Preparation And Properties Of Ti₃SiC₂/SiC Functionally Graded Materials

Ti₃SiC₂ has good toughness,but high temperature creep resistance and oxidation resistance above 1100?are poor.SiC is a high performance ceramic with excellent high temperature creep resistance and oxidation resistance,but poor toughness.In this paper,an FGM can be produced from Ti₃SiC₂ to SiC,and gradual transitional composition and structure can be achieved in FGM.On the one hand,due to the strong covalent bonds,the formation of dense SiC ceramics requires a high temperature?approximately 2000??and/or a high pressure?over 50 MPa?.On the other hand,optimal sintering temperature of Ti₃SiC₂ is from 1500 to 1600?.However,Ti₃SiC₂tends to decompose into TiC at temperatures above 1700?,even in a vacuum atmosphere.Therefore,it is difficult to prepare Ti₃SiC₂/SiC FGM,and a new densification method should be developed.This paper developed a local impregnation technique called positioning impregnation.The positioning impregnation process significantly densified layers with SiC volume fractions of more than 70%while only slightly affecting the densities of the other layers and preserving sufficiently weak interfaces between layers.Due to the densification effect of positioning impregnation,the density of the FGM hot pressed at1600?increased from 3.47 to 3.79 g/cm³,and the open porosity decreased from 11.2%to 1.98%;the density of the FGM hot pressed at 1700?increased from 3.50 to 3.83g/cm³,and the open porosity decreased from 10.8%to 1.58%.FGMs that were hot pressed at 1600 and 1700?and then subjected to impregnation showed not only high flexural strengths but also zigzag load-displacement behaviour.The flexural strengths of these FGMs were 436 and 485 MPa,respectively;in comparison,the values for the FGMs without impregnation that were hot pressed at1600,1700 and 1800?were 235,268 and 328 MPa,respectively.Moreover,the fracture toughnesses of these FGMs were 8.23 and 7.15 MPa·m^1/2,respectively;in comparison,the values for the FGMs without impregnation that were hot pressed at1600,1700 and 1800?were 6.77,7.05 and 4.65 MPa·m^1/2,respectively.The oxidation resistance of the FGM hot pressed at 1600?and then positioning impregnated was significantly improved,and a mass gain percentage of only 12.9%after oxidation at 1400?for 20 h was observed.This gain was 45%and 20%lower than those of the unimpregnated FGMs hot pressed at 1600 and 1800?,respectively,and 39%lower than that of the Ti₃SiC₂/30 wt%SiC composite.For the unimpregnated FGM,the central transition layers showed the best oxidation resistance,whereas the pure SiC and pure Ti₃SiC₂ layers at both ends showed poor oxidation resistance.With the increased Ti₃SiC₂ content in the layer,the resistivity decreased significantly firstly and then decreased slowly and finally tended to be a constant.The SE_T of all impregnated samples over X band were 43-52 dB.Therefore,the Ti₃SiC₂/SiC FGM prepared by positioning impregnation can be suitable for potential applications in excellent EM shielding or EM absorbing materials.Furthermore,adding?-Al₂O₃nanoparticles did not improve obviously the SE_T.However,the SE_T was sightly improved in the case of the pure SiC layer as the incident face.The SE_T enhanced in the case of the pure Ti₃SiC₂ layer as the incident face compared with that in the case of the pure SiC layer as the incident face.The SE_A was higher than the SE_R for all samples either the 100%Ti₃SiC₂ layer or the 100%SiC layer being used as the incident face.