Preparation And Microstructure Analysis Of CVD SiC Fiber

SiC fiber prepared by chemical vapor deposition is an ideal enhancer for the metal, polymer and ceramic matrix composites, due to its high strength, high specific modulus, good high temperature oxidation, etc. It can be widely used in aerospace and other high-tech fields. CVD SiC fiber has been commercial abroad, but foreign countries implement the technology blockade and embargo the relevant products to China. Therefore, it’s necessary to develop the preparation and research about CVD SiC fiber.In this paper, we explore a manufacturing process-the direct current heating CVD to prepare SiC fiber. The process parameters are:the H2flow with CH3SiCl3and CH3SiHCl2are1.7L/min and0.3L/min; flow rates of C2H2and H2are0.5L/min and0.2L/min; walk the wire voltage is2.20V; deposition power is1450V×313mA; water bath temperature is33℃; prepared carbon-coated SiC fibers with a diameter of about100μm; carbon coating thickness of about2μm; the average tensile strength of the fibers2820MPa; Weibull modulus is8.5.The cross-section microstructure of high performance SiC fiber, is characterized by scanning electron microscopy (SEM), Raman spectroscopy (Raman), energy dispersive X-ray spectrum(EDS). The result indicates that CVD SiC fiber is a composite fiber,which is composed of W core, W/SiC interface reaction layer, SiC deposition layer and carbon-rich coating. SiC grains are relatively small near the core of W. As the deposition process continues, the grain size grows up gradually, besides the crystallinity of grains as well as the crystal orientation also increased, which grows outward along the fiber radial direaction. SiC deposition layer contains free carbon, but it only exists in the internal SiC deposition layer. There is free silicon present in SiC fiber, however, it does not have a high degree of crystallinity.The SiC fiber is mainly composed of β-SiC crystallites, in which there are some stacking faults. The carbon coating on SiC fiber surface is composed of amorphous carbon and graphite crystal structure.We research the breakage process of SiC fiber and find that fiber breakage is brittle fracture, crack originates primarily from the W/SiC interface reaction layer, surface defects, and W-core inside, etc.The relationshiop of the fiber properties and residual stress is studied by using Raman spectroscopy. The results reveal that the internal stress in SiC fiber with low performance is compressive stress, and the area near the W core bears more stress than that near the fiber surface. While the internal stress in high performance SiC fibers is tensile stress, which increases with the distance from the W core increasing.There is compressive stress on the surface of carbon coating. The carbon coating layer with rougher surface would bear more surface stress and the defects are easier to be generated, leading to the decrease of the fiber strength. Conversely, smooth surface carbon coating will endure less surface stress, which is beneficial to the improvement of the fiber strength.