Non-oxide fibers

Silicon carbide fibers can be manufactured using the same process as boron fibers (see Section 5.2.7.2), if methyltrichlorosilane is used as the starting material. Deposition from the gas phase proceeds according to the following equation  

Disadvantage large fiber diameter limits textile processing in the manufacture of composites 

3rd step fixing of the fiber shape by intermolecular crosslinking reactions 

There are two further processes for the manufacture of SiC-fibers sintering of melt-spun SiC-suspensions and the chemical conversion of C-fibers by reaction with gaseous SiO. These processes have not yet achieved major commercial importance. 

Melt-spinning ol SiC-snspensinns and the chemical conversion ol C-fibers have minor commercial impoilancc 

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Spun non-oxide fibers (i.e., those made by the first three processes above) are produced in tows consisting of hundreds of filaments with diameters of 10 to 20 pm. Typical ranges of properties are given in Table ES-1. All of these fibers are based on SiC except for amorphous Si-B-N-C, a promising new fiber. 

The best non-oxide fibers have good creep resistance but are susceptible to degradation by formation of an amorphous silica layer upon oxidation. This layer offers some resistance to further oxidation, but prolonged exposure to oxidizing environments results in oxidative embrittlement of the composite. 

Non-oxide fiber coatings research should be focused on ... 

Impact. Durable coatings for non-oxide fibers would provide suitable lifetimes for many applications, such as thermally loaded gas turbine components and heat exchangers. 

Recommendation 4. The following areas should be investigated for non-oxide fibers ... 

Recommendations 2, 3, and 4 (listed in order of decreasing priority) are related to performance, which is also considered to be a high priority. Recommendation 2 is the most important in this category. The oxidation resistance of oxide fibers is attractive, but poor creep resistance is a significant limitation. Thus, Recommendation 3 addresses the need to improve this property. Recommendation 4 (regarding non-oxide fibers) is last in this category because the committee concluded that resources directed toward property improvement in fiber coatings and oxide fibers was more important. The committee is satisfied that the preliminary properties reported for Si-B-N-C amorphous fibers are sufficiently attractive to stimulate the research needed to verify them. 

In general, oxide fibers are poor thermal and electrical conductors and have higher coefficients of thermal expansion (CTEs) than non-oxide fibers. In addition, oxide fibers are usually more dense than non-oxide fibers. Non-oxide fibers generally have higher electrical and thermal conductivities, and SiC-based fibers are more conductive than Si(N)-based fibers. 

In the Tyranno family of non-oxide fibers, Ti was originally incorporated into the fiber during processing to create a very fine P-SiC grain size. In the Tyranno ZM fiber and its derivatives, zirconium (Zr) is included to improve its high-temperature properties and resistance to NaCl corrosion. 

TEMPERATURE AND TIME DEPENDENCE OF PROPERTIES OF NON-OXIDE FIBERS... 

The literature is rich with experimental data on the room-temperature strength of non-oxide fibers after exposure to high temperatures in air, inert atmospheres, and reducing atmospheres. 

Creep Behavior of Non-Oxide Fibers and Effects of Heat Treatment, Oxidation, and Aging... 

Rupture Behavior of Non-Oxide Fibers in Oxidizing and Non-Oxidizing Environments and the Effects... 

It should be noted that these comparisons are based on very small data sets that represent relatively short times. Future studies should focus on longer-term durability, particularly with coated fibers and in oxidizing environments. Rupture strength studies are needed for all non-oxide fibers. 

Finally, the crystallization behavior of the remarkably stable Si-B-N-C fiber must be studied under anticipated service conditions in order to determine the limits of the material and to establish its comparative advantages or disadvantages over other non-oxide fibers. 

In studies by Tressler and DiCarlo (1993, 1995), the target rupture strength and 1 percent creep strength defined for the best superalloy (PWA 1480 Ni-based superalloy) were compared to the corresponding data for fibers. Of the commercially available fibers, only Hi-Nicalon had a clear advantage over the superalloy when corrected to 20 volume percent fibers along the load axis. Developmental fibers, such as Hi-Nicalon S, Sylramic, and other non-oxide fibers, also promise to meet these minimum goals. 

Non-oxide fibers cause composites to be susceptible to oxidative degradation, particularly when the matrix is cracked. Thus, the limiting stress at the present time is the matrix cracking stress (on the order of 100 MPa [14.5 ksi]). 

This means that until more oxidation tolerant matrix-interface-fiber systems are developed, service conditions will be rather limited for non-oxide fiber-reinforced CMCs. CMCs do have a large weight advantage over superalloys, however, as well as low observability to radar. 

The Bayer Si-B-N-C fiber is reported to have impressive thermal stability (i.e., it remains amorphous at high temperatures) as well as exeellent ereep resistance (Baldus, 1997). To understand the performance eapabilities of this material in terms of time, temperature, and stress conditions, however, the crystallization behavior of the Si-B-N-C fiber must be studied under anticipated service eonditions. Continued study is also needed to establish its advantages or disadvantages in relation to other non-oxide fibers. Therefore, the committee recommends that detailed, high-temperature studies of the Bayer Si-B-N-C fiber, especially long-term durability studies, be done to verify preliminary results. 

New ceramic compositions and microstructures with thermomechanical and thermochemical properties beyond the capability of SiC will be required for the next generation of high-temperature materials. Developing and commercializing new non-oxide fibers that cost less and/or have improved properties are likely to be lengthy and expensive processes. Because non-oxide ceramic fibers have not been profitable so far, industry alone is unlikely to support further development. 

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