Research Of Residual Thermal Stress And Matrix Cracking Stress For CMC-SIC With Hybrid Fibers

Continuous carbon fiber (Cf) reinforced silicon carbide matrix (SiCm) compositeshave a broad application prospect in aviation, aerospace and energy fields. However,the residual thermal stress (RTS), produced due to the mismatch between the CfandSiCmwhen the composites cooled down from the preparation temperature to the roomtemperature, results in the matrix cracking and impact on the performance of thecomposites, especially, the matrix cracking stress of the composites. As far as weknow, there is no study about the relationship between the RTS and matrix crackingstress of the composites in domestic.In this thesis, the effect of hybrid ways on RTS distribution, microstructure featuresand the strength distribution were studied by three dimentional Cfand silicon carbidefiber (SiCf) reinforced SiCmcomposites in order to investigation the relationshipbetween the RTS and the matrix cracking stress. On the basis of the optimizationhybrid way, the effect of Cfand SiCfwith different hybrid proportions on the matrixcracking stress was studied. The relationship between matrix cracking stress and RTSwas established from the perspective of mesoscopic mechanics of composite materials.The behaviors of thermal expansion, thermal diffusion and oxidation were alsostudied in this thesis. The main research contents and results are as follows:1) The effect of hybrid ways and hybrid proportion between Cfand SiCfon theresidual thermal stress distribution was studied. The results show that:(1) Both the hybrid composites with Cfand SiCfalternative distribution[(xC×ySiC)/PyC/SiC] and the hybrid composites with Cfand SiCfcontacteddistribution [(xC-ySiC)/PyC/SiC] can reduce the axial residual thermal stress of SiCmcompared to that of C/PyC/SiC composite. For an example with the volume fractionratio of Cfto SiCfequal to1:2, the SiCmaxial stress in (xC×ySiC)/PyC/SiC and(xC-ySiC)/PyC/SiC composites is reduced to (349and227MPa, θ=45°;174and39MPa, θ=0°) from726MPa in C/PyC/SiC composite, respectively. It is advisable tochoose Cfand SiCfcontact mixed distribution form the change of SiCmaxial stress.The SiCmradial stress in (xC×ySiC)/PyC/SiC and (xC-ySiC)/PyC/SiC composites isincreased from100MPa to123and145MPa in θ=45°direction, respectively.Meanwhile, SiCmradial stress in (xC×ySiC)/PyC/SiC and (xC-ySiC)/PyC/SiCcomposites is reduced from100MPa to-43and-28MPa in θ=0°direction, respectively. It is advisable to choose Cfand SiCfalternative mixed distribution fromthe change of SiCmradial stress.(2) For an example with the Cfand SiCfcontact mixed distribution, the axial andradial residual stress of SiCmdecrease with the increase of SiCfvolume fractions.However, the change trend (increase or decrease) of the structural units’ axial stresswill be slow when the hybrid ratio of Cfto SiCfincreased from1:2to1:4, especially,the SiCmaxial tensile stress is reduced only from174to170MPa in θ=45°direction.2) The microstructure features and strength distribution of C/PyC/SiC,(C×2SiC)/PyC/SiC and (C-2SiC)/PyC/SiC composites were studied. The results showthat:(1) Both the amount and dimensions of SiCmmicrocracks in the two kinds ofhybrid composites are dramatically reduced compared to those of C/PyC/SiCcomposite. There are also some less than micron grade axial microcracks in SiCmaround the Cfin (C×2SiC)/PyC/SiC and (C-2SiC)/PyC/SiC composites, and themicrocracks only extend to Cfaccumulation region. The amount and dimensions ofSiCmmicrocracks in (C-2SiC)/PyC/SiC are less than those in (C×2SiC)/PyC/SiCcomposite. Statistics results show that: the SiC coating microcracks of the three kindsof composites obey uniform distribution. Compared to C/PyC/SiC composite, boththe SiC coating microcracks width of (C×2SiC)/PyC/SiC and (C-2SiC)/PyC/SiCcomposites are reduced from1to0.2μm, and the SiC coating microcracks density aredecreased from1090to46twig/m and50twig/m.(2) The flexural strength of (C×2SiC)/PyC/SiC and (C-2SiC)/PyC/SiC isincreased by439%and469%compared to that of C/PyC/SiC composite, repectively.The flexural strength of three kinds of composites obeys the weibull distribution, andthe weibull modulus is18.1,2.4and15.8, respectively. The strength deviationbetween the measured values and predicted values according to the weibulldistribution function is4.4%,1526%and0.19%. Obviously, the weibull modulus of(C×2SiC)/PyC/SiC with the Cfand SiCfalternative distribution is too low, whichresult in the large predicted deviation. As a result, the bybrid composites with the Cfand SiCfalternative distribution are the main research object in this thesis.3) The matrix cracking stresses of C/PyC/SiC and the bybrid Cfand SiCfalternativedistribution composites with different hybrid ratios were studied. A mathematicalrelationship model between the matrix cracking stress and residual thermal stress wasestablished based on the mesoscopic mechanics analysis method. The results show that:(1) The matrix cracking stress of C/PyC/SiC is only29±3MPa. The matrixcracking stresses of (3C-2SiC)/PyC/SiC,(C-SiC)/PyC/SiC,(C-2SiC)/PyC/SiC and(C-4SiC)/PyC/SiC, in which the hybrid ratios between Cfand SiCfis3:2,1:1,1:2and1:4are47±10,74±4,98±4and99±11MPa. The matrix cracking stress does notincrease obviously when the hybrid ratio is more than1:2.(2) The mathematical relationship model between the matrix cracking stress andSiCmresidual thermal stress shows that: The matrix cracking stress can be enhancedby either reducing the SiCmresidual tensile stress in matrix or improving the elasticitymodulus ratio of fiber to matrix. The matrix cracking stresses of (C-4SiC)/PyC/SiCand SiC/PyC/SiC are116±8and138±5MPa according to the mathematicalrelationship model, and the deviation between the calculated values and the measuredvalues is only15%for the two kinds of composites. Meanwhile, the residual stressesof SiCmin C/PyC/SiC,(3C-2SiC)/PyC/SiC,(C-SiC)/PyC/SiC and (C-2SiC)/PyC/SiCcomposites are calculated according to their matrix cracking stresses by themathematical relationship model, and the values are140,120,90and60MPa. Thechange trend is consistent with the calculated residual thermal stress, which means themathematical relationship model is practical and reliable.4) The fracture model of C/PyC/SiC,(C×2SiC)/PyC/SiC and (C-2SiC)/PyC/SiCcomposites show that: The fracture model of C/PyC/SiC is a noncumulative model(no Cfpull out); The fracture model of SiC/PyC/SiC is a cumulative model (SiCfpullout); The fracture model of (C-2SiC)/PyC/SiC, including the noncumulative modeland cumulative model, is a mix fracture model.5) The behaviors of thermal expansion and thermal diffusion of C/PyC/SiC and(C-2SiC)/PyC/SiC composites were studied. The results show that: The coefficients ofengineering thermal expansion and thermal diffusion of (C-2SiC)/PyC/SiC are largerthan those of C/PyC/SiC composite. The corresponding relationship between thephysical thermal expansion behaviors and residual thermal stress was obtained forC/PyC/SiC and (C-2SiC)/PyC/SiC composites. The results show that: Below thecomposites preparation temperature, the elastic release region of thermal stressdecreases from450to350oC compared to that of C/PyC/SiC composite, whichmeans the matrix cracking temperature difference increased. Above the compositespreparation temperature, the coefficient of thermal expansion of C/PyC/SiC increaserising rapidly due to the low in situ strength Cfmay fracture under the tensile stress. While for (C-2SiC)/PyC/SiC composite, the slope of coefficient of thermal expansionis lower than that of C/PyC/SiC because the thermal expansion of SiCmis restrainedby high in situ strength of SiCf. It is indicated that the change of thermal expansionbehavior is effected by the in situ strength of the fiber for CMC-SiC above thepreparation temperature.6) The behaviors of oxidation for C/PyC/SiC and (C-2SiC)/PyC/SiC composites werestudied. The results show that: The oxidation weightless rate of (C-2SiC)/PyC/SiC islower than that of C/PyC/SiC beacause both the amount and dimensions of SiCmmicrocracks in the (C-2SiC)/PyC/SiC are less than those in C/PyC/SiC composite.The strength retention after the oxidation test for the two kinds of composites showsthat: The lower oxidation weight loss rate, the higher strength retention for C/PyC/SiCcomposite; the strength retention is in accordance with the change trend of oxidationweight loss below1000oC for (C-2SiC)/PyC/SiC composite. The strength retentiondecreases with the rising temperature even though the (C-2SiC)/PyC/SiC compositeshows oxidation weight gain above1200oC, which is due to the grain growth ofHi-Nicalon SiCf.