Thermal shock and post-quench strength of lapped borosilicate optical glass

We have measured and modeled the thermal shock fracture and strength degradation of the commercially available BK-7 borosilicate crown optical glass as a function of surface finish prior to thermal shock testing. For surfaces lapped with alumina abrasives in the range 5–40 μm, the critical temperature drop for catastrophic fracture in thin disk samples increases with diminishing abrasive size, and changes from 123.7 ± 1.1 °C (for surfaces lapped with 40 μm abrasives) to 140.2 ± 2.8 °C (for surfaces lapped with 5 μm abrasives). There is little strength degradation for temperature drops smaller than critical, and post-quenching strength depends on surface finish. We correlate the measured thermal shock (critical) temperature drop with the glass thermal and mechanical properties and lapping-induced surface finish. We distinguish between ‘severe’ and ‘mild’ thermal shock conditions in terms of the applicable heat transfer coefficient and Biot number. We estimate that the depth of the strength-controlling cracks on the edge of the disk samples was about 33–42 μm.