This is the second of two issues of the MRS BULLETIN on interface materials and is entirely devoted to their mechanical and high-temperature behavior. Mechanical properties provide a rich area for investigating the effect of the local inhomogeneities near the interfaces, and their effect on the interrelation between the structure and chemistry on one hand, and the elastic and fracture behavior on the other.

Based on much experimental work on grain-boundary fracture it seems that, with the exception of “beneficial” segregants, the embrittlement potential of most impurities is governed by their propensity for segregation to the grain boundaries, which in turn is strongly influenced by the energies of the pure boundaries. To investigate the role of the grain-boundary structure in its fracture behavior, one must therefore consider the correlations between (1) the structure (i.e., the five macroscopic degrees of freedom) and the energy of pure grain boundaries, (2) impurity segregation and the grain boundary energy, (3) structure, impurity segregation and elastic response at the interface, and finally (4) the correlation between embrittlement and segregation. In addition, the mobility of dislocations near a crack tip also plays an important role. Unfortunately, relatively little knowledge has been accumulated on most of these complex interrelations even though their unraveling is widely recognized as the ultimate goal.

An interface can be defined as a surface that serves as a common boundary between two phases. Examples include the boundaries between two solids, two immiscible liquids, a solid and a liquid, a solid and a gas, and a liquid and a gas. Interfaces have been studied for decades by scientists of many different disciplines. One reason for this interest is that the atomic structure and the chemical composition at the interface can differ from that of the bulk material on either side of it. Consequently, the properties of the interface can differ greatly from those of either bulk phase, and chemical reactions can occur more readily at the interface than in the bulk.

All the interfaces listed in the previous paragraph are of interest to materials scientists. However, this article will only consider the grain boundary because it has received the most attention by researchers in materials science. Furthermore, we will only consider grain boundaries in metals; nonmetallic systems will be covered in other articles in this issue.

A grain boundary is an interface that exists where two single crystals are joined in such a way that their crystallographic orientations are not completely matched. Thus, any polycrystalline material contains many grain boundaries. They occur wherever the individual grains meet one another and can usually be observed by etching a polished cross section of the surface as shown in Figure 1. Grain boundaries first form in a metal as a result of the multiple nucleation sites that occur during solidification.

The presence and origin of elastic anomalies in superlattices, interfaces, composites, and nanocrystalline materials has been a subject of much interest and controversy in recent years. In particular, superlattices are being used as model systems to study the effect of interfaces on the mechanical properties of novel materials. Early claims of anomalously large enhancements of the biaxial and flexural moduli of Au/Ni and Cu/Ni superlattices (“supermodulus effect”) created considerable controversy and contradictory reports in the experimental and theoretical literature. To understand the mechanical properties of superlattices and their implication for other types of nanofabricated materials, it is important to look critically at the field.

Superlattices have been fabricated in different laboratories by a variety of preparation methods and have been characterized structurally and elastically to various degrees. Because of this, before addressing any sophisticated theoretical issues regarding elastic anomalies, it is important to understand in detail the experimental techniques and the possible pitfalls in the experimental determination of mechanical properties of thin films. Once the presence or absence of the effect is ascertained, the need arises to understand any possible experimental correlations with other structural and/or physical properties. These correlations can be used to address issues regarding the possible origins of the effect and their theoretical basis. It will be apparent from this article that slight changes in structural properties are correlated with relatively large elastic anomalies. Consequently, this points to the importance of precise, comprehensive, and detailed structural characterization. With our present understanding of the physics of superlattices, qualitative studies are no longer sufficient!

The importance of interface materials is based largely on their inherent inhomogeneity, i.e., that the chemical composition and physical properties at or near an interface can differ dramatically from those of the nearby bulk material. For example, the propagation of a crack along an interface — rather than through the surrounding bulk material — indicates a different mechanical strength near the interface. Also, the elastic response and thermal behavior near an interface can be highly anisotropic in an otherwise isotropic material, and can differ by orders of magnitude from those of the adjacent bulk regions. Typically, these gradients extend over only a few atomic distances.

Because relatively few atoms control the properties in the interfacial region, the inherent difficulty in the experimental investigation of buried interfaces is actually an advantage in the atomic-level study of solid interfaces by means of computer-simulation techniques. While the limitations of such simulations are well known, this article will attempt to demonstrate the unique insights they can provide on some aspects of the mechanical behavior of both buried and thin-film interfaces. While to date, relatively little simulation work has focused directly on the observation of crack extension, we will discuss two types of phenomena with particular relevance in the fracture behavior of interface materials, namely their elastic and high-temperature properties. We will conclude with an outlook, too optimistic perhaps, on how the complementary capabilities of continuum-elastic theory, atomic-level computer simulation, and experiment could (and probably should) be combined in a new strategy for tackling the difficult problem of interface fracture to elucidate the underlying complex interplay between elasticity, plasticity, and temperature.

Metal-ceramic interfaces play an important, sometimes controlling, role in composites, multilayer substrates, capacitors, electron tubes, and automotive power sources. Often bonding and adhesion between the ceramic and metal are critical to the components' performance. Interface geometry and chemistry play a dominant role in determining the mechanical and electrical integrity of composites. Furthermore, unique properties may be developed from multilayer ceramic-metal structures.

Systematic studies of metal-ceramic interfaces started in the early 1960s. Such studies were directed toward identifying general rules that govern bonding and interface behavior both theoretically and experimentally, including the thermodynamics of interfacial reactions and crys-tallographic relationships, and toward evaluating atomistic structure at the interface. This article summarizes results concerning the interrelation between atomistic structure and the macroscopic fracture resistance of metal-ceramic interfaces. More details are published in a recent conference proceedings.

Determining atomistic structures of metal-ceramics interfaces is, in general, complicated since the two materials that have to be matched exhibit different atoms (ions) and possess different crystal symmetries, crystal structures, and lattice parameters. The adjacent lattices are not commensurate, the two different structures can be described as being just quasiperiodic. However, examples exist where the lattice mismatch is small, and both components possess the same lattice symmetry. Ag/MgO and Nb/Al2O3 interfaces are examples that serve as model systems for experimental studies as well as theoretical calculations. The interfaces can be formed either by diffusion-bonding, internal oxidation, or epitaxial film growth.

Structural ceramics are necessarily polycrystalline and their usefulness is largely determined by the interfaces between the grains. The relationship between the structure and chemistry of different interfaces and the micro-structure can be illustrated by reviewing studies of interfaces in a wide range of materials including such classical ceramics as Al2O3, the current “hightech” polyphase ceramics exemplified by ZrO2-toughened Al2O3, and the composite materials of the future. Using transmission electron microscopy is essential for a complete understanding, but limitations to its use must be recognized. Only by understanding the factors that control the behavior of these interfaces will it become possible to further extend the application of interface engineering.

Structural ceramics are a group of materials that can be used for applications requiring their strength to persist at high temperatures or in conditions that would be particularly corrosive to alternative materials, which are usually metallic. Strength and strength-related properties such as toughness depend largely on the microstructural features of the processed material.

The microstructure is defined by the morphology and size of the grains and the interfaces between these grains. If the grains are in intimate contact, then the interface is a grain boundary of the type familiar from studies of metals.

The synthesis of nanometer-sized atomic clusters of metals and ceramics by means of the gas-condensation method, followed by their in situ consolidation under high-vacuum conditions, has resulted in a new class of ultrafine-grained interface materials. These nanophase materials, with average grain sizes presently ranging from about 5 to 25 nm, exhibit properties that are often rather different and improved relative to those of conventional materials. In addition, their processing characteristics appear in some cases to be greatly improved over their conventional coarser grained counterparts. The synthesis of nanophase materials by means of cluster assembly under controlled conditions should enable the design of materials, heretofore unavailable, with improved or unique properties. As such, it is likely that the combination of new capabilities to synthesize, characterize, anc) engineer the properties of materials based on the assembly of atomic clusters will significantly impact materials science and engineering in the coming years.

The assembly of matter by the consolidation of gas-condensed atomic clusters is probably as old as the universe itself, since this is thought to be the way condensed matter formed during the cooling period that followed the “big bang,” as evidenced by the structure of the earliest meteorites. The modern synthesis of ultrafine-grained materials by the in situ consolidation of nanometer-sized gas-condensed ultrafine particles or atomic clusters, however, was first suggested by Gleiter.