Fracture toughness of Al2O3 fibers with an artificial notch introduced by a focused-ion-beam

The present work was carried out to estimate the fracture toughness of two types of Al2O3 fibers (85Al2O3–15SiO2, Altex® (Sumitomo Chemical Co., Ltd) and α-Al2O3, Almax® (Mitsui Mining Co., Ltd)) and to elucidate the transition from the intrinsic defects-induced fracture to introduced notch-induced one. With an application of the focused-ion (Ga+)-beam micromachining method, a mode I type straight-fronted edge notch with a notch-tip radius around 25 nm was introduced in fiber specimen. The fracture toughness KIc was estimated for each fiber specimen based on the fracture mechanical approach in which the measured values of notch depth, fiber diameter, fracture strength and calculated correction factor were substituted. The fracture toughness values of the 85Al2O3–15SiO2 and α-Al2O3 fibers were estimated to be 1.86 ± 0.24 and 2.05 ± 0.13 MPa m1/2, respectively. The fracture toughness value was almost independent of the fiber diameter and notch depth in both fibers tested. From the obtained fracture toughness value and the measured fracture strength of the original fiber, the notch depth at the transition from intrinsic defects-induced fracture to notch-induced one, corresponding to the equivalent size of the intrinsic defects that determines the strength of the original fiber, were estimated to be 0.3 and 0.8 μm for 85Al2O3–15SiO2 and α-Al2O3 fibers, respectively.