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6 Interfacial Coatings
Pages 54-74

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From page 54...
... Once matrix cracks have formed, they remain open even after the operating stress is reduced below the matrix cracking strength. Matrix cracks allow oxygen ingress and, therefore, oxidation of the fiber coating, and, potentially the degradation of the fiber itself.
From page 55...
... Carbon coatings can form in-situ by the decomposition of Nicalon fibers, as has been observed in glass-ceramic matrix composites prepared by hot pressing, or they may be applied via CVD (chemical vapor deposition) techniques.
From page 56...
... has applied this boron sealing concept to enhanced CVI SiC/SiC composites. It was not clear, however, whether boron additives work fast enough to prevent the degradation of carbon fiber coatings when matrix cracks develop.
From page 57...
... volatilizes as boron hydroxides before a borosilicate glass forms that can seal the gap created by BN oxidation and volatilization. Figure 6-6 shows the depth of oxidation of the BN coating in SiC-Si matrix composites reinforced with Hi Nicalon fibers with a nominal coating thickness of 0.5,um (0.02 mile)
From page 58...
... Alternative Fiber Coatings Coating Compositions Several alternative fiber coatings have been evaluated in non-oxide fiber-reinforced composites, either to improve the oxidation resistance of composites or to improve the stability of carbon and BN coatings. Alternatives include layered coatings (Luthra et al., 1994; Brennan, 1997)
From page 59...
... Therefore, a two pronged approach should be pursued: The oxidation resistance of non-oxide fiber coatings must be improved, from the current calculated value of a few minutes (at 1,200C [2,192F]
From page 60...
... Source: Luthra, 1997b. Coating Processes and Vendors Chemical Vapor Deposition CVD is the most common method of depositing fiber coatings for composite systems because it is a conformal process that can deposit fairly uniform coatings on a wide variety of structures.
From page 61...
... OXIDE FIBER COATINGS The development of ceramic oxide composites has lagged behind the development of non-oxide composites because of the poor creep resistance of oxide fibers (compared to SiC
From page 62...
... Oxide fiber coatings are most commonly applied to oxide fibers. Using oxide fiber coatings on silicon-based non-oxide fibers has generally been avoided because these fibers have a tendency to react with the oxide fiber coatings, thus creating strongly bonded interfaces.
From page 63...
... No Coating/Porous Matrix Approach The cost of ceramic composites could be lowered and the overall processing simplified if the fiber coating step could be eliminated. This approach is being investigated by General Electric and researchers at the University of California at Santa Barbara (UCSB)
From page 64...
... Uncoated fibers in a porous matrix, however, may subject the fibers to corrosive species in the operating environment that could degrade fiber strength. These composite systems must be tested in their service environments to determine if the absence of a fiber coating accelerates the degradation of fiber strength.
From page 65...
... 04~. Tests on tin oxide fiber coatings in model composite systems indicated some crack deflection at the coating-fiber interface (Siadati et al., 1991; Venkatesh and Chawla,1992~.
From page 66...
... in hot pressed matrices of alumina and YAG have shown encouraging crack deflection results. Figure 6-14 is a transmission electron micrograph of a cross-section of a hibonite-coated sapphire fiber that shows the desired crack deflection along the basal plane (Cinibulk and Hay, 1996~.
From page 67...
... The latest class of layered oxides to be proposed as fiber coatings are layered perovskites, including potassium calcium niobate (KCa2Nb3O~0) and barium neodymium titanate (BaNd2Ti3O~0)
From page 68...
... Coating Processes and Vendors Immiscible Liquid Coating Technique The most promising oxide fiber coatings appear to be multicomponent oxides. Liquid-based techniques are generally the most viable for producing these complex oxide coatings.
From page 69...
... Figure 6-18 shows CaWO4 fiber coatings obtained by making multiple passes through an 69 immiscible layer formed by the coating precursor and pentadecane (Goettler et al., 1 997b)
From page 70...
... Research into maximizing the electrostatic potential between common oxide fibers and candidate coating particles using surfactant technology in order to obtain sufficiently thick coatings in a single pass of a fiber tow through a colloidal sol would be useful. The technique would still be limited by having to use fine particle slurries or colloidal sots of the desired coating composition.
From page 71...
... . Source: Hay, 1991 RECOMMENDATIONS AND FUTURE DIRECTIONS Non-Oxide Fiber Coatings CMCs reinforced with SiC fibers have used a wide variety of fiber-matrix interface coatings, but acceptable toughness (debond characteristics)
From page 72...
... Because occasional matrix cracking probably cannot be prevented, oxidation of fiber coatings is a serious concern limiting the widespread use of non-oxide composites. Therefore the committee recommends the following approach to inhibiting oxidative embrittlement of non-oxide CMCs: The oxidation resistance of non-oxide fiber coatings should be improved from the current calculated value of a few minutes (at 1,200C [2,192F]
From page 73...
... for ceramic composites if other creep-resistant oxide fibers become available. Research into an improved creep-resistant Nextel 610 fiber that incorporates additions of rare-earth oxides or garnet-based fibers, may make the p-aluminas more viable as fiber coatings.
From page 74...
... There fore, the committee recommends that research be done on surfactants that could yield higher charges on the coating particles and/or fibers. It may be possible to absorb soluble inorganic polymer precursors onto the fiber surfaces by heterocoagulation, which would eliminate the need to form a fine powder of the desired coating composition.


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