In-situ tribometry for polymer transfer film characterisation
File(s)
Author(s)
Yap, Kian Kun
Type
Thesis
Abstract
Polymers often exhibit low friction when sliding against metals, even without lubrication, making them ideal for tribological applications where the use of liquid lubricants is unfavourable. This low friction property is generally believed to be associated with the transfer of polymers to the metal counterface during shearing, forming lubricious transfer films. However, this process can lead to increased wear rates for polymers. This thesis explores the development and application of various in-situ pin-on-disc tribometry techniques to enhance the understanding and tribological performance of polymer transfer films. Spatiotemporal mapping and in-situ high-bandwidth microscopy were developed to study a polytetrafluoroethylene (PTFE)/stainless steel system. Insights from these in-situ techniques have enabled the achievement of near ultralow wear rates in the order of 10-6 mm3/Nm for unfilled PTFE. This is accomplished by promoting the formation of wear-reducing tribofilms on PTFE through humidity control and steel counterface modification. Such low wear rates were previously only attainable with fillers, which unfortunately reduce PTFE’s recyclability. Additionally, this thesis explores the feasibility of in-situ PTFE transfer film thickness measurement using optical profilometry. Traditionally, this measurement assumes identical refractive indices for transfer films and their bulk polymers. However, it was found that transfer films have higher refractive indices, leading to significant measurement errors under the traditional assumption. This thesis also investigates the tribology of polyimide (PI), a potentially more environmentally friendly alternative to PTFE. PI generally exhibits excellent wear performance across a wide temperature range, but its friction performance at room temperature requires improvement. The study identifies the occasional formation of back-transfer films on the PI surface at room temperature. Although these films are unstable, they effectively reduce friction and offer potential for improving PI’s friction performance in future applications. The outcomes of this thesis contribute to the advancement of polymer tribometry and support the development of sustainable polymers.
Version
Open Access
Date Issued
2024-07-19
Date Awarded
01/11/2024
Advisor
Masen, Marc
Wong, Janet
Sponsor
Imperial College London
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)