Design And Performance Research Of SiC/SiC Joints Reinforced By In-situ SiC Nanowires

SiC ceramics are widely used in aerospace,space optics,microelectronics,nuclear energy,transportation,etc.,due to their low density,high thermal conductivity,small thermal expansion coefficient,and good mechanical properties at ordinary high temperatures.However,the limitations of high-performance ceramic forming technology make it difficult to prepare large-sized and complex-shaped ceramic parts.Joining technology is one of the most effective methods for preparing large-sized and complex components by splicing relatively simple parts.The development of joining technology plays a very important role in promoting the expansion of application field of SiC ceramics.The ceramic joining technology should improve the high temperature resistance and wear resistance of ceramics as much as possible while solving the joining strength.Joining technology for high-temperature applications is the core technology that is urgently needed to be developed at present.The purpose of this subject is to develop a novel two-step joining technology to meet the requirements of high-temperature applications.Based on the traditional C-Si reaction bonding technology,it is proposed for the first time to introduce SiC nanowires with good thermal and mechanical properties into SiC joints in-situ.The following works were systematically investigated:1.Preparation and optimization of multi-component slurry.2.Analysis of growth mechanism of SiC nanowires.3.Analysis of microstructure and mechanical properties of C-Si reaction bonding joint with SiC nanowire reinforcements.Through the exploration of the above work,the main conclusions were as follows:1.The mixed solution of ethyl acetate and xylene was used as the solvent of this slurry system and the remaining components of the slurry were composed of castor oil,phenolic resin,polycarbosilane,inert SiC powder,carbon black and ferrocene.When castor oil content was4⁵wt%and the viscosity of slurry was maintained at 2000³000mPa.s(shear rate:20S^-1),the viscosity and uniformity of the prepared slurry were more suitable for preparing ceramic joining layer.2.During the pyrolysis of polycarbosilane,PCS-derived product underwent inorganic transformation to form amorphous SiC,and then gradually transformed into crystalline?-SiC and?-SiC with the increase of pyrolysis temperature.Gaseous molecules produced by pyrolysis of polycarbosilane mainly included H₂,CH₄,SiH₄ and CH₃SiH₃,which were the main raw materials for the growth of SiC nanowires.When no catalyst was added,the prepared SiC nanowires were randomly distributed and there was not pellet at the tip of nanowire?Vapor-Solid mechanism,VS?.When ferrocene was used as catalytic agent,the uniformity of distribution of SiC nanowires was improved and the nanowires displayed a needle-like morphology.Since there was obvious pellet at the tip of nanowire,it indicated that the growth mechanism was attributed to Vapor-Liquid-Solid mechanism?VLS?.3.The morphology and size of SiC nanowires inside the ceramic joining layer can be optimized by adjusting the pyrolysis temperature,size of inert SiC powder and PCS content.After densification joining,the thickness of ceramic joining layer was affected by PCS content and SiC powder size.The phase composition of joints was comprised of?-SiC and the mixture of?-SiC and free Si.The highest 4-point flexural strength of the joined samples at room temperature and 1200?was 289±60MPa and 368±44MPa,respectively,which can reach 81%of the matrix strength.