5. Interface

Interface is a very general term used in different fields of science and technology to denote the location where two entities meet. The term in composites refers to a bounding surface between the reinforcement and matrix across which there occurs a discontinuity in a parameter such as chemical composition, elastic modulus, coefficient of thermal expansion (CTE), and thermodynamic properties such as chemical potential. Interface (fiber/matrix or particle/matrix) is very important in all kinds of composites. This is because in most all composites, the interfacial area per unit volume is very large. Also, in most metal matrix composite (MMC) systems, the reinforcement and the matrix will not be in thermodynamic equilibrium, i.e., there will be a present thermodynamic driving force for an interfacial reaction that will reduce the energy of the system. All these items make the interface to have very important influence on the properties of the composite.