A search for the upper bound of the cleavage energy of silicon crystal

Griffith barrier of 2γ_s is the accepted fracture energy under vacuum at crack initiation in brittle materials. It was shown recently, however, that the Griffith barrier is only the lower bound of that energy, while the upper bound was found to be about 60% higher. This bound was found in specimens having relatively short, straight but slightly curved, crack fronts, leading to the conclusion that ‘shorter is tougher’ and stronger than predicted by Griffith. In the current investigation, we attempted to explore further, experimentally, the upper bound of the cleavage energy. This was done by cleaving silicon crystal specimens along its two low-energy cleavage systems by generating shorter precracks. We discovered that the upper bound energy in silicon is over 3 times the Griffith barrier under air for the above-mentioned crack systems. This is much above the theoretical predictions of the lattice-trapping effect and atomistic simulations. We further confronted Griffith's finding that σa₀ = Const, which became a keystone in fracture since it defines the fracture toughness and fracture energy.