Quasistatic indentation and spherical particle impact studies of turbine-grade silicon nitrides

Abstract

Abstract Quasistatic indentation studies and small-particle impact studies have been made of specimens of two specially developed silicon nitrides ST-2 and SN252. Indentation studies were made using diamond hemispheres of 2 and 4mm diameter, whereas the impact studies were made using samarium-cobalt spheres of 1 mm diameter with impact velocities of up to 500 m s−1. In the indentation studies, some preliminary experiments were conducted using tungsten carbide spheres of 1-4 mm diameter. They were, however, found to be inadequate as they deformed plastically and then fractured without causing any damage to the silicon nitrides. With the diamond indenters, true Hertzian cone cracks could not be obtained, as the specimens deformed plastically when the mean indentation pressure exceeded 12 GPa. As the mean indentation pressure was further increased, ring cracks sometimes formed at the edge of the plastic indentations; unlike the true Hertzian cone cracks, these ring cracks extended into the bulk almost normal to the indented surface. At still higher indentation pressures, median and lateral cracking occurred. It has been concluded from the observations that the two silicon nitrides are clearly stronger than those used by previous investigators. In the impact experiments, the projectiles themselves fractured for velocities greater than 10 m s−1. However, segmental ring cracks formed in ST-2 and in SN252 when the impact velocities were 410 and 460 m s−1 respectively. At the impact sites, plasticity was found to have occurred and it was consistent with calculated maximum impact pressure. A comparison of the response of single-piece specimens and split specimens for quasistatic indentation studies was also made and clear evidence has been provided showing that split specimens yield misleading results and that they should be avoided for such work.