An atomistic view of adhesion

Research on the modelling of industrial materials is particularly important for the automobile industry, in which there is a rather broad spectrum of materials interests. For a recent review of automotive materials needs, see Ref. 1. Processing and design of automotive materials often involves issues of adhesion. The nature of the strong bonds which can form between materials in intimate contact is therefore of importance. Because of the variety of materials found in these interfaces (metals, ceramics, intermetallics, and impurities), adhesion computations must be first-principles, self-consistent, quantummechanical calculations. This and the relatively low symmetry found in interfaces presents a significant challenge for computational methods. A number of years ago, the first such adhesion computations [2] revealed an unexpected universality. We found that the total energy versus interfacial spacing for a number of different metal contacts could be simply scaled onto a single curve. Subsequently [3], we found that this universality extended to a variety of materials in adhesion, cohesion, chemisorption, and to diatomic molecules, as shown in Fig. 1. First-principles computations were recently [4] carried out on Mo/MoSi 2 interfaces with and without interfacial impurities such as O, C, B, S, and Nb. Again, the universal behavior was exhibited in all cases. We found that the (equilibrium) interfacial spacings increased in proport ion to impurity covalent radii. All of the impurities were found to decrease the Mo/MoSi 2 adhesive energy, with S lowering it by a factor of two. Universal behavior was also found for A1Ni/Cr adhesion [5].