Sol-gel control of the micro/nanostructure of functional ceramic-ceramic and metal-ceramic composites
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Sol-gel control of the micro/nanostructure of functional ceramic-ceramic and metal-ceramic composites Philippe Colomban LASIR, CNRS, 2 rue Henri Dunant, 94320, Thiais and Materials Department, ONERA, BP 72, 92322 Chˆatillon, France (Received 28 July 1997; accepted 5 January 1998)
The problems encountered to tailor simultaneously various specific chemical or physical properties are discussed. Selected polymeric precursors used in association with fine powders allow the control of the nano/microstructure of composites and hence the preparation of functional (FGM) and hierarchical reinforced (HRC) composites, making it possible to combine several kinds of fibers, interphases, and matrices in the same composite (hot microwave absorbent), to control the fiber/matrix interface (long life times composites), to achieve net-shape sintering of 3D composite matrices, and to prepare thick films of metal-ceramic composites with tailored microwave absorption (radar stealthiness).
I. INTRODUCTION
A key technological challenge in the aerospace field is to design new materials satisfying specific and sometimes conflicting properties. For example, the replacement of heavy metal parts of aircraft engines by lighter ceramic matrix composites would help to decrease both NOx pollution and fuel consumption. The incorporation of long, woven, ceramic fibers embedded in a refractory matrix results in damage-tolerant materials exhibiting high toughness through a specific micromechanism at the fiber-matrix interface: the cracks are deflected, dissociated, and stopped at the interfaces and the resultant fibrous and pseudo-plastic fracture allows design of reliable parts.1 The key step in the advancement of the technology is the control of the reaction between the fiber surface and the embedding matrix. The sliding interfacial behavior results from the presence of a very compliant material between the fiber and the matrix, typically C or BN coating. Unfortunately, these latter compounds are unstable in the oxidizing— sometime wet— atmosphere of aircraft engines. Thermally and chemically stable interphases are therefore required. We will show how the sol-gel route and more generally the liquid precursor routes allow the tailoring of the micro- and nanostructure, especially at the fiber/matrix interface. Another problem in composites preparation is the difficulty to achieve a coherent and robust matrix at low temperatures. The matrix precursor must be introduced between the yarns fibers (their spacing is a few microns or less) through interyarn voids of a few millimeters. Furthermore, the presence of a fiber preform with invariable geometry inhibits the coherent shrinkage of the matrix. Only infiltration of gaseous or liquid precursors J. Mater. Res., Vol. 13, No. 4, Apr 1998
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is applicable for which the ceramic yield is low, which implies porosity and cracking after precursor pyrolysis unless a very slow and time expansive multisequenced proce
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