We have measured the frictional properties of single-crystal molybdenum disulfide in microscale contacts subject to high frequency shear oscillations. For each trial, an alumina sphere is loaded onto the surface of a 5 MHz quartz microbalance using an indenter probe.1 The surface is prepared by adhering and exfoliating a thin layer of MoS2 on the existing gold electrode with a process that preserves the high quality factor of the resonator. We monitor changes in the quartz’s resonance properties as functions of shear amplitude to quantify lateral contact forces. As the amplitude is decreased from its highest value, we observe a transition from slipping to stuck conditions at the interface associated with a maximum in the lateral elastic force. The contact stiffness at the lowest amplitudes is used to infer the contact area. We present trends in the threshold force and contact area versus load for three sphere sizes, with diameters of 50 µm, 100 µm, and 1 mm. Tests with the microspheres show that the friction on a single microscopic flake of MoS2 is four times smaller than on exposed gold regions, with coefficients of friction near 0.07 and 0.3, respectively. The shear strength of each material varies linearly with pressure. In contrast, the friction of the 1 mm sphere is much larger than that of the microspheres over the same range of loads while the shear strength remains constant. We find that this is due to the comparatively large contact areas and low pressures achieved with the largest sphere.
1 Borovsky, Bouxsein, O’Neill, Sletten, Trib. Lett. 65 (2017)