FORMATION OF A CHROMIUM-CARBIDE CONVERSION COATING ON SILICON-CARBIDE

The formation of a chromium-carbide conversion coating on SiC was achi eved using the pack-cementation technique. The conversion coating is i ntended to improve the corrosion resistance of SiC and its derivatives , such as SiC-base continuous fibers and composites, by forming a prot ective Cr2O3 scale upon exposure to high-temperature corrosive environ ments. Different pack chemistry and processing parameters were evaluat ed in the laboratory. Results indicated that the coating molphologies and compositions achieved were significantly affected by variation of these processing factors. In this paper, the conversion coating obtain ed from one of the systems investigated is reported. The coating consi sts of a multilayered structure with each of the sublayers containing a high-Cr concentration. In addition, the coating surface is relativel y dense and pore free compared to the underlying SiC substrate materia l. A dense and pore-free morphology is highly desirable for coating ap plications, especially on porous substrates. The multilayered coating structure consists of the following sublayers: Cr-23 C-6/Cr7C3/Cr7C3 Cr3Si/Cr5Si3Cx/SiC substrate.