Evaporation of NaCl Cleavage Planes in Vacuum

Abstract

Cleavage planes of NaCl (99.999%) were produced in air and heated in vacua of 10−5−10−6 Torr between 454° and 593°C. Reproducible variations in pit densities were observed between 106 and 102 cm−2. Evidence is presented showing that these variations are due to changes in surface contamination. Matching cleavage planes showed no correlation in their pit sites; slip lines and small-angle boundaries showed no perferred pitting. It is concluded that dislocation emergence points per se are not sufficient for the formation of thermal pits in vacuum of 10−6 Torr. The evaporation process caused the development of (1) pits with different slope angles and (2) slope-dependent variations of their shape and faceting behavior. There was no rounding of leading or trailing edges between surface sections of different inclinations. These observations cannot be explained in terms of evaporation theories which have been suggested before. A different interpretation in terms of surface energy is given. A numerical estimate for the required variations in surface energy is included.