Effects of Counterions on Crack Growth in Vitreous Silica

Abstract

Crack‐growth studies on vitreous silica were conducted in aqueous solutions as a function of pH in‐ the range 1 to 14. Slopes of crack velocity‐ K I curves were steeper in acidic and neutral solutions than in basic solutions. Of the four alkali‐metal hydroxide solutions studied, only lithium hydroxide behaved differently from the others and only in concentrated, ≥1M, solutions. At this concentration, the slope and position of the crack‐growth curves for LiOH were nearly identical to those obtained in acidic or neutral solutions. It is suggested that the influence of alkali‐metal hydroxides on crack growth is largely chemical in nature, involving short‐range forces associated with the breaking of bonds at the crack tip. Two mechanisms for chemical attack are considered: in basic solutions the siloxane bonds at the crack tip are believed to be attacked directly by the hydroxide ions in solution; in acidic or neutral solutions the siloxane bonds are believed to be attacked by water in the electrolyte. Lithium hydroxide behaves differently from the others because lithium ions associate with hydroxide ions, reducing the effective activity of the hydroxide ions at the crack tip.