Design, Preparation And Performance Optimization Of Wave-absorbing SiC_f/Si(B)CN Composites

With the development of all-wave radar detection technology, it is urgent for the high temperature parts of aircrafts to realize the radar stealth. Focusing on this issue, continuous Si C fibers reinforced ceramic matrix composites with good high temperature resistance, excellent force bearing and wave-absorbing capacity have been fabricated and applied in some developed countries; however, the related public reports are rare due to the technique blockade. Domestically prepared Si C/Si C composites mainly show the electromagnetic wave reflection performance, thus, it is imperative to develop the novel Si C fibers reinforced composites with good wave-absorbing property.Aiming to the combination of force-bearing and wave-absorbing capacity of Si C fibers reinforced ceramic matrix composites, the microstructure and electromagnetic property of Si C fibers, BN interface and Si(B)CN matrix were studied, respectively. Furthermore, the relationships between composition/microstructure and mechanical /electromagnetic properties of Si C fibers reinforced Si(B)CN matrix composites fabricated by precursor infiltration pyrolysis(PIP) and chemical vapor deposition/infiltration(CVD/CVI). The main research contents and results are as follows:(1) Each structural unit of the composite were theoretically designed for achieving the synergies between mechanical and microwave absorbing properties. Results show that the composites composed of Si C fibers, BN interface, Si BCN, Si CN or Si3N4 matrix are expected to meet the demands of high temperature withstanding, high specific strength, high specific modulus and excellent wave absorption capacity.(2) Relationships between the composition/microstructure and electromagnetic properties of Si C fibers were investigated. Results show that for the untreated carbon-rich Si C fibers, high temperature treatment can promote the formation of Si C nanocrystals and free carbon clusters, so as to improve dielectric property; for the treated carbon-rich Si C fibers, the oxygen content mainly influences the size and content of Si C and Si O2 microsrystaline, and Si C fibers with lower oxygen content have higher permittivity. The carbon content mainly influences the size and content of free carbon phase, and Si C fibers with higher carbon content have larger dielectric loss; both low temperature surface decarburization and depositing BN coating can improve the impedance mismatch between carbon-rich Si C fibers and free space, but the former treatment decreases the tensile strength of Si C fiber bundles by 29.25% and the latter increases it by 1.39%.(3) Effects of heat treatment and adding nano Si C particles on the microstructure and electromagnetic property of PIP Si BCN were investigated.(a) Results show that Si BCN ceramics pyrolyzed at 900 oC are amorphous and have low permittivity(2.75-j0.03) and low dielectric loss( 0.011); after annealed at 1350 oC, Si C nanocrystals of ceramics begin to crystallize; after annealed at 1650 oC, abundant Si C nanocrystals crystallize and disperse in the amorphous matrix, leading to the medium permittivity(9.11-j5.24) and medium dielectric loss(0.575) of the composite ceramics.(b) For Si BCN ceramics with added nano Si C particles, after annealed above 1350 oC, abandunt of Si C and Si3N4 nanocrystals are inducedly formed at hetero-interfaces between the added Si C particles and amorphous matrx due to internal stress concentration, which greatly increases dielectric propertiy; after annealed at 1650 oC, Si BCN ceramics with 20 wt.% of nano Si C particles have high permittivity(12.12-j28.62) and high dielectric loss(2.36), which show a great potential as the candidate material of high temperature absorbers.(4) Thermodynamic and kinetic study on CVD/CVI Si-C-N from the precursors system of Si Cl4-C3H6-NH3-H2 were done. And the relationships between deposition conditions, microstructure and electromagnetic property of Si-C-N were investigated.(a) Thermodynamic calculations show that when T=800~1000 oC, Ptotal=0.01 atm, [H2]/[Si Cl4]=10, both increasing [N]/[Si] and decreasing [C]/[Si] are beneticial to the co-deposition of Si3N4 and Si C phases.(b) Kinetic experiments show that Si C-Si3N4 composite ceramics prepared at T=800 oC and [Si]:[C]:[N]=4.25:9:1 are composed of amorphous Si3N4 and Si C nanocrystals(5 nm) dispersing uniformly in amorphous phase. Such a unique microstructure endows this ceramics with low permittivity(4.91-j2.15) but medium dielectric loss(0.438). Due to achieving both impedance matching and electromagnetic energy attenuation, this ceramics can be considered as an excellent candidate material of wave-absorbing ceramics matrx.(5) High-carbon and low-carbon Si C fibers reinforced Si(B)CN matrix composites with good mechanical and electromagnetic properties were prepared by PIP and CVI, respectively. The wave-absorbing capacity of the as-prepared Si C/Si(B)CN composites was further increased by several approaches.(a) Results show that the flexural strength, elastic modulus, fracture toughness, permittivity, dielectric loss and electromagnetic reflection coefficient of PIP Si C/Si BCN at room temperature are 169.7 MPa, 58.8 GPa and 8.2 MP·m1/2, 27.76-j30.58, 1.30 and-2.98 d B, respectively; after annealed at 1350 oC, the electromagnetic reflection coefficient of PIP Si C/Si BCN reduces to –6.04 d B, but at the same time the mechanical property decreases inevitably; after preparing wave-absorbing coating containing the high temperature absorbers as described in(4) on the surface of PIP Si C/Si BCN, the electromagnetic reflection coefficient obviously reduces to –12.13 d B, and the good mechanical property is maintained as well.(b) The flexural strength, elastic modulus and fracture toughness of CVI Si C/ Si C-Si3N4 at room temperature are respectively increased to 326.3 MPa, 84.2 GPa and 9.4 MP·m1/2. And the permittivity, dielectric loss and electromagnetic reflection coefficient are 7.17-j9.40, 1.30 and-5.15 d B, respectively. The lay up design of high-carbon and low-carbon Si C fiber cloth were done based on impedance matching model and modified Salisbury model. The electromagnetic reflection coefficient of this kind of composites greatly reduces to-22.97 d B, and is less than-10 d B in 8~18 GHz, accordingly.