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Title: Triboemission mechanisms
Author: Ciniero, Alessandra
ISNI:       0000 0004 7427 715X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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Reducing mechanical friction by improving lubrication in tribological contacts is still an important goal for researches around the word. The extensive use of lubricants in applications, ranging from heavy industry to micro-biology, makes the understanding of the mechanisms by which these lubricants behave an important challenge. The fact that certain boundary lubricating films form only on contacting surfaces implies that phenomena linked to wear may play a key role in this process. One of these phenomena is known as triboemission which refers to the emission of electrons, photons and other charged particles that occur when surfaces undergo wear. Despite the possibility that triboemission events may lead to the formation of protective boundary films and also may also be responsible for lubricant degradation, the mechanism behind this phenomenon is still not well understood. In this study, spatially resolved triboemission events were analysed in a sliding contact under vacuum condition. This was achieved using an in-house developed tribometer consisting of a system of electron multipliers coupled with a phosphor screen, which allowed the in situ visualization of emission events for the first time. A range of materials were assessed in order to understand the effects of mechanical and electrical properties on the triboemission behaviour. This showed unequivocally that conductive materials show negligible emission, while those with high electrical resistivity not only produce high levels of emission but also sustained after-emission, which has not been seen before. Moreover, following the application of certain image processing techniques, a strong correlation between the spatial distribution of emission events and the topography of surface wear features was revealed. This supports the theory that triboemission is a subset of fractoemission and arises due to the uneven distribution of charge on the opposing faces of wear cracks. It also raises the possibility of using triboemission to monitor wear in real time under high magnification. In addition to this, methods were developed to differentiate between negative, positive and neutral particles, showing that each of these is emitted simultaneously but with a different spatial distribution. Surface charge was also monitored simultaneously to emission detection, showing for the first time the interactions between triboemission and tribocharging – in particular, the generation of fractures causes emission events, which reverse the expected charging trend. This may explain tribocharging mechanisms. Finally, the use of this technique to monitor coating failure was investigated. Specifically, Diamond-Like-Carbon coating specimens were assessed in vacuum and atmosphere condition revealing that coating failure can be detected in real time by observing both emission and surface charge measurements. The results presented here shed light on poorly understood triboemission mechanisms, and helps to understand how the emission interacts with the surrounding environment.
Supervisor: Reddyhoff, Thomas ; Dini, Daniele Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral