Steve Vilt Thesis Defense

Graduate student Steve Vilt will defend his Ph.D thesis on August 2nd 2011 at 10 am in 135 Olin Hall.


Investigation of Interfacial Composition, Structure, and Topography on Tribometric Friction

Future innovations in a wide range of fields, from telecommunications to biomedical devices, will further push the boundaries of miniaturization and create a greater demand for microelectromechanical systems (MEMS). In MEMS devices, the operation of scale and large surface-to-volume ratio create unique tribological issues as surface forces rather than bulk forces begin to play a dominant role in device performance. Traditional lubrication strategies, such as oils, are insufficient for MEMS as the microscale dimensions prevent viscous liquids from fully integrating into the device and do not provide ample wear protection or allow proper functioning. This thesis focuses on advancing microscale friction by investigating the interfacial interactions of various thin film systems and developing novel lubrication schemes. Two- component mixed monolayers, in which the two components have different chain lengths, represent a way to integrate a mobile, liquid-like layer into a bound film. By altering the ratios of long and short chain molecules and inserting fluorocarbons into the film, I have investigated the lubrication effect of chain mobility and functional group exposure on monolayer friction. For mixed monolayers containing all hydrocarbons, once a critical thickness is reached, the tribological properties were indistinguishable from the pure monolayers, indicating that molecular mobility within these films and interfacial oleophilicity are insignificant. When a fluorocarbon base layer was present, however, load-dependent frictional properties were observed, which is explained by the mobile hydrocarbon chains resisting interaction with the fluorocarbon base layer. To gain deeper insight into the influence of adhesion, topography, and probe surface on microscale testing, I tested various substrate films with a variety of functionalized probes in several testing mediums. Collectively, the results show that adhesion and the testing medium are significant for high-energy surfaces, while probe topography strongly influences polymer friction. Once a well-formed hydrocarbon monolayer is present on the surface though, the influence of the probe surface and testing medium is insignificant. Lastly, I present a fundamental study on the frictional performance of a microscale ball bearing system and demonstrate that low friction rolling can be established at this size scale, and I identify critical parameters, such as the surface coverage of the spheres, which need to be satisfied for effective lubrication.