| Continuous silicon carbide fiber reinforced silicon carbide (SiC_f/SiC) composites have been considered for high temperature structural applications in aerospace, missile and new power plants due to their high strength, high modulus, excellent thermal stability, good fracture resistance, corrosion resistance, creep resistance, low activity, irradiation resistance and low afterheat. However, despite the much improved toughness compared with the bulk SiC ceramics, their applications are still rather limited due to the lack of toughness and low damage tolerance. As is well known, the fiber-matrix interface plays a vital role in controlling the ultimate composite properties, and the key method to overcome the inherent brittleness would be lie in tailoring the interface bonding.In this research, a series of coatings, e.g. PyC coatings and SiC coatings with different thicknesses, and (PyC/SiC)n multilayer coatings were designed and deposited onto the KD-I SiC fibers, prior to the fabrication and densification of 3D SiC_f/SiC composites using a precursor infiltration and pyrolysis (PIP) method. Microstructure and bonding strength of the fiber-matrix interface were characterized and evaluated together with the single filament tensile strength and multifilament tensile strength of the SiC fibers as well as the fracture toughness, flexural strength and elastic modulus of the composites. On that basis, we investigated the effects of interfacial structure on mechanical properties especially the fracture toughness of the composites, discussed the influence of different coatings on fiber strength and the mechanical properties of composites, and highlighted the influence of different coatings on oxidation susceptibility of the composites. Meanwhile, considering its potential application in flow channel insert (FCI) in the liquid blanket of the fusion reactor, the effect of interface coatings on thermal conductivity and electrical conductivity of the composites were also evaluated.The microstructures of SiC_f/SiC composites and particularly near the interface areas were systematically examined by using TEM, HRTEM, EDX and SAED. We explored the composition of interface area, morphology and degree of crystallization of each component, and the cracks propogating within the interface area. Moreover, the mechanisms behind the improved fracture toughenss of SiC_f/SiC composites with (PyC/SiC)n multilayer coatings were elucidated from microstructural aspects. Cracks were found to deflect and branch between interface coatings and fibers, between different sublayers, and/or even inside a certain sublayer, and a large amount of fracture energy was consumed. However, the propagation path of the main matrix crack was still within the first PyC sublayer.Single-fiber push-out test was conducted to evaluate the mechanical properties of the fiber/matrix interface, by quantifying the interface shear strength of KD-I/SiC composites with different interface coatings. The interface shear strength of composites increased from 72.1MPa to a maximum of 105.9MPa for the PyC coatings, the highest interface shear strength of 209.8MPa was obtained for composites with SiC coating. As for the (PyC/SiC)n multilayer coatings, the interface shear strength is found in the range between 93.5MPa and 150MPa. An appropriate strengthening to the interface bonding is expected to improve the fracture toughness of the composites. However, too strong interface bonding would inevitably decrease the mechanical properties of the composites. The brittle fracture behavior of composite reinforced with the as-received fiber is likely due to the extreme weak bond between fiber and matrix, which reduced the load transfer at the interface.The thickness of the interface multilayer coatings influenced the mechanical properties of the 3D KD-I/SiC composites. PyC coating effectively sealed the cracks and defects on the fiber surface, and narrowed the scattering of the fiber strength. Thus, the fracture toughness of the composites was substantially enhanced. On the other hand, CVD SiC coating did not cover the cracks and defects on the fiber surface, and decreased the mechanical properties of the SiC fiber. Replacing the single layer coatings with (PyC/SiC)n multilayer coatings improved the fracture toughness of the composites, to a larger extend than the single layer PyC coatings. For example, the fracture toughness reached 23.1MPa?m1/2 with 0.53μm of PyC single coating, which is 65% higher compared with that of as-received fiber composites; while the value of 28.1MPa?m1/2 was obtained with (PyC/SiC)2 multilayer coating, which is 101% higher compared with that of as-received composites.Various interface treatments had different influences on the oxidation resistance of the 3D KD-I/SiC composites. PyC coatings deteriorated the oxidation resistance, while SiC coatings were oxidized to form glass SiO2 and effectively protected the material and increased the oxidation resistance. For the (PyC/SiC)n multilayer coatings, the first sublayer with PyC helped improve the room temperature fracture toughness of the composites, but SiC is beneficial to the oxidation resistance of the composites.The investigation on the influences of the coating thickness on electrical and thermal conductivity of the 3D KD-I/SiC composites concluded that the introduction of PyC coating enhanced the transport properties, but not for the CVD SiC. Composites contain 0.38μm of SiC coating, 0.71μm of SiC coating and SiC/PyC/SiC multilayer coating were found to satisfy the requirements of potential FCI applications.
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