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.

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Ceramic coating is a very popular technology for improving the properties of structural materials. A titanium nitride (TiN) coating is a typical example and has been widely applied to cutting tools, electronic devices and many other fields utilizing its superior physical properties. This paper sought to produce a graded TiN coating on a Ti substrate by combining Supersonic Free-Jet PVD (SFJ-PVD) with a reactive plasma-metal reaction technique. The authors have developed SFJ-PVD as a new coating method in which a coating film is formed by depositing nanoparticles with very high velocity onto a substrate. SFJ-PVD can provide a high deposition rate and thick film coating. Gradually changing the nitrogen flow rate during deposition produces a graded TiN coating, in which composition changes gradually from pure Ti to TiN. A monolithic TiN coating is also produced with SFJ-PVD. XRD analysis of the graded TiN detected peaks for Ti, Ti 2 N and TiN, while only a TiN peak is observed in the monolithic TiN coating. EPMA analysis of a graded coating reveals a gradual compositional change from pure Ti to TiN. Few pores or cracks are observed in a graded TiN or in a monolithic TiN formed under the optimized conditions of SFJ-PVD.

The tensile fracture stress and strain of carbon fiber-reinforced carbon matrix composites (C–Cs) were examined as functions of the bulk density. When the density increased, the interfacial strength of the C–Cs monotonically increased, and the tensile fracture strain decreased. In contrast, the tensile fracture stress was improved and degraded in the regions of density lower and higher than 1.6 g/cm3, respectively. Two tensile fracture mechanisms of the examined C–Cs were identified with the transition at the density of 1.6 g/cm3. In the low-density region, load transfer capability across fiber–matrix interfaces was shown to have an important role, and in the high-density region, stress concentrations at matrix-crack tips were presumed to be a major factor for the tensile fracture of C–Cs. This suggests that the most important interfacial property for tensile fracture is not interfacial sliding but debonding stress.