Paper TR-ThP4
Tribological Investigations of Octadecylphosphonic Acid (ODP) and Octadecyltrichlorosilane (OTS) Self-Assembled Monolayers: A Comparative Study of MEMS-type Interfaces

Thursday, November 3, 2011, 6:00 pm, Room East Exhibit Hall

Microelectromechanical systems (MEMS) are critically-limited by interfacial phenomena such as friction and adhesion. One strategy to reduce friction between MEMS surfaces is to coat them with molecularly-thin self-assembled monolayer (SAM) coatings. Historically, silicon MEMS have been coated with silane-based SAMs, such as octadecyltrichlorosilane (OTS). However, continued progress in the development of MEMS may require new material systems to be employed. Therefore, in this study, we have investigated the frictional properties of octadecylphosphonic acid (ODP) monolayers deposited on aluminum oxide surfaces, across speed regimes. Measurements using an atomic force microscope (AFM) and separately using a nanoindenter-quartz crystal microbalance system were performed each with a microsphere-terminated probe. This allows for a comparative study between different velocity regimes using contacts with similar sizes, pressures, surface roughnesses, and interfacial chemistries. AFM colloidal probe friction measurements indicate that for a bare tip sliding on various substrates, ODP-coated alumina surfaces exert a lower friction force than either bare or OTS-coated alumina substrates. We also observed strong evidence of transfer of the ODP molecules to the tip when the tip is uncoated. The results presented in this study are significant contributions towards our goal of better understanding the frictional properties of phosphonate SAMs in pursuit of alternative MEMS materials.