Imaging the crystallization and growth of oxide domains on the NiAl(110) surface

Using low-energy electron microscopy to selectively image different surface phases, we have examined how the NiAl(1 1 0)surface undergoes oxidation. The clean surface was exposed to either a high or a low O-2 dose at 325 degreesC and then imaged at progressively higher temperatures. While the high-dose procedure does produce a relatively uniform film, oxide-free regions (pinholes) develop when the initially amorphous oxide crystallizes. This oxide film consists of one type of alumina that has two orientational domains, consistent with previous literature reports. Selective imaging of the domains just after crystallization shows that nuclei of both domains are present at high density. With time and temperature, the domains coarsen greatly, as do the oxide-free regions (pinholes). For a low O-2 dose, the substrate is only partially covered by oxide after annealing. Furthermore, the oxide is a mixture of two distinct types of oxide. That is, in addition to the same oxide type as produced at high dose, a different oxide type is also present. Like the high-dose oxide type, the new (low-dose) oxide type is also crystalline and has well-defined crystallographic relationships with respect to the substrate. (C) 2001 Elsevier Science B.V. All rights reserved.