Brittle fracture in silicon studied by an hybrid Quantum/Classical method (LOTF)
Brittle fracture is a nice example
of intrinsically multi-level problems in material science.
On the large scale one has to correctly take into account the
driving force of the crack that depends on the elastic
properties of the whole system. But simultaneously it is also
necessary to accurately describe the bond disruption processes that
take place at the atomic scale.
In strongly covalent materials the quantum precision
in the vicinity of the crack tip is particularly crucial.
In the case of diamond silicon
the use of purely classical potentials leads to qualitatively
wrong descriptions of the crack dynamics showing
for example plastic deformations, high fracture
energy release rates, unphysical crack tip structures,
large surface roughness
and even crack blunting and wrong crack propagation direction.
On the other hand full quantum calculations remain practically
untractable on the several thousands of atoms needed
to integrate the slowly decaying crack elastic field.
To face this problem we used the hybrid method LOTF in
which the parameters of an adaptable potential vary
with the time to capture the quantum forces evaluated
in some selected regions of the system.
The method was applied to (110)[1-10] and
(111)[1-10] cracks upon mode I opening, each simulation
box containing around 180 000 atoms.
The results evidence
that the local accurate treatment in the crack tip region
largely influences the structure and roughness
of the opened surfaces as well as the fracture propagation direction.