Interfacial modelling

Abstract

Protective oxide scales are typically transition metal oxides grown onto rough, machined surfaces. The quality of the oxide, judged by the extent to which it reduces subsequent oxidation and provides certain standards of mechanical, chemical and electrical behaviour, is determined by processes at interfaces both at the boundaries of the oxide and within the oxide. Understanding these processes is the key to controlling oxide quality and oxidation. This understanding is usually approached at two levels: atomistic studies (and here the state of the art has developed enormously) and macroscopic studies (elasticity theory plus empirical deformation maps and basic fracture mechanics). Such studies can be of value, at least as a framework. However, the link between atomistic and macroscopic can only be made successfully via an intermediate level, mesoscopic, in which the grain structure and other microstructures are explicit. This paper outlines two aspects of mesoscopic modelling, covering the processes at the metal-oxide interface in the presence of a vacancy flux and stress generation within a grain boundary network. Also described are approaches to metal-oxide adhesion, especially the image charge formalism which recognizes that the major energy term is electrostatic, illustrated by the difference between non- reactive metals on MgO and NiO. For NiO there is evidence for a thin intermediate layer of a higher oxide.

Keywords:

• oxide scales
• atomistic modelling
• mesoscopic modelling