THERMODYNAMICS AND KINETICS OF REACTIONS AT INTERFACES IN COMPOSITES

Composites are complex engineering systems in which the constituent materials are not in thermodynamic equilibrium during initial fabrication, during production of components or in use. Diffusion, phase transformations and roughening at interfaces can occur at any of these stages. By the use of thermodynamic and kinetic concepts, changes in the interface morphology in composite systems can be predicted and thus controlled. Two features which govern interfacial reactions, compositions, phases and structures are described: (1) surface energy effects at interfaces, including nucleation, and (2) stress effects accompanying diffusion at interfaces. Through both theory and experiment, small departures from equilibrium, even without the formation of new phases, are shown to cause large-scale changes in diffusion near the interface and interface morphology. These theoretical concepts are used to examine reactions in the AlSiC system and control of phase formation and interface structure in Al2O3Cr2O3Cr metal composites. © 1990.