CVI SiC matrix

Webb, J.E., Singh, R.N., Lowden, R.A. (1996), Thermal shock damage in a two-dimensional woven-fiber-reinforced-CVI SiC-matrix composite , /. Am. Ceram. Soc., 79(11), 2857-2864. 

Amercom, Inc., and 3M have made small and large [(— 8" diameter x 6 long 20 X 180 cm)] tubes. Both Nicalon SiC fiber and Nextel fiber have been used with an SiC matrix. 3M Company prepares a product called Siconex which consists of Nextel fibers in a CVI SiC matrix. 

The CVI SiC matrix also possesses the superior properties of ceramics over metals ... 

For the N22 CMC system, remaining open porosity in the CVI SiC matrix was filled by room-temperature infiltration of SiC particulate or slurry casting, followed by the melt-infiltration (MI) of silicon metal near 1400°C. This yielded a final composite with 2% closed porosity within the fiber tows and 0% porosity between the tows. The final composite system (often referred to as a slurry-cast MI composite) typically displayed a thermal conductivity about double that of a full CVI SiC composite system in which the CVI matrix process was carried to completion. Also the composite did not require an oxidation-protective over-coating to seal open porosity. Decreasing the porosity of the hybrid matrix also increased the N22 CMC elastic modulus, which in turn contributed to a high proportional limit stress. However, since the filler contained some low-modulus silicon, the modulus increase was not as great as if the filler were completely dense SiC. 

To address the temperature issues related to excess silicon, all the same constituents in the N24-C system are used for potential N26 CMC generations, but remaining open pores in the CVI SiC matrix are filled by silicon-free ceramics, rather than by melt infiltration of silicon. In particular, for the N26-A CMC system, a SiC-yielding polymer from Starfire Inc. [13] is infiltrated into the matrix porosity at room temperature and then pyrolyzed at temperatures up to 2912°F (1600°C). This polymer infiltration and pyrolysis (PIP) process was repeated a few times until composite porosity was reduced to 14 vol,%, At this point, the total CMC system is then thermally treated at NASA to improve its thermal conductivity and creep-resistance. Thus although more porous than the other CMC systems, the N26-A system has no free silicon in the matrix, thereby allowing long-time structural use at 2600°F... 

As predicted by CMC theory, some of the Table 4 properties will change significantly with fiber content and test direction. Nevertheless, for the selected fiber content and panel test conditions, qualitative trends can be observed in the as-fabricated CMC mechanical properties at room temperature. For example, regarding elastic modulus, variations between CMC systems can be correlated to the content of the high modulus CVI SiC matrix and to the existence of matrix porosity and/or an outside debonding interphase, both of which... 

Y. Katoh, K. Ozawa, C. Shih, T. Nozawa, R. J. Shinavski, A. Hasegawa, and L. L. Snead, "Continuous SiC Fiber, CVI SiC Matrix Composites for Nuclear Applications Properties and Irradiation Effects," J. Nucl. Mater., in press (2013). 

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