Effect of surface preparation on elastic precursor decay in shocked pure lithium fluoride

To understand the mechanisms for elastic precursor decay in pure 〈100〉 lithium fluoride (LiF) crystals under shock loading we have examined the role of large dislocation densities produced near the crystal surfaces by sample preparation. After unsuccessfully trying various methods to produce flat and undamaged samples, we chose to harden the surfaces by inward diffusion of magnesium fluoride (MgF2). A combination of heat treatments and other procedures was developed to ensure hardened or doped surfaces with a maximum depth of 600 μm and an undoped interior. Sample characterization was carried out using dislocation etch pit, hardness, and electron microprobe measurements. A 0.24-mm sample, with MgF2 completely diffused through, gave a 16.7-kbar precursor amplitude. A 2.93-mm sample with a front diffusion layer of 0.14–0.39 mm and a back diffusion layer of 0.00–0.32 mm and undoped interior gave a precursor amplitude of 2.1 kbar. These experiments indicate that the surface damage layer does not control precursor decay in pure LiF crystals.