IN-SITU MICROSCOPY OF CRACK HEALING IN BOROSILICATE GLASS

In situ environmental scanning electron microscopy studies of semi-mac ro indent cracks in borosilicate glass indicate that crack healing occ urred at humidities as low as 8% relative humidity and at temperatures as low as about 400 degrees C. The crack morphology changes observed in situ include slow crack regression (at low temperatures) and multip le crack pinch-off (at higher temperatures). Subsurface crack morpholo gy changes were observed using conventional scanning electron microsco py of the fractured healed specimens. Subsurface healing included pinc h-off into voids which appear quasi-circular in cross-section. In addi tion, crack debris were observed to hinder the crack healing process, which has important implications for fatigue of ceramic materials, whe re debris generation is frequently reported in the literature.