Crack propagation in alkaline-earth fluorides

Abstract

The fracture of CaF₂, SrF₂, and BaF₂ crystals by {111} cleavage requires critical fracture energies (γ_IC) which increase with decrease in cation size, as would be predicted. The γ_IC of single crystals, however, can be affected by the crystallographic direction of crack propagation and by annealing treatment. The introduction of grain boundaries can also result in some increase in γ_IC, particularly for finer grain sizes. Slow crack growth, which is sensitive to the environment, is observed but differs from that noted in other brittle materials in that it is often accompanied by crack arrest. This observation, as well as the stair‐step crack motion during slow crack growth in concentrated HF, indicates that dislocation motion at the crack tip is associated with the slow‐crack‐growth phenomenon. Because of the extremely strong influence of the stress intensity on crack growth velocity, the effects of delayed failure should be minimal in CaF₂, SrF₂, and BaF₂.