Low-temperature Sintering And Characterization Of SiC-based Ceramic

Silicon carbide?SiC?ceramic is a promising high-temperature structural material,being used in a wide range of industrial applications including metallurgy,microelectronics,automotive,gas turbines and space shuttles,because of its high thermal conductivity,excellent thermomechanical and tribological properties,outstanding oxidation resistance.In addition,silicon carbide has a small neutron absorption cross section,low induced radioactivity under neutron irradiation,making it becoming one of the most important structural materials in generation of nuclear fission and nuclear fusion reactors in future.However,one of the main obstacles associated with engineering applications is the very high processing temperature to achieve fully dense bulk material due to the strong covalent bonding nature and low self-diffusion coefficient of SiC.At the present stage,the sintering aids are mainly introduced by the ball-milling method to promote the sintering.However,through the ball-milling method,an issue often encountered is that the additives are difficult to homogenously disperse with SiC powders,which may have an influence on the sintering process and homogeneity of performance.Therefore,in order to further improve the sintering process and performance of SiC ceramic,a novel processing route is explored in the current work.The phase of Al₄C₃ was in-situ synthesized on the surface of SiC powders and a portion of aluminium could not be reacted completely instead of dispersing in Si C homogeneously to form SiC-Al₄C₃-Al powders by using a molten salt technique.Then the SiC powders with different mass of Al₄C₃ were sintered at 1500°C and 1700°C by SPS,respectively.Typically,at 1700°C,the bulk SiC ceramic presented a relative density of 99.6%when the molar ratio of Al to SiC is 1:3 in molten salts.However,the thermal conductivity is about 94.4 W/?m·K?.The Al could be solution in SiC and the formation of?/?interfaces in SiC grains as a result of the?to?phase transformation leads to the increased phonon scattering at the interfaces,contributing to the decrease of the thermal conductivity.In the same way,the Y₃Si₂C₂ coating was in-situ synthesized on the surface of SiC powders to form SiC-Y₃Si₂C₂ core-shell structure by using a molten salt technique.Such Y₃Si₂C₂ coating could effectively improve the sintering,mechanical and thermal properties of SiC-based ceramics fabricated by the spark plasma sintering.Microstructure analysis indicated that a moderate thickness and homogeneous Y₃Si₂C₂ coating on SiC powders could significantly improve the densification and properties of the SiC ceramics.Typically,at1700°C,the bulk SiC ceramic presented a mean grain size of 2.5 um and relative density of99.5%when the molar ratio of Y to SiC is 1:4 in molten salts;the Young's modulus,indentation hardness and fracture toughness measured by indentation test were 451.7 GPa,26.3 GPa and7.9 MPa·m^1/2,respectively;the thermal conductivity is about 145.9 W/?m·K?.Excellent thermal and mechanical properties could be associated with the fine grain size,optimized phase composition and improved grain boundary structure.The main densification mechanism was analyzed.