Micro-scale deformation of gypsum during micro-indentation loading

Abstract: The role of plastic deformation and fracture during small-scale, low-rate contact scenarios in gypsum is investigated. Results indicate that the hardness decreases with increasing load, on all axes, ranging from 216kg/mm2 at 0.25N to 91kg/mm2 at 4.91N on the (010) plane and from 122kg/mm2 at 0.98N to 75kg/mm2 at 4.91N on the (011) and (110) cleavage planes. Higher hardness values for lower loads are attributed to dislocation saturation and localized necking, which leads to densification and work-hardening. Transmitted light microscopy reveals that the decrease in hardness for increasing load is related to an increase in subsurface cracking along the (010) and (011) cleavage planes, enabling the indenter to penetrate deeper into the crystal. Coupled with an increase in subsurface cracking is an increase in surface smoothness as a result of increased plastic material flow beneath the indenter, with crack densities decreasing from 34% at a 0.25N load to 6% at a 4.91N loading. Raman spectroscopy reveals that the higher-hardness material comprises water-reduced forms of calcium sulphate: hemihydrate and α-CaSO4. Local temperatures required for the dehydration of gypsum range from 118 to 142°C, indicating such temperatures were reached during plastic flow, and related heat generation, of the material beneath the indenter under relatively quasi-static conditions. Evaporated water generates voids in the original crystal structure and enables the sulphate to deform more easily. [Copyright &y& Elsevier]