Ceramic Fibers and Coatings Advanced Materials for the Twenty-First Century (1998) / Chapter Skim
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4 Ceramic Fiber Processing
4 Ceramic Fiber Processing
Pages 37-48
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From page 37... ...
Therefore, the following discussion of non-oxide fiber processing will be limited to techniques applicable to these 37 materials, including CVD (chemical vapor deposition) , chemical conversion, extrusion/sintering, and preceramic polymer processing.
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From page 38... ...
Several reviews of the many variations of this process are available in the open literature (Lipowitz, 1991, 1997a; Lipowitz et al., 1993; Laine et al., 1995~. Although preceramic polymer processing is simple in principle, producing fibers with high tensile strength and high thermal stability requires high purity polymers, high quality spinning, and minimal impurities and mechanical damage introduced during spinning, curing, pyrolysis, and sizing (Haider and Clark, 1986; Salinger et al., 1988; Freeman et al., 1993~.
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From page 39... ...
39 = o ~ oF v v ca of c)
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From page 40... ...
, which are all high molecular weight, linear molecules capable of molecular orientation along the fiber axis, as well as of partial crystallization during drawing, which leads to high strength. The fragile nature of preceramic polymer fibers limits spinning and drawing speeds, imposing an economic penalty on throughput of the high capital-cost spinning line.
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From page 41... ...
Increasing the sintering temperature leads to increasing grain size, density, elastic modulus, strength, and creep resistance over a wide temperature range. OXIDE FIBER PROCESSING Commercial polycrystalline oxide fibers used to reinforce ceramic composites are produced by spinning and pyrolyzing chemically-derived precursors.
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From page 42... ...
to remove volatile components of the precursor, producing ceramic fibers. Heat treatment above 800°C (1,472°F)
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From page 43... ...
The relative amounts of each type of A1 complex in solution vary with the type and amount of anion ligand present, concentration, heat treatment time and temperature, and preparation route. In 43 general, lower anion levels result in higher levels of polymerized or clustered aluminum species.
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From page 44... ...
Source: 3M Company The downward forces from the draw wheel and gravitation must, therefore, equal the restraining forces generated by the viscosity and surface tension of the spin dope plus the inertial drag of the fiber as it is accelerated. Low viscosity leads to insufficient rheological force to resist gravitation; fiber diameter is reduced to the point where fracture occurs by necking.
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From page 45... ...
However, a series of cubic alumina spinets, commonly called transition aluminas, form during heat treatment of alumina precursors. The nature of the transformation (i.e., temperature, porosity, grain size)
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From page 46... ...
BSR creep experiments on polycrystalline YAG fibers have demonstrated creep resistance at 100 to 200°C (212 to 392°F) higher than the useful operating temperatures for commercially available oxide FIGURE 4-7 Nextel 610 fiber showing small grain size resulting from the addition of nucleation agents.
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From page 47... ...
The committee recommends that efforts be made to determine the effect of grain size, microstructure, and flaw population on the strength and creep resistance of oxide fibers. The utility of oxide fibers in composites can also be enhanced by using heat treatments to produce in-situ interface coatings.
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From page 48... ...
. The committee recommends that research on ceramic oxide fiber processing include fiber surface modifications and the development of in-situ coatings.
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