Large increase in fracture resistance of stishovite with crack extension less than one micrometer

Abstract

The development of strong, tough, and damage-tolerant ceramics requires nano/microstructure design to utilize toughening mechanisms operating at different length scales. The toughening mechanisms so far known are effective in micro-scale, then, they require the crack extension of more than a few micrometers to increase the fracture resistance. Here, we developed a micro-mechanical test method using micro-cantilever beam specimens to determine the very early part of resistance-curve of nanocrystalline SiO2 stishovite, which exhibited fracture-induced amorphization. We revealed that this novel toughening mechanism was effective even at length scale of nanometer due to narrow transformation zone width of a few tens of nanometers and large dilatational strain (from 60 to 95%) associated with the transition of crystal to amorphous state. This testing method will be a powerful tool to search for toughening mechanisms that may operate at nanoscale for attaining both reliability and strength of structural materials.

Picosecond amorphization of SiO 2 stishovite under tension

“…With a typical speed on the order of v = 5 × 10 3 m/s, a crack propagates by a distance of h = v τ = 10 nm within this time frame. This distance is consistent with the experimentally measured thickness of transformation zones, and thus, the disordering dynamics found here are sufficiently fast to account for experimentally observed transformation toughening ( 8 , 9 ). The critical tension of −30 GPa in stishovite is similar to that (−18 GPa) for tensile amorphization of CaSiO 3 and MgSiO 3 perovskites ( 17 ).…”

“…Because NPS is made by simply applying high pressure to Earth-abundant silica glass, it could provide a sustainable supply of high-performance ceramics in the future. Subsequent micromechanical tests using microcantilever beam specimens showed that the underlying toughening mechanism is effective even when a crack length is on the order of nanometers ( 9 ). Electron microscopy and x-ray absorption near-edge structure spectroscopy identified the existence of amorphous silica within subnanometers from fracture surfaces ( 8 ).…”

“…Namely, under high tensile stress in front of a crack tip, metastable stishovite undergoes structural transformation to amorphous silica that is more stable under ambient conditions. The stishovite-to-amorphous transformation is accompanied by a volume expansion of ~100%, and the swollen transformation zone screens and self-heals the crack ( 8 , 9 ). Although plausible, for this transformation-toughening mechanism ( 10 ) to operate, the amorphization of stishovite must proceed rapidly to catch up with the crack tip that is moving fast at a speed above nanometers per picosecond (or 10 3 m/s).…”

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

“…6,7 H V of this material was reported to be [29][30] GPa, which is comparable to that of the single crystal 17 (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 6 as that of the single crystal. 17 A transformation toughening 18 at the nanoscale is activated in nanocrystalline bulk stishovite 7,19 : tensile stress at the crack tip causes solid-state amorphization that accompanies huge volume expansion (60%-95%), which produces compressive stress against crack extension. Hence, the nanocrystalline bulk form of stishovite is the hardest oxide as well as a very tough ceramic.…”

3D multiscale-imaging of processing-induced defects formed during sintering of hierarchical powder packings

“…Seidel and Rödel 43 revealed that the crack tip toughness of alumina was 2.3 MPa·m 1/2 , while the fracture toughness measured by using macroscopic crack was 3.6 MPa·m 1/2 . Yoshida reported K 0 of 2.8 MPa·m 1/2 by using micro-cantilever beam specimens 44 . The fracture strength is estimated to be 310–380 MPa by using for a circular crack and K 0 of 2.3–2.8 MPa·m 1/2 for alumina.…”