40207_14 - CERAMIC COMPOSITES M.F. Amateau 14 The...

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CERAMIC COMPOSITES 14 M. F. Amateau 14.1 INTRODUCTION Ceramics generally are compounds of metallic or non-metallic elements and other non-metals such as oxygen, nitrogen, carbon and boron. Compared to metals these compounds have higher melting temperatures, higher Young’s moduli and hardness, lower densities and lower electrical and thermal conductivities. Engineered ceramics are used in thermal and structural applications requiring high temper- ature resistance, high hardness and chemical inertness. Applications that exploit the ther- mal structural properties of ceramic commonly include cutting tool inserts, wear resistant components, ballistic armor, heat exchangers, burner tubes, prosthetics, dental implants, heat engine components and ther- mal barrier coatings. The dominating characteristics of ceramics that limit these and other engineering applications are their lack of plastic behavior at room temperatures and their low tolerance to flaws, i.e. low fracture toughness, that lead to catastrophic failure. Reinforcing ceramics with particles, whiskers, platelets, discontinuous fibers and continuous fibers significantly improves their strength, toughness and apparent ductility. Composite design can also be used to tailor other important properties such as high tem- perature strength and thermal shock resistance, wear resistance and low friction, thermal and electric conductivity and thermo- elastic proper ties. Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7 The mechanical characteristics of ceramic composites are profoundly influenced by the mechanisms of strengthening and toughening. Successful design of ceramic composites requires an understanding of the role of the constituents, including reinforcement, matrix and interphase, on the these mechanisms. In addition to the properties and form of the con- stituents, the characteristics of the processing method can also have a major impact on mechanical and physical properties. 14.2 CONSTITUENT MATERIALS 14.2.1 REINFORCEMENTS Fibers used in ceramic matrix composites fall into three general categories based on their diameter: monofilaments, textile fibers and whiskers. In addition, reinforcements in the form of particulates and platelets are used in ceramic composite designs. The strengthening role of the reinforcements in ceramic compos- ites is significantly different from that in polymer and even metal matrix composites. In metal and polymer composites the reinforce- ments contribute directly to the increase of strength and stiffness by carrying a significant portion of the load. In ceramic composites the reinforcements usually increase the strength indirectly by increasing toughness of the matrix. The load carrying capabilities of the reinforcements are of secondary interest at most. This places special importance on the whiskers as reinforcements for ceramic com- posites.
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308 Ceramic composites Whisker reinforcements Whiskers are a very fine filamentary form of a material. They are usually 1 pm or less in diameter and up to 200 pm in length. They are
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40207_14 - CERAMIC COMPOSITES M.F. Amateau 14 The...

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