Effects of water on chemomechanical instabilities in amorphous silica : nanoscale experiments and molecular simulation

Abstract

We elucidate the tensile failure mechanism of amorphous silica and the effects of water on the process, combining: (a) atomic force microscope (AFM) bending tests, (b) molecular dynamics (MD) simulation and (c) molecular orbital (MO) simulation. Bending tests of silica nanowires provide validation for the predictions of the simulations, in which we study the failure of dry silica using MD and define a representative system to be studied with the more chemically accurate MO method. We used the AFM to perform bending tests on silica nanowires of diameter D < 1 [mu]m, which have very high surface-to-volume ratio and no microscopic flaws. No size effects on elastic modulus were observed down to 130 nm. For 500 nm wires, water reduces the strength from 10.5 GPa in air to 6.5 GPa in water, results comparable to those reported for micrometer-scale fibers. By probing the strength of silica at this scale, we bring experiments to the length scales accessible to atomistic simulation. Using classical MD, we found that crystalline silica fails globally by crack nucleation, but amorphous silica displays plastic deformation due to the formation of local defects, which cascade into larger compound defects. We extend to amorphous systems the instability criterion for material failure and use the Lanczos iteration method to isolate unstable modes. Failure of these modes create local defects, which are used to define a simpler representative system. We studied the water effect on these defects using a semi-empirical MO method, showing first that a water dimer is sufficient to lower the strength of a single Si-O-Si bond. Next, we use a representative system to describe the failure mechanism near instability.

(cont.) We found that water reduces the tensile strength by both reducing the athermal failure strain and the energy barrier for failure. In summary, we demonstrate experimentally that the tensile strength of amorphous silica is governed by the nanoscale crack initiation event, after which the system fails in a brittle manner. Using a multiscale approach, we describe the nanoscale mechanism through MD simulation and the effect of water through MO simulation, bridging the gap between breaking a single bond and breaking a macroscopic body in tension.