This issue of the MRS Bulletin provides an up-to-date look at ongoing research activities within the field of functionally gradient materials (FGM). The term FGM, now widely used by the materials community, originated in Japan in the late 1980s as a description for a class of engineering materials exhibiting spatially inhomogeneous microstructures and properties. Of course, gradient materials are not something new. It must be recognized that humans have extensively utilized materials containing microstructural gradients (either those found in nature or those created through processing) since the earliest days of craftsmanship and engineering construction. Indeed, there are examples of graded materials developed long ago, such as case-hardened steel, which are still in common use today. Contemporary examples of these materials serve in technologically significant applications, as, for example, in thermal-barrier coatings for gas turbines. Nevertheless, what is new and exciting about FGMs is the realization that gradients can be designed at the microstructural level to tailor a material for the specific functional and performance requirements of an intended application. In addition, recent advances in processing are opening the possibility for the extension of the gradient materials concept to new materials systems and engineering problems.

The recent resurgence of interest in gradient materials has been driven by the need for improved materials, capable of meeting the demanding performance requirements established by emerging technologies such as the aerospace plane, ceramic engines, and nuclear fusion.

Space planes require high-performance heat-resistant materials which can withstand ultrahigh temperatures and extremely large temperature gradients. To meet these needs, functionally gradient materials (FGMs) were proposed about 10 years ago in Japan.

Figure 1 shows a conceptual diagram of functionally gradient materials, taking into account the relaxation of thermal stress. For the surface that contacts high-temperature gases at thousands of degrees, ceramics are used to provide adequate heat resistance. For the surface that provides cooling, metallic materials are used to furnish the necessary thermal conductivity and mechanical strength. In addition, the composition of these materials is formulated to provide optimum distribution of composition, structure, and porosity to effectively relax thermal stress.

Since fiscal 1987, an R&D project entitled “Research on Fundamental Techniques to Develop Functionally Gradient Materials for Relaxation of Thermal Stress,” which aimed to develop ultra heat-resistant materials, had been carried out with special coordination funds from the Science and Technology Agency. The five-year project had two phases; Phase I was carried out from 1987 to 1989, and Phase II from 1990 to 1991.

The application of functionally graded materials (FGM) principles is widespread in Europe, despite the lack of national or European programs coordinating the efforts of the individual institutions and researchers. In the following article, the status of research in Europe will be outlined by examples of research projects from various application fields. More detailed information on European FGM research is available from the proceedings of the FGM '94 symposium held in October 1994 in Lausanne, Switzerland. The symposium was organized by B. Ilschner through the Materials Department of the Swiss Federal Institute of Technology of Lausanne.

Functionally gradient materials (FGMs) display continuously or discontinuously varying compositions and/or microstructures over definable geometrical distances. The gradients can be continuous on a microscopic level, or they can be laminates comprised of gradients of metals, ceramics, polymers, or variations of porosity/density. Several processing techniques have been explored for the fabrication of FGMs for structural applications, e.g., powder metallurgy, thermal spraying, in situ synthesis, self-propagating high-temperature synthesis, reactive infiltration, etc. Physical and chemical vapor deposition (CVD) techniques are also being explored to process FGM films with nanometer level gradients in composition. This article addresses the issues related to thermal-spray processing of FGMs and will only peripherally compare the advantages and limitations of thermal spray versus other processing techniques as reported in the literature.

In thermal spraying, feedstock material (in the form of powder, rod, or wire) is introduced into a combustion or plasma flame. The particles melt in transit and impinge on the substrate where they flatten, undergo rapid solidification, and form a deposit through successive impingement. Thermal spraying has been traditionally employed to produce a variety of protective coatings of ceramics, metals, and polymers on a range of substrates. More recently, the process has been used for spray-forming structural components.

Arc spray, combustion, and plasma are the major techniques comprising thermal spray. These classifications are based on the type of heat source and the method by which feedstock is injected. Arc-spray processes use electrically conductive wire as feedstock, while combustion methods use powder or wire.

Powder metallurgical (P/M) processing of FGMs provides a wide range of compositional and microstructural control, along with shape-forming capability. Oxide/metal systems are desirable because this materials combination can be used to easily tailor materials properties. However, there are many problems to be investigated which pertain to each of the processing steps; process innovations will often be required to realize the versatility of this route. In this article, I briefly review the present status of the powder-processing method.

Powder metallurgical fabrication of FGMs involves the following sequential steps with a selected material combination of metals and ceramics: determination of the optimum composition profile for an effective thermal-stress reduction; stepwise or continuous stacking of powder premixes according to the predesigned composition profile; compaction of the stacked powder heap and sintering with or without pressurizing. Besides the conventional powder metallurgical routes, a spray deposition method, using mixed powder suspensions and a slurry stacking method, have been developed to form continuously graded stacking. A powder spray stacking apparatus has been devised, which is fully automatic with computer control. Deposited compacts were cold isostatically pressed (CIP) and consolidated by hot isostatic pressing. Their microstructures show that this process provides fine compositional control with desired profiles.

Differential temperature sintering by laser-beam heating has been studied to add versatility to the P/M process. The surface of the green compacts is scanned with a laser beam using a predesigned scanning pattern to ensure homogeneous heating over the entire surface.

Bamboo and certain other plants have excellent characteristics based on functionally gradient structures. Figure 1 shows a cross section of bamboo, corn, and barley culms. The collection of solid dots in bamboo is called the bundle sheath, which acts as fibers for the plant. These bundle sheaths are shown in the magnified photograph of corn and barley. These two plants are made of composite materials reinforced by fibers of the bundle sheath; the fiber strength is about 10 times that of the matrix. Furthermore, the fiber distribution is dense in the outer region and sparse in the inner region. It can be said that the fiber distribution forms a gradient structure. The fibers in barley are sparse in the narrow cross section and are different from the fibers in corn and bamboo. The number of honey-comb-shaped cells in barley is higher on the outside than the inside region. Therefore, barley also has a gradient structure. These relations of the microscopic gradient structure are displayed graphically by the cross-sectional photographs. Figures 1a and 1b present the fiber contents of bamboo and corn, respectively, with respect to radius measured from the outer radius. Figure 1c gives the gradient distribution of the cell number for barley with respect to radius. These gradient features probably relate to the bending-stress distribution caused by wind, rain, and snow—the so-called environmental loads.

When a discontinuity in material properties exists across a bonded interface, stresses are generated as a result of any thermal or mechanical loading. These stresses significantly affect strength and failure characteristics and may be large enough to prevent successful fabrication of a reliable joint. The use of an interlayer material to successfully reduce mismatch stresses, thereby preventing joint failure or improving joint strength and reliability, requires knowledge of failure mechanisms and of the effects of interlayer properties on the critical stress components.

The origin of residual stresses developed during cooling of a ceramic-metal joint from an elevated fabrication temperature is illustrated qualitatively in Figure 1. Away from edges, the in-plane (parallel to interface) stresses are typically compressive in the ceramic and tensile in the metal. These stresses can cause cracking perpendicular to the interface, leading to spalling or delamination failures. Such failures are frequently observed in thin-film and coating geometries. Where the interface intersects a free edge, large shear and axial (perpendicular to the interface) stresses are generated. The edge stresses are typically tensile within the ceramic and tend to promote crack propagation within the ceramic parallel and adjacent to the interface. This is the most commonly observed failure mode in bonded structural components.

For the design of functionally gradient materials (FGMs), necessary material properties, such as thermal-expansion-coefficient and Young's modulus in the specific region, are optimized by controlling the distribution profiles of composition and microstructures, as well as micropores in the materials. For this purpose, our research team employs the inverse design procedure in which both the basic material combination and the optimum profile of the composition and microstructures are determined with respect to the objective structural shape and the thermomechanical boundary conditions. Figure 1 shows the inverse design procedure for FGM, in which the final structure to be developed, as well as the boundary conditions, are specified first. After the fabrication method and an allowable material combination are selected from the FGM database, the estimation rules for the material properties of the intermediate compositions are determined based upon the micro-structure. Then, the temperature distribution and the thermal-stress distribution are calculated with the assumed profiles of the distribution functions for the constituents. Other possible combinations and different profiles are also investigated until the optimum is obtained.

In today's highly demanding technological environment, one of the main challenges in new material design is combining seemingly irreconcilable thermomechanical properties in the same component (e.g., high heat and corrosion resistance, high strength in elevated-temperature applications and high resistance to wear, and high toughness in load-bearing elements). In many cases, the problem may be solved by using coatings or by layering dissimilar materials. From a structural viewpoint, a major disadvantage of these techniques, particularly in ceramic coating of metals, has been the resulting high thermal and residual stresses and relatively poor bonding strength. Thus, in thin films, coatings, and layered materials, surface cracking and debonding or delamination have been common forms of mechanical failure. One effective way of reducing residual and thermal stresses and enhancing bonding strength has been to eliminate material-property discontinuities by grading the material composition near the interfaces or through the coating. These new materials, with continuously varying compositions or volume fractions, are known as functionally graded materials (FGMs).

In developing FGMs, research on the mechanics, and particularly on the fracture mechanics of these inhomogeneous materials, is needed to provide technical support to materials scientists and to manufacturing and design engineers. In the past, fracture mechanics has been useful both as a screening tool during material processing and as a design and maintenance tool for service-life assessment. Broadly speaking, fracture mechanics involves studying the effect of the applied loads, the component/flaw geometry, and the environmental conditions on the fracture of engineering materials.

Various methods are used in functionally gradient materials technology to control the composition and structure of a composite.

The chemical vapor deposition (CVD) method yields a deposit with source gases by applying variou s forms of energy (heat, light, plasma, etc.) to the gases after they are introduced into a CVD reactor. Hydride, bromide, and chloride are generally used for source materials. By continually changing the mixture ratio of source gases or by controlling the CVD conditions such as deposition temperatures, gas pressure, and gas-flow rate, CVD permits relatively easy syntheses of various FGMs (to a maximum thickness on the order of a centimeter).

A characteristic feature of soft biological tissue is a low compression modulus at small strains. On the other hand, the compression modulus of artificial elastomers, such as vulcanized rubber, generally increases with decreasing strain, according to the theory of rubber elasticity. In this paper, I will consider that the low compression modulus of biological tissue is possibly brought about by the organic and inorganic FGM structure of the tissue. An example of a bivalve hinge ligament is given here.

The tissue of the hinge ligament of a bivalve (Spisula sachalinensis) is a hybrid material, composed of a soft cross-linked protein matrix and inorganic calcium carbonate crystals (aragonite). In this example, small rectangular strips were cut from the ligament. The aragonite can then be easily removed from the tissue by treating it with dilute acetic acid (to decalcify the sample). At this point, mechanical anisotropy and swelling were measured and discussed, based on the anisotropic, fine structure of the aragonite.

Figure 1 shows the coordinates for a bivalve hinge ligament. The x axis and y axis are the growth direction and the tangential direction against the growth line, respectively. The z axis is approximately normal in direction to the shell surface. X-ray diffraction of a long strip cut along the x axis showed a typical fiber pattern characteristic of the aragonite structure. The crystallographic c axis of the aragonite coincided with the x direction.

This article focuses exclusively on the application of functionally gradient materials (FGMs) as structural components in high-heat-flux environments. Preliminary data suggest that FGMs can equal or surpass the properties of fiber-reinforced composites (FRCs) in these applications at a fraction of the material costs. An optimistic value of $1 per foot for 100-μm-diameter Saphikon alumina fiber yields a per-pound cost of $48,000. In contrast, a costly alumina powder used for FGMs might be in the range of .$20 per pound. While many counter-arguments can be advanced, this price differential cannot be easily overcome.

One relative difficulty with FGMs is their inherent compositional flexibility which, ironically, is normally considered an advantage. Determination of the optimum material combination for a given application should precede any effort directed toward determining the “best” processing route. This combination will then specify the processing technique. This would “short circuit” the expensive learning curve historically associated with FRCs.

“Processing for processing's sake” is no longer a viable option in today's composite research market. Without necessary advances in the predictive modeling of structure, composition (and, concurrently, cost) and behavior, FGMs may remain an “on-the-horizon” advanced composite.

FGMs have been fabricated using the combustion synthesis (or self-propagating high-temperature synthesis (SHS)) process by exploiting a rapid and exothermic chemical reaction, in order to synthesize some (or all) of the constituents in an FGM to simultaneously increase its density. The thermal energy required to drive the process is derived from this internal, chemical source, rather than from an external and usually expensive source (e.g., a furnace). The combustion synthesis process is a powder-based process that has been used to synthesize over 300 compounds, and is particularly useful in preparing materials such as highly refractory ceramics and high-temperature intermetallics that are difficult to prepare by other synthesis methods. In addition, the process can be used to prepare ceramic-metal and ceramic-intermetallic composite materials. As a result, only slight modifications of the combustion synthesis are required to prepare functionally gradient materials from these same combinations of materials.

Sample preparation begins by the creation of a series of mixtures from the powders that will react to form the constituent materials of the FGM sample. Each of these mixtures contains a slightly different percentage of reactants, so that each mixture will yield its own (predetermined) volume fraction of each of its constituents, following the combustion synthesis process. Prior to the combustion step, the samples are assembled by stacking layers of each of the reactant powder mixtures in appropriate amounts, in such a way that the multilayered powder mixture will faithfully produce the composition gradient that is required in the resultant FGM.