Cleavage surface energy of the ((111)) plane of strontium fluoride

Abstract

Due to its excellent optical transmission in the 2–6‐μm wavelength region, SrF₂ is an attractive candidate window material for high‐energy CO and chemical lasers. Fracture strength, an essential parameter used to evaluate window materials, is largely controlled by the specific surface energy of potential cleavage planes in brittle ionic crystals such as SrF₂. This quantity has been measured previously for the ((111)) plane of CaF₂ and BaF₂, but not for SrF₂. In order to obtain this information, a modification of the Obreimov‐Gilman cleavage technique has been used to determine the specific surface energy of SrF₂ ((111)) under a variety of conditions and two methods of calculation of the experimental value have been examined and compared. An intrinsic value of 260±10 erg/cm² deduced from the data at 298°K is below theoretical estimates, but corresponds most closely to surface energies calculated using Pauling's ionic radii. An intrinsic value of 430±75 erg/cm², obtained by an alternative method of calculation, is in excellent agreement with surface energies deduced from a Born‐Mayer potential and Huggins radii. However, this alternaive method of calculation markedly increases the apparent scatter of the data, rendering its validity questionable. The apparent surface energy of annealed SrF₂ decreases linearly with the longarithm of crosshead speed from 0.002 in./min to 2.0 in./min. This effect is attributable, at least qualitatively, to energy dissipation through dislocation multiplication by the multiple cross glide mechanism in the region of the crack tip. While vacuum (10⁻⁶ Torr) annealing at 800°C and variation in the direction of crack propagation are found to have a negligible effect on γ, cleavage at 77°K or in the presence of a DMF/DMSO solution leads to significant increases in γ. These increases may be attributed, respectively, to crack blunting and energy dissipation by dislocation multiplication and step formation.