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BORON, HIGH SILICA, QUARTZ AND CERAMIC FIBERS 8 Anthony Marzullo 8.1 INTRODUCTION Boron, high silica, fused quartz and ceramic fibers are used in demanding industrial, auto- motive, electronic, aircraft and aerospace environments. Boards made of ceramic fibers such as alumina, alumina-silica or zirconia are used as supplemental linings to insulate high temperature furnaces. Ceramic tiles contain- ing alumina and silica fibers protect the aluminum skin of USA's space shuttle orbiter during re-entry, where the tile's surface may be aerodynamically heated to 1260°C (Banas, McCormick and Creedon, 1991). Other appli- cations of these fibers involve reinforcing polymer-, glass-, ceramic-, or metal-matrix composites. If the composite is well designed, it will be tougher than the matrix material by itself. As an example, the fracture toughness of Sic is 1.5 MPa m-2, but 8-15 for Sic reinforced with Sic fibers (Richerson, 1992). Fibers can be classified according to struc- ture, diameter (or cross sectional width) and length. In general, a material is classified as a fiber if its diameter or cross sectional width is less than 0.0254 m (0.010 in) and 1ength:diam- eter ratio is greater than 1O:l. Most commercial fibers meet these requirements easily. A fiber is called a whisker if its microstructure is pre- dominantly single crystal. Fibers or whiskers can be subclassified as continuous or discon- Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7 tinuous. Continuous fiber or whisker is capa- ble of being manufactured to an indefinite length. Discontinuous fiber can be chopped continuous fiber, or a short fiber called staple. Discontinuous whisker is manufactured to a definite length in batch type processing, while continuous whisker is produced by melt processes such as the laser-heated floating- zone process. Summarizing, there is continuous fiber (indefinite length), continuous whisker (single crystal, indefinite length), discontinuous fiber (chopped continuous fiber or staple fiber) and discontinuous whisker (single crystal, definite length). 8.2 MANUFACTURE Melt, vapor deposition and chemical processes are used to manufacture fibers and whiskers. Fibers and whiskers that are made by melt processing include continuous quartz fibers by drawing from a fused quartz rod, alu- mina-silica staple by atomizing a molten ceramic stream and continuous alumina whiskers by drawing from a melt. Vapor depo- sition processes include chemical vapor deposition of silicon carbide or boron onto a tungsten core to make continuous fibers and the vapor-liquid-solid (VLS) process to make discontinuous whiskers. Chemical processing called sol-gel technology is used to create alu- mina-based continuous fiber. Other chemical processes include the creation of polymer
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Continuous fibers 157 precursor fiber that is later pyrolyzed into sili- con, carbon and nitrogen-based continuous fiber and the reaction between silica and car- bon to make discontinuous whiskers.
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