Presentation Information
[S08-19]Nanoscale Surface Groove Generation Mechanisms in Quartz Asperities: Effect of Lattice Orientation
*SHENG LI1, Eiichi Fukuyama1 (1. Kyoto University)
Surface grooving is an important feature on the wear of fault surfaces and originates from asperity fracture during shear slip. It represents a key phenomenon in fault surface roughness evolution and gouge particle formation. Clarifying the mechanism of surface grooving is important for revealing the nature of fault friction, seismic energy dissipation, and fault rupture dynamics. Here, to elucidate the formation mechanism of nanoscale surface grooving, we conducted molecular dynamics simulations of multiple 3-D nanoscale wear processes, using α-quartz—one of the most abundant minerals in crustal faults—as a representative geomaterial. Our simulation involves a pair of blocks with two identical semi-spherical α-quartz asperities on each surface, which are not initially in contact. The periodic boundary condition was applied in two horizontal directions, while the vertical boundaries were fixed in displacement (Aghababaei et al., 2016). Therefore, the normal force dynamically increases with loading distance. We investigated two asperity radii: 8.10 nm and 16.98 nm. To account for the anisotropic structure of α-quartz, we rotate the lattice by 90° around b axis from that used in the previous study (Li & Fukuyama, 2025, JPGU). All simulations were performed using LAMMPS (https://www.lammps.org). The equations of motion were integrated using the velocity Verlet algorithm (Martys & Mountain, 1999) with a fixed timestep of 0.0025 ps. A shear loading velocity of 20 m/s was applied by considering the computation efficiency. Our results indicate that the formation of surface grooving strongly depends on crystallographic orientations. In Li & Fukuyama, macroscopic cracks did not nucleate at the asperity root. Damage was confined within the asperity above the substratum, without the formation of grooves. In contrast, in the present case, the grooves are generated in both asperity sizes. Initially, tensile cracks develop at approximately 45° in the asperity roots within substrata during frictional collisions, preceding any internal asperity damage. As increasing loading displacement, cracks inside substratum propagate further, accompanied by the formation and crack propagation of internal damage bands within the asperity (area near contact interface). However, the substratum cracks arrest when shear load is further applied, preventing complete groove formation. Consequently, this incomplete grooving process ultimately terminates with the internal failure of the asperity during collision. Here, we infer the damage process observed in the present case as the incomplete grooving process. In comparison, previous studies reported the formation of complete grooves, without mentioning the propagation of internal cracks (Aghababaei et al., 2016). Moreover, in those cases, the asperity remained almost entirely elastic, in sharp contrast to the damaged asperity observed in our study. At present, capturing the complete grooving process would require even more extensive computations. However, the current results clearly indicate that the generation of grooves in anisotropic quartz cannot be explained solely by the balance between elastic strain energy and fracture surface energy (Aghababaei et al., 2016). The evolution of crack-tip stress fields should be further investigated both within asperities and in the quartz substratum to explain the mechanism of groove formation.