Fabrication And Performance Of Continuous Nitride Fiber Reinforced Boron Nitride Wave-Transparent Ceramic Matrix Composites

To meet the urgent requirement for high-speed,long endurance and accurateguiding radomes,and with the aim of developing long-term high-temperature durable,low dielectric constant and ablation-resistant wave-transparent composites,newly nitride fiber reinforced boron nitride composites were fabricated using liquid borazine as precursor and the properties of the composites were investigated.Systematic study of the cross-linking and pryrolysis of liquid borazine lead to the foundation of the relationship between the microstructure and properties of the derived boron nitride ceramic with the pyrolysis temperature.The influence of high-temperature heat treatment on the mechanical properties of SiNO and Si₃N₄ fibers was studied.Two kinds of wave-transparent composites,SiNO_f/BN and Si₃N_4f/BN composites,were fabricated through precursor infiltration and pyrolysis?PIP?process using SiNO fiber and Si₃N₄ fiber as the reinforcements and borazine as the precursor of the BN matrix,and the mechanical properties and interfacial bonding characteristics were investigated profoundly.Digital image correlation was applied to monitor and analyze the crack emergence and propagation of the composites.Interface modification was further employed in the composite by introducing a chemical vapor deposition?CVD?fabricated BN coating interphase between the reinforcing fibers and the BN matrix.And the studies of thermal-physical,dielectric and ablative performance of the composites were carried out.The cross-linking and pyrolysis characteristics of borazine were investigated.Liquid borazine was able to be consolidated at a relative low temperature of 90?,with its ceramization nearly completed after pyrolyzed at 800?.The crystallinity of the BN products increased with the pyrolysis temperature,with its microstructure gradually transformed from amorphous phase to well crystallized h-BN,and large layering crystals appeared above 1800 oC.Dense,pure and homogeneous bulk BN ceramics were fabricated by using polyborazylene powers following the molding,pre-sintering and precursor infiltration and pyrolysis densification processes.The comprehensive mechanical,thermalphysical and dielectric properties were obtained.It was found that the thermal conductivity,coefficient of thermal expansion and dielectric of the derived BN ceramics all increased with the increase of fabrication temperature.The thermal resistance of the reinforcing fibers were studied in terms of morphology,microstructure and mechanical strength after high-temperature heat treatment.Both SiNO and Si₃N₄ fibers were amorphous at room temperature.The SiNO fiber kept its amorphous structure up to 1400 o C,while it suffered from partly fiber decomposition.The SiNO fiber retained a high tensile strength of 0.77 GPa?a residual ratio of 61.1%?after heat treatment at 1100 o C in air,which was superior than silica fiberthat undergone a same heat treatment.The Si₃N₄ fiber was also capable of remaining amorphous up to 1400 o C while no notable fiber decomposition was observed.After heat treatment at 1300 oC in air,the retained fiber strength was as high as 0.66 GPa?a residual ratio of 58.9%?,which was superior than SiNO fiber and silica fiber.The SiNO_f/BN and Si₃N_4f/BN composites were fabricated by PIP route and the effects of fabrication temperature on the microstructure,mechanical and fiber-matrix interfacial properties were studied.The 1000 oC fabricated SiNO_f/BN displayed good bending strength,elastic modulus and fracture toughness of 138.2 MPa,28.6 GPa and5.18 MPa?m1/2,respectively.The composite retained 61.0%?84.3 MPa?of its raw strength when tested at 1100 o C.The bending strength of the composites increased at first and then decreased as the fabrication temperature increased,which was effectively explained in terms of in-situ elastic modulus and hardness of the reinforcing fibers and matrix,the fiber-matrix interfacial bonding strength and porosity.As the fabrication temperature increased from 800 o C to 1200 o C,the interfacial shearing strength increased gradually,which was induced by the increased thermal mismatch between the reinforcing fibers and the matrix as well as the interfacial reaction observed by TEM analysis.The 1200 o C fabricated Si₃N_4f/BN composite also had desiring bending strength,of 132.6 MPa.It retained 55.4%?73.4 MPa?of its raw strength when tested at 1300 o C.Thus,the Si₃N_4f/BN composite displayed superior high-temperature mechanical properties than the SiNO_f/BN and Si O2f/Si O2 composite.Digital image correlation?DIC?technique was applied to observe and calculate the critical strains of three types of crack propagation within the SiNO_f/BN composite,the fiber tow de-bonding crack,fiber tow fracturing crack and intra-fiber tow fiber debonding crack.A cohesive element model was generated to simulate the deformation and damage in 2.5D SiNO_f/BN composite during diametral compression.By applying the cohesive law using the DIC obtained critical strains,the model correctly simulated the crack initiation and propagation within the composite.A CVD BN coating was deposited on the surface of the Si₃N₄ fibers using liquid borazine as precursor and the effects of deposition temperature on the microstructure,growth mechanism,composition and crystallinity of the BN coating were studied.It was suggested that the growth of the BN coating was controlled by surface reaction mechanism when the deposition temperature was lower than 1100 oC,which generated dense and well adhered BN coating on the fiber surface.However,when the deposition temperature was higher than 1100 o C,the quality of BN coating decreased due to the obvious gas phase nucleation.The effects of BN coating thickness on the interfacial bonding strength and mechanical properties of the composites were studied.With a CVD BN deposition temperature of 1100? and a thickness of 290 nm,the interfacial shearing strength between Si₃N₄ fiber and BN matrix decreased from 131.42 MPa to110.6 MPa,while the corresponded bending strength and fracture toughness of the Si₃N_4f/BN composite were improved by 11.4% and 22.6%,respectively.The CVD BN displayed a layering structure that promoted the propagation of cracks within it and thus optimized the interfacial bonding strength between the reinforcing fibers and the matrix.The thermal-physical,dielectric properties and of the oxyacetylene ablation performance of the SiNO_f/BN and Si₃N_4f/BN composites were studied.With the increasing of fabrication temperature,the thermal conductivity,CTE,dielectric constant and loss tangent of the two composites both increased,yet remained at admirable levels.The oxyacetylenen flame tests results suggested that the BN matrix composite showed more smooth ablation surface as well as lower linear and mass ablation rates than the Si O2f/Si O2 composite.The molten Si O2 formed during ablation acted as a protective layer that hindered the quick diffusion of oxditive flame into the composite.However,molten Si O2 is generally insufficient in strength to withstand the high velocity scouring of the heat flow,resulting in rough ablation surface in Si O2f/Si O2 composite.In comparison,the BN matrix in BN based composites largely decreased the ablation rates by gasfication ablation and also constrained the flowing and gas-shearing of the molten Si O2,leading to the good ablation performance of the composites.