Carbothermal Reduction Synthesis And Sintering Of SiC-TiC（TiB₂） Composite Powders

Silicon carbide (SiC) has been widely used in machinery, chemical engineering,energy and many other industries due to its excellent combination of properties suchas high temperature strength, good oxidation resistance, high thermal conductivity,extreme chemical stability, etc. However, the application fields of SiC ceramics islimited by its low room-temperature strength, poor fracture toughness and high flawsensitivity. An effective method to solve the problems is to introduce transition metalboride or carbide into SiC matrix as strengthening phase. Generally, SiC-TiC andSiC-TiB2composites are prepared by directly mixing SiC and TiC or TiB2powdersand then hot-pressing or pressureless sintering. However, mechanical mixing of thepowders makes inhomogeneity of the SiC-TiC(TiB2) composites，resulting in anunfavourable inference on the properties of SiC-TiC(TiB2) composites. Based on theabove, this work intends to prepare SiC-TiC(TiB2) composite powders, andinvestigate synthesis, particle growth and sintering mechanism of SiC-TiC(TiB2)composite powders.SiC-TiC composite powders were synthesized in Ar by carbothermal reductionmethod using silica sol, carbon black and titanium oxide as raw materials. The effectsof reaction temperature and titanium oxide content on the phase composition andmorphology of SiC-TiC composite powders were investigated. The thermodynamicanalysis and calculation were performed systematically during the reaction, and thegrowth mechanism of SiC-TiC composite powders was discussed. The results showthat the suitable condition for synthesizing SiC-TiC composite powders is at1600℃for1h. In the reaction process, TiC formed ahead SiC, and the formation of TiCinhibits the grain growth of SiC particles. When TiC content is about10%, theformation of SiC is controlled from vapor-solid mechanism to the combinationvapor-solid and vapor-vapor mechanism. The composite powders are mainlycomposed of round-like particles, short rod-like particles and a small amount ofwhiskers. With the increase of TiC content in the composite powders, the growth ofSiC whiskers is inhibited and the morphology of SiC gradually transfers from longfiber to short rod-like and round-like particles.SiC-TiC composite powders were synthesized in vacuum using SiC, TiO2andcarbon black as starting materials. The phase composition and morphology of SiC-TiC composite powders were investigated and the antioxidation performance of SiC-TiCcomposite powders was discussed. Results show that the suitable condition forsynthesizing SiC-TiC composite powders is at1350℃for1h. The size of in-situcreated TiC particles is about0.1~0.2μm. In the oxidation process, TiC reacts inpriority with O2to form TiO2, then SiC reacts with O2to form SiO2.SiC-TiB2composite powders were synthesized in Ar by carbothermal reductionusing silica sol, carbon black, TiO2and B4C as raw materials. The effects of reactiontemperature and titanium oxide content on the phase composition and morphology ofSiC-TiB2composite powders were investigated. The growth mechanism of SiC-TiB2composite powders was discussed. The results show that the suitable condition forsynthesizing SiC-TiB2composite powders is at1600℃for1h. In the reaction process,TiB2formed ahead SiC, and the formation of TiB2alters the growth mode of SiC.When TiB2content is about10%, the formation of SiC is controlled from vapor-solidreaction to the combination of vapor-solid and vapor-vapor reaction. The compositepowders are mainly composed of a small amount of round-like particles, shortrod-like particles and lot of whiskers. With the increase of TiB2content in thecomposite powders, the growth of SiC whiskers is inhibited. And the number ofround-like, plate-shaped and short rod-like particles in the composite powdersincreases noticeably, which results in the variety of the morphology.SiC-TiB2composite powders were synthesized in vacuum using SiC, TiO2，B4Cand carbon black. The effects of reaction temperature on the phase composition andmorphology of SiC-TiB2composite powders were investigated and the antioxidationperformance of SiC-TiB2composite powders was also discussed. Results show thatthe suitable condition for synthesizing SiC-TiB2composite powders in vacuum is at1300℃for1h. In the oxidation process, TiB2reacts in priority with O2to form TiO2and B2O3，then SiC reacts with O2to form SiO2and CO.The sintering behavior of SiC-TiC composite powders synthesized bycarbothermal reduction using silica sol, carbon black and titania powders isinvestigated. The effects of the volume fraction of AlN and Y2O3on density, hardnessand microstructure of SiC-TiC composites were discussed. Results show that theSiC-TiC composite powders reveal good sinterability when the AlN-Y2O3adds isabout10vol%. The sample hot-pressed at1900℃for1h under25MPa, possesses arelative density of98.9%and Rockwell hardness of93.2HRA.SiC-TiB2composites with20vol%TiB2were fabricated by in situ reactionbetween TiO2, C and B4C，with AlN-Y2O3as sintering aids. The effects of the volume fraction of AlN and Y2O3on the density, mechanical properties, electrical propertiesand microstructure of the SiC-TiB2composites were determined. Results show thatSiC-TiB2composites possess the good comprehensive performance when the contentof sintering aids is10vol%, showing a relative density of99.1%, flexural strength of641MPa and Rockwell hardness of91.8HRA, respectively. The resistivity of theSiC-TiB2composites reaches17.5mΩ·cm at10vol%AlN-Y2O3. Pull-out of crystalparticles is observed from the fracture morphology of the composites. Tightly bondedgrains contributed to the excellent combination of properties of SiC-TiB2composites.