Grain-Boundary Sliding in Ice Ih: Tribology and Rheology at the Nanoscale

Using nonequilibrium molecular dynamics simulations, we investigate the process of grain-boundary (GB) sliding in ice I-h. We focus on the Sigma 35 symmetric tilt boundary, which has been observed experimentally in polycrystalline samples, and employ the explicit-proton TIP4P/Ice model to describe the interactions between the water molecules. In all cases, the sliding process closely resembles that observed in viscoelastic substances. After an initial linear elastic regime, the stress-strain response passes through a yield-stress maximum that triggers the onset of rheological response through grain sliding, followed by a final relaxation toward a stationary sliding regime. To assess the role of the molecular structure and dynamics of the GB region, we impose various sliding velocities and directions, as well as appraise different temperatures. In particular, we contrast two cases: one in which the GB interface features the presence of a liquid-like layer at 250 K and another in which there is not, at 150 K. In all cases, we find that the liquid-like layer significantly facilitates GB sliding, acting as a boundary lubricant. Both the yield stress as well as the steady-state stress required to maintain sliding, which can be interpreted in terms of effective static and dynamic frictional forces, respectively, are significantly lower than those obtained at 150 K. Whereas in the latter case, the GB region undergoes large-scale amorphization in the frictional process; the thickness of the liquid-like layer at 250 K only increases moderately, reflecting its effectiveness in facilitating the sliding process. The present results provide valuable information regarding the viscous relaxation processes and the role of the disordered GB layer in the frictional behavior at the nanoscale during GB sliding in ice I-h.