Friction is independent of time and contact area in quartz

Nir Z. Badt, David L. Goldsby, Christopher Walker, & George M. Pharr

Submitted September 11, 2022, SCEC Contribution #12293, 2022 SCEC Annual Meeting Poster #147

Friction between two rock surfaces in slide-hold-slide (SHS) experiments has been shown to increase with the duration of hold time, where the surfaces are held in stationary or quasi-stationary contact prior to re-sliding. Rock analogues demonstrate time-dependent contact area growth, leading to the interpretation that the dependence of friction on the hold time for rocks is due to an increase in the real area of contact - contact ‘quantity’ - with time (or slip). Recent studies, however, suggest that contact ‘quality’ (e.g., the density of chemical bonds on a frictional interface) may also be important in rock friction. Here we present results from SHS experiments on single crystals of quartz and calcite conducted with a nanoindenter. In these experiments, samples were indented with a diamond Berkovich tip to a load that resulted in an indentation depth of 200 nm. This load was then held constant while the lateral load was ramped to induce sliding at 1 μm/s. The sliding velocity was then set to zero for holds 1, 10, 100 and 1000 s in duration, followed by reloading to a velocity of 1 μm/s. Contrary to previous SHS experiments on quartz rocks, our tests on quartz show no dependence of peak friction after the holds on hold time or contact area. During the holds, contact area increases with time (or observed slip), as evidenced by the increase in measured contact stiffness. During the reload to sliding, however, contact area decreases from a maximum obtained during the hold before reloading, to a diminished value at the point when the peak friction is measured. In contrast, calcite, a much ‘softer’ material than quartz, shows a dependence of peak friction on time and contact area. These results suggest that compared to ‘softer’ materials like calcite and rock analogues (e.g., acrylic plastic), contact quality may control sliding friction for ‘harder’ materials like quartz. Our study provides a rationale for exploring quartz-on-quartz and other like-on-like sliding contacts, using the same nanoindentation apparatus, to determine the relative contributions of contact quality and contact quantity on rock friction behavior.

Key Words
quartz friction, contact area, hold time

Badt, N. Z., Goldsby, D. L., Walker, C., & Pharr, G. M. (2022, 09). Friction is independent of time and contact area in quartz. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)