The NASA Glenn Research Center has developed advanced silicon carbide (SiC) fiber-reinforced SiC-matrix composite systems that can operate for hundreds of hours under stress at temperatures to 1450 °C (2640 °F) and above. These SiC/SiC systems are designed to be lightweight (~30 percent of metal density) and, in contrast to monolithic ceramics and carbon-fiber-reinforced ceramic composites, to reliably retain their structural properties for long times under aggressive high-temperature environments. The key to developing these systems was to first understand the process-structure-property relationships for their potential commercial constituents: fiber, fiber architecture, fiber interphase coating, and matrix, and then to use this understanding to develop advanced processes that significantly improve the thermostructural performance of the constituents.
These advancements were achieved in a series of steps. First, Glenn researchers gained an understanding of the materials and process relationships for available SiC fibers. Then, they developed processes that convert commercial Sylramic fibers into Sylramic-iBN SiC fibers that display thermal stability to over 1700 °C, high tensile strength, high creep-rupture resistance, and high thermal conductivity, and that possess a thin in situ grown boron nitride (BN) surface layer for added environmental durability (ref. 1). Further capability was derived by understanding how the Sylramic-iBN fibers can be arranged in certain fiber architectures to provide thin-walled backside-cooled SiC/SiC components with desirable in-plane and through-the-thickness thermostructural properties (ref. 2).
Next, the interphase coatings for the Sylramic-iBN fibers were addressed. Because commercial chemical vapor infiltration (CVI) for conventional BN coatings is done at relatively low temperatures, Glenn needed to develop advanced processes that produce BN-based coatings that are more thermally, dimensionally, and environmentally stable, but still retain the prime interphase requirement of weak bonding with the Sylramic-iBN fiber (ref. 2).
Finally, although SiC matrices produced by CVI can provide SiC/SiC composites with good thermal conductivity and creep-rupture resistance, these properties are not optimum because current commercial CVI processes are typically performed near 1000 °C, resulting in nonstoichiometric matrix microstructures containing free silicon and high porosity. These problems were minimized through the development of (1) annealing treatments that effectively eliminate the free silicon, and (2) polymer-infiltration and pyrolysis (PIP) processes that fill the pore content with stoichiometric SiC, thereby allowing temperatures above silicon’s melting point (1410 °C) (refs. 2 and 3).
The table lists some key thermostructural properties for two advanced SiC/SiC systems fabricated into thin-walled panels using Glenn’s constituent technologies. System A was produced by the more conventional two-dimensional approach of fabric layup, whereas System B uses a 2.5-dimensional architecture with a low content of Sylramic-iBN fibers running through the panel thickness. Both systems display excellent in-plane strength and creep-rupture properties up to1450 °C, but System B shows much higher through-the-thickness strength and thermal conductivity because of the use of state-of-the-art Sylramic-iBN fiber through the panel wall. These Glenn technologies are available for transfer under a NASA Space Act Agreement. Approaches for scale up, and further improvements have been identified.
| Property | System A | System B |
|---|---|---|
| Upper-use temperature, °C | 1450 | 1450 |
| Architecture | 2-dimensional, 0°/90° layup |
2.5-dimensional, woven |
| Matrix | CVI+PIP SiC | CVI+PIP SiC | Elastic modulus at room temperature, GPa | 190 | 190 |
| Proportional limit strength at room temperature, MPa | 150 | 150 |
| Ultimate strength at room temperature, MPa | 380 | 360 |
| Through-the-thickness strength at room temperature, MPa | 10 | 25 |
| Through-the-thickness conductivity, W/m-K
|
23 12 |
36 26 |
| Creep strain at 69 MPa for 300 hr at 1450 °C, percent | ~0.2 | ~0.2 |
| Rupture life at 69 MPa in air at 1450 °C, hr | >500 | >500 |
Last updated: October 11, 2006
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