Surface and frictional properties of two-component alkylsilane monolayers and hydroxyl-terminated monolayers on silicon

To investigate the effect of chain mobility and functional group exposure on friction, the tribological properties of one-component (pure) and two-component (mixed) alkylsilane monolayers on silicon, with variance in the ratios of long- and short-chain molecules, are presented. The tribological properties of these monolayers were measured with a ball-on-flat tribometer at 9.8 mN load and a speed of 0.1 mm/s. The molecular precursors investigated were n-hexyltrichlorosilane (C6), n-dodecyltrichlorosilane (C12), n-hexadecyltrichlorosilane (C16), n-octadecyltrichlorosilane (C18), and n-docosyltrichlorosilane (C22). Created from these molecules were the following four mixed monolayers: C6/C18, C12/C18, C16/C18, and C12/C22 in which the ratio of long- and short-chain molecules was varied in order to create different levels of mobility and average chain cant in the upper liquidlike region of the film. For the two-component monolayers, once a critical thickness is reached, the tribological properties were indistinguishable from the pure monolayers although the oleophilicity was much higher for the mixed films. Altering the terminal group of the monolayer to a hydroxyl, however, resulted in an increased coefficient of friction and significant adhesive forces between the monolayer and the stainless steel probe of the microtribometer. This tribological behavior can be explained by the low surface energies of the methyl and methylene groups and the increased adhesion that occurs with hydroxyl-terminated monolayers due to stronger interfacial interactions. Collectively, the results indicate that monolayer thickness and surface energy greatly affect the tribological properties of a monolayer but that molecular mobility within the film and interfacial oleophilicity are less important at these loads and speeds.