Microstructural Evolution Of The Self-healing Continuous Fiber Ceramic Composites Under Simulated Environment

Continuous fiber reinforced SiC-matrix composites(CMC-SiC) are considered as the most promising thermo-structural materials due to their high thermal stability,low density,high specific strength,high specific modulus,good oxidation-ablation resistance,especially improved flaw tolerance and non-catastrophic mode of failure. However,the bad oxidation resistance of CMC-SiC in high temperature,high load and oxidation environment limits their long-term application in aero-engine.Consequently CMC-SiC with self-healing function should be developed,which can be named CMC-MS(Multilayer Self-healing Ceramic Matrix Composites).Since the CMC-SiC was used at a high temperature under a complex environment containing O₂,H₂O and corrosives,the mechanical property will be controled by a series of thermo-physical chemistry reactions(such as surface oxidation,phase transformation and atomic diffusion) which occurred under the effects of temperature and environment.So,it is important to study the microstructural evolution of the CMC-SiC components annealed in the temperature/environment coupling conditions.In this thesis,the mechanical and structural evolution of Hi-Nicalon^TM SiC fiber annealed in O₂-Ar-H₂O atmospheres were investigated.Much of analysis and discussion will then reveal the factors related to the residual strength and microstructural evolution of the annealed fiber.Then,the location and chemical state of Al element in Al-SiC fiber were explored and the effect of Al during the structure formation process was discussed.Bisides,a CVD B-C ceramic coating used as a self-healing modification component of CMC-SiC was also annealed in pure Ar and O₂-Ar-H₂O atmospheres respectively to explore the crystallization and oxidation.Finally,two kinds of CMC-MSs were fabricated.The self-healing performance,oxidation resistance and mechanical properties of the two CMC-MSs were demonstrated.The main subjects and results are summarized as follow:1.Hi-Nicalon fibers were exposed in O₂/Ar / H₂O atmosphere for 1 hour at 1300℃, 1400℃,1500℃,1600℃,respectively.Results indicated that residual tensile strength increased with increasing temperature from 1300℃to 1500℃,then decreased after annealing in 1600℃.In conclusion,the control effects of water vapor on formation and structural evolution of the passive film were remarkble.The change of residual strength of the annealed fiber was influenced by the competition of negative and active effects. Below 1500℃,the active effects were dominated factors.Above 1600℃,the negative effects such as rapid oxidation,volatility,film-melting,and fihn/fiber reactions occurred and became dominated reactions.After these processes,fiber's structure was evidently destroyed.2.The role,location and chemical state of Al element in Al-SiC fiber were explored. The results and conclusions were as followed:(1)The structure and orientation of the crystals and defects in Al-SiC fiber were investigated by HRTEM and structural simulation technic.(2) Al can lower the densification temperature and restrain the grain growth of SiC crystals during the sintering process,but these effects were limited by sintering temperature,oxidative crosslinking time and grain size of the SiC crystals.(3) The Al containing compound which mainly existed as the intergranular phase between SiC crystals were Al₂O₃,Al₂OC and Al₄SiC₄.Finally,a model was built to illustrate the structure of Al-SiC fiber.3.Microstructure of CVD B-C coatings deposited under various parameters was examined by electron microscopy and spectroscopy.Then,structural evolution of the coating annealed in different environments was also investigated.The results and conclusions were as followed:(1) The B-C coatings produced by CVD can be divided as crystallized coatings and amorphous coatings.(2) A two-layer B-C coating was annealed in an Ar atmosphere for 2 hour at 1600℃to 2000℃.After annealing,the amorphous boron carbide had partly transformed to B₄C crystals,and the crystallization was controlled by the diffusion of C and B.TEM showed an evident grain growth with the increase of annealing temperature.The B₄C crystals produced near the layer/layer interface mainly grew epitaxially along the(012) direction.(3) The protection effect of B-C coating could be destroyed in a short time because the rapid oxidation of BC_x and volatilization of B₂O₃,so it was unsuitable to make B-C coating as an outer coating. Thus,a SiC/B-C/SiC structure was fabricated and annealed.Results showed that there were different oxidation mechanisms under different annealing temperatures:the oxidation mechanism of low temperature annealing was controlled by the diffusion of the O₂/H₂O gas through the defects in the SiC surface coating,and then controlled by the oxidation of the B-C coating;in the middle temperature district,reaction was controlled by the local oxidation of SiC surface coating,the diffusion of the O₂/H₂O gas through the surface coating defects,and then controlled by the oxidation of the B-C coating and the partial formation of B₂O₃·xSiO₂;in the high temperature district, reaction was controlled by the rapid oxidation of SiC coating,the diffusion of the O₂/H₂O gas through the surface defects and oxidation product SiO₂,the rapid oxidation of B-C and substantive formation of B₂O₃·xSiO₂,orderly.Weight-change data was also used to explain these mechanisms,all the information indicated that the SiC/B-C/SiC coating possess excellent performance after annealing in the O₂/H₂O/Ar atmosphere under different temperatures.4.Two kinds of CMC-MSs with good properties were fabricated.One called coating-modified C_f/SiC composite,another called matrix-modified C_f/SiC composite. The antioxidation properties and self-healing funtions of each CMC-MS were examined under different temperature regions.The results and conclusion were as follows:(1) At least 4 types of the microcracks can be observed in both two composites,and the defects in CMC-MS structure had a linear distribution.(2) The PyC coating had a similar epitaxy with the carbon fiber after annealing,which indicated that the coherent structure was the key factor to form a strong bond between them.And the microcracks can hardly deflect at the fiber/PyC interface.(3) Micro-defects in the coating-modified composite could be healed by SiO₂ after high temperature oxidation.Limitted by the fluidity of SiO₂,it was difficult to heal a large-size deflect.(4) By contrast,much of the fabrication and oxidation induced deflects had been healed by the B₂O₃ fluidity in matrix-modified composite,assisted by SiO₂ and B₂O₃·xSiO₂.