Ceramic Fibers and Coatings Advanced Materials for the Twenty-First Century (1998) / Chapter Skim
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3 State of the Art in Ceramic Fiber Performance
3 State of the Art in Ceramic Fiber Performance
Pages 20-36
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From page 20... ...
They also represent the two extremes of creep resistance (Saphikon) and creep rupture strength (SCS-6~.
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From page 21... ...
This fiber had the best creep resistance of all the polycrystalline ceramic fibers fabricated to date. The developmental Dow-Corning Sylramic SiC fiber is a B-doped, fully crystalline fiber with a p-SiC grain size on the order of 0.1 to 0.5,um (0.004 to 0.02 mile)
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From page 22... ...
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From page 23... ...
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From page 24... ...
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From page 25... ...
However, surprisingly few reports of fast fracture strength as a function of temperature for these and other recently developed nonoxide fibers are available. The trends in fast fracture strength of both oxide and non-oxide ceramic fibers as a function of iCeramic materials often fail wellbelow theirinherent theoretical strengths as a result of critical flaws.
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From page 26... ...
1,200 1,400 1,600 1,800 2,000 2,200 Heat-treatment temperature (K) FIGURE 3-6 Tensile strength of SiC fibers after heat treatment in argon for one hour.
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From page 27... ...
That is, reducing the oxygen content of SiC fibers increases O their resistance to microstructural degradation associated with high-temperature exposure. High temperature thermal exposure in argon shows very little affect on the room-temperature strength of the Dow Corning Sylramic fiber or the Carborundum fiber.
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From page 28... ...
.2 Recent developments that have resulted in low oxygen content polymerderived fibers and near stoichiometric or stoichiometric SiC fibers have dramatically improved the creep resistance of nonoxide fibers. These results are illustrated in a composite figure (Figure 3-14)
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From page 29... ...
Increasing the volume fraction of p-SiC to near stoichiometry, with a concomitant doubling of grain size, drastically increases the creep resistance of the ~Nicalon S fiber. The Dow Corning and Carborundum fibers are comparable in terms of microstructure in the final product, except that the grain size of the Carborundum material is three to ten times the size of the Dow Corning fiber.
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From page 30... ...
The CVD p-SiC fibers have grain sizes on the order of 4 to 10 times larger than the HiNicalon fiber, but creep rates are 10 to 100 times those of Hi-Nicalon. Lowering the boron content of the Sylramic fiber appears to improve creep resistance, but the effect has not been quantified.
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From page 31... ...
The role of boron additives has not been thoroughly studied, but there is some evidence that boron degrades creep resistance and rupture strength. In general, the onset of measurable creep is accompanied by a degradation in rupture strength suggesting that improvements in creep resistance would lead to improvements in rupture resistance (as predicted by the Monkman-Grant relationship)
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From page 32... ...
TEMPERATURE AND TIME DEPENDENCE OF PROPERTIES OF OXIDE FIBERS Strength and Stiffness as a Function of Test Temperature Figure 3-1 illustrates the trends in tensile strength, as a function of temperature, for the small diameter ceramic fibers that were available in the late 1980s (Pysher et al., 1989~. The tensile elastic modulus was also measured as a function of temperature (Figures 3-2a and 3-2b)
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From page 33... ...
For example, DiCarlo and coworkers have investigated the time dependent mechanical properties of ceramic fibers (Yun et al., 1995a, 1995b,1994~. Tensile tests on single fibers have been used to study creep, and the BSR technique has been used as a screening test to compare the creep behavior of various fibers (Sabol and Tressler, 1990~.
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From page 34... ...
However, heat treatments that coarsen the microstructure, thereby improving creep resistance, should increase the rupture strength of polycrystalline oxides, as indicated by the Monkman-Grant relationship.
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From page 35... ...
Therefore, specific data relating heat treatment and grain growth to strength and creep rupture are not available. PERFORMANCE CHARACTERISTICS COMPARED TO PERFORMANCE GOALS Detailed comparisons of fiber properties and performance goals requires a knowledge of the thermo structural requirements for a CMC component (e.g., a combustor liner for an aircraft engine)
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From page 36... ...
For oxide ceramics, the role of microstructure in controlling creep rate and creep rupture strength must be determined, particularly for multiphase microstructures. The effects of thermal aging on creep rupture strength have been studied for the Hi-Nicalon, Dow Corning, and Carborundum fibers, but no systematic studies of the effects of heat treatment or aging on the creep rupture strength of more recently developed fibers, such as Hi-Nicalon S
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