Research On Properties Optimization Of Self-healing CMC-SiCs Fabricated By Liquid Silicon Infiltration

Continous fiber reinforced SiC ceramic matrix composite（CMC-SiCs）is one of the promising candidates for application in nuclear fusion reactor and thermal protection systems,such as aircraft engine and aerospace vehicle,due to the low density,good mechanical strength,high thermal stability and good oxidation resistance.Liquid silicon infiltration（LSI）method is a quick densification method and can fabricate ceramic matrix composites with porosity less than 5%.However,the high process temperature（1600oC）of LSI lead to the CTE mismatch between the fiber and matrix,which brings the increased thermal resildual stress（TRS）and TRS induced matrix micro-crack density.The existence of matrix micro-crack restricts the mechanical peoperties and oxidation resistance of CMC-SiCs.Therefore,using LSI method to fabricate CMC-SiCs with synergistic improvement of mechanical properties and performance still needs further improvement.The introducing of self-healing phases such as B-bearing(B₁₂（C,Si,B）₃ and SiB₄)and Ti₃Si（Al）C₂ ceramic into porous CMC-SiCs（self-healing CMC-SiCs）by LSI method was proposed in this paper.Through comparing the microstructure,mechanical property and oxidation resistance of CMC-SiCs with different meterials systems,the effect of the self-healing matrix and PyC interphase thickness on the failure mode and oxidation mechanism of self-healing CMC-SiCs was systematically studied.The main contents and results are as follows.（1）The effect of Si-B-C matrix on the microstructure,room-temperature and high-temperature mechanical properties,thermophysical properties and oxidation resistance of C/SiC composites was studied.The results show that the LSI formed Si-B-C matrix uniformly distributes in the inter-bundle pores,leading to the porosity of 4%and density of 2.2g/cm³.The porous SiC matrix distributes in the inter-bundle pores of C/SiC composites fabricated by CVI,leading to the porosity of 11%and density of 2g/cm³.The different microstructures result in the different failure modes:the compressive failure mode of C/SiC-SiBC composites includes matrix shear failure and fiber kinking,the compressive failure mode of C/SiC composites includes matrix delamination,matrix shear fracture and interphase failure;the interlaminar shear failure mode of C/SiC-SiBC composites is across-layered fracture and the interlaminar shear failure mode of C/SiC composites is layered fracture.The improvement of matrix density gives rise to the compressive strength,in-plane shear strength,thermal expansion and thermal diffusion properties of C/SiC-SiBC composites compared with that of CVI fabricated C/SiC composites.The higher process temperature of LSI leads to the larger TRS of C/SiC-SiBC,which reduces the load transfer efficiency of matrix to fiber,resulting in the lower flexural strength of C/SiC-SiBCcomposites compared with CVI fabricated C/SiC composites.At elevated temperature,flexural strength of C/SiC-SiBC increases as TRS relieving,reaching the value of C/SiC composites at 1200oC.The oxidation behaviors of C/SiC-SiBC and C/SiC composites in the temperature range from 800oC to 1200oC are assessed.The results show that the retained flexural strength ratio of C/SiC-SiBC composites after oxidized at 800oC,1000oC and 1200oC for 10 hours are all about 80%,while it is 47%,73%and 88%for C/SiC composites.With the introducing of Si-B-C matrix,the oxidation resistance of C/SiC composites is improved through the healing of matrix micro-crack at 800oC.The oxidation mechanism of C/SiC-SiBC composites is as follows:at intermediate temperature region（800¹000oC）,oxidation is controlled by the competition between carbon-oxygen reaction rate and healing speed of matrix mico-crack;at high temperature region（1000¹200oC）,oxidation is controlled by the diffusion rate of oxygen through the borosilicate glass.（2）The effect of PyC interphase thickness on the mechanical properties and oxidation behavior of C/SiC-SiBC composites was studied.The results show that the flexural strength,fracture toughness and tensile strength of C/SiC-SiBC composites are dramaticlly increased with the increase of PyC interphase thickness.The thicker PyC interphase is helpful to relieving of TRS and affords appropriate interphase bonding strength which can promote interphase debonding and fiber pull-out,consequently improving the mechanical properties of C/SiC-SiBC composites.The C/SiC-SiBC composites with 450 nm PyC interphase attain the flexural strength of 412MPa,fracture toughness of 24MPa·m^1/2 ans tensile strength of 336MPa.The less TRS results in lower matrix micro-crack density and better oxidation resistance of C/SiC-SiBC composites.The retained flexural strength ratio of C/SiC-SiBC composites with 450 nm PyC interphase after oxidized at 800oC,1000oC and 1200oC for 10 hours are all above 100%.（3）The effect of Si-B-C matrix on the microstructure,mechanical properties and oxidation resistance of SiC/SiC composites was studied.The results show that the matrix density of SiC/SiC-SiBC composites is improved by introducing Si-B-C matrix,resulting in the higher compressive strength and interlaminar shear strength of SiC/SiC-SiBC composites than that of CVI fabricated SiC/SiC composites.The degradation of in-situ strength of SiC fiber in the process of LSI brings the lower flexural strength and fracture toughness of SiC/SiC-SiBC than SiC/SiC composites.After oxidized at 800oC,1000oC and 1200oC for 100 hours,the fiber and interphase of SiC/SiC composites keep intact and the retained flexural strength ratios maintain above 90%.The oxidation mechanism of SiC/SiC composites is as follows:at intermediate temperature range（800¹000oC）,the oxidation is controlled by oxygen diffusion through SiC coating defects;at high temperature range（1000¹200oC）,the oxdiation is controlled by oxygen diffusion through SiO₂ glass of SiC coating.Due to the unhealing of matrix micro-crack and the slow oxidation of near coating BN interphase and SiC fiber,the retained flexural strength ratio of Si C/SiC-SiBC conposites after oxidized at 800oC for100 hours is only 75%.After oxidized at 1000oC and 1200oC for 100 hours,the fiber and interphase are protected due to the healing of matrix micro-crack and the retained flexural strength ratios of SiC/SiC-SiBC conposites are all about 100%.The oxidation mechanism of SiC/SiC-SiBC composites is as follows:at intermedial temperature range（800¹000oC）,oxdiation is controlled by the fiber and interphase reaction with oxygen;at high temperature range（1000¹200oC）,oxidation is controlled by the diffusion rate of oxygen through the borosilicate glass.（4）The effect of Ti₃Si（Al）C₂-based matrix on the microstructure,mechanical properties,thermophysical properties and oxidation resistance of SiC/SiC compsoites was studied.The results show that the Ti₃Si（Al）C₂-based matrix（including Ti₃Si（Al）C₂,TiSi_xAl_y and residual Al and Si）are distributed uniformly in the inter-bundle pores of porous SiC/SiC composites and results in the decreased porosity of SiC/SiC-TiSiAlC composites of 3%.With the introducing of high ductile phase of Ti₃Si（Al）C₂,the fracture toughness of SiC/SiC-TiSiAlC composites is eaquel to that of CVI fabricated SiC/SiC composites.The laminate-structured Ti₃Si（Al）C₂ phase is helpful to absorb TRS in SiC/SiC-TiSiAlC composites and results in the cooperatively bearing load of fiber and matrix,consequently contributing to the good flexural strength of SiC/SiC-TiSiAlC composites.The compressive strength,interlaminar shear strength,in-plane shear strength and thermal diffusivity of SiC/SiC-TiSiAlC composites are improved due to the increased matrix density.The retained flexural strength ratio of SiC/SiC-TiSiAlC composites after oxidized at 800oC,1000oC and 1200oC for 100 hours are56%,55%and 71%,respectively.Due to the porous oxide and low-melt-point Al/Al₂O₃after TiSiAlC matrix oxidized at temperature range of 800¹200oC,the fiber edge and BN interphase of SiC/SiC-TiSiAlC composites are corroded and oxidized,which resulting in the low retained flexural strength ratio.The retained flexural strength of SiC/SiC-TiSiAlC composites depends on the degree of fiber damage.The oxidation mechanism of SiC/SiC-TiSiAlC composites is obtained:at intermediate temperature region（800¹000oC）,the oxidation is controlled by the fiber and interphase reaction with oxygen;at high temperature region（1000¹200oC）,the oxidation is controlled by the diffusion rate of oxygen and Al/Al₂O₃ through the matrix micro-crack.（5）The effect of matrix phase on mechanical peoperties of dense CMC-SiCs fabricated by LSI was studied through comparing the flexural strength and fracture toughness of dense CMC-SiCs with different material systems.The flexural strengths of dense CMC-SiCs with the same matrix,such as C/SiC-SiBC and SiC/SiC-SiBC,C/SiC-TiSiAlC and SiC/SiC-TiSiAlC,are compared.The results show that if the matrix is brittle phase,the higher CTE match degree between the fiber and the matrix,the less TRS and the higher flexural strength,which means the flexural strength of dense CMC-SiCs is impacted by the TRS value;if the matrix is tough phase,which can absorb TRS,the flexural strength of dense CMC-SiCs is not impacted by the TRS value.The fracture toughnesses of dense CMC-SiCs with the same fiber,such as C/SiC-SiBC and C/SiC-TiSiAlC,SiC/SiC-SiBC and SiC/SiC-TiSiAlC,are compared.The results show that when the matrix is brittle phase,the fracture toughness of dense CMC-SiCs is lower;when the matrix is toughness phase,the fracture toughness of dense CMC-SiCs is higher,which means the fracture toughness of dense CMC-SiCs is depend on the fracture toughness of the matrix and unrelated to the reinforcement.The porosity of dense CMC-SiCs with different material systems is lower than that of CVI fabricated CMC-SiCs and the compressive strengths of dense CMC-SiCs with different material systems are higher than CVI fabricated CMC-SiCs,which means the compressive strength is closely connected to the matrix density.