Strengthening And Toughening Design Of Dense CMC-SiC And Its Multifunctional Properties

Continuous fiber reinforced silicon carbide ceramic matrix composites（CMC-SiC）were once thermal structure materials but now are developed as structural-functional materials.In order to obtain CMC-SiC with excellent mechanical properties,electromagnetic shielding properties and oxidation resistance,it is necessary to obey a comprehensive design criteria considering strengthening and toughening,high conductivity and crack self-healing.The poor oxidation resistance of C/SiC can be improved by introducing self-healing phase into the matrix.The poor electromagnetic shielding property of SiC/SiC can be improved by introducing high conductivity phase into the matrix.However,the introduction of modified matrix will affect the strength and toughness of the composites.The difficulty lies in the synergistic improvement of mechanical and functional properties.Through melt infiltration（MI）,modified phases can be formed in CMC-SiC prepared by chemical vapor infiltration（CVI）.Compared with the CMC-SiC prepared by CVI,the matrix of CMC-SiC prepared by CVI combined with MI is much more dense,which is called dense CMC-SiC.Through the oxidation of carbon phases in C/SiC,the contribution of each structural unit to the mechanical and electromagnetic shielding properties can be specified.Through interface or matrix design,good modified CMC-SiC with excellent mechanical,electromagnetic shielding properties and oxidation resistance can be obtained.Through establishing the relationships among interface or matrix design,thermal residual stresses（TRS）,interfacial micro-mechanical behavior and macro-mechanical properties,the influence mechanism of interface or matrix design on the strength and toughness of dense modified CMC-SiC can be revealed.The main results are as follows.（1）The evolution of microstructure,mechanical properties and electromagnetic shielding properties of T-300 C/SiC in oxidation environment at 700℃was studied.The contribution of each component to the mechanical and electromagnetic shielding properties was also studied.As the carbon phases content was reduced from 41.7 vol.%to 0 vol.%,the effective fiber volume fraction,electrical conductivity and the ability of reflecting and absorbing electromagnetic wave in the composite decreased.Finally,the flexural strength,fracture toughness and total shielding effectiveness of C/SiC decreased by 95.5%,97.4%and 44.2%,respectively,indicating that the mechanical properties mainly depend on the carbon fibers coated by Py C,while the electromagnetic shielding performance depends on both carbon fiber and SiC matrix.As the carbon phases exhausted,the electromagnetic shielding effectiveness of porous SiC matrix is still above 20 d B,which is related to the existence of a large of number of3C-SiC/2H-SiC interfaces in the columnar crystals,where an electric dipole region can be formed to generate polarization loss.（2）The influence mechanism of self-healing matrix Si BC on the strength,toughness and thermal expansion behavior of T-300 C/SiC and AMO-SiC/SiC（A-SiC/SiC）were studied.With the introduction of Si BC matrix,the axial TRS of matrix in C/SiC and A-SiC/SiC increased from 141±7.5 MPa（tensile）and-27±7 MPa（compressive）to 154±6 MPa（tensile）and 47±9MPa（tensile）,respectively;the interfacial shear strength increased from 15±4 MPa and 8.4±1MPa to 22±4 MPa and 32±3,respectively.The increase of TRS is detrimental to the loading and of fibers.The increase of interfacial shear strength lead to decreased length of pulling fibers.Therefore,with the introduction of Si BC,the flexural strength,tensile strength and fracture toughness of C/SiC decreased by 12%,42%and 19%,respectively,while those of A-SiC/SiC decreased by 13%,59%and 34%,respectively.The performance degradation of A-SiC/SiC was more significant than that of C/SiC,indicating the thermal damage of AMO-SiC fibers.Different form A-SiC/SiC,there are matrix cracks caused by high thermal residual stresses in C/SiC system,which can provide space for matrix expansion.Therefore,with the introduction of Si BC matrix,the thermal expansion coefficient of C/SiC decreased while that of A-SiC/SiC increased.（3）The influence mechanism of Py C interphase thickness(e _{Py C})on the strength and toughness of C/SiC-Si BC was studied.Due to the anisotropy of carbon fiber in coefficient of thermal expansion（CTE）,the fiber in C/SiC system will buckle under the axial residual compressive stress,which makes the fiber suffer additional compressive stress in the radial direction.As the e _{Py C} in C/SiC-Si BC increased from 50 nm to 400 nm,the TRS of matrix and fiber decreases from 160±4 MPa（tensile）and-480±15 MPa（compressive）decreased to 124±29MPa（tensile）and-372±87 MPa（compressive）,respectively,which weakened the buckling degree of carbon fibers.Accordingly,the interfacial shear strength of C/SiC-Si BC decreased from 76±13 MPa to 8±1 MPa.The decrease of TRS and interfacial shear strength are beneficial to the loading and pulling out of fibers.As a result,the flexural strength and tensile strength of C/SiC-Si BC increased by 94%and 247%,respectively.As the e _{Py C} increased from 200 nm to400 nm,the fracture toughness of C/SiC-Si BC increased by 32%.The tensile strength,flexural strength,fracture toughness and total shielding effectiveness of C/SiC-Si BC(e _{Py C}=400 nm)are260±39 MPa,562±26 MPa,23.7±0.5 MPa·m^1/2 and 39.3 d B,respectively.After 60 times of thermal shock under different temperature difference,the retention rate of flexural strength and total shielding effectiveness were more than 80%and 95%,respectively,which indicates that C/SiC-Si BC(e _{Py C}=400 nm)is a structural functional material with excellent mechanical properties,electromagnetic shielding performance and environmental resistance.（4）The damage mode and strengthening and toughening mechanism of ZMI-SiC/SiC（Z-SiC/SiC）and AMO-SiC/SiC（A-SiC/SiC）modified by Ti₃Si（Al）C₂,a phase with plastic deformations,were studied.The introduction of Ti₃Si（Al）C₂ aggravated the thermal mismatch in SiC/SiC and increased the interfacial shear strength.However,the introduction of Ti₃Si（Al）C₂ also changed the damage mode of SiC/SiC from vertical cracking（perpendicular to fibers）of matrix to hoop cracking（parallel to fibers）of SiC matrix due to the plastic deformations of Ti₃Si（Al）C₂ which improved the ability of modified matrix to resist crack propagation.Accordingly,the cracks propagated along the transition layer（composed of amorphous carbon and Si O₂）in SiC matrix,which released the TRS and weakened the radial compressive stress of fiber.Consequently,the strength and toughness of modified SiC/SiC with Ti₃Si（Al）C₂ are better than that of modified SiC/SiC without Ti₃Si（Al）C₂.As a high conductive phase,the introduction of Ti₃Si（Al）C₂ also improved the conductivity and shielding effectiveness of composite.The flexural strength,fracture toughness and total shielding effectiveness of Z-SiC/SiC-Ti₃Si（Al）C₂ are 499±43 MPa,23.7±1.2 MPa·m^1/2 and 43.0±0.3 d B,respectively,which indicates that it is a structural functional material with excellent mechanical and electromagnetic shielding properties.