Characterization of Surface and Frictional Properties of Two-Component Alkyltrichlorosilane Monolayers

To investigate the affect of chain mobility and functional group exposure on friction, we report the tribological properties of two-component monolayers composed of long and short chain alkyltrichlorosilanes on silicon. The tribological properties of these monolayers were measured with a ball-on-flat tribometer at various loads and 0.1 mm/s speeds. The ratio of long and short chain molecules was varied, creating different levels of mobility and average chain cant in the upper liquid region of the film. Once a critical thickness is reached, the tribological properties for the mixed monolayers were indistinguishable from the pure, one-component monolayers although the oleophilicity was much higher for the two-component films. Two-component monolayers created from a combination of shorter perfluoroalkyl silanes and longer alkylsilane, however, showed tribological properties that were dependant on molecular mobility and the applied load. Collectively, the tribological behavior can be explained by the differences in interfacial free energies between hydrocarbon chains near hydrocarbon versus fluorocarbon surfaces, in addition to local repulsion between the hydrocarbon and fluorocarbon chains as quantified through molecular simulations.