Preparation Of Boron-containing Silicon Nitride Ceramic Matrix Composites By Boron In Situ Nitridation

Silicon carbide(SiC)fiber reinforced ceramic matrix composites are advanced absorbing composite materials with high strength,good thermal stability,corrosion resistance,oxidation resistance and other excellent performance.Its have potential applications in the field of aerospace and energy.In this study,SiC fibers are used as the reinforcement and microwave absorbing agent and silicon nitride(Si3N4)is used as the matching matrix.The SiC/Si3N4 composite is fabricated by reactive sintering,BN synthesized by boron in situ nitriding in the matrix to adjust the interfacial properties between the fiber and the matrix.The effects of boron content on the nitriding rate,phase composition,microstructure,mechanical properties,machinability and dielectric properties of the matrix were studied.At the same time,the effect of boron in situ nitriding on the properties of the composite was researched.The results show that ZrO2 can improve the nitridation rate of the matrix,The nitriding rates were 89.91%,89.15%,87.22%after 1390? nitriding with boron boron 0,5 and 10 mass fraction mass fraction.However,The nitriding rate was only 59.11%,62.43%and 64.60%after nitriding without ZrO2 addition by boron 0,5 and 10 mass fraction,it reached 87.91%,85.02%,83.08%after nitriding at 1470?.The curved BN coats the Si3N4 grains to form a shell-core structure.The in-situ nitridation of boron to form BN affects the ?-Si3N4 and effectively improves the machinability and dielectric properties of the matrix.With the increase of boron mass fraction,the porosity of matrix gradually increases while the bending strength decreases.Boron can improve the dielectric properties and machinability processing performance.For the matrix with ZrO2 and 10%mass fraction boron,the dielectric constant is 3.36-3.81 and loss tangent is 3.62×10-3-1.38×10-2 at a frequency range of 2-18 GHz.While the matrix without ZrO2 achieves full nitriding until 1470?.The dielectric constant and loss tangent are 3.26-3.57 and 1.01 ×10-3-1.07×10-2,respectively.The BN improves the binding force of the fiber to the substrate.The fracture surface of composite without boron show a brittle fracture behavior without fibers being pulled out.Oppositely,the composite with boron addition has obvious SiC fiber extraction,dissociation and debonding,which shows obviously pseudoplastic fracture behavior.In addition.In addition,the dielectric constant and loss tangent of composite increase with the increasing the volume fraction of SiC fibers.