Universal behavior in adhesion of some high temperature materials

Abstract

Processing and design of materials often involves issues of adhesion. The nature of the strong bonds which can form between materials in intimate contact is therefore of considerable interest. Because of the variety of materials found in these interfaces (metals, ceramics, intermetallics, and impurities), adhesion computations must be first-principles, self-consistent quantum-mechanical calculations. A number of years ago, the first such computations revealed an unexpected universality. It was found that the total energy versus interfacial spacing for a number of different metal contacts could be simply scaled onto a single curve. Subsequently it was found that this universality extended to a variety of materials in cohesion, chemisorption, and to diatomic molecules. More recently, it was shown that the universal behavior extends to Mo/MoSi2 adhesion with and without monolayers of C, O, B, S, and Nb interfacial impurities. Impurity effects were found to be large and strongly dependent on impurity atom type. For example, S lowered the adhesive energy by approximately a factor of two. For the ceramic/metal interfaces MgO/Ag and MgO/Al with and without C and S impurities, universal behavior was again observed as were substantial impurity effects. For Al2O3/Cu, surface relaxation effects were found to be substantial, lowering the work of adhesion by a factor of three. The bimetallic interface NiAl/Cr accurately exhibited universal behavior, as expected. In all these cases electron density distributions help to illustrate the nature of the bonding, which varies from pure metallic to partially ionic - partially covalent.