Friction characteristics are critical mechanical properties of clay, playing a pivotal role in the structural stability of cohesive soils. In this study, molecular dynamics simulations were employed to investigate the shear behavior of undrained montmorillonite (MMT) nanopores with varying surface charges and interlayer cations (Na+, K+, Ca2+), subjected to different normal loads and sliding velocities. Consistent with previous findings, our results confirm that shear stress increases with normal load. However, the normal load-shear stress curves reveal two distinct linear regions, indicating segmented friction behavior. Remarkably, the friction coefficient declines sharply beyond a critical pressure point, ranging from 5 to 7.5 GPa, while cohesion follows an inverse trend. The elevated friction coefficient at lower pressures is attributed to the enhanced formation of hydrogen bonds and concomitant changes in density distribution. Furthermore, shear strength was observed to increase with sliding velocities, normal loads, and surface charges, with Na-MMT exhibiting superior shear strength compared to KMMT and Ca-MMT. Interestingly, the friction coefficient shows a slight decrease with increasing surface charge, while ion type exerts a minimal effect. In contrast, cohesion is predominantly influenced by surface charge and remains largely unaffected by ion type, except under extreme pressures and velocities.

The interfacial friction performance of the fiber-reinforced polymer (FRP) pile-sand interface plays an essential role in determining the load capacity of the foundation. It is necessary to identify its friction behavior in the marine environment due to the unique pile-soil interaction characteristics, which have not been well established at the nanoscale. The cross-linked epoxy resin and crystalline silica substrate are utilized to investigate the nanoscale friction characteristics at different normal stresses and sliding velocities in the dry, pure water, and salt water systems using the molecular dynamics (MD) simulation. The obtained coefficients of friction in three systems are ranked as dry > salt water > pure water systems. There is a tendency for the coefficient of friction to decrease with increasing normal stress, which is consistent with the experimental results. The interface roughness is validated by an analytical equation based on Archard's elastic deformation friction theory. The increase in normal stress shortens the distance between the silica and the epoxy and increases the van der Waals force between the two layers, resulting in the increase of maximum friction force in three systems. The higher normal stress induces the more pronounced stick-slip motion of the silica substrate in the dry system, which can be reduced in the pure water system due to the lubrication effect of water, while the NaCl ions weaken the lubrication effect. On the other hand, the lubrication effect of the water molecules has more beneficial at lower sliding velocity because the diffusion of water molecules is more intense, which leads to the reduction of the amplitude of friction force in the stick-slip motion. By considering the effects of sliding velocity, normal stress, water, and NaCl ions, this study provides the fundamental insight for the friction process of FRP pile embedded in sand.

Ground vibration during earthquakes can lead to loss of soil strength and structural damage. Rubber-soil mixtures (RSM) show promise in mitigating the residual ground deformation under dynamic loading. The influence of clay minerals on soil frictional strength and system stability is essential in the context of earthquake mechanics. This study employs molecular dynamics (MD) simulations to investigate the friction behavior of the rubber-clay interface within the RSM system. The results indicate a direct correlation between normal stress and friction force, with denser soil systems exhibiting higher friction forces, analogous to natural soils. The increase in friction can be achieved by compacting the rubber and clay components in the RSM systems. The inclusion of rubber in the RSM significantly reduces the stick-slip motion at the montmorillonite-rubber interface, providing a damping effect that reduces the intensity of the stick-slip vibration during sliding. The friction force between the montmorillonite and rubber exhibits a velocity enhancement behavior. The higher the sliding velocity, the less the adaptation time for interfacial atoms, resulting in a higher friction force. The rubber/montmorillonite surface exhibits a higher friction coefficient at higher sliding velocities, effectively limiting the buildup of shear stress responsible for initiating stick-slip behavior. Comparisons with experimental data validate the accuracy of the calculated mechanical properties, work of adhesion, and friction coefficients. These results contribute to a better understanding of the friction behavior within the RSM system, facilitating its application in improving seismic resistance.