【作者】 李建平；

【导师】 成来飞； 刘永胜； 叶昉；

【作者基本信息】 西北工业大学 ， 材料学， 2018， 博士

【摘要】 Si-B-N系陶瓷包括Si₃N₄陶瓷、BN陶瓷、SiBN陶瓷及其复合材料,具有耐高温、高比强、抗氧化、抗烧蚀和电磁透波等性能,是高温透波材料的重要候选材料。Si₃N₄陶瓷综合性能优异,具有高强度、低密度、耐高温和化学性能稳定等特性,但是介电常数偏高,且现有制备工艺对材料性能调控空间较小。BN陶瓷具有优异的高温稳定性和低介电常数,但是力学性能较低。SiBN陶瓷具备更为优异的综合性能,但是有关块体SiBN陶瓷的研究鲜见报导。而且,单一陶瓷材料的脆性大,易发生灾难性损毁,可靠性差。因此,有必要通过材料的优化设计和发展新的工艺来制备力学、透波性能优异的Si-B-N系陶瓷及其复合材料。本文首先研究了CVI工艺制备的Si₃N₄陶瓷的微结构与性能;然后,在CVI工艺制备的Si₃N₄陶瓷和BN陶瓷的性能基础上,研究了CVI工艺制备的SiBN陶瓷的微结构与性能;最后,采用CVI工艺制备了BN纤维增强Si-B-N系陶瓷基复合材料（BN_f/Si-B-N复合材料）和Si₃N₄纤维增强Si₃N₄陶瓷基复合材料(Si₃N_4f/Si₃N₄复合材料),表征其微结构与性能。主要研究内容和结果如下:（1）采用3D打印结合CVI工艺制备Si₃N₄复相陶瓷,研究其微结构与性能。3D打印制备的多孔Si₃N₄基片具有β-Si₃N₄柱状晶互相搭接形成的疏松多孔结构,弯曲强度随Si₃N₄粉体中细粉含量增加而增大;添加25 wt.%细粉所制得多孔Si₃N₄基片的密度、孔隙率、弯曲强度、介电常数（10 GHz）和介电损耗（10 GHz）分别为0.99 g/cm³、68.65%、5 MPa、1.72和0。Si₃N₄复相陶瓷由β-Si₃N₄构成的Si₃N₄骨架和CVI工艺制备的非晶Si₃N₄基体组成,随着非晶Si₃N₄基体含量的提高,Si₃N₄复相陶瓷的弯曲强度先升后降,介电常数增大;沉积12 h得到的Si₃N₄复相陶瓷的密度、孔隙率、弯曲强度、介电常数和介电损耗分别为2.02 g/cm³、26.07%、35 MPa、3.60和0,表明Si₃N₄复相陶瓷具备良好的透波性能。热处理使Si₃N₄复相陶瓷中非晶Si₃N₄基体转变生成α-Si₃N₄,复相陶瓷的弯曲强度因非晶Si₃N₄的分解、气孔率的提高和热膨胀失配而降低,介电常数稍有上升。与传统的冷压成型-热压烧结工艺相比,3D打印结合CVI工艺可在较大范围调控Si₃N₄复相陶瓷的孔隙率,使其获得良好的透波性能,兼具一定的力学性能。（2）计算了SiCl₄-BCl₃-NH₃-H₂-Ar体系的沉积相图,制备并研究SiBN陶瓷的微结构、高温稳定性和介电性能。热力学计算表明,BN+Si₃N₄两相区位于NH₃过量的区域,低于1000 ^oC时两相的理论沉积效率较高;以热力学计算结果为指导,采用CVI工艺在碳纤维布上成功制备了非晶SiBN陶瓷,其含有N-Si键和N-B键,以N-B键为主;非晶SiBN陶瓷在低于1600℃时保持非晶态,在1600℃晶化生成α/β-Si₃N₄,热稳定性高于非晶态Si₃N₄和非晶态BN陶瓷;含有8.23 vol.%SiBN的Si₃N₄-SiBN复相陶瓷的密度、孔隙率、介电常数和介电损耗分别为1.92 g/cm³、41.91%、3.65和6.1×10^-3,表明制备的SiBN陶瓷很有潜力作为高温透波材料候选。（3）研究了CVI工艺制备BN_f/Si-B-N系陶瓷基复合材料（包括BN_f/BN、BN_f/SiBN和BN_f/Si₃N₄纤维束复合材料,以及3D BN_f/Si₃N₄复合材料）的微结构与力学/介电性能。BN_f/BN、BN_f/SiBN和BN_f/Si₃N₄纤维束复合材料的拉伸强度分别为68.8 MPa、30.6 MPa和26.2 MPa,表明纤维与基体间的模量匹配程度越高,纤维起到承载作用就越大,复合材料力学性能即越优异;纤维在制备温度下会发生损伤,制备温度越高损伤越大;三种复合材料的界面结合强度偏高,断裂模式均为脆性断裂。三种纤维束复合材料在X波段的介电常数实部<4.0,介电损耗<0.04,电磁透波性能良好。所制备的3D BN_f/Si₃N₄复合材料的密度、介电常数和介电损耗分别为1.89 g/cm³、3.68和2.6×10^-3,透波性能优异;弯曲强度和模量分别为52.6 MPa和34.8 GPa,因界面结合强度偏高,复合材料呈脆性断裂特征。（4）采用CVI工艺制备Si₃N_4f/Si₃N₄复合材料(包括Si₃N_4f/Si₃N₄纤维束复合材料和2D Si₃N_4f/Si₃N₄复合材料),研究其微结构与力学/介电性能。Si₃N₄纤维由非晶Si₃N₄相组成,纤维表面不均匀地分布胶层,纤维中C元素含量为1.13 wt.%,纤维束拉伸强度为800 MPa;对Si₃N₄纤维进行等效沉积温度热处理可使其C元素含量降至1%以下,800 ^oC热处理后纤维束强度下降19%,介电常数变化不大,介电损耗可能因表面胶层分解残留的微量碳而升高。对Si₃N₄纤维进行表面胶层处理可使其C元素含量降至1%以下,纤维束拉伸强度和介电性能几乎不发生变化。采用BN作为Si₃N_4f/Si₃N₄复合材料的界面相,可使纤维/基体实现弱界面结合,提高复合材料强度和韧性;Si₃N_4f/Si₃N₄纤维束复合材料的拉伸强度随界面厚度增加而增加,界面厚度为2220 nm时,Si₃N_4f/Si₃N₄纤维束复合材料拉伸强度为163.2 MPa。2D Si₃N_4f/Si₃N₄复合材料的弯曲强度随Si₃N₄基体含量的提高先稳定上升后趋于平稳,复合材料的密度、孔隙率、弯曲强度和断裂韧性分别为2.26 g/cm³、10%、62.7 MPa和2.66 MPa·m^1/2;2D Si₃N_4f/Si₃N₄复合材料的介电常数随基体含量的提高而增大,当基体含量从11 vol.%升至28 vol.%时,复合材料的介电常数实部从2.7增至3.6,介电损耗维持在0附近,透波性能良好。更多还原

【Abstract】 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 overlappingβ-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 ofβ-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 formα-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 formα/β-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.更多还原