Experimental investigation on thermal healing of subsurface damage in borosilicate glass

Subsurface damage (SSD) is the fracture and deformation near the surface of brittle optical materials, caused by surface lapping or grinding. The existence of SSD dramatically influences the performance of optical glass and reduces the laser-induced damage threshold. Subsurface cracks of borosilicate glass can be spontaneously healed when heated under appropriate conditions. In this paper, thermal healing experiments of borosilicate glass (BK7) subsurface cracks are conducted on typical cracks induced by an indentation process, and the effects of the Beilby layer, temperature, crack depth, and water vapor pressure are studied. A semi-empirical relation is obtained through the regression of experimental results to describe the variation of subsurface crack length. Finally, a healing experiment is performed on the subsurface damage formed by grinding. The detection results show both the damage density and maximum damage depth have been reduced after heat treatment, demonstrating the effectiveness of the thermal healing method on eliminating glass subsurface damage.