Nanoscale Stick-Slip Behavior of Na-Montmorillonite Clay

Clay minerals are platelike particles that play a critical role in problems involving swelling, deformation, and failure. Fundamental understanding of these phenomena and the parameters that influence them requires studies at the nanoscale. The nanoscale mechanism of the sliding of clay sheets at different states of hydration and hydrostatic stress was studied here using molecular dynamics in an isobaric–isothermal ensemble. The hydration state lay in the range of crystalline swelling ($0–400{\mathrm{mg}}_{\mathrm{water}}/{\mathrm{g}}_{\mathrm{clay}}$), and the hydrostatic pressure was varied in the range 5–12 GPa. Under hydrostatic pressures as high as several gigpascals, the mobility and the molecular structure of water are comparable to those of supercooled water. Despite high hydrostatic stresses, the failure was located in the shear-dominated zone. Examination of the molecular structure of interlayer water, number of hydrogen bonds, and their configuration during shear loading showed evidence of stick-slip phenomena similar to those found in thin films. The number of hydrogen bonds between interlayer water molecules increased as the clay–water system approached the slip point. Hydration state and hydrostatic stress influenced the stress–strain behavior of system and the average shear stress. The hydration states, in which the maximum average shear stresses occurred, coincided with the formation of the second, third, and fifth water layers. The shear strength decreased with the increase of water content above $340{\mathrm{mg}}_{\mathrm{water}}/{\mathrm{g}}_{\mathrm{clay}}$. The nanoscale cohesion and friction angle of the layers were calculated using the Mohr–Coulomb failure criterion and were found to be in good agreement with previous studies.