Atomistic insights into scratch-induced structural evolution of silica glass

Understanding the mechanism of scratch damage is vital to developing better scratch-resistant glasses. To this extent, employing molecular dynamics simulations and experiments, we investigate the scratch damage of silica glass against a rigid diamond indenter. The glass surface is pre-indented to a constant depth and then dragged to simulate a linear scratch, and the structural impact in the indent-to-scratch transitioning phase is examined. We observe that despite the differences in length and timescales, the simulated values of indentation hardness and coefficient of friction exhibit excellent agreement with experimental values from nanoindentation and atomic force microscopy experiments. Interestingly, analysis of the subsurface deformation in the scratched region reveals densification and shear flow, in contrast to pure densification, as in the case of indentation. Furthermore, similar percentages of recovery from experiments and simulation reveal that the reversible component of plastic deformation owing to densification is comparable in both cases. Finally, in contrast to the common hypothesis, we demonstrate that while the bond angles and lengths recover significantly, the ring structure does not recover upon annealing, although they exhibit some relaxation. Thus, the present study sheds new light on the crucial role of the medium-range structure of glasses subjected to scratch deformation.