Microcraters formed in glass by projectiles of various densities

Microcraters were produced in soda lime glass and fused quartz by polystyrene-divinylbenzene (1.06 g/cm3), aluminum (2.7 g/cm3), and iron (7.8 g/cm3) projectiles with masses between 0.5 and 200 pg and velocities between 0.5 and 15 km/s. Crater morphology varies primarily with the projectile velocity and density and secondarily with the target material. With increasing projectile velocity at normal incidence on soda lime glass the morphological sequence is as follows: (1) a dent, (2) a narrow melted lip ringing the central pit, and (3) an extensive spallation zone around the central pit. The sequence in fused quartz is similar, except that radiate fractures and spallation occur around the dent before a melted lip forms and the spallation zone is more irregular. For both targets the threshold velocities for the morphological transitions increase as the projectile density decreases. The ratios of central pit diameter and depth to projectile diameter and the ratio of depth to central pit diameter increase with increasing impact velocity. At a given velocity the ratio of pit depth to projectile diameter increases as the projectile density increases and the target density decreases. The deeper craters have a bulbous shape, the pit diameter below the Surface being greater than that at the lip. This shape is most evident in fused quartz impacted by iron projectiles. In the craters formed by polystyrene-divinylbenzene at velocities exceeding 8.5 km/s a convex surface forms in the bottom of the central pit. Projectile remnants are found in the crater for ranges of velocity that depend on target and projectile properties. One-dimensional shock wave analysis contributes to the understanding of the sequence in crater morphology.