The effect of environment on silica fracture: Vacuum, carbon monoxide, water and nitrogen

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Abstract
A quantitative fracture model is presented for slow crack growth in amorphous silica. The model is based on semiempirical quantum molecular orbital calculations using the Austin method AM1. The fracture of three-, four-, five- and six-member silica rings is modelled in four cases to determine the reaction pathways under strain in vacuum, carbon monoxide, water and nitrogen (N2). A low-energy reaction path was found for the fracture event through a ring contraction mechanism in the case of vacuum, carbon monoxide and nitrogen. Ring opening through hydrolysis was found to be the lowest-energy pathway in the case of water-enhanced fracture. The calculated transition-state energy barriers were used to estimate the crack velocity V as a function of stress intensity K I for all four environmental effects in amorphous silica assuming a Bell—Dean distribution of ring sizes. The quantum mechanics fracture model and experimental V—K I curves exhibit the same sequence of environmental effects. The contractions of the ring during fracture are related to the fractal nature of the fracture surface. The characteristic atomic dimension a 0 of fracture emerges from the analysis of contracted ring structures. Theoretical calculations of a 0 for silica, silicon, alumina and zinc sulphide match experimental values to within 10—20%.

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