Irradiation effects on Amosic-3 silicon carbide composites by Si ions implantation

SiC_f/SiC composites were irradiated to over 100 dpa with 300 keV Si ions at 300 ℃. Here, electron microscopy and Raman spectroscopy were utilized to study the microstructural evolution. The Raman spectra of fiber and matrix showed that the crystal structure was seriously damaged. TEM images revealed that the fiber underwent grain nucleation and growth in lower fluence region, accompanied by an amorphous layer near the damage peak area. Also, the matrix went through recrystallization, and the columnar grains turned into equiaxial ones. Moreover, stacking faults and massive amorphous islands were observed in high resolution TEM images. Following irradiation at 300 ℃, the matrix swelled, but the fiber and interphase shrunken along the axis. And, more remarkably, the hardness of fiber and matrix decreased to different extents, a result that was explained by the generation of amorphous islands and breakdown of covalent bonds, and recrystallization might be responsible for this.     

Microstructure response of Amosic-3 SiC/SiC composites under self-ion irradiation

Amosic-3 SiC/SiC composites were irradiated at 300 °C using 6 MeV Si ions to peak doses of 13 and 55 displacements per atom (dpa). The loss of amorphous carbon packets and the growth of SiC grains were simultaneously observed in Amosic-3 SiC fibers, using a combination of transmission electron microscopy (TEM) and Raman spectroscopy. A mechanism based on the grain growth theory was proposed to expound the relationship between the loss of carbon packets and the growth of SiC grains. Small and curved SiC grains can absorb surrounding carbon packets to grow themselves; at some point, these grains further grow at the expense of adjacent small SiC grains until their grain boundary became straight. TEM images were found to support the above mechanism.     

Enhancement of the microwave absorption properties of PyC-SiCf/SiC composites by electrophoretic deposition of SiC nanowires on SiC fibers

The employment of coating technique on the silicon carbide fibers plays a pivotal role in preparing SiC fiber-reinforced SiC composites (SiC_f/SiC) toward electromagnetic wave absorption applications. In this work, SiC nanowires (SiCNWs) are successfully deposited onto the pyrolytic carbon (PyC) coated SiC fibers by an electrophoretic deposition method, and subsequently densified by chemical vapor infiltration to obtain SiCNWs/PyC-SiC_f/SiC composites. The results reveal that the introduction of SiCNWs could markedly enhance the microwave absorption properties of PyC-SiC_f/SiC composites. Owing to the increasing of SiCNWs loading, the minimum reflection loss of composites raises up to −58.5 dB in the SiCNWs/PyC-SiC_f/SiC composites with an effective absorption bandwidth (reflection loss ≤ −10 dB) of 6.13 GHz. The remarkable enhancement of electromagnetic wave absorption performances is mainly attributed to the improved dielectric loss ability, impedance matching and multiple reflections. This work provides a novel strategy in preparing SiC_f/SiC composites with excellent electromagnetic wave absorption properties.     

Improved thermal conductivity and mechanical properties of SiC/SiC composites using pitch-based carbon fibers

Pitch-based carbon fibers were assembled in horizontal and thickness directions of SiC/SiC composites to form three-dimensional heat conduction networks. The effects of heat conduction networks on microstructures, mechanics, and thermal conductivities were investigated. The results revealed the benefit of introducing heat conduction networks in the densification of composites. The maximum bending strength and interlaminar shear strength of the modified composites reached 568.67 MPa and 68.48 MPa, respectively. These values were equivalent to 18.6% and 69.4% increase compared to those of composites without channels. However, channels in thickness direction destroyed the continuity of fibers and matrix, creating numerous defects. As the volume fraction of heat conduction channels rose, the pinning strengthening effect of channels and influence of defects competed with each other to result in first enhanced mechanical properties followed by a decline. The in-plane thermal conductivity was found anisotropic with a maximum value reaching 86.20 W/(m·K) after introducing pitch-based carbon unidirectional tapes. The thermal conductivity in thickness direction increased with volume fraction of pitch-based carbon fibers and reached 19.13 W/(m·K) at 3.87 vol% pitch-based carbon fibers in the thickness direction. This value was 90.75% higher than that of composites without channels.     

Effect of in situ grown SiC nanowires on microstructure and mechanical properties of C/SiC composites

Hexagonal-shaped SiC nanowires were in situ formed in C/SiC composites with ferrocene as catalyst in the densification process of polymer impregnation and pyrolysis. The effect of SiC nanowires on microstructure and properties of the composites were studied. The results show that the in situ formed SiC nanowires were hexagonal, mostly with diamer of about 250 nm, and grew by the vapor–liquid–solid (VLS) mechanism. The C/SiC composite with nanowires shows higher bulk density and flexural strength than the one with no SiC nanowires, and the high temperature flexural strength behavior of C/SiC composites with SiC nanowires was evaluated.     

SiC coating with high crack resistance property for carbon/carbon composites

A novel SiC coating with a relatively high crack resistance property (crack extension force (G_C): 12.0 J·m^?2) and outstanding thermal shock resistance was achieved merely by pack cementation. Compared with the conventional SiC coating with Al₂O₃ addition (AOSC2), SiC coating with Al–B–C additions (ABSC2) possesses refined and denser microstructure owing to different effects in promoting SiC densification under different additions. Therefore, the improvement in microstructures results in superior mechanical capabilities, antioxidation performance (900 °C), and thermal shock resistance (between 1500 °C and room temperature).     

Microstructure evolutions of SiC whiskers at high temperature and its effects on silicon carbide ceramics

SiC whisker with a single-crystal structure is promising in enhancing the strength and toughness of advanced structural ceramics, owing to its excellent properties. However, studies on its microstructure evolution at high temperature (>2000 °C) are scarce. Herein, SiC whiskers were calcined at 2100 °C, and XRD, SEM, and TEM were employed to analyze microstructure evolutions. Compared with raw whiskers, XRD results indicated serious annihilation of stacking faults after calcination. The annihilation led to the fracture of whiskers and the formation of β-SiC grains, and then partial grains underwent the phase transformation to form hexagonal prism and triangular prism α-SiC grains with diameters of about 10 μm, according to SEM and TEM results. Furthermore, SiC ceramics containing different whisker contents were innovatively fabricated by pressureless solid-state sintering. The flexural strength and fracture toughness of SiC ceramic containing 10 vol% whiskers were 540 MPa and 5.1 MPa m^0.5, resulting in 38% and 11% higher values than those without whiskers, respectively.     

Effects of fabrication processes on the properties of SiC/SiC composites

Precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI) were used to fabricate SiC/SiC composites on a four-step 3D SiC fibre preform deposited with a pyrolytic carbon interface. The effects of fabrication processes on the microstructure and mechanical properties of the SiC/SiC composites were studied. Results showed the presence of irregular cracks in the matrix of the SiC/SiC composites prepared through PIP, and the crystal structure was amorphous. The room temperature flexural strength and modulus were 873.62 MPa and 98.16 GPa, respectively. The matrix of the SiC/SiC composites prepared through CVI was tightly bonded without cracks, the crystal structure had high crystallinity, and the room temperature bending strength and modulus were 790.79 MPa and 150.32 GPa, respectively. After heat treatment at 1300 °C for 50 h, the flexural strength and modulus retention rate of the SiC/SiC composites prepared through PIP were 50.01% and 61.87%, and those of the composites prepared through CVI were 99.24% and 96.18%, respectively. The mechanism of the evolution of the mechanical properties after heat treatment was examined, and the analysis revealed that it was caused by the different fabrication processes of the SiC matrix. After heat treatment, the SiC crystallites prepared through PIP greatly increased, and the SiOxCy in the matrix decomposed to produce volatile gases SiO and/or CO, ultimately leading to an increase in the number of cracks and porosity in the material and a decrease in the material load-bearing capacity. However, the size of the SiC crystallites prepared through CVI hardly changed, the SiC matrix was tightly bonded without cracks, and the load-bearing capacity only slightly changed.     

Preparation of SiC/SiC composites with high toughness by reaction of the SiCP+C double-cladding layer with liquid silicon

SiC/SiC composites prepared by liquid silicon infiltration (LSI) have the advantages of high densification, matrix cracking stress and ultimate tensile strength, but the toughness is usually insufficient. Relieving the residual microstress in fiber and interphase, dissipating crack propagation energy, and improving the crystallization degree of interphase can effectively increase the toughness of the composites. In this work, a special SiC particles and C (SiC_P +C) double-cladding layer is designed and prepared via the infiltration of SiC_P slurry and chemical vapor infiltration (CVI) of C in the porous SiC/SiC composites prepared by CVI. After LSI, the SiC generated by the reaction of C with molten Si combines with the SiC_P to form a layered structure matrix, which can effectually relieve residual microstress in fiber and interphase and dissipate crack propagation energy. The crystallization degree of BN interphase is increased under the effects of C-Si reaction exotherm. The as-received SiC/SiC composites possess a density of 2.64 g/cm³ and a porosity of 6.1%. The flexural strength of the SiC/SiC composites with layered structure matrix and highly crystalline BN interphase is 577 MPa, and the fracture toughness reaches up to 37 MPa·m^1/2. The microstructure and properties of four groups of SiC/SiC composites prepared by different processes are also investigated and compared to demonstrate the effectiveness of the SiC_P +C double-cladding layer design, which offers a strategy for developing the SiC/SiC composites with high performance.     

Preparation of diamond/SiC composites by the liquid silicon infiltration method and their microstructure and properties

Diamond/SiC composites have long been recognized as advanced materials for thermal management as they exhibit excellent thermal and mechanical properties. The objective was to investigate and understand the phase composition, diamond graphitization behavior, microstructure, and properties of diamond/SiC composites developed following the liquid silicon infiltration process. The results revealed that the incorporation of α-SiC particles increased the degree of uniformity of the microstructure of the diamond/SiC composites. The acoustic mismatch model was used to analyze the samples before and after diamond graphitization to evaluate the interfacial thermal resistance of the composites. The results indicated that the interfacial thermal resistance of the graphitized composites was 11.9 times higher than the interfacial thermal resistance of the un-graphitized composites. Finally, the correlation between the diamond content of the composites and their thermal and mechanical properties was investigated.     

SiC/SiC复合材料及其应用

日本开发的Nicalon和Tyranno两种品牌的SiC纤维占有世界上绝对性的市场份额。SiC/SiC复合材料典型的界面层是500 nm厚的单层热解碳(PyC)涂层或多层(PyC-SiC)n涂层,在湿度燃烧环境及中高温条件下界面层的稳定性是应用研究的重点。SiC/SiC复合材料,包括CVI-SiC基体和日本开发的Tyranno hex和NITE-SiC基体等,具有耐高温、耐氧化性和耐辐射性的特点,在航空涡轮发动机部件、航天热结构部件及核聚变反应堆炉第一壁材料等方面正开展工程研制应用。     

Mechanical properties degradation and microstructure evolution of 2D-SiCf / SiC composites in combustion gas environment

Based on the turbine high-temperature combustion gas simulation test platform, the long-term combustion gas environment exposure test of the 2D plain woven SiC_f/BN/SiC composites under two combustion conditions was carried out. Uniaxial tensile test, fracture morphology characterization and non-destructive testing analysis revealed the degradation and microstructure evolution of composites after exposure to combustion gas environment. The results show that the degradation of 2D-SiC_f/SiC composites after exposure to combustion gas environment is manifested as a decrease in static toughness, and the interphase transition is the mesoscopic cause of the decrease in static toughness of the composite.     

Micromechanical properties of vapour-exposed SiCf/BN/SiC ceramic-matrix composites

SiC_f/BN/SiC Ceramic-Matrix Composites are candidate materials for aero-engines, but their interphase stability after potential low-temperature water-vapour exposures during flight cycles is not well known. The examination of these composites exposed for 50, 250 and 500 h at low temperature (65 °C) and 95% relative humidity was therefore performed, in order to understand if resulting oxidation products affected the mechanical properties at the micro-scale. The composites were subject to fibre push-out tests to compare pristine from degraded composites. It was found that whilst the sample exposed for 50 h had no significant change from pristine, the samples exposed for 250 and 500 h had a clear decrease in interfacial shear strengths measured. Parallel studies also revealed that whilst damage was strongly localised, the diffusion of water within the composite was not fully complete at exposure times between 50 and 250 h. The permeability of the CMC was shown to be affected at longer exposure times where differences in mechanical performances even between tows and within tows were noted.     

The improvement of mechanical properties of SiC/SiC composites by in situ introducing vertically aligned carbon nanotubes on the PyC interface

In order to improve the mechanical properties, vertically aligned carbon nanotubes (VACNTs) were in situ introduced on the pyrocarbon (PyC) interfaces of the multilayer preform via chemical vapor deposition (CVD) process under tailored parameters. Chemical vapor infiltration (CVI) process was then employed to densify the multilayer preform to acquire SiC/SiC composites. The results show that the growth of VACNTs on PyC interface is highly dependent to the deposition temperature, time and constituent of gas during CVD process. The preferred orientation and high graphitization of VACNTs were obtained when temperature is 800?℃ and C₂H₄/H₂ ratio is 1:3. The bending strength and fracture toughness of SiC/SiC composites with PyC and PyC-VACNTs interfaces were compared. Compared to the SiC/SiC composite with PyC interface, the bending strength and fracture toughness increase 1.298 and 1.359 times, respectively after the introduction of PyC-VACNTs interface to the SiC/SiC composites. It is also demonstrated that the modification of PyC interface with VACNTs enhances the mechanical properties of SiC/SiC composites due to the occurrence of more fiber pull-outs, interfacial debonding, crack branching and deflection     

Mechanical and optical property assessment of irradiated SiC with displaced atoms

Explicit method to emulate neutron radiation effects on key ceramic materials is lacking, with most literatures stressing direct comparison between radiation parameters while underestimating the defect-property correlation. Herein, the evolutionary correlation between defects of displaced atoms and properties in 6H-SiC was systematically investigated, aiming to found the basis for performance assessment of neutron irradiated material. The point defect accumulation, defect cluster growth, and homogeneous crystalline-amorphous (c-a) transition were recognized, with different stages of Young’s modulus reduction and optical absorption enhancement accordingly triggered. Specifically, the relaxation process accompanying the c-a transition was believed to induce the final reduction stage of Young’s modulus and account for the conflict between existing research results. Average lattice disorder, regarded as the average density of displaced atoms, was proved to be the success reference in the property assessment approach, paving the way for the extension into high temperature radiation where more defect types coexist.     

Micromechanical model of time-dependent damage and deformation behavior for an orthogonal 3D-woven SiC/SiC composite at elevated temperature in vacuum

This paper presents a micromechanical model to predict the time-dependent damage and deformation behavior of an orthogonal 3-D woven SiC fiber/BN interface/SiC matrix composite under constant tensile loading at elevated temperature in vacuum. In-situ observation under monotonic tensile loading at room temperature, load–unload tensile testing at 1200 °C in argon, and constant load tensile testing at 1200 °C in vacuum were conducted to investigate the effects of microscopic damage on deformation behavior. The experimentally obtained results led to production of a time-dependent nonlinear stress–strain response model for the orthogonal 3-D woven SiC/SiC. It was established using the linear viscoelastic model, micro-damage propagation model, and a shear-lag model. The predicted creep deformation was found to agree well with the experimentally obtained results.     

Improved antioxidative and mechanical properties of SiC coated C/C composites via a SiO2-SiC reticulated layer

To improve the oxidation resistances of SiC coated C/C composites by a pack cementation (PC) method at high temperature and alleviate the siliconization erosion of molten silicon on C/C substrate during the preparation of SiC coating, a SiO₂-SiC reticulated layer with SiC nanowires was pre-prepared on C/C composites through combined slurry painting and thermal treatment before the fabrication of SiC coating. The presence of porous SiO₂-SiC layer with SiC nanowires was beneficial to fabricate a compact and homogeneous SiC coating resulting from synergistic effect of further reaction between SiO₂ and pack powders and the reinforcement of SiC nanowires. Therefore, the results of thermal shock and isothermal oxidation tests showed that the mass loss of modified SiC coating was only 0.02 % after suffering 50-time thermal cycles between room temperature and 1773 K and decreased from 5.95 % to 1.08 % after static oxidation for 49.5 h in air at 1773 K. Moreover, due to the blocking effect of SiO₂-SiC reticulated layer on siliconization erosion during PC, the flexural strength of SiC coated C/C composites with SiO₂-SiC reticulated layer increased by 64.8 % compared with the untreated specimen.     

Microstructure and properties of diamond/SiC composites prepared by tape-casting and chemical vapor infiltration process

This article reported a novel method for preparing diamond/SiC composites by tape-casting and chemical vapor infiltration (CVI) process, and the advantages of this method were discussed. The diamond particle was proved to be thermally stable under CVI conditions and the CVI diamond/SiC composites only contained diamond and CVI-SiC phases. The SEM and TEM results showed a strong interfacial bonding existed between diamond and CVI-SiC matrix. Due to the strong bonding, the surface HRA hardness could reach up to 98.4 (HV 50 ± 5 GPa) and the thermal conductivity (TC) of composites was five times higher than that of pure CVI-SiC matrix. Additionally, the effects of diamond particle size on microstructure and properties of composites were also investigated. With the increasing of particle size, the density and TC of composites with the size 27 μm reached 2.940 g/cm³ and 82 W/(m K), respectively.     

SiC含量对反应烧结SiC-Si3N4复合材料性能的影响

以粒度均≤0.044mm的工业Si粉和α-SiC粉为原料,酚醛树脂为结合剂(占总粉末质量的6.5%),配成SiC含量(质量分数)分别为10%、30%、50%和70%的4组试样,经200MPa冷等静压成型后,在N2气氛中(压力为1.25MPa)于1395℃反应烧结制备了SiC-Si3N4复合材料,并采用SEM、XRD和EDS等测试手段对试样进行了观察和测试。结果表明:随着粉料中SiC含量的增加,烧后试样的体积密度下降,显气孔率提高,抗折强度降低,以SiC加入量为10%的试样性能最优;4组试样经800℃～室温空冷热震15次后的抗折强度保持率均在90%以上,表明材料具有良好的抗热震性能。     

Strengthening/toughening of 2D C/SiC composites by coating

SiC and SiC_w/SiC coatings were prepared on two-dimensional carbon fiber reinforced silicon carbide ceramic matrix composites (2D C/SiC), and strengthening/toughening of the composite by the coatings was investigated. After coating, the density of the C/SiC composites was increased effectively and the mechanical properties were improved significantly. Compared with SiC coating, SiC_w/SiC coating showed the more significant effect on strength/toughness of the composites. Coatings had two effects: surface strengthening and matrix strengthening. The latter was the dominant effect. The surface strengthening can increase the crack initiation stress, while the matrix strengthening can enhance the crack propagation resistance. The former effect increased the strength and the latter effect increased the toughness.