Mechanisms for silanol formation on amorphous silica fracture surfaces

Amorphous SiO₂ fracture surfaces created under different partial pressures of water vapor (P_H2O) were analyzed using temperature-programmed static secondary ion mass spectroscopy. The results were used to develop an atomistic model for the formation of a fracture surface. It was found that substantial reconstruction of the SiO₂ fracture surface took place immediately after the fracture event. Formation of the fracture surface was modeled as three individual steps - rupture of Si-O-Si bonds to form dangling Si*and Si-O*bonds, reconstruction and relaxation of the surface to form both strained and unstrained siloxane bonds, and, lastly, reaction of H₂O molecules with strained siloxane bonds to form surface silanol groups. The final concentration of surface silanol groups was found to have only a weak dependence on the p_H2O in the ambient atmosphere during the fracture process. It was also found that the number of strained siloxane bonds on the SiO₂ fracture surface could be substantially reduced by heat treatment of the glass under vacuum.