Fracturing and friction are fundamental processes with microscopic origin and macroscopic consequences, but our basic understanding of the processes is still limited. The development of a better understanding of both fracturing and friction is important for many practical applications, spanning from understanding the strength of technological materials to determining the occurrence of earthquakes. Significant effort has been made to study fracturing and friction phenomena using modeling techniques spanning from the atomic to the macroscopic scale. For example, billion-particle molecular dynamics models can now routinely be applied to address material deformation processes. However, our understanding of the effect of various types and symmetries of chemical bonds on the qualitative behavior of nanoscale cracks and surface contacts is still in its infancy.
I have addressed the difference in behavior in systems with ionic bonds and with covalent bonds using the sum of an LJ potential and a Coulomb potential to model the ionic bonds in NaCl, and a three-body SW potential for covalent bonds in Si. These two materials are assumed to represent members of two different material classes, classified according to the symmetry of the interactions. The two material classes behave differently in response to tensile and shear stress. While NaCl undergoes plastic deformations by slip transformations which preserve its crystal structure, Si has a more random atom arrangement and behaves more like a liquid when compressed.