Humidity dependent frictional properties of single crystal quartz at low to high slip velocities

• Dynamic friction of quartz decreases drastically at >∼5 μm/s under dry conditions. • Dynamic friction of quartz would be higher if measured under humid conditions. • Growth of capillary water has a significant influence on quartz friction. The frictional properties of synthetic quartz were investigated under humid (relative humidity, RH of 5–80 %) and nominally dry (≤3 % RH) conditions at a constant normal stress of 1.5 MPa and at various slip velocities of 0.005–105 mm/s. In slide–hold–slide tests performed after an initial sliding phase at 105 mm/s, the amount of frictional aging was found to increase with RH. The steady-state friction of quartz shows negative velocity dependence; steady-state friction decreases for an increase in slip velocity under all tested humidities. Notably, steady-state friction increases with RH in the specific range of velocity from ∼10 μm/s to 1 mm/s. The steady-state value of friction in quartz under humid conditions can be viewed as a combination of its intrinsic dry friction and the increased friction resulting from moisture-assisted strengthening mechanisms. The humidity dependence observed for aging and steady-state friction, respectively, is pronounced at low humidity levels (≤∼20 % RH) but becomes moderate at higher humidity levels (>∼20 % RH) for both cases. This trend suggests that similar fundamental frictional mechanisms govern the humidity dependence of frictional aging and steady-state values of friction at slow- to high-velocities. Transmission electron microscopy of the fault zone under steady-state sliding shows that fault-zone material mainly consists of amorphous silica, domains of which are composed of ∼0.5 μm-thick ultrathin layers. The formation of capillary water bridges around asperities with nanometer-scale surface roughness composed of amorphous silica grains might explain the humidity-dependent friction behavior of quartz.