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.
Exposure to ionizing radiation is known to alter the mechanical behavior of ionic crystals by impeding the motion of dislocations, leading to increases in such parameters as yield strength and hardness. The influence of irradiation upon fracture strength is not as well understood, particulary for the alkaline earth fluorides, which find important technological use as ir laser windows. In order to better assess these effects, changes in the apparent specific surface energy of the ((111)) planes of SrF2 at 298 °K induced by Co60 γ-gamma irradiation, as determined by a modified Obreimov-Gilman cleavage technique, have been measured. The apparent surface energy is found to increase approximately linearly with the logarithm of total exposure to Co60 γ irradiation in the range from 9.6×105 to 4.3×107 R. This effect is consistent with the decreased dislocation mobility and size of the loops generated at the crack tip, the increased concentration of dislocation sources, and the lengthening of the easy-glide region of the stress-strain curve.
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