Preparation,Modification And Properties Of C_f/SiC Composites With Layered Carbides Interphase

Due to high temperature resistance,corrosion resistance,oxidation resistance,radiation resistance and low radioactivity,carbon fiber reinforced silicon carbide matrix（C_f/SiC）composites have wide application prospect in aerospace industry and advanced nuclear systems.The interphase,which is the third phase between the matrix and fiber,plays a crucial role in mechanical properties,physical properties and chemical stability of the composites.Traditional interface materials such as PyC,h-BN and SiC,are unable to meet the requirement because of the intrinsic drawbacks,which limiting the application of SiC matrix composites in advanced nuclear systems.Owing to special layered structure and electronic density of states,MAX phase materials have fracture energy absorption mechanisms and good irradiation resistance.In this paper,Ti₂AlC phase is proposed as the nuclear-grade interphase to improve the irradiation resistance of silicon carbide matrix composites.For the first time,a uniform and thickness-controllable TiC/Ti₂AlC constituent gradient coating was obtained on the surface of carbon fibers via microwave-assisted molten salts method.Preparation process was further optimized by surface treatment of carbon fibers,raw materials ratio and temperature adjustment,thus TiC/Ti₂AlC coating with thickness of⁴00 nm was fabricated at 900°C for 1 h.The inner layer is TiC,and the outer layer is mainly Ti₂AlC.The reaction pathway for TiC/Ti₂AlC coating is also determined with thermodynamic of Ti-Al-C.However,the in-situ growth of TiC/Ti₂AlC coating causes catastrophic damage to carbon fibers.Thus the tensile strength of carbon fibers is dramatically decreased to 78±13 MPa.In order to improve the performance of the as-prepared TiC/Ti₂AlC-coated carbon fibers,a pyrolytic carbon layer was pre-fabricated on carbon fibers.And then the tensile strength of the fibers is significantly increased to 550±72 MPa.Moreover,interfacial debonding of the coating is obvious with the additional nano-graphite in mixture powders.When the molar ratio of Ti,Al and C is 2:1.1:1,the tensile strength of the carbon fibers is increased to424±107 MPa.C_f/SiC composites with layered carbides interphase were obtained via PIP process.The results indicated that LHBPCS has better wettability and impregnation ability.However,the long-term high-temperature treatment causes the decomposition of Ti₂AlC phase,forming a double-layer TiC interphase eventually.Amorphous carbon and Si,Al were also detected among the nanocrystals.The tensile strength of C_f/SiC composites is 92±21 MPa without interface modification.With the increasing of additional nano-graphite in mixture powders,the tensile strength of C_f/SiC composites is increased first,and then decreased.When the molar ratio of Ti,Al and C is 2:1.1:0.5,the tensile strength of C_f/SiC composites is 180±63 MPa.Interfacial debonding and pulling of carbon fibers are improved.The irradiation resistance of C_f/SiC composites was evaluated by Si⁵⁺ion irradiation.The results indicated that with the increasing of irradiation dose,the structure of carbon fibers is destroyed,and the volume shrinkage occurs.When the irradiation dose is 20 dpa,the relative height is-193.31±27 nm.The surface composition of SiC matrix is changed after ion irradiation,accompanied by a little volume shrinkage.The layered carbides interphase swells,and relative height nonlinearly increases with increasing of irradiation dose.When the irradiation dose is20 dpa,the relative height is 115.3±23.2 nm.Though layered carbides interphase are under compressive stress from SiC matrix and carbon fibers,the structure is intact and no interfacial debonding occurs,which showing a good irradiation resistance.