Fracture-induced amorphization of polycrystalline SiO2 stishovite: a potential platform for toughening in ceramics

Abstract

Silicon dioxide has eight stable crystalline phases at conditions of the Earth's rocky parts. Many metastable phases including amorphous phases have been known, which indicates the presence of large kinetic barriers. As a consequence, some crystalline silica phases transform to amorphous phases by bypassing the liquid via two different pathways. Here we show a new pathway, a fracture-induced amorphization of stishovite that is a high-pressure polymorph. The amorphization accompanies a huge volume expansion of ~100% and occurs in a thin layer whose thickness from the fracture surface is several tens of nanometers. Amorphous silica materials that look like strings or worms were observed on the fracture surfaces. The amount of amorphous silica near the fracture surfaces is positively correlated with indentation fracture toughness. This result indicates that the fracture-induced amorphization causes toughening of stishovite polycrystals. The fracture-induced solid-state amorphization may provide a potential platform for toughening in ceramics.

Microscopic evidence of stishovite generated in low‐pressure shock experiments on porous sandstone: Constraints on its genesis

“…However, the observation of concentrated tiny crystals of stishovite within this glass matrix as well as the absence of cracks due to volume expansion (Nishiyama et al. ) excludes amorphization of stishovite that usually starts already at 560 °C (Xue et al. ; Brazhkin et al.…”

“…The amorphization of stishovite is accompanied by a volume expansion of ~100%, which would have led to cracking of the glass matrix (Nishiyama et al. ). Amorphization of stishovite can therefore be excluded, which means that the bulk postshock temperature of the sandstone must have been lower than the amorphization temperature of stishovite that was determined to be around 560 °C by different heating experiments at ambient pressure (Xue et al.…”

Synthesis of Al2O3/SiO2 nano‐nano composite ceramics under high pressure and its inverse Hall–Petch behavior

“…Silica is used as optical fibers and lenses; alumina (α‐Al 2 O 3 ) is widely used as a structural material because it is a hard oxide with Vickers hardness ( H V ) of 17‐20 GPa . Silica is also a potential structural material because stishovite, which is a high‐pressure polymorph of silica with a rutile structure, is the hardest known oxide ( H V ~30 GPa) at ambient conditions. In general, the enhancement of the hardness of structural polycrystalline ceramics can be achieved by reducing grain size following the Hall–Petch relation and/or by making composites in which the second phase is harder than the matrix.…”

“…On the other hand, recently, nanocrystalline bulk stishovite with grain size of ~200 nm was reported to be very hard and tough . H V of this material was reported to be 29‐30 GPa, which is comparable to that of the single crystal ( H V =29 GPa: the average of the hardness parallel to the c ‐axis and that perpendicular to this axis), while the fracture toughness is six to seven times as high as that of the single crystal .…”

D03+hcp mixed phase with nanostructures in Mg85Zn6Y9 alloy obtained by high-pressure and high-temperature treatments

“…In the case of ceramics, many recoverable structural transitions by high pressure and high temperature (HPHT) conditions are known to make highly functional materials, as described for example in Ref. [9]. Therefore, HPHT treatments between 7 and 20 GPa at several temperatures were performed and the effect of the HPHT treatments on the crystal and micro-structures of the recovered Mg 85 Zn 6 Y 9 were investigated using XRD and field emission type scanning electron microscope (FE-SEM).…”