High-temperature Performance And Influence Mechanisms Of ZrC-modified C/C-SiC Composites

Due to the high specific strength,high temperature resistance and excellent antioxidation/ablation resistance,carbon fiber-reinforced silicon carbide ceramic matrix composites(C/C-SiC)can be used in thermal protection systems,propulsion systems and highperformance braking systems.Liquid silicon infiltration(LSI)is an attractive technique to fabricate C/C-SiC composite,which presents a lot of advantages,such as the simplicity of processing,short fabrication time,low cost and near-net shape manufacturing.During the fabrication,liquid silicon can climb into the inner pores of the C/C preform under the capillary force and reacts with its adjacent carbonaceous material,forming in-suit SiC matrix.Therefore,the content and distribution of SiC matrix in the C/C-SiC composite are strongly dependent on the microstructure of the C/C preform.Furthermore,the comprehensive properties of the C/CSiC composite would be significantly affected by the morphology,the distribution and the porosity of the micro-pores in C/C preform.The application of C/C-SiC composites often relates to complex high-temperature environments,which requires better comprehensive properties in oxidation,high-temperature strength,thermal shock resistance and ablation.For the purpose of satisfying higher requirements under high-temperature extreme environments,it is necessary to modify the matrix composition of the C/C-SiC composite.As a typical ultrahigh temperature ceramic,ZrC possesses high melting point(3540?),low density(6.73 g/cm3)and superior chemical innerness,which is preferred to be chosen as the ceramic component for matrix modification.Therefore,ZrC-modified C/C-SiC composites were fabricated in this work by a hybrid process,a combination of slurry infiltration(SI)and LSI.By controlling the content of ZrC nanoparticles in the phenolic resin(PF)solution,the effects of ZrC nanoparticles on the microstructure of C/C preforms and C/C-SiC composites as well as the composites' hightemperature properties were researched.This work mainly focused on the high-temperature properties of ZrC-modified C/C-SiC composites.Combined the high-temperature properties of the composites with their microstructures,the related influence factors and mechanisms were revealed.The main contents and results of this work are summarised as follows:1)The distribution pattern of the micro-pores in ZrC-containing C/C preforms and SiC matrix distribution in the ZrC-modified C/C-SiC composites were characterized and their influence mechanisms were studied.The microstructures of the preforms were characterized by Scanning Electron Microscopy(SEM).SEM results showed that the ZrC-containing resin matrix was uniformly coated on the fiber surface without obvious agglomeration.When the introduced ZrC content was less than 5 wt.%,the fiber-matrix bonding strength in the preform was high,and the microcracks of the preform were mainly distributed in the transverse direction and interlayer debonding.When the introduced ZrC content was more than 5 wt.%,the fibermatrix bonding strength in the preform decreased,and network cracks in the preform with increasing number and decreasing crack gap were formed.The results about the morphologies of the ZrC-modified C/C-SiC composites showed that the SiC/Si matrix in the composite gradually changed from a zonal distribution to a network distribution with increasing the ZrC content.Meanwhile,the uniformity of the SiC/Si matrix was significantly improved with increasing the ZrC content.2)The high-temperature flexural strengths and the related influence mechanisms of ZrCmodified C/C-SiC composites were studied under vacuum and atmospheric environments,respectively.Results showed that the flexural strength and modulus of the ZrC-modified C/CSiC composites were significantly improved in comparison with the original ones.Under vacuum condition,composite with the 10 wt.%ZrC nanoparticles(CSZ10)showed the best high-temperature strength,and the value could be reached to be 323.0 MPa,352.3 MPa and 161.6 MPa at 800?,1200 ? and 1600 ?,respectively.Under ambient atmosphere,the hightemperature strength of all composites showed different decrease due to the oxidation of the composites,but the ZrC-modified composites showed better flexural strength and antioxidation compared with the original ones.The improvement of high-temperature strength of ZrC-modified C/C-SiC composites was mainly attributed to the decrease of residual Si,the improvement of homogeneity of SiC matrix,the modification of fiber-matrix bonding strength and the decrease of residual thermal stress.The microstructure observation results showed that the uniformity of the SiC matrix and high-temperature structure stability were improved after the introduction of ZrC nanoparticles,which contributed to the high-temperature strength and anti-oxidation resistance of the ZrC-modified C/C-SiC composites.3)The thermal shock resistances of ZrC-modified C/C-SiC composite were performed by water quenching.The flexural strength of the composites after thermal shock at different temperature differences was measured by three-point bending test at room temperature.Results indicated that ZrC-modified C/C-SiC presented non-catastrophic fracture behaviors and possessed better flexural strength.Meanwhile,the flexural modulus of all composites decreased after water quenching.The change of the mechanical properties of the composites after thermal shock was mainly caused by thermal shock damage,which consisted of matrix cracking,interface debonding,fiber broken and matrix peeling-off.SEM observations found that the thermal shock damage in the composites was dependent on the temperature difference,which was corresponding to the thermal stress induced during water quenching.When the temperature difference was less than 500?,matrix cracking would be induced by the thermal stress;when the temperature difference was less than 1200?,partial matrix would be peeled off except for matrix cracking;when the temperature difference was greater than 1200?,the debonding between the fiber and the matrix would be generated.4)The ablation behavior of ZrC-modified C/C-SiC composites under high-temperature oxidation environments were numerically and experimentally investigated,respectively.Based on the ablation mechanisms and the heat transport theory of ZrC-modified C/C-SiC composites at high-temperature oxidation environments,a surficial ablation model with moving boundary of ZrC-modified C/C-SiC composites was developed.The liner ablation rate,oxide thickness and the temperature distribution of the ablation model were predicted through the computational procedures developed in this work.The calculation results showed that the linear ablation rates of ZrC-modified C/C-SiC composites would be decreased with increasing the content of SiC,Si or ZrC,and the linear ablation rate decreased obviously when the SiC content was increased.The ablation behavior of the ZrC-modified C/C-SiC composites was also experimentally characterized by oxyacetylene flame,and the experimental results showed that the linear ablation rate of composite with the 10 wt.%ZrC nanoparticles(CSZ10)decreased by 42%compared with composite without matrix modification.By comparing the numerical and experimental results,it indicated that the ablation model developed in this work is reasonable and the computational procedures had a rather high calculation accuracy,which could be used to predict the ablation behavior of the model with varying C/Si/SiC/ZrC phase contents.