Laser thermal shock testing of neutron-irradiated sapphire

Half of a set of sapphire disks was exposed to fast neutrons (0.8-10 MeV) at a fluence of 10²² neutrons/m². Each 25-mm-diameter × 1-mm-thick disk was then exposed to a 10.6 μm CO₂ laser (121 W/cm²) while the central 12.7-mm-diameter region of the disk was shielded from the laser. +c-Plane disks that had not been exposed to neutrons survived 76% longer than a-plane disks that had not been exposed to neutrons. Neutron irradiation had no significant effect on time to failure of c-plane sapphire. However, neutron irradiation increased the survival time of a-plane sapphire by 30% - a result that was significant at the 99.9% confidence level. c-Plane disks were expected to fail in tension at the center of the disk. The calculated tensile stress at the mean failure time was ∼700 MPa and the center temperature was ∼400°C. By contrast, a-plane disks failed near the boundary between the shielded central region and the exposed outer annulus. The radial stress at this location is tensile and the hoop stress is compressive. Failure origins were at surface scratches. Rhombohedral twinning was observed in many a-plane disks, but there was no fractographic evidence that r-plane twinning caused failure. The mechanism by which neutron irradiation increases the time to failure of a-plane disks is unknown.