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.