Surface Chemistry and Slow-Crack Growth Behavior of Fluorozirconate Glasses

Abstract

The surface chemical reactions which might contribute to stress corrosion in fluorozirconate glasses have been investigated and the results are related to the slow crack growth behavior of these glasses. The surface studies indicate that aqueous environments are especially aggressive. It is found that the penetration and bulk diffusion of molecular water precedes any hydrolysis or dissolution of the glass structure. The chemisorption of water to produce oxide or hydroxide species is not prevalent except in the case of the basic aqueous environments. The most notable feature of the stress intensity-crack velocity diagrams is a distinct and reproducible stress-intensity threshold at KI = .15–.20 MPa-m1/2 in the environments which contain water. This threshold is observed for both gaseous and liquid environments even though the surface chemistry and corrosion in water vapor and liquid water are very different. The wet nitrogen atmospheres also exhibit a velocity plateau whose magnitude increases systematically with the water activity; it approaches the plateau observed in pure liquid water at ~10−3 m/s. Altogether, it appears that the crack growth mechanism may be independent of any chemisorption reactions at the crack tip. Perhaps, the formation of a coordinate bond between molecular water and the barium or zirconium cations weakens the coulombic forces of attraction with the fluorine anions, and thereby, influences the energy necessary for crack propagation. This interaction also appears to be the first step in the corrosion reaction. It is further suggested that the stress-intensity thresholds and velocity plateaus may be due to stress-enhanced water diffusion ahead of the crack tip where it modifies both the material and stress distribution.