Peak Friction Does Not Depend on Contact Area in Quartz
Abstract
The size and number of stationary and quasi-stationary frictional contacts in rock analogues have been shown to increase with time due to deformation of asperities. This increase in the real area of contact, or contact 'quantity', has been suggested to control friction in sliding contacts in rocks. Recent studies, however, find that contact 'quality', e.g., the density of chemical bonds on a frictional interface, may also be important in sliding friction. Here we present results of slide-hold-slide experiments on single crystals of quartz and halite conducted with a nanoindenter capable of applying a constant normal load and ramping to a lateral load that induces sliding. Indentations were first made with a diamond Berkovich tip to an indentation depth of 200 nm, after which sliding was induced at a velocity of 1 μm/s. The sliding velocity was then set to zero for holds 1, 10, 100 and 316 s in duration, followed by reloading to a sliding velocity of 1 μm/s. Contrary to previous slide-hold-slide experiments on quartz rocks, our tests do not show an increase in friction, relative to the pre-hold friction, with hold time or contact area. The contact area was determined from measurements of contact stiffness made at a rate of 100 Hz throughout the tests and does not necessarily increase with hold time. Post-hold peak friction corresponds to the diamond tip being pulled away from the surface and a subsequent decrease in contact area. In contrast, experiments on single crystals of halite show an increase in friction with hold time which scales with contact area. These results suggest that compared to 'softer' materials like halite and rock analogue materials, contact quality may control sliding friction for 'harder' materials like quartz. Our study provides a rationale for exploring 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.
- Publication:
- AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFM.T32A..07B