Strength assessment of Al2O3 and MgAl2O4 using micro- and macro-scale biaxial tests

The application of ceramics in advanced functional applications often requires thicknesses below a few hundred micrometers, rendering an assessment of the materials' strength particularly challenging. In this work, different testing methods are combined to elucidate the effect of the volume and the surface area of the specimens under tensile loading on the fracture strength of Alumina of different purities and Spinel. A ball-on-3-ball test has been implemented into a micro-indentation system permitting as novelty a high control and acquisition of loads and displacements to study the biaxial fracture stress of thin specimens. In addition, ring-on-ring tests are carried out for thicker specimens. Weibull statistics is applied to analyze the fracture stresses. Considering all individual data sets obtained using the two testing methods, the effective volume approach appears most suitable for the materials tested. This conclusion is confirmed by fractographic analysis, where pores could be identified as the main failure initiating defect. The combination of a micro-indentation system and a ball-on-3-ball test as micro-and macro-scale biaxial tests represents an easy, fast and reliable methodology to investigate small scale ceramic materials.