Surfaces formed by subcritical crack growth in silicate glasses

The topography of surfaces formed in glass by subcritical crack growth was investigated by a method of mapping using atomic force microscopy. The objective of the study was to determine how well “upper” and “lower” surfaces matched after having been formed by a crack moving at slow velocity. The question arose, were features left in the fracture surfaces of silicate glasses that would indicate the formation of cavities during the fracture process? Studies were performed on silica glass and soda-lime-silicate glass. Fracture surfaces were formed either in water or in moist environments at velocities that ranged from 10−2 m/s down to 10−10 m/s. This procedure covered almost the entire range of velocities used for subcritical crack growth experiments in glass. Opposing fracture surfaces formed during our studies were found to “match” over the entire range of velocities and for all environments studied. For silica glass, the surfaces were found to match to an accuracy of better than 0.3nm normal to the fracture surface and 5 nm within the fracture surface. For soda-lime-silicate glass, surfaces were found to match to an accuracy of 0.5nm to 0.8nm normal to the fracture surface and 5 nm within the fracture surface. Within these limits, no evidence for cavitation was found in either glass.