THERMAL ROUGHENING OF TUNGSTEN SURFACES STUDIED BY FIELD EMISSION

Surface self-diffusion measurements of tungsten atoms and other defects were carried out by the field emission fluctuation method at various positions along the zones (011)-(112) and (011)-(001). In the first zone, consisting of (011) terraces and closepacked steps 2-dimensional diffusion attributed to atoms on terraces and 1-dimensional diffusion attributed to kinks diffusing along steps was found. At 875 K kink diffusion decreases sharply, then increases again and become 2-dimensional at 950 K, suggesting disorder and eventual disappearance of steps. This is accompanied by a decrease in atom diffusion and eventual merging of both regimes above 1000 K. On the zone (011)-(001) steps are not closepacked and only 1-dimensional was seen, although two regimes were also found. The lack of 2-dimensional diffusion is attributed to the inability of atoms to climb none closepacked steps. Evidence for roughening was also found in most locations of this second zone but at much lower temperatures than along (011)-(112)