Analysis of the role of water condition on the friction performance of calcareous sand from the nano-scale

The strength of calcareous sand, which is closely related to the coastal stability, is controlled by the interparticle frictions. Therefore, it is essential to evaluate the friction behavior of soil particles under different water conditions to predict their potential failure. This work aims to study the role of water on the friction behaviour of calcite, the main mineral in calcareous sand, at the nano-scale. Using molecular dynamics (MD) simulations, the friction behaviour of calcite slabs under different water layers (WL) and sliding directions were compared and analysed. From the results, it is found that there is a nonlinear decrease in friction force as the water layers between calcite slabs increases. This is because that thin water films serve to weaken the direct attraction between the upper and bottom slabs, leading to the friction force reduces to a stable level. Meanwhile, the friction behaviour of calcite is affected by the sliding direction, particularly in low water layers (< 2WL), which is due to different atoms arrangement on the surfaces. Higher friction is observed when sliding in the x-direction ([100] crystal orientation) compared with that sliding in the y-direction ([010] crystal orientation). In addition, the stick-slip phenomenon is observed for the low water layers (< 2WL) and becomes less pronounced with increasing water layers which shows a smoother friction force evolution at relatively higher water layers (> 4WL). The results contribute to a deeper understanding of the role of water on the friction behaviour of calcite at the nano-scale.