Dehydroxylation and formation of KU-1 silica glass surface defects during annealing

Formation of KU-1 silica glass surface defects under annealing is considered. Based on experimental data obtained by annealing of silica glass samples in the temperature range of 800–980°C, it is shown that internal stress forming due to dehydroxylation of the surface zone is the main cause of surface defects. To determine the depth of this zone for different annealing conditions, a formula is suggested. It shows that the internal stress is less than the critical one and the silica glass surface is not changed if the annealing temperature is 850°C or higher due to the fast relaxation rate. At a lower annealing temperature, the surface defects are formed in a few tens of hours during annealing.