title: Studies on the friction of structured interfaces, the static friction peak in reciprocating sliding and the role of friction in triboelectrification
Creators: name: Bin Jaber, Saad
Publisher: name: University of Glasgow
Year: 2023

description: This project investigates the frictional behaviour of contacting surfaces at different test conditions to better understand the contact mechanism. It aims to implement novel techniques and theoretical approaches to acquire more control over the friction response of contacting surfaces. It has been suggested that surface structuring is beneficial for controlling the friction response. This project utilises photochemical etching (photolithography) to produce micro-trenches (square-wave features). Five different sample categories were prepared with different micro-feature widths. These structured features were placed in contact with unstructured silicon surfaces. The silicon samples possess idealised surface conditions with sub 1 nm roughness and flatness of 1 µm. The tests were carried out on a bespoke friction rig. This research topic is directed to examine the applicability of Amontons’ first and second laws of friction on these structured features with nanoscale roughness. It has been found that the COF of these structured surfaces is independent of the manipulation of the feature contact area and normal load. This means that the real contact area was not varied with the different feature contact areas. To validate this assumption, two theoretical approaches using the Bush, Gibson and Thomas (GBT) model and Boundary Element Method (BEM) were implemented. These approaches estimate the real contact area by inserting experimental parameters and roughness data of the contacting surfaces. The models’ results showed that the real contact area was independent of the changes in the feature contact area. The experimental and model results indicate that the nanoscale roughness on the raised features was sufficient for the friction laws to be obeyed. The second part of the thesis concerns the static friction peak in reciprocating sliding. It has been noticed that the peak has not been observed in the studies of friction hysteresis loops. The purpose of studying this aspect of friction is to determine the reason for the absence of the static friction peak in commonly published hysteresis loops. Experiments were performed on the unstructured silicon samples using the friction rig. The tests showed that the peak was apparent in the initial cycles. It then started to decay towards the end of the test. It is believed that sliding roughened the surfaces resulting in a lower real contact area. The reduction in the real contact area decreased static friction towards kinetic friction leading to burying the peak. This assumption was verified by an adhesive contact model. It is worth noting that the only variable in the model was the roughness data, to inspect the impact of wear through changes in surface roughness. The model showed a similar trend as the experiment indicating that wear was responsible for the disappearance of the peak. The reason for not observing the peak in the literature is most probably because the reported data was recorded well after the peak is buried. The final research topic aims to investigate the frictional and electrical behaviour of triboelectric layers via performing a triboelectric nanogenerator study (TENG). There has been a lack of studies on the role of friction in triboelectrification. To perform the tests, electrical measurements were incorporated into the friction rig to simultaneously measure friction and electrical output to find whether friction can be correlated with the electrical output. Another main goal of this topic is to systematically compare the electrical output of the two main modes of TENG (sliding and vertical contact separation modes). In a sense, a direct comparison of contact electrification and triboelectrification for exactly the same material pair and test conditions etc. was performed. The friction and output voltage showed almost the same trend suggesting that they are influenced by the same source. It has also resulted that the existence of friction in the sliding mode was advantageous. It helps acquire more triboelectric charges giving rise to higher outputs. It should be noted that triboelectric layers in the sliding mode are susceptible to wear requiring careful attention and future study.