What can we learn about the cleavage fracture of Silicon and Diamond with

ab initio calculations?

 

Ruben Perez

 

Departamento de Fisica Teorica de la Materia Condensada

Universidad Autonoma de Madrid, E-28049 Madrid, Spain

 

Pablo Pou and Peter Gumbsch

 

IZBS, Universitat Karlsruhe

 

The catastrophic failure of materials, and particularly the brittle fracture of Si and Diamond, is ultimately determined by events on the atomic scale. Total-energy pseudopotential calculations provide the accurate description of the interatomic interactions needed to characterize the atomically sharp crack tip and its movement by breaking individual bonds between atoms. Fixed-boundary ab initio simulations,

complemented with system size scaling, show that bonds break continuously and cracks propagate easily on {111} and {110} planes provided crack propagation proceeds in the <-110> direction. In contrast, if the crack is driven in a <001> direction on a {110} plane the bond breaking process is discontinuous and associated with pronounced relaxations of the surrounding atoms, which results in a large lattice trapping. The different lattice trapping for different crack propagation directions can explain the experimentally observed cleavage anisotropy. While very similar in the behaviour, differences between Si and diamond cracks are discussed in terms of the possible surface graphitization.