マイクロカンチレバー試験片による高硬度・高靭性ナノ多結晶スティショバイト（SiO₂）の破壊抵抗及び強度評価

抄録

We developed a micro-mechanical test method using micro-cantilever beam specimens to determine the early part of R-curve of nanocrystalline stishovite that exhibits fracture-induced amorphization. Stishovite is a high-pressure phase of silicon dioxide (SiO₂), and has the high hardness (HV=30 GPa) and excellent fracture toughness of 13 MPa·m^1/2. The metastable stishovite transforms to amorphous by fracture. In analogy to the transformation toughening of zirconia, the fracture-induced amorphization can be a toughening mechanism of stishovite. The fracture resistance measurement by using micro-cantilever beam specimens revealed very steep rising R-curve of stishovite whose narrow transformation zone of a few tens of nanometers is very effective to increase the fracture resistance by a few micrometer crack extension. Steep initial slopes are desirable for R-curves because materials that achieve high toughness at small crack length also achieve high strength. The fracture strength of stishovite evaluated by micro-cantilever testing was more than 6 GPa, and it indicates that the critical stress for amorphization of stishovite is much higher than that value.