Investigation On The Growth Mechanism And High-temperature Degradation Mechanism Of Continuous W-core SiC Fibers

Continuous SiC fibers prepared by chemical vapor deposition（CVD）are considered an ideal reinforcement for metal matrix and metal matrix composite materials due to their high strength,stiffness,and excellent creep resistance in a wide temperature range（room temperature～1000℃）.Among them,Ti-matrix composites reinforced with CVD-SiC fibers have shown great potential for applications in high thrust-to-weight ratio aerospace engines and hypersonic aircraft due to their high specific strength and stiffness.Continuous W-core SiC fibers are produced using a hot-wire CVD apparatus with a continuous W wire as the core material,growing a SiC sheath on the W core through the cracking of MTS CH₃SiCl₃and/or（DCMS CH₃Si HCl₂）reaction gases in hydrogen（H₂）and/or argon（Ar）atmosphere.However,due to the complexity and variety of the CVD fiber reactor configuration（the length and diameter of reactor,ventilation mode,etc.）and deposition conditions（gas species and ratio,temperature,etc.）,as well as the sensitivity of SiC fiber growth to its phase structure and by-products（free Si or C）,W-core SiC fibers often exhibit significant differences in microstructure and large fluctuations in strength.Understanding the microstructure nature that affects the tensile strength of continuous W-core SiC fibers is crucial for understanding their growth mechanism and further improving and stabilizing the performance of SiC fibers.In addition,investigating the dynamic evolution behavior of W-core SiC fiber microstructure and the corresponding strength changes at high temperatures is important for understanding the high-temperature degradation mechanism and confirming the applicable temperature range.Firstly,in this thesis,SiC fibers with different tensile strengths were prepared to explore the intrinsic connection between the tensile strength and fracture mechanism of SiC fibers with the spatial distribution state of their composition,crystal structure,grain size,and configuration,elucidating the microstructural characteristics that dominate the tensile strength of SiC fibers.Secondly,W-core SiC fibers with a strength of 3500MPa were selected as the research object to investigate the structural evolution of the W/SiC interface,SiC sheath,and surface C coating and the corresponding tensile strength changes during vacuum heat treatment at 1100～1400℃for 2h,revealing the high-temperature degradation mechanism of W-core SiC fibers at the micro-/nano-scale.The following main conclusions were obtained:1.Continuous W-core SiC fiber with average strength of about 3200 MPa produced by CVD method at deposition temperature of 1300℃and gas flow ratio of CH₃SiCl₃/CH₃Si HCl₂/H₂=2.5/0.6/7.6.To analysis its micro-/nano-structure,it is found that high crystallinityβ-SiC（111）columnar grain parallel to the fiber radial in the internal area of the SiC fiber.There is no obvious amorphous SiC or free C,and the columnar grain including high density stacking fault.Along the radial direction of the fibers,the crystal quality ofβ-SiC columnar grains gradually deteriorates,and the growth of columnar crystals was hindered by free C in the outer region of fiber close to C coating,and transform into the composite structure of nanocrystalline and amorphous co-exists.Base on this,by lowering the deposition temperature to 1200℃or increasing H₂content to CH₃SiCl₃/CH₃Si HCl₂/H₂=2.5/0.6/13.8,the content of free C can be reduced in the outer region of the fiber,and the crystallinity of SiC grains can be improved.The high qualityβ-SiC columnar grain growth was ensured in the outer region of SiC sheath,and the tensile strength of SiC fibers was increased to about 3800 MPa and 3500 MPa,respectively.This indicates that high crystalline qualityβ-SiC columnar grains is the primary body for bearing load,and the high quality columnar grains obtained from internal to outer along the radial direction of fibers can make the crack propagation step higher and the propagation path longer along theβ-SiC columnar crystals,which is helpful to improve the tensile strength of SiC fibers.2.The effects of high-temperature heat treatment（HT）on microstructure evolution and tensile strength degradation of W core SiC fibers were specifically studied by HT at 1100～1400℃/2 h.Even after 1400℃/2 h HT,the integrity of the surface C coating was well maintained,thus avoiding the appearance of the surface dominant fracture mode.In addition,defect annihilation and crystallinity increase of SiC sheaths appeared during HT process,but no excessive coarsing of grains was observed in this temperature range,which should contribute to the improvement of mechanical properties,and the degradation of tensile strength should not be ascribe to the microstructure evolution of SiC sheath.The rapid growth of the reaction zone at the W/SiC interface induced by the high-temperature HT,which can be attributed to the rapid thickening of each sublayer in the W₂C/W₅Si₃interfacial layer,and the W/SiC interface region as the crack original point,dominated the failure of the fiber.When the thickness of W/SiC interfacial reaction layer exceeds the critical thickness of 360 nm,the properties of SiC fibers inevitably deteriorate.With HT temperature gradually increasing,the interfacial reaction layer continuously thickens,and the proportion of mirror fracture zone increases,which eventually leads to the rapid degradation of tensile strength of SiC fiber.