Study On The Relation Between Properties And Microstructures Of Continuous SiC Fibers

Continuous SiC fibers prepared via polymer-derived methods have excellent thermal stability and good oxidation resistance.They are ideal reinforcements for ceramic matrix composites.Composites reinforced by SiC fibers are key strategic materials in high-tech field.In recent years,although some progress has been made in the preparation of continuous SiC fibers in China,the und erstanding of phase evolution involved in fiber preparation and their detail microstructuresare not clear.The present study investigates the microstructures of four kinds of SiC fibers and the relationship between the microstructures and their macro-mechanical properties and oxidation resistance using X-ray diffraction,scanning electron microscope,transmission electron microscopy and energy dispersive spectrum.The main contents and conclusions are as the following:(1)The SiC grains on the surface of fiber sintered at 1800 ? are highly crystallized with a size of about 200 nm.Fracture morphology shows a small amount of transgranular fracture characteristics,the ?-SiC grains near the surface of the cross-section of the fiber with high crystallinity are densely bounded but the core are loose.The surface and cross-section of fibers sintered at 1500 ? and 1300 ? all show fine grain structure characteristics.The fibers are compact and have no obvious morphological difference.The high tensile strength fibers sintered by 1300 ? have compact structure and no obvious defects.(2)TEM samples of the SiC fibers were prepared by metal embedding method,and their cross-section microstructures were characterized.TEM observation shows that the cross-section structure of the fiber sintered at 1800 ? is gradient distribution: From the surface to the core,the grain size of ?-SiC decreases gradually from 240 nm to 100 nm.but the graphite shows an increase.The morphological distribution change from straight on the fiber surface to turbostratic in the core.There are a few holes and graphite phases in the core of the fiber sintered at 1800 ?,thus the tensile strength is 1.7 GPa.The fibers prepared at 1500 ? and 1300 ? are mainly consist of ?-SiC nanocrystals and turbostratic carbon embedding in the SiCxOy amorphous phase.On the cross-section,these fibers have uniform structure and no holes,so the tensile strength can maintain at 2.7 GPa?3.8 GPa.The fiber sintered at 1300 ? with high tensile strength is mainly composed of amorphous phase and a large number of nanocrystalline ?-SiC,and defects were not observed.It is believed that the control of graphite content in and the reduction of defects in the preparation of fibers are the key to obtain SiC fibers with excellent properties.(3)It is found that fibers sintered at 1800 ? and 1300 ? are oxidized after heating at 1000 ?,the oxide layer is formed on the surface,and with the prolongation of time,the oxide layer becomes thicker but the strength of the fibers decreases.After oxidizing for 1 h,the thickness of oxide layer reaches 100 nm,but no obvious defects were observed.The strength retention rates of the fiber sintered at 1800 ? and 1300 ? were 87.3 % and 80.8 %,respectively.After oxidizing for 10 h,although the voids between SiC grain boundaries in the surface layer of fiber sintered at 1800 ? act as oxygen diffusion channels,the strength retention rate can still reach 55%.while the thickness of the oxide layer of the fiber sintered at 1300 ? can reach 430 nm after oxidizing for 10 h,and the carbon content is high,the degree of oxidation is aggravated,resulting in the loss of strength.After oxidizing for 50 h,the oxide layer thickness on the surface of the two fibers increased greatly,and the fibers were easy too break to measure.Compared with fiber sintered at 1300 ?,fibers sintered at 1800 ? with high crystallinity have excellent high temperature stability.