submerged conditions, a pin-on-disc tribometer is developed, whose transparent pin allows the contact to be viewed while sliding is occurring. This equipment is first used to detect the triboplasma caused by the discharge of air molecules on tribocharged surfaces under ambient conditions. Here, a range of polymer materials were tested (chosen due to their known ability to induce chemical reactions through rubbing) and maps showing plasma distributions were successfully obtained. Results show for the first time, the pronounced transient variations in plasma generation and have enabled a reasonably complete picture of polymer charging behaviour to be built up. Specifically, it is shown that the charge carrying ability of debris plays an important role and that electrical resistivity and friction are not the primary factors controlling charge and triboplasma generation, as previously thought. A fluorescence microscope coupled to this tribometer enabled viewing of the sliding contact submerged in a lubricant that contains a fluorescent dye and this was successfully used to detect in contact chemical changes due to the emission of charged particles. However, there is still some level of uncertainty regarding the detected chemical changes i.e. it is still a question whether the observed changes were a consequence of shear activated aggregation or electrons/radical initiated chemical reaction. This setup has potential for further improvement in order to detect and monitor chemical changes during rubbing. Finally, various drivers for tribofilm formation are studied, such as the generation of fresh surface, shearing, mixing are studied on a mini traction machine using new and existing methods. It is concluded that fresh surface generation and mixing do not play a significant role in tribofilm growth whereas shearing does improve the mechanical properties and morphology of the tribofilm. Next, shear stimulated emission from oxide surfaces were induced in order to study links between tribo-film formation and electron emission, since emission has been correlated with oxide thickness. Results show that counter specimen oxide thickness, and therefore possibly electron emission, plays a role in the initial growth of ZDDP films. However, further work is needed in this area. Another first of its kind test involved artificially stimulating electron emission from an additive/mineral oil covered steel surface with ultraviolet light to stimulate photoelectron emission. This was done in order to assess whether electron emission resulted in film growth. ToF SIMS analysis of steel surface following irradiation showed that electron emission resulted in very subtle changes to oil-steel interface. However, further high temperature UV tests confirmed that indeed electrons have the potential to cause tribofilm growth.