We present density functional theory based atomistic calculations predicting that slow fracturing of silicon is possible at any chosen crack propagation speed under suitable temperature and load conditions. We also present experiments demonstrating fracture propagation on the Si(110) cleavage plane in the ∼100 m=s speed range, consistent with our predictions. These results suggest that many other brittle crystals could be broken arbitrarily slowly in controlled experiments.

Fracture experiments to evaluate the cleavage energy of the ð110Þ½11 0 and ð111Þ½1 12 cleavage systems in silicon at room temperature and humidity give 2:7 AE 0.3 and 2.2 AE 0.2 J=m 2 , respectively, lower than any previous measurement and inconsistent with density functional theory (DFT) surface energy calculations of 3.46 and 2.88 J=m 2 . However, in an inert gas environment, we measure values of 3.5 AE 0.2 and 2.9 AE 0.2 J=m 2 , consistent with DFT, that suggest a previously undetected stress corrosion cracking scenario for Si crack initiation in room conditions. This is fully confirmed by hybrid quantum-mechanicsmolecular-mechanics calculations.

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Evaluating the cleavage energy of brittle crystals is challenging due to the need to generate a controlled deformation of only a few microns; to introduce a straight and planar, atomistic thick precrack, perfectly aligned with the plane of propagation; to align the external loading with that plane; and above all, to grip the specimen while avoiding premature cleavage. In addition, achieving a relatively slow crack upon initiation is another requirement that needs special treatment. Consequently, we designed an experimental method that fulfills the above requirements. It consists in gluing a thin, rectangular specimen, with an atomistic sharp precrack, inside a rectangular hole in an aluminum loading frame, using thin layers of epoxy resin. Crack initiation and propagation occur upon heating the assembly on top of an electrical heating stage by only a few °C, due to the coefficients of thermal expansion mismatch between the specimen and the aluminum frame. Finite element analysis is used to evaluate the strain energy release rate. We describe it in full through cleavage experiments of two low energy cleavage systems of silicon crystal under pure Mode I, under room conditions and argon gas at atmospheric pressure.