We investigated the 'application to refractories of a synthesis technique to produce sintered composites," namely, the application of the sintering reaction BCxN + xSi -> BN + xSiC to composite refractories based on ZrO_2 + C. Using this reaction, we devised a new method to convert densely packed raw material mixes of ZrO_2-C-BCN-Si into ZrO_2-C-BN-SiC composites containing small amounts of third components, that is, AN and SiC. Compared with ZrO_2 + C refractors that contain no additives or conventional ZrO_2 + C refractories in which the third components are merely admixed, those composites proved to excel in several properties. The main results of our investigation are as follows: The most frequent pore size of a material containing 1 percent BCN + 1 percent Si was as small 0.5 mum, while those of materials containing no additives or 0.8 percent BN + 1.4 percent SiC exceeded 2 mum. The hot bending strengths of the materials containing 1 percent BCN + 1 percent Si were higher than those of the other materials tested, 2 to 3 times as high as that of the material containing no additives, and 1.5 to 2 times as high as the material containing 0.8 percent BN + 1.4 percent SiC. Especially when fired at 1400 deg C, the composition containing 1 percent BCN + 1 percent Si showed the highest value. As for elasticity retention after the spalling test, the highest rate was shown by the material containing 1 percent milled BCN + 1 percent Si. We assumed this was attributed to the good pore dispersion of materials containing 1 percent BCN + 1 percent Si because of the fineness of their pores, and that pore dispersion was further improved by grinding the BCN. As for oxidation resistance, materials containing 1 percent BCN + 1 percent Si and 0.8 percent BN + 1.4 percent SiC were the best, exhibiting better oxidation resistance than those containing no additives or 2 percent Al. The very fine pore sizes of the material containing 1 percent BCN + 1 percent Si seems to be one of the reasons why the material exhibited excellent oxidation resistance in spite of its higher apparent porosity. The slag corrosion resistance of the material containing 1 percent BCN + 1 percent Si was comparable to that of those containing no additives, 0.8 percent BN + 1.4 percent SiC, or 2 percent Al. As for slag infiltration, the material containing 1 percent BCN + 1 percent Si was the best among the five kinds of samples studied. It showed the most stable microstructure with a dense reaction layer forming and the graphite remaining free from oxidation.
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