Self-propagating high-temperature synthesis of functionally graded materials as thermal protection systems for high-temp
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F. Maglia and U. Anselmi Tamburini Dipartimento di Chimica fisica, IENI-CNR, Università di Pavia, Viale Taramelli 16, 27100 Pavia, Italy (Received 30 July 2002; accepted 18 November 2002)
Self-propagating high-temperature synthesis was used to prepare boride-based functionally graded materials (FGMs) as thermal barriers for space re-entry vehicles. FGMs are characterized by inhomogeneous spatial composition, resulting in different spatial characteristics. In this work, the FGMs were composed of a ceramic [i.e., MB2 (M ⳱ Ti, Zr, Hf)] and a metallic (i.e., NiAl) side, joined together by composite layers of graded stoichiometries of the two components. Thus, in the same material, the boride end gives thermal insulation, while the intermetallic end offers an easy junction to the structure of the space aircraft. The prepared FGMs showed good adhesion between the layers and global compactness after preparation and thermal tests. The microhardness along the samples was measured, and their insulating capabilities were evaluated.
I. INTRODUCTION
Self-propagating high-temperature synthesis (SHS) is a preparation method that takes advantage of the exothermicity of the reactions involved. Fast reaction times, low energy requirements, simplicity of the experimental apparatus, and the high purity of the product are welldocumented characteristics of this method.1–5 Functionally graded (or gradient) materials (FGMs) have attracted an increasing deal of attention since their concept was introduced by Niino in 1984.6 FGMs can be considered second-generation composite materials. As a matter of fact, classical composite materials are inhomogeneous from the microscopic point of view, but homogeneous from the macroscopic one. FGMs are inhomogeneous both microscopically and macroscopically by resulting in a property variation due to their spatial compositional gradient. They are intended to improve junctions of different materials by reducing stress singularities due to mismatch of elastic or thermal properties, unwanted reflections at the interface, and poor adhesion. During the 1990s, FGMs evolved from exotic to industrially oriented materials used for applications such as thermal protection systems, hard metals for mining or cutting tools, biomaterials, and acoustic devices.7
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J. Mater. Res., Vol. 18, No. 2, Feb 2003 Downloaded: 14 Apr 2015
A variety of methods have been used to synthesize FGMs, such as chemical vapor deposition, plasma spray, magnetron sputtering, physical vapor deposition, dynamic ion mixing, chemical vapor infiltration, and reaction bonding,8–10 as well as SHS11 and various powder metallurgy techniques.12 SHS appears to be particularly appropriate for the preparation of thick FGM or mass production, due to its short operational time and low energy consumption. Moreover, SHS is suitable for preparing a wide range of ceramic and intermetallic compounds, porous materials, and metastable and complex p
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