Microstructure And Properties Of Si-B-N Ceramics And Ceramic Matrix Composites

Si-B-N ceramics,including Si₃N₄,BN and SiBN ceramics and their ceramic matrix composites,are important candidate materials for high temperature wave-transparent materials,due to the high temperature durability,high specific strength,oxidation resistance,ablation resistance and Electromagnetic wave transparent.Among them,silicon nitride ceramics have excellent comprehensive properties,such as high strength,low density,high temperature resistance,chemical stability,etc.However,dielectric constant of Si₃N₄ is a little higher.BN ceramics have excellent high temperature stability and low dielectric constant.However,the mechanical properties of BN ceramics are much lower.SiBN ceramics have better overall performance,but the bulk SiBN ceramics have not been reported.Moreover,the single ceramic material is brittle and prone to catastrophic damage,leading to poor reliability.Therefore,it is necessary to prepare Si-B-N ceramics and composite materials with excellent mechanical and wave transparent properties through optimized design of materials and development of new processes.In this paper,the microstructure and properties of Si₃N₄ ceramics prepared by CVI process are studied firstly.Then,based on the properties of Si₃N₄ ceramics and BN ceramics prepared by CVI process,the microstructure and properties of SiBN ceramics prepared by CVI process are studied.Finally,BN fiber reinforced Si-B-N ceramic matrix composites?BN_f/Si-B-N?and Si₃N₄ fiber reinforced Si₃N₄ ceramic matrix composites(Si₃N_4f/Si₃N₄)were prepared by CVI process to investigate the microstructure and properties.The main research contents and results are as follows:?1?The microstructure and properties of Si₃N₄ composite ceramics prepared by 3D printing combined with CVI process were studied.The microstructure of the porous Si₃N₄substrate prepared by 3D printing is a loose porous structure formed by overlapping?-Si₃N₄columnar crystals,and the bending strength increases with the increase of the fine powder content in the Si₃N₄ powder.Density,porosity,flexural strength,dielectric constant?10 GHz?and dielectric loss?10 GHz?of porous Si₃N₄ substrate prepared by adding 25 wt.%fine powder were 0.99 g/cm³,68.65%,5 MPa,1.72,and 0,respectively.Si₃N₄ composite ceramics consist of Si₃N₄ framework composed of?-Si₃N₄ and amorphous Si₃N₄ matrix prepared by CVI process.With the increase of amorphous Si₃N₄ matrix content,the bending strength of Si₃N₄ composite ceramics first rises and then decreases,and the dielectric constant increases.The density,porosity,flexural strength,dielectric constant and dielectric loss of Si₃N₄ composite ceramic deposited for 12 h were 26.07%,2.02 g/cm³,35 MPa,3.60 and 0,which demonstrates that Si₃N₄composite ceramics have good wave transparent performance.The heat treatment changes the amorphous Si₃N₄ matrix in Si₃N₄ composite ceramics to form?-Si₃N₄.The flexural strength of the composite ceramics decreases due to the decomposition of amorphous Si₃N₄,the increase of porosity and thermal expansion mismatch,and the dielectric constant increases slightly.Compared with the traditional cold press forming-hot pressing sintering process,3D printing combined with CVI process can be used to tune the porosity of Si₃N₄ composite ceramics in a large range,so that it has good wave transparent performance and certain mechanical properties.?2?The deposition phase diagram of SiCl₄-BCl₃-NH₃-H₂-Ar system and the microstructure,high temperature stability and dielectric properties of the prepared SiBN ceramics were studied.The thermodynamics shows that the BN+Si₃N₄ two-phase region exists in the NH₃ excess region and the theoretical deposition efficiency of the two phases is higher when the temperature is lower than 1000°C.The amorphous SiBN ceramics were successfully prepared on carbon fiber cloth by CVI process under the guidance of thermodynamic analysis,which contained N-Si bond and N-B bond,and N-B bond is the main bond.Amorphous SiBN ceramics remain amorphous at less than 1600°C and crystallize at 1600°C to form?/?-Si₃N₄,and the thermal stability is higher than that of amorphous Si₃N₄ and amorphous BN ceramics.The density,porosity,dielectric constant and dielectric loss of Si₃N₄-SiBN composite ceramics with8.23 vol.%SiBN were 1.92 g?cm^-3,41.91%,3.65 and 6.1×10^-3.?3?The microstructure,mechanical and dielectric properties of BN_f/Si-B-N ceramic matrix composites prepared by CVI process were studied,including BN_f/BN,BN_f/SiBN and BN_f/Si₃N₄ mini-composites,and 3D BN_f/Si₃N₄ composites.Tensile strength of BN_f/BN,BN_f/SiBN and BN_f/Si₃N₄ mini-composites were 68.8 MPa,30.6 MPa and 26.2 MPa,respectively,indicating that the higher the modulus matching between the fiber and the matrix,the more the fiber plays the bearing role,resulting in better mechanical property.The fiber will be damaged at the preparation temperature,and the higher the preparation temperature,the greater the damage.The interfacial bonding strength of the three composite materials is relatively high,and the fracture modes are all brittle fractures.Dielectric constant and dielectric loss in X-band of BN_f/BN,BN_f/SiBN and BN_f/Si₃N₄ mini-composites are all lower than 4.0and 0.04,respectively.The real part and imaginary part of dielectric constant,and dielectric loss of the prepared 3D BN_f/Si₃N₄ composite were 3.68,9.6×10^-3 and 2.6×10^-3,respectively,indicating excellent wave transparent performance.The bending strength and modulus of 3D BN_f/Si₃N₄ were respectively 52.6 MPa and 34.8 GPa,and the composites exhibited brittle fracture characteristics due to strong interface bonding.?4?Si₃N_4f/Si₃N₄ composites were prepared by CVI process and their microstructure and properties were studied,including Si₃N_4f/Si₃N₄ mini-composites and 2D Si₃N_4f/Si₃N₄composite.Si₃N₄ fiber is composed of amorphous Si₃N₄ phase,with sizing agent unevenly distributed on the surface.C element content of Si₃N₄ fiber is 1.13 wt.%.The tensile strength of the fiber bundle is 800 MPa.The heat treatment at preparation temperature reduces the content of C element to less than 1%.The strength of the fiber bundle after heat treatment at800°C decreased by 19%.Heat treatment doesn't change the dielectric constant,but enlarge the dielectric loss because of the free carbon decomposed from the surface sizing agent.The surface coating treatment reduces the content of C element in the fiber to below 1%,and its influence on the tensile strength and dielectric properties of the fiber is negligible.The use of BN as the interfacial phase of the Si₃N_4f/Si₃N₄ composite material enables the fiber/matrix to achieve weak interfacial bonding and improve the strength and toughness of the composite.The tensile strength of Si₃N_4f/Si₃N₄ mini-composites increases with the increase of the thickness of the interface layer.When the interface thickness is 2220nm,the tensile strength of the composite is 163.2 MPa.With the increase of matrix content,the flexural strength of 2D Si₃N_4f/Si₃N₄composites first increased and then become stability.The density,porosity,flexural strength and fracture toughness of 2D Si₃N_4f/Si₃N₄ composites were 2.26 g/cm³,10%,62.7 MPa and2.66 MPa·m^1/2,respectively.The dielectric constant of 2D Si₃N_4f/Si₃N₄ increases with the increase of the matrix content.When the matrix content increases from 11 vol.%to 28 vol.%,the real part of the dielectric constant of 2D Si₃N_4f/Si₃N₄ gradually increases from 2.7 to 3.6,while the imaginary part is maintained at around 0,which showed good wave transparent performance.