In Situ Synthesis Of Al₂O₃-SiC Composite Ceramic Powders By Molten Salt Method

Al₂O₃ ceramics has merits like high hardness,corrosion resistance and low cost.It can be used in aerospace,medicine and other fields.But the fault of brittleness blocks its utilization.Adding different secondary phases into the matrix is a way to ameliorate the brittleness of ceramics.SiC is one of the most commonly used second phases,and the high melting point of Al₂O₃ and SiC makes they are difficult to sinter,so it is necessary to explore a way to synthesize Al₂O₃-SiC composite powders with low cost.Molten salt method is a promising way to fabricate inorganic powders with high melting point at low temperature.Therefore,this thesis prepared Al₂O₃-SiC composite powder by molten salt method.Effects of temperature,holding time,salt-reactant ratio,C/SiO₂ ratio on phase and properties of the product were studied.The synthesis mechanism be studied by thermodynamic.It gives a basis for further optimizing the morphology of the synthesized products.Based on the former work,the effects of catalysts（Fe,Y₂O₃）on the microstructure of the synthesized powders were studied.The conclusions are below:Al₂O₃-SiC composite powders were synthesized at 1300℃for 4 h,using silica fume,Al and carbon black as raw materials and NaCl-KCl as molten salt.The results prove that Al₂O₃-SiC content rises with an increase in temperature.The proper salt/reactant is 2:1,and n（C）:n（SiO₂）is 4:1.The SEM images imply that the morphology of the product comprises Al₂O₃-SiC particles and SiC whiskers.The analysis of chemical reaction thermodynamics and growth mechanism shows that the molten salt method can accelerated the reactions for the formation of Al₂O₃-SiC at lower temperature.SiC whiskers follow gas-solid growth mode.Al₂O₃-SiC composite powder was synthesized by molten salt method at 1300℃for 4 h with silicon dioxide,carbon black and aluminum powder as raw materials and NaCl and KCl as molten salt medium.Al₂O₃ and SiC exist as particles.The microstructure and thermodynamics analysis show that the template mechanism plays a dominant role in the synthesis process.The Al₂O₃-SiC ceramics sample was fabricated by sintering at 1500°C for 4 h,using raw powders,i.e.,the powder maintaining at 1300°C for 4 h.The porosity,bulk density,and compressive strength were 42.3%,1.81 g·cm^-3,and 16.4 MPa,respectively.Al₂O₃-SiC composite powder was prepared by molten salt method at 1250℃for4 h with Fe as catalyst,silica fume,Al and carbon black as raw materials.The proper amount of Fe was 3 wt%.Fe effects the morphology of the product.SiC is characterized by whisker,with a diameter of 30-100 nm and a length of several microns.During the synthesis process,Fe promotes the formation of SiC whisker by forming Fe/Sialloy.The growth mechanism of SiC whisker is solid-liquid-gas-solid.With Y₂O₃ as catalyst,Al₂O₃-SiC composite powder can be synthesized at 1300℃for4 h,and the optimal addition of Y₂O₃ is 3 wt%.The addition of Y₂O₃ can accelerate the conversion of oxygen.SiC exists as whiskers,and the growth mode is gas-solid.