Quantitative stress/strain mapping during micropillar compression

Micropillar compression is increasingly used as a method to examine small length-scale mechanical properties since it minimises the strain gradients that are unavoidable during nanoindentation into a flat surface. It also simplifies the data analysis since it is assumed that the compression is uniaxial. But how valid is this assumption when misalignments in the microscale setup are generally unavoidable? In order to investigate this, the stress and strain tensors of the micropillar should be measured during the compression. To accomplish this, electron backscatter diffraction (EBSD) mappings were obtained before, during and after consecutive compressions of a GaAs micropillar inside of a high-resolution scanning electron microscope. Elastic strain and the corresponding stress tensors were determined from the EBSD measurements using a cross-correlation technique. The results show that the von Mises stress compares well to the engineering stress determined from the nanoindenter load–displacement data. Even with rotations of almost 3° at maximum load, the negligible shear components in the strain tensor within the pillar indicates that the assumption of uniaxial compression can be made. Due to its high spatial resolution and strain sensitivity, in situ strain–stress mapping during micromechanical testing is a very promising route for the investigation of deformation phenomena on the sub-micron scale.