Microstructure Effect Of Carbon Fibers On The Growth Of Carbides And The Formation Mechanism Of Carbides

Depending on the excellent thermal preformance and high-temperature structural properties,C/C composites are considered as the most promising candidate for high-temperature structural material used in aerospace industries.However,C/C composites are easy to be oxidized under high temperature aerobic environment,resulting in the degradation of their properties,and thus restricts their application as high-temperature structural material.Up to now,the surface coating technology is the most effective method to improve the anti-oxidation of C/C composites.And the reactive melt infiltration?RMI?was considered as one of the most commonly used process for the low cost,effective time-consuming and near net shaping among various processes of oxidation protective coating fabrication.In RMI process,the formation of carbide coatings on C/C compositions was mainly involved in the direct reactions of various structural carbon fiber and carbon matrix in C/C compositions with one metal or various metals.However,the effect of carbon material microstructure on the formation of carbide and the formation mechanism of carbides with various morphologies via the direct reaction between carbon materials and metals are still not clear.Focusing on the above-mentioned problems,in this thesis,isotropic carbon fibers?IF?and anisotropic pitch-based carbon fibers?AF?with different microstructure were firstly prepared using isotropic and anisotropic pitch as raw materials,and then carbides were synthesized by directly reacting Si or transition metal Zr,Hf and Ta powders with these various structural carbon fibers.The growth behaviors and the formation mechanisms of SiC whiskers and carbide coatings on carbon fibers were investigated.Meanwhile,the effect of the microstructure of carbon fibers on the SiC whisker nucleation and the atomic diffusion during the formation of carbide coatings was discussed.In order to investigate the influence of the presence of various metals on the formation of carbides,the growth of carbide coatings and SiC whiskers was carried out via the further reaction of carbide coated carbon fibers with metals,and their formation mechanisms were also discussed.The main research and conclusions in this work were exhibited as follows:?1?Using optically isotropic and anisotropic pitch as raw materials,isotropic pitch-based carbon fibers IF1000,IF2000 and IF3000 as well as anisotropic carbon fibers AF1000,AF2000 and AF3000 were prepared through the process of melt-spinning,oxidative stabilization and subsequent heat-treatment at 1000,2000 and 3000?,respectively.The grain size in isotropic carbon fibers is small and slightly increases with the heat-treatment temperature from 1000 to 2000?.The carbon layer orientation of isotropic carbon fibers shows turbostratic and disordered structure.After graphitization at3000?,isotropic carbon fibers are partly short-range ordered and showing the isotropic characteristics.Their carbon layers still display the turbostratic and bent orientation.While graphite crystals in anisotropic carbon fibers show better orientation,and their crystallite size and crystal orientation increase with the heat treatment temperature.The crystallite sizes of the carbon fibers increase markedly after the graphitization at 3000?,and their carbon layers are highly preferred orientation along the axial and radial of carbon fibers,exhibiting the highly anisotropic features.?2?In the range of 1000-1400?,the formation and growth of SiC whiskers were investigated by the direct reaction silicon powders with carbon fibers heat treated at different temperatures.It was found that SiC whiskers preferred to grow on the surface of carbon fibers IF1000,IF2000,AF1000 and AF2000,but few SiC whiskers could be observed on the surface of carbon fibers IF3000 and AF3000 heat-treated at 3000?.It indicates that the microstructure and crystallite size of carbon fibers have significant influence on the nucleation of SiC whisker.Carbon materials possessing minor curvature of carbon layers,La_?100?in nanometer scale and the distribution range of d₁₀₀ value near to the d₂₀₀=0.217 nm of cubic phase SiC,is in favour of the nucleation of Si C whisker.Highly oriented graphite fibers AF3000 possess larger crystal size and d₁₀₀ value is close to 0.213 nm are disadvantage to the nucleation of Si C whiskers.Although,graphite fiber IF3000 has small crystal size,larger carbon layer curvature,the d₁₀₀ value of these short-range ordered carbon layers is near to 0.213 nm,resulting in that the nucleation of Si C whiskers on IF3000 is also difficult.Using the mixture of Si/SiO₂ as silicon source and selected carbon fibers beneficial to the formation of SiC nucleus as growth substrate,flexible C/Si C_W composites were successfully prepared with the introduction of additional carbon source into the reaction system.It significantly reduced the carbon consuming in carbon fibers during the formation of SiC whiskers,and thus SiC_W/carbon fibers composite reserved 68%of the tensile strength of original carbon fibers.It provides an effective route for the low-cost synthesis and application of flexible C/SiC_W composites.?3?In the reaction temperature range of 1000-1400?,SiC whiskers was hard to be formed on carbon fibers?IF2000 and AF2000?which is beneficial to the formation of Si C nucleus by reacting them with high-purity silicon powders.It can be attributed to low mobility of high-purity silicon on carbon fibers at lower reaction temperature.The formation of Si C whiskers is observed on carbon fibers IF1000 and AF1000 by reaction high-purity silicon powders with these carbon fibers containing a certain content of oxygen and hydrogen.The results show that the formation and growth of SiC whiskers in this temperature range is closely related to the formation of SiO gases in the reaction system.However,SiC whiskers However,SiC whiskers can be formed by direct reaction high-purity silicon powders with carbon fibers?IF2000 and AF2000?being beneficial to the formation of SiC nucleus in the temperature range of 1550-1650?.Carbon atoms diffusion through the SiC crystal nucleus provided the carbon sources for the growth of Si C whiskers.Bur,Si C whiskers was still hard to be formed on carbon fibers IF3000 and AF3000.It further confirms that the microstructure and crystallite size influence of carbon fibers on the nucleation of SiC whiskers.?4?Using Zr,Hf and Ta powders as metal sources,continuous and uniform carbide coatings were prepared on carbon fibers by reacting carbon fibers with these metal powders via solid state reaction at the temperature of 1000-1400?.A confirmatory experiment that one end of a bundle of carbon fibers was embedded in metal powders was designed to study the formation mechanism of carbide coating.The results show that Zr,Hf and Ta atoms diffuse on the surface of carbon fibers and the surface diffusion of transition metal atoms along the surface of carbon fibers is the critical factor for the formation of continuous carbide coatings.Carbide coatings possessing suitable thickness,good integrity and great combination state with carbon fiber matrix can be prepared on carbon fibers by controlling the reaction temperature and time,which can remarkably improve the oxidation resistance of the carbon fibers.The crystal size and ordering degree of graphite crystallite in carbon fibers have significant effect on the formation and growth of carbide coatings formed on their surface,carbide coatings preferred to grow on the surface of carbon fibers having larger crystal size and better crystal orientation.?5?According to the formation of ZrC/HfC multiple coating by reacting HfC coated carbon fibers with zirconium powders,it confirms that carbon diffuses from the carbon fiber to the surface of carbide coating,which provides nucleation sites and carbon sources for the formation of new carbide coatings.The formation of transition metal carbide coatings on the surface of carbon fibers is in relation to the bi-directional diffusion of metal and carbon atoms inside the carbide coating.Meanwhile,duplex carbide coated carbon fibers were prepared successfully by reacting carbide coated carbon fibers with different metals.The formation process of duplex carbide coatings is controlled by the diffusion of carbon atoms.However,when the reactions between silicon powders and transition metal carbide coated carbon fibers were carried out at high temperature,Si powders react easily with transition metal carbides to dissolve out carbon and form amorphous carbon coated silicide alloys.?6?Using transition metal carbide coated IF1000 and AF3000 fibers,the mixture of Si/SiO₂ powders and thermal carbon black as growth substrate,silicon source and carbon source,respectively,the growth of SiC whiskers on the surface of carbide coated carbon fibers was achieved.Compared with that of uncoated carbon fibers?IF1000 and AF3000?,the growth behavior of SiC whiskers on carbide coated carbon fibers was significantly improved,which can overcome the restriction of carbon material structures on the growth of SiC whiskers.The results show that the grain size of carbide coating is the crucial factor for the formation and growth SiC whiskers.Due to that carbon atoms can diffuse in carbide coatings,carbon diffuses from the carbon fiber to the surface of carbide coating provides nucleation sites and carbon sources for the formation of SiCwhiskers.