Fracture toughness testing of nanocrystalline alumina and fused quartz using chevron-notched microbeams

Abstract

We show that chevron-notched samples offer an attractive approach to the measurement of fracture toughness in micron-scale samples of brittle materials and use the method to characterize quartz and nanocrystalline alumina. Focused ion beam milling is used to carve bend bars of rectangular cross-section a few micrometres wide and containing a notch with a triangular ligament. Load-controlled testing is conducted using a nanoindentation apparatus. If the notch is appropriately machined, cracks nucleate and propagate in a stable fashion before becoming unstable. Sample dimensions are measured using a scanning electron microscope, and are used as input in finite element simulations of the bars' elastic deformation for various crack lengths. The calculated compliance calibration curve and the measured peak load then give the local fracture toughness of the material. Advantages of the method include a low sensitivity to environmental subcritical crack growth, and the fact that it measures toughness at the tip of a sharp crack situated in material unaffected by ion-milling. The approach is demonstrated on two materials, namely, monolithic fused quartz and nanocrystalline alumina Nextele 610 fibres; results for the latter give the intrinsic grain boundary toughness of alumina, free of grain bridging effects.

Strength and toughness of structural fibres for composite material reinforcement

“…However, the methodology entails a large experimental scatter owing to the uncertainty in the determination of the exact defect size that caused the failure and the maximum stress attained during the experiments. The focused ion beam (FIB) opens revolutionary opportunities for testing materials at the microlevel by selective removing/carving material and generating complex geometries suitable for testing [1,2].…”

Evaluation of the Mechanical Properties of Naturally Grown Multilayered Oxides Formed on HCM12A Using Small Scale Mechanical Testing

“…In addition, the outer oxide layer fractures in an intercrystalline mode, which indicates that the grain boundaries contribute to lower strength. Mueller et al observed similar slight differences between two values of K Ic measured in alumina fibers and suggested that it could be because of the differences between transgranular and intergranular fracture [64]. Since the oxide layer is growing while exposed to flowing LBE, it is likely that small amounts of LBE migrate into the grain boundaries.…”

On measuring fracture toughness under load control in the presence of slow crack growth

“…In Ref [37] microscopic chevron-notched cantilever specimens, with dimensions W and B from ≈2 to ≈5 m were produced by focused ion beam milling in fused quartz and nanocrystalline alumina (grain size ≈60 nm). Testing was conducted under load-control, at room temperature and 20-50% relative humidity using a nanoindentation instrument at loading rates ≈2 N/s.…”