Strength and toughness of nanocrystalline SiO2 stishovite toughened by fracture-induced amorphization

The finding of “fracture-induced amorphization” in nanocrystalline SiO$_2$ stishovite lead to a proposal of a new type of transformation toughening by the direct transition from crystal to amorphous state, which is different from the classical martensitic transformation of zirconia. Here, we investigated strength and toughness of nanocrystalline stishovite by using micro-cantilever beam specimens of different sizes. The maximum strength of 6.3 GPa gave the estimate of the lower bound of critical stress for amorphization, which was much higher than the critical transformation stress of zirconia. The crack growth resistance curve (R-curve) rose steeply with crack extension of only a few μm, and reached to a plateau value of 10.9 MPa m$^1/2$. We discussed the effects of grain size, microstrain, and dislocation density on the critical stress, the transformation zone width, and thereby, the fracture toughness.