Extrinsic size dependent plastic deformability of ZnS micropillars

ZnS has been widely used as an infrared optical material due to its superb optical properties, but its mechanical properties are less well understood. In this study, we investigated the size-dependent mechanical behavior of vacuum hot-pressed ZnS micropillars with a diameter of 3, 1.5, and 0.5 μm by in-situ microcompression tests in a scanning electron microscope. A clear size effect was observed. The 3 μm micropillars showed the highest variability of stress-strain behavior. Whereas the 0.5 μm pillars exhibited higher flow stress and improved strain-hardening capability to a compressive strain of ~10%. Post-mortem microscopy analyses coupled with ASTAR crystal orientation mapping suggest that plastic deformation is accommodated by stress-induced wurtzite-to-sphalerite phase transformation and the formation of abundant stacking faults and deformation twins. Meanwhile, the substantial strain hardening derives from preexisting stacking faults, twin boundaries, and grain boundaries that hinder the propagation of partial dislocations. This study provides a fresh perspective on the design of ceramic materials with high strength and plastic flow ability.