Initial characterization of coated carbon-carbon composites

All the coatings used in this work had a SiC conversion layer of carbon-carbon composite substrate. The only difference between the coatings is the coating thickness. The light coating is about 150 um thick, while the moderate coating is 300um thick. The coating thickness, however, is not uniform around the surface of the sample and contains cracks. The phases Si, B4C, and –SiC appeared in all HP24 conversion coatings. These observations will be discussed.

The phases that appear in the conversion coating were a direct result of the pack cementation process. In the process Si vapor was passed through a CC137E substrate to convert carbon into SiC. The Si reacted with the carbon fibers and matrix to form SiC. The SiC that formed was mainly the cubic polymorph with a lattice parameter of 4.37 A. Thicker conversion coatings such as the HP24 moderate show x-ray diffraction peaks from hexagonal polymorphs of a-SiC. Fine B4C particles are found to outline the converted fibers in SEM micrographs. The B4C outlines the fibers because the inhibitor does not interact significantly with the Si vapor. Often the boron carbide is seen to follow the outline of the fibers from the CC composite substrate into the HP24 coating. The hexagonal B4C grains vary from 0.1 to 1.0 um in diameter. The interaction of the Si with open porosity results in the deposition of free Si. The free Si appears white in optical micrographs but is very hard to detect in SEM micrographs since the contrast is similar to that from SiC. The Si has a coarse microstructure which depends on the prior porosity and often exhibits the size and shape of carbon fibers and extends at various angles from the substrate to the surface of the coating. The Si appears to have been oxidized to a small extent during coating fabrication. The Si also fills some cracks and blocky pores assuming these shapes.

Cracks are observed in the planar-surface and transverse-cross sections of the HP24 coatings. A planar-surface, shows that cracks are spaced regularly from 200 to 600 um on the surface. In addition the surface has 10-20 um cavities which are aligned in a parallel manner. The cross section in the same figure shows that the large surface cracks extend from the surface to the CC composite substrate. The cracks are roughly 0.9 mm apart.

The cracks present in the initial microstructure result from the thermal expansion mismatch between the substrate and coating phases. Note that a mismatch of greater than 0.5% occurs between CC composite fibers and SiC at temperatures greater than 1000C. The temperature used in the pack process is not known, but is probably higher than 1000C. A mismatch of 0.5% would produce a stress on the order of 35000 p.s.i. on the sample configurations used in this work. This stress is calculated from a formula based on thermal expansion coefficients, thickness of the coating and substrate, and the elastic modulii of the constituent phases. The modulus of rupture of bulk SiC can range from 25 to 75 k.s.i. and, therefore, cracking is likely to occur.

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