9:15 AM - 9:30 AM
[SCG50-02] Micro to macro transition in fluorite friction
Keywords:firction, fluorite, direct shear friction experiment, rotary shear friction experiment
Friction, which affects the strength and sliding behaviors of faults, is influenced by physical processes at various scales, ranging from the atomic to the macroscopic scale. On the macroscopic scale, friction has been thought to be determined by adhesion and deformation of asperities (i.e., true contacts[SI1] ), which is controlled by the yield stress of solid materials. The deformation of asperities plays a major role in the frictional sliding of rough surfaces, and is important in understandings of frictional behaviors of fault planes. In this study, we used fluorite (CaF2) as an analogue of fault zone materials, which has a much lower yield stress than those of silicate minerals such as quartz. We conducted friction experiments on single crystalline or powdered samples of fluorite. Cylindrical specimens of fluorite with diameter of 2 mm or 3 mm, which were cored from natural fluorite single crystals, were subjected to the direct shear tests. Powdered samples were tested using a rotary shear friction apparatus. All experiments were conducted at room temperature under dry or water saturated conditions.
The direct shear friction experiments were a normal stress of approximately 10 [MPa] and a sliding velocity ranging from 100 - 1000 [μm/s]. The rotary shear friction experiments were conducted at normal stresses of up to 200 MPa and sliding velocities ranging from 1 - 1000 [μm/s]. The results showed that the frictional stresses for both the single-crystal and powder samples was proportional to the normal stresses and independent of the apparent contact area, in accordance with the Amontons-Coulomb friction laws, and that there was no significant velocity dependence. The single-crystal samples showed low friction coefficients (μ) of 0.2-0.3 at steady-state friction, whereas the powdered samples showed μ = 0.5-0.6, which is close to the commonly known values of the Byerlee's law.
It is suggested that ordinal macroscopic frictional mechanisms determined by the plastic deformation of asperities operated during friction experiments of the powdered samples, whereas atomic-scale mechanisms on the crystalline surfaces, or asperity deformation mechanisms different from those observed in the powdered samples, were dominant during frictional sliding of the single-crystal samples.
The direct shear friction experiments were a normal stress of approximately 10 [MPa] and a sliding velocity ranging from 100 - 1000 [μm/s]. The rotary shear friction experiments were conducted at normal stresses of up to 200 MPa and sliding velocities ranging from 1 - 1000 [μm/s]. The results showed that the frictional stresses for both the single-crystal and powder samples was proportional to the normal stresses and independent of the apparent contact area, in accordance with the Amontons-Coulomb friction laws, and that there was no significant velocity dependence. The single-crystal samples showed low friction coefficients (μ) of 0.2-0.3 at steady-state friction, whereas the powdered samples showed μ = 0.5-0.6, which is close to the commonly known values of the Byerlee's law.
It is suggested that ordinal macroscopic frictional mechanisms determined by the plastic deformation of asperities operated during friction experiments of the powdered samples, whereas atomic-scale mechanisms on the crystalline surfaces, or asperity deformation mechanisms different from those observed in the powdered samples, were dominant during frictional sliding of the single-crystal samples.