Preparation And Properties Of Laminated SiC Multiphase Ceramics

Among various advanced structural ceramic materials,SiC ceramics have attracted much attention due to their high strength,high hardness,low thermal expansion coefficient,high thermal conductivity,good oxidation resistance and so on.Unfortunately,low toughness and poor thermal shock resistance limit their wide application.Based on bionics principle,laminated SiC/BN multiphase ceramics and laminated SiC/Graphite multiphase ceramics were prepared by tape casting and then sintered by reactive sintering and hot pressing sintering,respectively.The influencing factors of the material preparation process were discussed;The effects of liquid silicon infiltration on the properties of laminaed SiC/BN multiphase ceramics were studied;The mechanical properties,oxidation resistance and thermal shock resistance of laminated SiC/Graphite composites were analyzed.The tape-casting substrates were fabricated by tape casting.Studies have shown that the slurry is suitable for casting when the content of binder and solid phase content of slurry were 25wt%and 0.375g/ml.Morever,the water bath temperature and stirring time were 45?and 48 h.The matrix layer and the interface layer were stacked alternately in the graphite mould and then the green body were degreased at 650?.After degreasing at650?,the binder and plasticizer were almost all removed,and the samples remained intact.The laminated SiC/BN multiphase ceramics was prepared by reactive sintering,first sintered at 1900?,and then siliconized at 1700?.The thickness of the composite substrate layer and the interface layer are 400?m and 40?m,respectively.The effect of liquid silicon infiltration on material properties was investigated,The relative density of the composite increased from 77%before siliconizing to 94%,and the porosity decreased from 18%to 3%.The flexural strength and fracture toughness of the laminated ceramics after siliconizing were 235MPa and 7.1MPa·m^1/2,respectively,and the strength?108MPa?and toughness(3.3MPa·m^1/2)before the siliconization were increased by 117%and 115%,respectively.However,the inclusion of 16%free Si in the laminated SiC/BN composite results in low strength.The laminated SiC/Graphite multiphase ceramics with 5wt%MgO-Y₂O₃ sintering aid were prepared by hot pressing sintering.The thickness of the composite substrate layer and the interface layer are 300?m and 30?m,The relative density of the composites was89%.The flexural strength and fracture toughness were 282MPa and 9.48MPa·m^1/2,respectively.The flexural strength was slightly lower than that of monolithic SiC?330MPa?,but the toughness was nearly 2 times higher than that of monolithic ceramic(3.56MPa·m^1/2).The R-curve behavior of laminated SiC/Graphite composites was found to have an R-curve rise behavior.The significant increase in toughness and the rise in the R-curve were mainly attributed to the deflection of cracks by the graphite interface layer,thus absorbing more fracture energy.The laminated SiC/Graphite multiphase ceramicswere oxidized at 1100?and 1300?for 1h,5h and 10h.It was found oxidative weight loss at 1100?and oxidation weight gain at 1300?.The residual strength after oxidation at 1300?were higher than that of samples at 1100?.When laminated SiC/Graphite composites were oxidized at 1100?,the oxidized surface failed to form a dense SiO₂ oxide layer.In addition,the oxidation of graphite in the interface layer caused the material properties to drop rapidly.When laminated SiC/Graphite composites were oxidized at 1300?,a dense SiO₂ oxide layer was formed rapidly on the oxidized material surface.The formation of dense SiO₂ oxide layer can slow down the oxidation of graphite and fill the pores left by the oxidation of graphite,heal the defects and improve the mechanical properties of the material to some extent..