| SiC fiber reinforced SiC matrix(SiC/SiC) composites exhibit excellent properties such as low density, high specific strength, high specific mod ulus, high temperature resistance, oxidation resistant and corrosion resistance. Currently, SiC/SiC compistes are known to have widespread application prospect in aerospace, military and nuclear industry. The research of the SiC/SiC composites is attracting more and more attention. Among potential fabrication processes of the SiC/SiC composites, polymer impregnation and pyrolysis(PIP) process is one of the main preparation processes of the SiC/SiC composites and has been became a hot research topic because of its advantages in low cost and large-scale components fabrication with complicated shapes. However, there were few reports about the research on the high- temperature, oxidation and long-time performances of the SiC/SiC composites by PIP process. In this thesis, the SiC/SiC composites were fabricated by optimized PIP process. The flexural and tensile properties were tested and analyzed at room temperature and 1300℃. The fatigue and creep tests were carried out of the SiC/SiC composites in high- temperature and oxidation environment and damage mechanisms were deeply discussed. At last, the stress, strain and life of the SiC/SiC composites component at the working condition of aero-engine were simulated and analyzed by finite element method.The PIP process was optimized by using liquid Polycarbosilane with-CH=CH2 groups(LPVCS), pyrocarbon(Py C) interphase, thermal molding process and environmental barrier coating(EBC). The density was increased, the costs and fabricated cycles were reduced and the SiC/SiC composites exhibited outstanding performances. The effects of LPVCS or PCS, with or without Py C interphase, different molding pressures and EBC coated or uncoated on the density, porosity, flexural strength, fracture toughness, tensile strength, and high-temperature oxidation resistance of the SiC /SiC composites. The results showed that when LPVCS was used as precursor polymer, the Py C interphase was about 500 nm thick, after 9 cycles of impregnation, crosslinking and pyrolysis process under the thermal molding pressure of 3MPa, the density and porosity of the SiC/SiC composites were 2.16g·cm-3 and 10.6% respectively, the flexural strength and fracture toughness were 637.5MPa and 29.8 MPa·m1/2, and the tensile strength and Young’s modulus were 336.8MPa and 94.8GPa respectively. Mullite/ Er2 Si O5+Er2O3 were coated on the surface of the SiC /SiC composites as the environmental barrier coatings(EBC). Durability and oxidation resistance at high-temperature of SiC/SiC composites were enhanced by EBC. However, cracks appeared on EBC after suffering in high-temperature and oxidation environment for 20 h, and breakdown occurred when to 100 h. Further research should be needed to improve the properties of EBC.The bending and tensile tests of the SiC/SiC composites were carried out at 1300℃ and the micro morphology was observed and analyzed. The flexural strength and fracture toughness of the SiC/SiC composites at 1300℃ were 470.2MPa and 20.7 MPa·m1/2 respectively. The tensile strength and Young’s modulus were 226.1MPa and 64.4GPa respectively. The tensile strength and Young’s modulus of the SiC/SiC composites with EBC were 339.7MPa and 85.5GPa, which were close to room temperature performance. According to the Weibull statistical theory, the ratio of tensile strength to flexural strength at 1300℃ was about 0.50. High-temperature flexural and tensile fracture processes of SiC /SiC composites were analyzed. The fracture displacement and failure stain of SiC/SiC composites increased significantly at 1300℃ and The failure mode exhibited pseudo plastic characteristics. High-temperature and oxidation damage of SiC/SiC composites were consist of interphase degeneration, matrix oxidation and fiber damage. Py C interphase was oxidized at 550℃. Above 1000 oC, the SiC fiber and matrix began to be oxidized and gradually the surfaces of the SiC/SiC composites were sealed by glassy phase of Si O2 and C-O-Si. The glassy phase had excellent effects of anti-oxidant, and the oxidation of the SiC/SiC composites slowed down with the temperature and oxidation time increasing.High-temperature fatigue tests of the SiC/SiC composites were carried out and the fracture morphology was observed Fatigue run-out of SiC/SiC composites was defined as 105 cycles. Fatigue limit of SiC /SiC composites with EBC was 60 MPa at 1300℃, and it was 40 MPa for SiC /SiC composites without EBC at 1300℃. The fatigue fracture surface was divided into oxidized region and non-embrittled region when the fatigue stress was higher than fatigue limit. The fatigue fracture surface was flat and exhibited brittle characteristic when the fatigue stress was lower than fatigue limit. The effect of fatigue stress, temperature and frequency on fatigue performance of SiC/SiC composites was investigated. Fatigue stress exhibited significant impact on fatigue performance of SiC/SiC composites. Fatigue damage increased and fatigue life shortened with the increasing of the fatigue stress. Temperature had little effect on fatigue performance of the SiC/SiC composites between 1000℃-1300℃. The influence of frequency on fatigue performance of SiC/SiC composites could be divided into two cases. Fatigue life was prolonged with the increasing of the frequency for EBC coated SiC/SiC composites. Fatigue life was shortened with the increasing of the frequency for SiC/SiC composites without EBC due to serious oxidation. Damage evolution and mechanism of high-temperature fatigue performance of SiC/SiC composites were proposed based on the experimental data and morphological observation. The groups of transverse cracks were initiated damage for high-temperature fatigue. During the fatigue processes, the damage consisted of friction of interphase, wear of fiber, oxidation and creep of SiC fiber and matrix. The cracks continuously extended by cracks connecting, interfacial deboning and fiber bridgeing, finally resulting in the fracture failure.High-temperature creep tests of the SiC /SiC composites were carried out and the fracture morphology was observed. Creep strength limit of SiC /SiC composites with EBC was 60 MPa at 1300℃, and it was 40 MPa for SiC/SiC composites without EBC at 1300℃. Fiber pullouts shortened with the decrease of the creep stress. The creep fracture surface was flat and exhibited brittle characteristic when the creep stress was lower than fatigue limit. At 1300℃ and the same creep stress level, the creep strain rate of the SiC/SiC composites with EBC was lower 1-2 magnitudes than that without EBC. At the same creep stress level for the SiC/SiC composites with EBC, the creep strain rate at 1100℃ was lower 1-2 magnitudes than that at 1300℃. Creep life prediction of SiC/SiC composites were modeled basing on the experimental data fitting by empirical formulae, Monkman- Grant relationship and Larson- Miler parameter. The creep mechanism and damage evolution of the SiC/SiC composites were investigated. When temperature<1100℃, the main damage mechanism occurring during creep was periodic fiber fracture and the creep behavior was governed by the SiC fibers. When temperature>1100℃, matrix micro cracking is the dominant mode of damage and the creep behavior was governed by the SiC matrix.The high-temperature fatigue- creep interaction tests of the SiC/SiC composites were carried out and the damage mechanisms of the high-temperature fatigue- creep interaction were proposed. At the first a few fatigue- creep interaction cycles, the damage of the SiC/SiC composites was governed by creep mechanism. The strain and the strain rate were rapidly increased and decreased respectively, and a large number of micro cracks were developed in the SiC matrix. After a period of fatigue- creep interaction cycles, the damage of the SiC /SiC composites was governed by fatigue mechanism. The matrix cracks propagation and fiber bridging finally resulted in the failure of the SiC/SiC composites.The stress and strain of the component fabricated by SiC /SiC composites under working conditions were analyzed by finite element software. The fatigue and creep lives of the component were predicted basing on the experimental data. Based on the micro finite element model of SiC/SiC composites, the stress and strain were calculated and analyzed. |
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