A method of fabricating composites of pretreated silicon carbide fibers in silicon carbide matrices provides for the formation of improved boron nitride fiber/matrix interfacial layers. As explained below, these BN layers help to deflect matrix cracks away from the fibers.
In order to impart strength and fracture toughness to a fiber/matrix composite material, one must formulate it so that under high stress, it fails in a fiber-pullout mode rather than a brittle-fracture mode. This requirement translates to a need for a fiber/matrix interfacial layer of a material (denoted the "interphase") that is weakly bonded to both the fibers and the matrix to insure that matrix cracks are deflected away from the fibers.
One of the most widely used fiber/matrix interphases is boron nitride, which, heretofore, has usually been deposited on fibers or fiber preforms by chemical vapor deposition (CVD). One of the drawbacks of the CVD BN process is that when a tow (a bundle that normally consists of between 500 and 800 fibers with diameters between 10 and 15 µm) is formed into a woven fiber preform prior to CVD BN coating, many of the fibers are in contact with each other and therefore do not become completely coated with BN. On subsequent fracture of the composite, those fibers that are in contact with no BN between them tend to fail in a brittle manner, with little or no fiber pullout. This situation is exacerbated if the SiC fibers have relatively rough surfaces (as some commercial SiC fibers do). In addition, ordinary CVD BN fiber coats are not particularly crystalline, and have been found to be very susceptible to oxidation in hot, moist environments like those in gas turbine combustors.
In the present method, thin layers of either (1) carbon-rich SiC containing some boron or (2) highly crystalline BN are formed in situ on the SiC fibers, before further processing; these layers prevent the SiC fibers from remaining in contact with each other. Because these thin surface layers are not thin enough (and, in the case of highly crystalline BN are not weak enough) to act by themselves as crack-deflecting layers, it is necessary to deposit BN overcoats (with or without Si doping) by chemical vapor infiltration to obtain the desired crack-deflection property. The remaining processing steps, following conventional practice, include the formation of SiC overcoats by chemical vapor infiltration, infiltration by a slurry that contains SiC particulate, and infiltration by molten silicon at a temperature of ≈1,420 °C to realize a dense matrix.
The subprocess to form the boron-doped carbon or highly crystalline BN begins with a heat treatment of the fibers at a temperature of 1,800 °C in either an inert or nitrogen gas. In the case of a nitrogen atmosphere, boron (which is present in typical commercial SiC fibers as a sintering aid) diffuses to the fiber surfaces to form thin (≈100-nm thick) layers of very crystalline BN. The added advantage of diffusing the boron out of the fibers is that the resistance to creep of the fibers at high temperatures is increased. In the case of an inert atmosphere, the surfaces of the fibers become reduced, with resultant formation of thin (≈50 to ≈100 nm thick) carbon-rich surface layers, which also contain some boron from diffusion out of the bulk.
When the fibers that have been thus heat treated are subsequently woven into cloth preforms or are heat-treated in cloth form, the highly crystalline BN or carbon-rich surface layers prevent the SiC fiber cores from touching. While carbon-rich fiber surface layers may prevent the fibers from touching, they are not desired because they do not exhibit sufficient resistance to oxidation as that of the BN layers.
The highly crystalline layers of BN are strongly bonded to the fiber surfaces, and prevent the propagation of cracks from the matrix into the fibers (see figure). By keeping the fibers out of contact, the highly crystalline layers also prevent interlocking of the rough fiber surfaces. The highly crystalline nature of these layers also contributes to stability under stress in an oxidizing environment.
This work was done by James A. DiCarlo and Hee Mann Yun of Glenn Research Center and John J. Brennan of United Technologies. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.
Inquiries concerning rights for the commercial use of this invention should be addressed to
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Commercial Technology Office,
Attn: Steve Fedor,
Mail Stop 4 —8,
21000 Brookpark Road,
Cleveland, Ohio 44135.
Refer to LEW-16864.