Combustion-synthesized functionally gradient refractory materials
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Yoshinari Kaieda National Research Institute for Metals, 2-3-12 Nakameguro, Meguro-ku, Tokyo 153, Japan (Received 23 December 1992; accepted 23 March 1993)
Functionally Gradient Materials (FGM's) are soon to be used in a variety of important commercial applications; joining and thermal barrier coatings are two of the most widely studied. FGM's of the TiC/NiAl and the TiC/Ni 3 Al systems were fabricated using a one-step, self-propagating high-temperature synthesis (SHS) and densification method. It was observed that ignition of the starting mixture for these two systems was affected by the initial sample temperature and the external pressure that was applied to the sample during the ignition stage. Quality of the final product (e.g., porosity, grain size, cracking and microcracking, etc.) depends on a number of factors during this one-step operation. Reaction temperature control is important and is necessary to minimize residual porosity of the final product. Particle size of reactant powders, as well as applied pressure, also has an effect on the resulting microstructure. If careful reaction temperature control is achieved, along with optimum reactant powder size and applied pressure, an FGM of minimal porosity is obtained without residual macrocracks. Further, this method can easily be used to fabricate an FGM with a highly precise composition and material properties gradient. Finally, this process results in FGM's of similar quality when compared to those prepared by existing fabrication methods at only a fraction of the cost. Most importantly, it is expected that this process can be scaled up with relative ease.
I. INTRODUCTION Functionally gradient materials (or "FGM's") represent a second generation of composite materials. While conventional composites are microscopically inhomogeneous, but macroscopically homogeneous, FGM's are intentionally inhomogeneous on both the microscopic and macroscopic length scales. They are characterized by a spatial variation in composition, introduced during fabrication, that results in a similar gradient in material properties. Coating/substrate interfaces and other transition layers thus become "smeared" or "fuzzy," rather than discontinuous or "steep" as is usually the case with conventional interphase boundaries. The benefits include improved thermal shock-, impact-, and wear-resistance, reduced mechanical and electromechanical fatigue, and beneficial residual stresses (a sort of "prestressing"). Important applications include thermal barrier coatings, turbine engine components, the National Aerospace Plane, disk brakes, hot gas valves and tubes, and certain piezoelectric devices. FGM fabrication methods include chemical vapor deposition1 and plasma spray techniques,2 as well as magnetron sputtering, physical vapor deposition, dynamic ion mixing, chemical vapor infiltration, reaction bonding,3 SHS, 4 and various powder metallurgy 2026
http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 8, Aug 1993
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techniques.5 Many of these methods are, how
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