Investigation of sapphire strength across the scales

The strength of sapphire across several length scales has been studied by micro-cantilever bending tests on specimens with the a-plane perpendicular to the main loading direction. The combined machining of two types of FIB (Ga and plasma Xe) is used to produce specimens with a wide range of dimensions, with specimen heights ranging from a few microns to several tens of microns. All specimens exhibit a fracture surface along m-planes. These micro-bending strength results are compared with macroscopic bending results obtained on the same sample with the same orientation. A marked size effect is observed on the measured strength, ranging from around 20 GPa at the micrometric scale down to 650 MPa for macroscopic specimens. A comparison with data from literature on sapphire samples of different size is made. Statistical models are found unable to capture the size effect on the entire range of sample sizes. Graphical abstract