Response of seven crystallographic orientations of sapphire crystals to shock stresses of 16–86 GPa

Abstract

Shock wave profiles of sapphire (single-crystal Al2O3) with seven crystallographic orientations (c, d, r, n, s, g, and m-cut) were measured with time-resolved VISAR (velocity interferometer for a surface of any reflector) interferometry at shock stresses in the range 16–86 GPa. Shock propagation was in the direction normal to the surface of each cut. The angle between the c-axis of the hexagonal representation of the sapphire crystal structure and the direction of shock propagation varied from 0 for c-cut up to 90° for m-cut in the basal plane. Based on published shock-induced transparencies for three directions of shock propagation, shock-induced optical transparency correlates with the smoothness of the mechanical shock wave profile. The ultimate goal was to find the direction of shock propagation for which shock-compressed sapphire is most transparent as a window material. In the experiments particle velocity histories were recorded at the interface between a sapphire crystal and a LiF window. In most cases measured wave profiles are noisy as a result of heterogeneity of deformation. Measured values of Hugoniot elastic limits (HELs) depend on direction of shock compression and peak shock stress. The largest HEL values (24 GPa) were recorded for shock loading along the c-axis and perpendicular to c along the m-direction. Shock compression along the m- and s-directions is accompanied by the smallest heterogeneity of deformation and the smallest rise time of the plastic shock wave. m- and s-cut sapphires most closely approach ideal elastic-plastic flow, which suggests that m- and s-cut sapphires are probably the sapphire orientations that remain the most transparent to the highest shock pressures. Under purely elastic deformation sapphire demonstrates very high spall strength, which depends on both load duration and peak stress. Plastic deformation of sapphire causes loss of its tensile strength.