Mechanical healing of lithium fluoride single crystals at elevated temperatures

Abstract

The mechanical healing of lithium fluoride (LiF) single crystals at elevated temperatures is studied. The LiF single crystal is cleaved into two halves and healed by compression at temperatures 25, 150, 300, 400, and 500 °C, respectively. The curves of compressive stress versus compressive strain are found to be separable into three regions for both healed and uncracked (virgin material) samples. The curves of fracture stress versus compressive stress consist of a) two regions if the temperature is equal to or larger than 300 °C and b) one linear region for a temperature equal to or less than 150 °C for healed samples; in addition, they are divided into two regions for temperatures 25 and 500 °C for uncracked ones. The compressive stress required to recover the same fracture stress decreases with increasing temperature. For the same compressive stress and temperature, the fracture stress of the healed sample is smaller than that of the uncracked one. The micrographs reveal the presence of many long and narrow stripes in the healed samples subjected to a small compression and many laminales in the healed ones subjected to a large compression. The two regions of the curves of fracture stress versus compressive stress for healed samples can possibly be accounted for by the morphology of the fracture surface.