Tribological Properties of LB films Measured under Zero Applied Load

We examined the pull-off and friction forces between an atomic force microscope (AFM) probe and submicron-scale asperities covered with Langmuir Blodgett (LB) films. First, we used a focused ion beam to produce several kinds of asperity arrays on two silicon substrates. The average radius of curvature was 95 to 800 nm, depending on the asperity array. An LB film of stearic acid (C17H35COOH) or fluoro-carboxylic acid (C6F13C11H22COOH) was deposited on each substrate. The friction force was measured at each asperity array under a zero applied load. The pull-off force was measured at each asperity array and also at controlled relative humidities of 3 to 73%. The results showed that the pull-off and friction forces were almost proportional to the radius of the curvature of the asperity peak with both stearic acid and fluoro-carboxylic acid LB films. The gradient of the friction force against the pull-off force, which corresponded to the friction coefficient, was 0.021 and 0.14 for the stearic acid and fluoro-carboxylic acid respectively. The pull-off force increased with higher relative humidity on the fluoro-carboxylic acid. We concluded that differences in the pull-off and friction forces between the two kinds of LB films were mainly caused by differences in the stiffness of the LB film