| dc.description.abstract | In recent years, the goal of understanding lubrication on the nano/microscale has gained prominence due to the rise of microelectromechanical systems (MEMS) that have applications in microfluidics, sensors, and switches. Traditional lubricants are insufficient at the length scales of microscale devices due to the high strain rates (~107 s-1) that components can be subjected to and the greater importance of surface forces. Here, I have used several molecular level approaches, including monolayers with various surface groups and ionic liquid films, to minimize the friction and wear between two surfaces on the microscale. Monolayers are prepared from the assembly or adsorption of special classes of molecules onto a substrate to generate a wide range of interfacial properties that can be tailored by manipulating the composition of the precursor molecule. I have found that at low loads (10 mN) methyl-terminated n-alkanethiolate self-assembled monolayers (SAMs) exhibit a 3-fold improvement in coefficient of friction over SAMs with hydroxyl- or carboxylic acid-terminated surfaces. For monolayers prepared from both n-alkanethiols on gold and n-alkyl trichlorosilanes on silicon, a critical chain length of at least 8 carbons is required for beneficial tribological performance at an applied load of 10 mN. I show that the durability of monolayers derived from n-alkyltrichlorosilanes on silicon increases exponentially with the chain length of the silane precursor and that monolayers with hydroxyl surfaces exhibit reduced stabilities due to stronger tip-surface interactions. Evidence for disruption of chemisorbed alkanethiolate SAMs with chain lengths n ≤ 12 is shown through EIS analysis of tribology wear tracks, and X-ray photoelectron spectroscopy (XPS) was used to show that tribological damage to monolayer films formed from alkyl trichlorosilanes consisted of the loss of molecular components from the surface. For monolayers derived from n-octadecyltrichlorosilane, a critical load was identified to be approximately 250 mN (200 MPa), above which failure of films occurred within 100 cycles of testing. Monolayers with the capability for cross linking exhibited much greater stabilities than monolayers where cross linking was limited or prevented. Collectively, these results demonstrate that the mechanical durability of monolayers when subjected to a tribological load is greatly enhanced by maximizing dispersion interactions among chains, achieving cross linking, and minimizing tip-surface interactions. | |
