Preparation And Performance Of Aluminosilicate Fiber-reinforced Oxide Ceramic Matrix Composites

Owing to high toughness, strength, modulus, inherent oxide resistance and excellent dielectric properties, continuous oxide fiber-reinforced oxide matrix composites（Oxide/Oxide CMCs）, have been considered as one of the most important high-temperature structural and functional materials. In this study, import continuous aluminosilicate（AS） fiber and homemade oxide（SiO₂ and Al₂O₃-SiO₂） sols were used as raw materials, and their properties were studied firstly. Continuous AS fiber-reinforced oxide（AS_f/Oxide） composites were fabricated by the sol-gel process. The effects of fabrication temperature on microstructure, mechanical and interfacial properties of the composites have been investigated. In order to improve the composite performance, interface engineering of AS_f/Oxide composites have been carried out, employing Py C and fugitive carbon（FC） interphases. Finally, for high-temperature applications, mechanical, thermal and dielectric properties of the optimal AS_f/Oxide composites have been investigated.The composition, structure and strength evolutions of AS fiber after high-temperature heat-treatments have been investigated. The results showed that the fiber was mainly composed of gamma Al₂O₃ and amorphous SiO₂. The mullitization reaction occurred in the fiber after heat-treated at 1100°C. Then mullite grains would grow up gradually with the temperature continuing rising. The original fiber surface was smooth and homogeneous. After heat-treated at high-temperatures, it turned rough gradually, and then apparent grains and micro-pores formed on the fiber surface. The phase transformation and grain growth both had great effects on the fiber tensile strength. As the temperature increased, the fiber strength distribution increased while the single fiber tensile strength decreased gradually. The strength retention of the fiber was above 73.26% after heat-treated below 1300°C, while that of the fiber decreased to 33.69% after heat-treated at 1400°C.The particles in the SiO₂ sols were very small, and the homogeneity level was very high. The gels were composed of amorphous SiO₂. After calcined at 1400°C, the SiO₂ glass was crystallized into cristobalite. After sintered above 1000°C, the sintering degree of the SiO₂ ceramic was very high and the densification degree was above 90.0%. The Al₂O₃-SiO₂ sols were composed of spherical SiO₂ and rodlike boehmite particles. The particles in the sols were large. The gels were composed of boehmite and amorphous SiO₂. The mullitization reaction occurred after calcined at 1300°C. After sintered above 1200°C, the sintering degree of the Al₂O₃-SiO₂ ceramic was very high and the densification degree was above 80.0%. The elastic modulus and hardness of the SiO₂ and Al₂O₃-SiO₂ ceramic were both increased as the sintering temperature increased.The fabrication process and properties of AS_f/Oxide composites have been studied. The results showed that the density of the composites increased while the porosity decreased as the heat-treatment temperature increased. Besides, the flexural strength, interlaminar shear strength and fracture toughness of the composites all increased firstly and then decreased. Those of AS_f/SiO₂ composites fabricated at 900°C all reached the maximum, and the corresponding values were 119.7±7.5（MPa）, 10.8±0.7（MPa） and 4.0±0.4（MPa·m1/2）. It was the same with AS_f/Al₂O₃-SiO₂ composites fabricated at 1100°C, and the corresponding values were 90.0±6.8（MPa）, 11.1±1.0（MPa） and 3.6±0.2（MPa·m1/2）. Matrix cracks would penetrate fibers in AS_f/SiO₂ composites fabricated above 1100°C. It was the same with AS_f/Al₂O₃-SiO₂ composites fabricated above 1200°C. It has been proved by the fiber push-in test that the interfacial shear strength of the composites increased remarkably. Moreover, diffusion reactions between the fiber and the matrix were confirmed by TEM analysis.High-temperature mechanical properties of AS_f/Oxide composites have been studied. The results showed that the flexural strength of the composites increased firstly and then decreased as the temperature increased, while the elastic modulus decreased all the time. When the temperature reached 900°C, the flexural strength of AS_f/SiO₂ composites reached the maximum, and the corresponding value was 132.2±14.5（MPa）. It was the same with AS_f/Al₂O₃-SiO₂ composites when the temperature reached 1000°C, and the corresponding value was 122.7±7.9（MPa）.Interface engineering of AS_f/Oxide composites have been carried out. The results showed that the deposition process of CVD Py C coatings was controlled by surface reaction when the deposition temperature was 1000°C. The crystalline degree of the coatings was high, and the coating thickness was optimal after deposited below 5h. The strength retention of coated fibers was high. After introducing Py C interphases, the interfacial bonding in AS_f/Oxide composites was weakened remarkably, and mechanical properties of the composites were improved. TEM analysis displayed that Py C interphases were turbostratic carbon, and the physical and chemical compatibility among Py C interphases, fiber and matrix were very good. After the removal of Py C interphases, a gap appeared between the fiber and the matrix. The load transfer was mainly achieved by the mechanical interlocking and the friction at the interface. Consequently, fiber toughening mechanisms could be actualized. Mechanical properties of AS_f/Oxide composites with FC interphases were mainly determined by the gap width. When the original Py C interphases were deposited for 3h, mechanical properties of AS_f/FC/SiO₂ composites fabricated at 1200°C were improved remarkably, and the corresponding flexural strength and elastic modulus were 106.4±6.7（MPa） and 23.3±3.0（GPa）, respectively. It was the same with AS_f/FC/Al₂O₃-SiO₂ composites fabricated at 1300°C, and the corresponding flexural strength and elastic modulus were 100.9±7.2（MPa） and 30.0±1.4（GPa）, respectively.High-temperature mechanical properties of AS_f/Oxide composites with FC interphases have been studied. The results showed that when the temperature reached 1100°C, the flexural strength and elastic modulus of AS_f/FC/SiO₂ composites were 106.5±15.8（MPa） and 14.5±5.3（GPa）, respectively. However, when the temperature reached 1200°C, those of AS_f/FC/Al₂O₃-SiO₂ composites were 83.0±5.1（MPa） and 20.2±1.3（GPa）, respectively. The composite fracture process was investigated by the digital image correlation（DIC） method. The results indicated that the initial crack was induced by the load lower than the maximum value, and the crack propagation rate turned slower at high temperature.Thermal physical properties, dielectric properties and thermal stability（thermal aging and thermal-shock resistance） of AS_f/Oxide composites have been studied. The results showed that coefficients of thermal conductivity（CTC） of AS_f/SiO₂ and AS_f/Al₂O₃-SiO₂ composites at room temperature（RT） were 0.28 W/（m·K） and 0.30 W/（m·K）, respectively. Both of them decreased firstly and than increased with the temperature increasing. Coefficients of thermal expansion（CTE） of AS_f/SiO₂ and AS_f/Al₂O₃-SiO₂ composites at RT were 2.42×10-6/K and 2.35×10-6/K, respectively. The CTE of AS_f/SiO₂ composites increased firstly with the temperature and then stabilized to 5.19×10-6/K when the temperature was above 210°C. However, that of AS_f/Al₂O₃-SiO₂ composites increased gradually with the temperature, and the average value was 5.03×10-6/K between RT and 1000°C. Dielectric constant and dielectric loss of AS_f/SiO₂ and AS_f/Al₂O₃-SiO₂ composites at RT and 9.375 GHz were 3.63, 0.28×10-2 and 4.75, 0.19×10-2, respectively. Both of them showed frequency independent and increased with the temperature. When the temperature reached 1200°C, both of them increased to 4.38, 5.53×10-2 and 5.50, 2.55×10-2, respectively. The flexural strength retention of AS_f/SiO₂ composites was 34.7%（or 58.6%） after heat-treated at 1200°C for 10h（or thermal-shock tested at 1200°C for 20 times）. However, that of AS_f/Al₂O₃-SiO₂ composites was 78.0%（or 74.6%） after heat-treated at 1300°C for 10h（or thermal-shock tested at 1300°C for 20 times）. It can be concluded that AS_f/Oxide composites are ideal high-temperature structural and functional materials due to the advantages of appropriate CTC, low CTE, excellent dielectric properties and thermal stability. Comparatively speaking, AS_f/SiO₂ composites provided better dielectric properties, while AS_f/Al₂O₃-SiO₂ composites provided better thermal stability and higher serving temperature.