Study On Properties And Simulation Of SiC_f/SiC Composites

SiC_f/SiC composites have become one of the most promising high-temperature structural materials for aero-engine hot-end components due to their excellent properties,such as low density,high specific modulus,high specific strength,high temperature resistance,oxidation resistance and ablation resistance.The thermal structural components of aero-engine will be working under environments with complex stresses and thermochemical reactions,which requires high mechanical and thermophysical properties of the selected materials.Therefore,in order to apply SiC_f/SiC composites to aero-engine thermal structural components,the testing and analysis of their material-level performances should be very important,which is directly related to the subsequent component evaluation.In this paper,SiC_f/SiC composites was prepared by PIP process using KD-II SiC fibers as the reinforcement and Py C as the interphase.The physical properties,mechanical properties,damage mechanism and evolution law of the composites under the coupled thermal/mechanical/oxygen environment were studied in order to accumulate the performance data of KD-II SiC_f/SiC composites,provide data and theoretical support for their applying to thermal structural components in high performance aero-engine.The following are the main research contents and results:(1)SiC_f/SiC composites for mechanical property tests were manufactured by by ten PIP cycles using three-dimensional four-directionnal and three-dimensional orthogonal KD-?SiC fiber fabric as the reinforcements,LPVCS(liquid polycarbosilane)as the ceramic precursor,Py C(cracked carbon)as the fiber-matrix interface phase.The pyrolysis temperature is 1200?,and the protective atmosphere is Ar(argon).Before the first PIP cycle,the impregnated fabrics are hot-molding pressed with 1MPa and cured at 300?.The composites for thermal conductivity tests are prepared by several PIP cycles with three-dimensional four-directionnal fabrics as the reinforcements.Among them,the hot molding pressure is 1-9mpa,and the pyrolysis temperature is 1200-1800?,respectivly.The matrix modified samples are prepared by filling with submicron sized SiC particles by EPD(electrophoretic deposition)method,after which 8 PIP cycles are processed.During EPD,the voltage is set to 30V and the deposition time is 15min.(2)Tensile and bending properties at room temperature of two three-dimensional fabric reinforced SiC_f/SiC composites were studied.The bending strengths of 3D4d and3D orthogonal composites are 895.4MPa and 716.4MPa,respectively.The uniaxial tensile strength,Young's modulus and proportional limit of 3D4d composites are441.4MPa,138.9GPa and 258MPa,respectively,while the corresponding tensile properties of 3D orthogonal composites are lower than those of 3D4d composites.The reason is that the fiber content along the tensile direction of 3D orthogonal composites is lower.What's more,the fracture modes of the two composites are different.The cracks in the 3D4d composites originate in the matrix enrichment zone at the intersection areas of yarns,and the cracks propagate along the yarn-matrix interface and deflect inside the yarn.The cracks in the 3D orthogonal composites originate from the defects in the Z-direction yarns or the matrix and interphase in the warp yarns,and then the composites are mainly loaded by the weft yarns.(3)The room temperature compression and interlaminar shear properties of two 3D fabric reinforced SiC_f/SiC composites were studied.The axial compressive strength and interlaminar shear strength of 3D4d composites are 418.7MPa and 71.8MPa,respectively,which are lower than those of the 3D orthogonal fabric reinforced composites,which are 498.8MPa and 114.7MPa,respectively.Compression failure modes of 3D4d composites include fiber breakage,fiber-matrix interphase cracking,yarn-matrix interface cracking and delamination,matrix breakage.And interlaminar shear failure modes include yarn-matrix interphase cracking and delamination,and fiber bending fracture at yarn intersection zones.Compression failure modes of 3D orthogonal composites include weft yarn-matrix interface cracking and delamination,warp/weft yarn shear failure,warp yarn fiber-matrix interphase cracking and matrix breakage.The interlaminar shear failure mode is shear failure between multilayer weft yarns.(4)Tensile and bending properties at 1350?in air of two 3D fabric reinforced SiC_f/SiC composites were studied.3D4d composites coated with CVD SiC exhibit the highest uniaxial tensile strength,which is 473.8 MPa,while that of 3D orthogonal composites is slightly lower(358.9 MPa).The SiC surface coating protects the composites from O₂ corrosion well.The oxidation reaction mainly occurs on the surface of the coating.The Si O₂ generated delays diffusion process of O₂ into the material.Tensile strength of the composite without surface coating decreases significantly at high temperature,and the difference between the oxidation zone and the non-oxidation zone in the furacture surface is obvious.Similarly,the bending properties at high temperatures of the composites without surface coatings also decreased significantly.At this condition,the Py C interphase is oxidized and consumed first,and then the exposed surfaces of fibers and matrix are oxidized.The Si O₂ forms a brittle interface of strengthening chemical bonding between the fiber and matrix.The tensile strength of the composites is greatly affected by the loading rate under atmospheric environment of1350?.For 3D4d composites,the tensile strength is 473.8 MPa when the loading rate is 5MPa/s,however,215.7 MPa when the loading rate is 0.05MPa/s.The rate dependence of high temperature tensile strength of the composites was explained by a delayed failure mechanism.The delayed failure parameters of SiC_f/SiC composites are calculated.The results show that the two kinds of fabric structural composites are highly sensitive to delayed failure.(5)The fatigue properties of 3D4d SiC_f/SiC composites under different fatigue stress levels(60 MPa,80 MPa,120 MPa and 160 MPa,respectively)at 1350?in air were studied,and the damage characteristics of fatigue failure were analyzed.The fatigue life decreases significantly with the increase of the stress level.In the testing range,the fatigue damage mechanism of the composites is oxidation induced brittle fracture damage,and the area of oxidation zone decreases with the increase of stress level.(6)The thermal shock resistance of 3D4d SiC_f/SiC composites under atmosphere is studied.The thermal shock temperature range is from room temperature to 1350?,and the maximum number of thermal shocks cycles is 50.The flexural strength of the composites begin to decrease after 10 thermal shock cycles and become stabilized after30 cycles,and the strength retention is still 63.5%after 50 cycles.The thermal shock damage mechanism of the composites is that the low thermal conductivity of the composites results in a larger temperature gradient and thermal stress in the interior of the composites when they are heated or cooled.The thermal stress causes the cracking of the coatings or interfaces with weak bonding,and the air entering the interior of the composites from the cracks causes oxidative damage.(7)Thermal conductivities of 3D4d SiC_f/SiC composites are studied experimentally.The results show that fiber volume fraction and porosity have little effect on the thermal conductivity,while the crystallinity of the matrix and fibers has great influence on the thermal conductivity of the composites.The higher the crystallinity of the matrix or fiber,the better the thermal conductivity of the composites.(8)The UMAT user's subroutine of Abaqus software is applied to simulate the tensile fracture process of the 3D4d composites by finite element method.The simulated strength,stress-strain curve and progressive damage process are in good agreement with the experimental phenomena and results.Thermal conductivities of the composites are also simulated by finite element method.The results show that the heat transfer mainly occurs in the components with high thermal conductivities,while the components with low thermal conductivities are neglected.Based on the porperties tested above,a static finite element simulation of a combustor liner under its working conditions is carried out,and the stress distribution and deformation are obtained.The results show that the thermal stress and deformation in the combustor liner are lower than the failure limit of the composites.