Preparation Of SiC Nanofiber Reinforced SiC Ceramic Matrix Composites By Hot Pressing Sintering

SiC fiber reinforced SiC ceramic matrix composites(SiCf/SiC)have the characteristics of low density,stable high temperature performance,strong resistance to external impact loads,and insensitivity to cracks.It is a lightweight new high-temperature thermal structural material that integrates structural loading and resistance to harsh environments,and has attracted widespread attention.To obtain high strength and toughness SiCf/SiC composites,it is important to have high-quality SiC fibers and advanced ceramic sintering processes.SiC nanofibers(SiCnf)have excellent properties,such as high strength,high modulus,high temperature resistance,high oxidation resistance,and high chemical resistance,and are ideal reinforcing materials for ceramic matrix composites.However,most of the SiCnf as a reinforcement is formed in situ during the ceramic sintering process,and the growth quality is affected by the size and distribution of pores or gaps.There are shortcomings such as difficulty in preparing interfacial coating,"bridging" effect and low content of the SiCnf.In this paper,SiC powders synthesized by silicon powders and carbon black are used as the raw materials and SiC nanofibers are used as reinforcements,SiC ceramic matrix composites(SiCnf/SiC)with controllable reinforcement content were prepared by hot pressing sintering process.Moreover,boron nitride(BN)coating is prepared on the surface of SiC nanofibers by dip-coating method.The effects of reinforcement content,sintering temperature and BN interface coating on the properties of SiCnf/SiC composites were investigated.The main research contents are as follows:(1)SiC powder was prepared by direct carbonization of Si at high temperature using silicon powder and carbon black as raw materials.After the SiC powder was decarbonized,separated by high-speed stirring,and left to stand to take the upper suspension and treated with NaOH solution,a high-purity SiC nano powder with an average particle diameter of about 50 nm was obtained.The SiC ceramics prepared by SiC nanopowders have excellent relative density and mechanical properties,and the grain size distribution in the ceramics is uniform.It was found that the grains were pulled out.(2)SiC nanofiber reinforced SiC ceramic matrix composites were prepared by hot-pressing sintering using SiC nanofibers as reinforcements.The effects of different SiC nanofiber content and different sintering temperatures on the properties of SiC ceramic matrix composites were investigated.When the content of SiC nanofibers is 15 wt%,the comprehensive properties of SiC ceramic matrix composites are optimal,and the flexural strength and fracture toughness are 678.2 MPa and 8.33 MPa*m1/2,respectively.Reinforcement in ceramics is dispersed uniformly,and nanofibers are pulled out and bridged.When the sintering temperature is 1850℃,the cross-sectional structure is gradually dense,the pores are reduced,and a small amount of SiC nanofibers are pulled out.the SiC ceramic matrix composite is strengthened and toughened by pulling out nanofibers,deflection of cracks by nanofibers,and bridging of nanofibers during ceramic fracture and a large amount of fracture energy is consumed.(3)BN coating was prepared on the surface of the SiC nanofibers by dip-coating method.Different concentrations of precursor solutions were prepared by using boric acid and urea as precursors and polyvinylpyrrolidone(PVP)used as a film-forming agent.After SiC nanofibers are dipped in the precursor solution and dried,BN coatings were prepared at different pyrolysis temperatures.As the precursor solution concentration and pyrolysis temperature increase,the thickness of the BN coating increases.Under the conditions of precursor concentration of 0.5 mol/L and pyrolysis temperature of 1000℃,the thickness of the BN coating on the surface of SiC nanofibers was uniform.XRD and infrared analysis showed that the coating on the surface of SiC nanofibers is h-BN.Compared with the non-BN interface layer,the SiC ceramic matrix composite material with the BN interface layer has more fibers pulled out,and the bending strength and fracture toughness have been greatly improved.The presence of BN interface layer improves the mechanical properties of SiC ceramic matrix composites.In summary,SiC ceramics matrix is reinforced with SiC nanofibers coated with BN coatings,and the mechanical properties of SiC ceramic matrix composites have been greatly improved,but they have not manifested obvious ductile fracture effects.In the future,further optimization research is needed in the design and preparation of the interface layer,increasing the volume fraction content of SiC nanofibers,avoiding fiber breakage,and maintaining the high aspect ratio of SiC nanofibers.