Microstructural Evolving And Failure Mechanism Of 3D C/SiC In Environment Simulating Hot-Section Component Service In Aero-Engine

Continuous carbon fiber reinforced silicon carbide matrix composites (C/SiC)were a high-temperature lightweight structural materials which were strategic innational defense and have the potentials in many fields including aerospace, space,and nuclear reactor. C/SiC were non-catastrophic with pseudo-plastic behavior andhad some advantage such as high strength-weight ratio, which made it a key candidateto aim at use with oxidation-protection and long life at high temperature when appliedas hot-section component in aero-engine with high thrust-weight ratio.The complicated multi-factor environments in hot-section component inadvanced aero-engine were selected as objective environment in this dissertation.Multi-scale characterization including optical microscopy (OM), scanning electronmicroscopy (SEM), transmission electron microscopy (TEM) and high resolutiontransmission electron microscopy (HRTEM) were made use to investigatemicrostructural evolving of 3-dimensional C/SiC (3D C/SiC) served in thermo-physical and chemical simulated atmosphere and simulated stress condition. Therelationship between the multi-scale structural change and properties change, and thecorresponding response to environment in 3D C/SiC were systematically explored, sothe failure behavior were clearly understood and microstructural controlling unit andelement were obtained. These failure and microstructure investigate induced theessential failure model and mechanism which were used to predict and analysis thefailure behavior in environments much more complicated. The prediction fitted wellwith experimental phenomena. The main results are as follows:1. "Fiber cluster" were first proposed to be a main kind of structural unit tocontrol strength and toughness of continuous fiber-reinforced ceramic composite(CFCC). The competition among fiber cluster, single fiber and fiber yarn formed themulti-scale microstructural control criterion which was successfully applied inprocessing, improving properties, mechanical behavior and failure mechanism for 3DC/SiC and which made the performance of C/SiC used in this thesis improved largely. 2. Biomimetic "basic structural model" and "function failure mechanism" ofCFCC were first proposed and introduced to 3D C/SiC, which were successfullyapplied in failure analysis and materials design and optimization. The definition anddetailed content of biomimetic "basic structural model" and "function failuremechanism" were given. The biomimetic model and mechanism became a simplifiedand unified model and mechanism for in-service analysis of 3D C/SiC in allcomplicated simulated environments and became a main part of new strengtheningand toughening theory of ceramic matrix composites.3. The microstructure of 3D C/SiC in different simulated environments weresystematically studied, especially in TEM and HRTEM. Many microscopicmechanism and new phenomena were observed directly and statistically whichformed some bases for analysis of environmental behavior evolving.(1) Notch-like oxidation mechanism of C/SiC was first observed in TEM andthen were modified in understanding and model. The form mechanism of notch-likeoxidation mechanism was discussed in detail. This mechanism occurred frequently inthe environment with high temperature chemical medium and influenced heavily thebehavior of 3D C/SiC.(2) SiO₂ amorphous films were found in TEM in 3D C/SiC served in H₂O at700℃, which corrected the react temperature between SiC and H₂O and led to newunderstanding of the relationship between environmental behavior and temperaturechange for 3D C/SiC in H₂O-containing atmosphere.(3) Interfacial zone were defined. The interactions of the interfacial zone withcracks were systematically investigated whose physical models were established,which advanced the understanding in toughening mechanism and mechanicalbehavior of C/SiC.4. Considering the relationship between microstructure evolving and propertiesevolving, the environment behavior of 3D C/SiC in pure oxygen with different partialpressure, pure water, pure molten salt, and O₂-containing coupled atmosphere withdifferent partial pressure were comparatively studied and the failure map of 3D C/SiCin O₂-containing atmosphere. Also below were obtained: (1) The oxidation temperature zone and kinetics determining factor in pure O₂.(2) The microstructural controlling unit and element—coating, heat history andcarbonaceous constituent.(3) The dominant factor in thermo- physical and chemical environment—oxygenand salt (Na₂SO₄). H₂O-O₂-Na₂SO₄ coupled environment became the most severeservice condition for 3D C/SiC.(4) The failure essence of 3D C/SiC in O₂-containing atmosphere—the attack ofoxygen (and SO₃) upon carbonaceous constituent.5. From structure, the effects of two structural characteristic of 3D C/SiC onmechanical behavior, which were weak restrict of SiC matrix at the crossover pore orcrossover between fiber yarns and strong weak-interface bond, were analysis detailed.Based these two structural characteristic, the monotonic tensile behavior, fatiguemechanism and creep behavior of 3D C/SiC under different parameter and variedtemperature were systematically investigated and compared from many aspectsincluding macroscopic fractograph, pull-out and fracture characteristic of single fiber,fiber cluster and yarn, crack characteristic, debonding and sliding in interfacial zone,and texture/structure change of interfacial zone and constituents. The microstructuralcontrolling unit and element and corresponding failure mechanism were obtained. Thefatigue failure and creep failure were both multi-scale and the difference betweenthem were discussed. They were similar in mechanism and microstructuralcharacteristics, and different only in order and proportion of every type.6. Based on the failure analysis and biomimetic model and mechanism discusseddetailed above, it was established that relative weight change in different temperaturezone was the most direct and basic characterization parameter of C/SiC in serviceenvironments. The biomimetic model and mechanism also were used to predict thefailure behavior of 3D C/SiC when in the coupled environments between stressconditions and themo- physical and chemical conditions, and the prediction resultswere accordant with experiments.