During the 20 years I have been working in the field of metal matrix composites, I have always been drawn to the study of interfaces. Any problems the materials researcher or developer encounters will eventually be tied to some issue involving the interface. The interface controls the in-situ fiber strength and hence the axial strength of the composite. The transverse strength of composites is also controlled directly by the strength of the interface. It follows that in order to optimize a fiber/matrix reinforcement system, one must also optimize the interface. It is no accident, then, that so soon after the two excellent issues on interfaces edited by D. Wolf and S. Yip (MRS Bulletin, September and October 1990), we launch into the subject again. This time, we approach the interface from the point of view of integrating the fields of materials science and engineering, i.e., integrating the structure-processing-properties-materialssynthesis relationships.

The concept of interface design is relatively new. Until recently, we simply controlled damage and limited fiber degradation by controlling processing variables, and contented ourselves with what nature allowed. For the most demanding applications, nature was not permitting reliable structures.

Our emphasis in this issue of the MRS Bulletin is the development of the skills and strategies to design interfaces to a specifie application. In the future, we will be asking materials to perform structurally at temperatures approaching 3000°C in some propulsion applications. The success of such applications will depend on how well we design the interfaces.

The key to controlling and predicting the properties of metal matrix composites lies in understanding and controlling the interface. When properly designed, the interface between reinforcing fibers and the matrix or protective coating can act as a mechanical fuse through a controlled delamination mechanism.

Controlled delamination, in effect, results in the decoupling of fibers from early damage due to stress concentrations in the vicinity. The delamination event must precede the crack bridging and frictional pull-out mechanisms that have been so effectively demonstrated in ceramic matrix composites. The delamination event, therefore, is the necessary precondition, and so analysis of composite toughening must start with a definition of the conditions for interface debonding.

This decoupling can be expressed in terms of cohesive strength of the interface, shear strength of the interface, and fiber fracture stress. In a related but alternative manner, debonding can be expressed in terms of the intrinsic work of fracture of the interface as compared to the transverse work of fracture of the fiber.

Mechanical properties of interfaces between dissimilar or similar materials (e.g., grain boundaries) have become the focal point of research in several fields, including composite materials (metal, ceramic and intermetallic matrix composites), tribology, and solid state devices. This is not surprising because the interfaces between dissimilar materials are sites for mechanical stress concentrations and often nucleate the overall failure process.

Interfaces of interest in composite materials exist between fibers and their diffusion barrier coatings or between the fibers and the surrounding matrix material. In the field of tribology, interfaces exist between various types of functional (magnetic, conducting, optical, electrical), protective (thermal barrier, corrosion, wear resistant), or decorative coatings and their underlying substrates. And, finally, metal/ceramic interfaces are of interest in multilayer devices and magnetic disks and head technology. In all the above applications, mechanical properties of the interface (tensile and shear strength, toughness, etc.) often control the overall functionality of the coated part. Therefore, improving the mechanical properties of the interface for a prolonged life of the coated part is of fundamental interest. However, in ceramic and metal matrix composites, where the fiber/coating interface is used to deflect impinging cracks from the matrix, it is often desirable to impair the strength of the interface.

Surface phenomena, including wetting, have surely fascinated humans foi millennia. The breakup of a falling stream of water and the behavior of froths, both driven by surface energy, certainly caught the interest of people long before records were kept. The Phoenicians are credited with inventing that remarkable wetting agent, soap, in about 600 B.C., and a 2000-year-old soap factory was unearthed in the ruins of Pompeii.

It is hard to pinpoint the first scientific studies of surfaces and wetting, but it is a good guess that the earliest scientists, or natural philosophers, were interested. In 1805 Young noted that in a system consisting of a liquid and a solid phase, the former will wet the latter to a degree dictated by the surface energies of the system. If a liquid drop rests on a solid substrate, as shown in Figure 1, the wetting angle, θ, characterizes the wetting of solid by liquid. The fundamental relationship between wetting angle and surface energy in equilibrium was given by Young as:

where γsv is the surface energy of solid, γsl is the solid-liquid interfacial energy, γlv is the surface energy of liquid, and θ is the wetting angle.

The advent of new materials processing and fabrication techniques has made it possible to control atomic-scale structure to an extent only dreamed of as recently as 20 years ago. This improved structure control takes us one step closer to synthesizing materials with a combination of properties tailored to the intended application. It is this ability to control structure that will accelerate the pace of technological advancement which is limited by the rate of materials development.

Mechanical performance of materials restricts their use, even for nonstructural applications. Magnetic composites used in recording heads are an example. The mechanical and tribological properties of these materials are a major consideration when designing reliable components.

Another example is electronic materials, where the strength of the interface between the polymer packaging and the semiconductor controls the lifetime of the semiconductor device. The semiconductor device undergoes thermal cycling as it is used, straining the polymer/semiconductor interface. Environmental impurities such as moisture can enter through cracks in the interface and degrade the semiconductor, causing device failure.

It is generally agreed that the mechanical performance of continuous fiber-reinforced composites will depend to a large extent on the nature of the interface between the fiber and the matrix. In the case of many inorganic composites, compositional and/or morphological gradients can arise over an extended region between the matrix and reinforcing fiber as a result of processing conditions, mechanical forces, or chemical interactions. The term “interphase” has been applied to such a region.

There have been numerous theoretical and experimental treatments of this subject in an effort to quantify the stress and strain in the interphase region, and to correlate the presence of such forces with the mechanical performance of the attendant composites. Much of the experimental work has emphasized controlling the nature of the interphase through careful control of the processing conditions or by the introduction of special interface layers, often by applying a coating to the reinforcing fiber prior to incorporating it into the matrix.