Microstructure And Oxidation Resistance Of SiC/(Ti,Nb)C Ceramic Composites

Ultra high temperature ceramics（UHTCs）have been widely studied for their excellent comprehensive high temperature physical and chemical properties.However,the low fracture toughness of carbides limits their application as structural materials in extreme environments.Si C/（Ti,Nb）C based multiphase ceramics sintered by spark plasma sintering（SPS）and the effects of Si C/content and carbides in（Ti,Nb）C solid solution were studied on the densification,microstructure,mechanical properties and oxidation resistance.The results showed that x wt.%Si C/（Ti Nb）C composite ceramics（x=0,5,10,20）were sintered compactly,and the densities were 99.4%,99.7%,99.1%and 98.4%respectively.The effect of Si C/content on the density is not obvious.With the increase of Si C/content,the flexural strength of Si C/（Ti Nb）C composite ceramics increased firstly and then decreased,and reached the maximum value of 706 MPa at 5%Si C.The comprehensive mechanical properties of 10%Si C/（Ti Nb）C were the best,with bending strength of 606 MPa,fracture toughness of 6.6 MPa·m^1/2,elastic modulus of 486 GPa and Vickers hardness of 22.4 GPa.The results showed that the 10%Si C/（Ti Nb Ta）C,10%Si C/（Ti Zr Nb Ta）C and10%Si C/（Ti Zr Nb Ta Mo）C composite ceramics prepared by the same sintering process were sintered compactly,and their densities were 98.3%,97.8%and 96.5%respectively,which were slightly lower than those of 10%Si C/（Ti Nb）C.With the increase of Ta,Zr and Mo in turn,the density of the composite ceramics decreased.The addition of Ta led to the segregation of Nb and Ta,and the addition of Zr improved the segregation of Nb and Ta.10%Si C/（Ti Zr Nb Ta Mo）C multiphase ceramic was characterized by inhomogeneous element distribution and serious segregation due to the addition of Mo.Among the mechanical properties,10%Si C/（Ti Nb Ta）C had the best comprehensive mechanical properties,with bending strength,fracture toughness,elastic modulus and Vickers hardness of 651 MPa,7.9 MPa·m^1/2,482 GPa and 21.7 GPa,respectively.At the same composition and oxidation temperature,the growth rate of oxide layer thickness decreased with the progress of oxidation,and oxidation time had little effect on the composition and proportion of oxide on the surface of oxide layer.Under the same oxidation time and same composition,the thickness of the oxide layer increased with the increase of oxidation temperature,and the proportion of Ti O₂,Ti Nb₂O₇ and Si O₂ in the oxide layer increased with the increase of oxidation temperature.Under the same oxidation temperature and time,the content of Si C had a certain influence on the oxide thickness.The oxide layer of 20%Si C/（Ti Nb）C composite ceramics was thicker,because the oxide density on the surface of the oxide layer was not as dense as that of5%Si C/（Ti Nb）C and 10%Si C/（Ti Nb）C composite ceramics.The content of Si C had influence on the composition and proportion of oxide on the surface of oxide layer.Under several oxidation conditions,Ti and O elements were enriched on the surface of10%Si C/（Ti Nb）C oxide layer,which indicated that the surface Ti O₂ oxide contributes to the surface densification.The change of Si C content would not change the oxidation mechanism of the composite ceramics,but would change the oxidation resistance of the composite ceramics.