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DEVELOPMENT OF SIC-BASED CERAMICS WITH IMPROVED OXIDATION AND HEAT RESISTANCE FOR ADVANCED APPLICATIONS

M. A. Vartanyan

Abstract

Silicon carbide is a promising material for heat-loaded units due to its high strength at elevated (up to 1500 °C) temperatures and good thermal conductivity. However, when heated in oxidising media SiC tends to lose its properties, and development of SiC-based materials able to withstand such conditions is a challenging issue. The present research discusses development of a heat- and oxidation-resistant structural ceramics based on SiC with oxide sintering aids. Sintering aids were chosen in ternary systems CaO-Al2O3-Y2O3 and MgO-Al2O3-Y2O3 and corresponded to eutectic compositions with melting points 1675 and 1775 °C respectively. Experimental samples were produced by pressureless sintering at 1900 °C in a protective atmosphere and subjected to testing of thermal properties (thermal coefficient of linear expansion and thermal resistance) and oxidation behavior. Thermal expansion of the material was determined under constant heating rate in temperature range 20 ? 800 °? in air, under these conditions SiC showed no signs of corruption. Thermal resistance was estimated by thermal cycling (800 °C ? water flow) and oxidation resistance was calculated as specific added weight when heated at 1900 K in ambient air for 15, 30 and 60 min, after heating a detailed study of samples microstructure and phase composition was carried out. In both cases introduction of oxide components improved thermal and oxidative resistance without compromising dimensional stability of the material. Surface of the samples after 30 min. exposition at 1900 K got covered by a dense SiO2 film that prevented further oxidation. However in presence of CaO-containing additive such film chipped off and the oxidised layer grew into the material; while in presence of MgO-containing additive surface layer was fully formed after 30 min. heating and showed no change in further testing. Results of thermal cycling also showed better effect of MgO-containing additive. Both calculated and experimental number of cycles before disruption for such samples was higher than that for CaO-doped materials (8 ? 9 and 3 respectively). This suggests that a major factor that determines high-temperature behaviour of SiC-based ceramics is interaction between SiO2 and cations in the additive: whenever such interaction becomes possible, either as phase transitions of SiO2 or as other physico-chemical processes, the properties of the materials are lower than those of the material where such interaction is suppressed.

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DOI resolver: doi.org/10.5593/sgem2019/6.1/s24.020

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