Improved formulations and processes have been invented for manufacturing ceramic fibers that exhibit structural stability and retain tensile strength at temperatures up to 1,200 °C, or even 1,300 °C in some cases. The final compositions of these fibers are given in the following table:
Element /Weight Percent:
- silicon / 30 to 60
- boron / 2 to 8
- carbon / 18 to 40
- oxygen* / <20
* Alternatively, as explained in the text below, fibers can contain nitrogen instead of oxygen.
These fibers are more stable at high temperatures than are the silicon carbide- and silicon nitride-based fibers made by the older processes described next.
Fibers containing the same elements in proportions different from those listed above have been made by older processes that involve (1) synthesis of precursor organic polymers, (2) extruding (melt-spinning) the polymer masses into fibers, and (3) treating the fibers by curing, sintering, and/or pyrolysis. In cases in which the polymer fibers are pyrolized directly, the fibers can deform or even melt during pyrolysis; this is a disadvantage in engineering applications in which fibers of specified shape are required.
The improved process includes the same basic steps as those described above. In addition, the improved process includes a step in which the precursor polymers are cured (their molecules are cross-linked) to prevent melting or deformation during the subsequent pyrolysis.
In a typical case, the improved process begins with heating a reaction mixture of an organoborohalide and an organohalosilane to synthesize a polyorganoborosilane, which is the precursor polymer. The polymer is then heated to a temperature between 80 and 200 °C (chosen to be somewhat above the softening or melting temperature of the polymer). The heated polymer is extruded into fibers by use of a standard spinneret, and the fibers are spun onto spools.
The precursor polymer fibers are cured in one of the following three ways:
- The fibers are oxidized slowly in air, starting at room temperature, gradually heating up to 150 °C during 3 or 4 days, and holding at 150 °C for about 1 day. Cross-linking is effected, and the level of oxygen incorporated in this step is between 5 and 25 weight percent.
- A better and faster oxidation-and-curing procedure involves irradiation of the fibers with ultraviolet light in air for 1 to 48 hours.
- A procedure for curing in the absence of oxygen involves exposure of the fibers to hydrazine vapor, preferably under a dry nitrogen or argon atmosphere for about 16 hours, followed by irradiation with ultraviolet light for 6 to 16 hours. In this procedure, nitrogen (instead of oxygen) is incorporated into the polymer.
The cured fibers are pyrolized by heating them from ambient temperature to about 1,300 °C in an inert atmosphere (argon or nitrogen). After initial heating, the fibers are held between 1,000 and 1,300 °C for as much as 1 hour. The products of pyrolysis are black fibers. The figure summarizes results of tensile tests of representative SiwBxCyOz fibers made in this process and of commercial silicon carbide- and silicon nitride-based fibers, showing that the fibers of this invention retain tensile strength better at high temperature.
This work was done by Salvatore R. Riccitiello, Ming-ta S. Hsu, and Timothy S. Chen of Ames Research Center.
This invention has been patented by NASA (U.S. Patent No. 5,223,461). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
Ames Research Center; (650) 604-5104
Refer to ARC-11956