Mechanical properties of ceramic matrix composites are highly dependent on the microstructure and homogeneity of the interphase. This study examined the influence of process factors, such as deposition temperature, propylene partial pressure, retention time, and hydrogen partial pressure, on the microstructure and homogeneity of PyC interphase deposited on the surface of carbon fiber cloth. Various characterization techniques were used to characterize the microstructure and texture of the PyC interphase, and the inherent correlation between microstructure, uniformity, and process parameters was analyzed. The results show that increasing temperature and propylene partial pressure lead to a more regular texture in the interphase, while decreasing hydrogen partial pressure has the opposite effect, and retention time has little effects. Additionally, an increase in deposition temperature and propylene partial pressure leads to an uneven distribution of interphase thickness, and overhigh propylene partial pressure directly leads to the production of carbon black. Increasing retention time helps to improve interphase homogeneity. The interphase uniformity of medium and high textures firstly drops and then increases when the hydrogen partial pressure increases, but the interphase uniformity of low texture is less impacted. Ultimately, the study clarified the growth method of the PyC interphase. Furthermore, the research delved into the process factors influencing the texture and uniformity of the PyC interphase, which provided a basis for the fine regulation of PyC interface phase in the future.

Silicon carbide ceramics are important engineering materials, but their application is limited by the inherent brittleness. Two-dimensional graphene, with its excellent properties, can be used as a second phase to improve the performance of silicon carbide ceramics. However, due to poor dispersion of graphene in the ceramic matrix, it is a challenge to fully exploit the modifying effect of graphene in composite materials. To address these challenges, SiC-based ceramic materials incorporating graphene nanosheets (GNPs) were synthesized using ceramic organic precursor polycarbosilane and industrial expandable graphite as starting materials. The precursor intercalation technique was employed to fabricate SiC/GNPs ceramic composites with GNPs volume fraction of 1%, 3%, and 5%. The GNPs were uniformly arranged in an array-like parallel fashion in the SiC ceramic matrix, showing excellent orientation. With the GNPs content increasing, the spacing between GNPs within the array decreased, indicating tunable microstructural topology. The addition of GNPs greatly enhanced the fracture toughness of SiC ceramics. When the GNPs content was 3%, the relative density of the samples reached 98.5%, the bending strength reached 445 MPa, and the fracture toughness (K_IC value) peaked at 5.67 MPa·m^1/2, surpassing pure SiC ceramics by 40%, which was primarily attributed to crack deflection and bridging induced by the GNPs. However, further increase in GNPs content led to a decrease in fracture toughness to 4.37 MPa·m^1/2. These SiC-based ceramic composites with a graphene array have potential application in design and development of novel “structure-function integration” SiC-based ceramic devices.

Low-velocity impact is an inevitable problem in the service of ceramic matrix composites structures in high speed aircraft. Therefore, the damage type and residual bearing capacity after low-velocity impact are critical factors for ensuring the safety of the aircraft structures. In this study, two-dimensional braided SiC/SiC composite laminates were taken as the research objects, and low-speed impact tests under different energies were carried out. The damage morphology of SiC/SiC composites was observed by computed tomography, and the damage mechanism of SiC/SiC composites during the impact process was revealed by analyzing the load history curve and strain history curve. Post-impact residual strength tests were carried out on specimens with barely visible damage and the effect of barely visible damage on the residual strength of SiC/SiC composites was investigated. The results showed that under low-velocity impact load, surface damage of specimens mainly included no surface damage, barely visible damage, semi-penetrating damage and penetrating damage. Internal damage of specimens mainly included cone cracks, yarn breakage and delamination. The residual properties of SiC/SiC composites were found to be severely affected by low velocity impact damage. The residual compressive strength of the specimen with barely visible damage was 81% of that of the undamaged specimens, and the residual tensile strength was only 68% of that of the undamaged specimens.

AlN-SiC multiphase ceramics possess robust mechanical strength, high thermal conductivity and good oxidization resistance, and show great potential as the matrix material of fiber reinforced ceramic matrix composites. In this work, AlN-SiC multiphase ceramics were fabricated via low temperature reactive melt infiltration of Si-Al alloy into porous C-Si₃N₄ preforms. Influence of Si-Al source on the melt infiltration process was studied, and impact of residual silicon on the mechanical and thermal properties of the AlN-SiC ceramics was investigated. It was found that an Al-O layer was in-situ formed at the interface between Si-Al melt and C-Si₃N₄ preform, when Si-Al powder was used as the infiltration medium. This seriously retarded the melt infiltration process and made the penetration of Si-Al melt into the C-Si₃N₄ preform hardly possible. However, when Si-Al ingot was used as the infiltration medium, a well infiltration of Si-Al melt into the C-Si₃N₄ preform occurred, which led to the formation of dense AlN-SiC ceramics. Mechanical and thermal property measurements indicated that the strength of the AlN-SiC ceramics was significantly improved as the residual silicon content in it was reduced, while a reverse trend was observed for the thermal conductivity. AlN-SiC ceramics with 4%(in mass) residual silicon showed a high strength of 320.1 MPa, nearly comparable to that of conventional reaction bonded SiC, although its thermal conductivity was modest (26.3 W·m^-1·K^-1). The fundamental reasons for the above phenomena were discussed. This study is of great significance for the preparation of SiC_f/AlN-SiC composites by low temperature reactive melt infiltration.

Controlling the structure and properties of low-density C/C porous preforms is the key to the preparation of C/C-SiC composites with excellent friction and wear properties. In this study, C/C porous preforms prepared by chemical vapor infiltration were subjected to high temperature heat-treatment at 2100 ℃. C/C-SiC composites were prepared by reactive melt infiltration. Effects of high temperature heat-treatment of C/C porous preforms on microstructures, thermal properties and tribological properties of C/C-SiC composites were investigated. The results showed that the porosity and graphitization degree of the C/C porous performs increased after high temperature heat-treatment at 2100 ℃. The C/C-SiC composites have a higher density (2.22 g/cm³), and the porosity is reduced from 5.1% to 3.4%, the phase content of SiC ceramic is increased by 11.9%. The mean free path of phonons is larger when C/C porous preforms have a higher degree of graphitization, resulting in thermal conductivity at room temperature being increased by 2.1 times, and the thermal conductivity at 1200 ℃ being increased by 0.2 times. Wear surface of C/C-SiC composites forms a continuous and stable friction film, which is attributed to the fact that the PyC after high temperature treatment is softer and easier to be extruded into a film. Thus, the friction coefficient is more stable, and the wear rate is reduced under the test loads of 3, 6 and 9 N, by 47.8%, 41.9% and 11.7%, and the average friction coefficients are 0.47, 0.38 and 0.39, respectively. Therefore, high temperature heat-treatment of the C/C porous preforms can improve the thermal conductivity of the C/C-SiC composites, which exhibits a more stable friction coefficient and more wear-resistant.

To study the mechanical properties at high temperature of pre-stressed ceramics which were prepared by using higher expansion coefficient materials as coating and lower expansion coefficient materials as substrate, zirconia and alumina were chosen as the coating and the substrate, respectively, to fabricate ZrO₂-Al₂O₃ (marked as ZcAs) pre-stressed ceramics with sandwich structure. Meanwhile, Al₂O₃-ZrO₂ pre-stressed ceramics (marked as AcZs, which has the similar section ratio between substrate and coating to ZcAs), ZrO₂ ceramics and Al₂O₃ ceramics were set as the reference samples. Combining the results of bending strength at different temperatures with the results of Vickers indentation, the existence form of residual stress and its influence on crack propagation behavior were clarified as well as the temperature dependence of residual stress. Results show that the residual tensile stress exists in the surface layer of ZcAs, while the compressive stress exists in the substrate. On the contrary, the compressive stress exists in the surface layer of AcZs and tensile stress exists in the substrate. Due to tensile stress promoting while compressive stress inhibiting crack growth, flexural strength of ZcAs is 13.2% lower than that of Al₂O₃, and AcZs possesses strength 25.0% higher than that of ZrO₂ at room temperature. In addition, both tensile stress and compressive stress are decreased with the increase of temperature, which is mainly attributed to the relaxation of pre-stress caused by high temperature.

Continuous silicon carbide fiber reinforced silicon carbide composite (SiC_f/SiC) is a key material for the advanced aero-engines. It is required to possess excellent high-temperature creep resistance for SiC_f/SiC to meet the long-term service lifetime of the aero-engines. Here, tensile creep behaviors of a plain woven Cansas-II SiC_f/SiC (2D-SiC_f/SiC) were investiged in the temperature of 1200-1400 ℃ with the stress levels of 80 to 140 MPa. Its microstructure and fracture morphology were observed, and composition was analyzed. Results show that creep-rupture time of 2D-SiC_f/SiC is more than 500 h and steady-state creep rate is 1×10^-10-5×10^-10 /s at stresses lower than the proportional limit stress (σ_PLS). The creep behaviors are controlled by matrix and fibers. The creep-rupture time is significantly reduced, and the steady-state creep rate is increased by an order of magnitude when the stress is higher than the σ_PLS. The matrix, fibers and interfaces of the composite are greatly oxidized, and the creep behaviors are mainly controlled by the fibers.

Silicon carbide ceramic matrix composites have been widely used in aerospace, friction brake, fusion fields and so on, and become advanced high-temperature structural and functional composites, due to their high specific strength and specific modulus, excellent ablation and oxidation resistance, and high conductivity and good thermal shock resistance. This paper reviews the latest research progress in preparation and property of silicon carbide ceramics matrix composites (CMCs) with high thermal conductivity. Researchers have improved the thermal conductivity of silicon carbide CMCs, including by introducing highly thermal conductive phases for reinforcing heat transport, such as diamond powders, and mesophase pitch-based carbon fibers (MPCF), by optimizing the interface between pyrolytic carbon (PyC) and silicon carbide matrix for reducing interfacial thermal resistance, by heat-treating for obtaining silicon carbide matrix with higher crystallinity and better thermal conductivity, and by designing preform structure for establishing continuous thermal conduction path. Meanwhile, research interests on silicon carbide CMCs are to explore new preparation with high efficiency and low cost through optimising their influencing factors, and to obtain isotropic highly thermal conductivity with dimensional stability and physical properties through deep understanding their thermal conductive mechanism, and flexible method based on the structure-activity relationship.

Silicon carbide fiber reinforced silicon carbide (SiC_f/SiC) composites have become the preferred candidate for structural applications in advanced nuclear energy systems, because of their low neutron toxicity, neutron irradiation tolerance and high-temperature oxidation resistance. In recent years, both academia and industry either domestic or abroad have carried out a lot of researches on SiC_f/SiC composites for nuclear application, and numerous important achievements have been made. This paper summarized and analysed some critical directions of SiC_f/SiC composites for nuclear applications, including nuclear-grade SiC fibers, fibre/matrix interfaces, composite processing, modeling and simulation, corrosion behavior and surface protection, joining technology, as well as radiation damage. The key issues and potential solutions of SiC_f/SiC composites for nuclear applications have been pointed out in account to the requirements, anticipating to be beneficial to promoting further researches and final applications.

Porous design of SiC composites with lightweight, high strength and low thermal conductivity can be obtained by constructing porous silicon carbide nanowires (SiCNWs) network and controlling chemical vapor infiltration (CVI) process. The SiCNWs network with an optimized volume fraction (15.6%) and uniform pore structure was prepared by mixing SiCNWs and polyvinyl alcohol (PVA) firstly. SiCNWs reinforced porous SiC ceramic matrix composite (SiCNWs/SiC) with a small uniform pore can be obtained by controlling the CVI parameters. The morphology of the grown SiC matrix, from the spherical particles to the hexagonal pyramid particles, can be influenced by the CVI parameters, such as temperature and reactive gas concentration. The strength of the SiCNWs/SiC ceramic matrix composites reaches (194.3±21.3) MPa with a porosity of 38.9% and thermal conductivity of (1.9± 0.1) W/(m·K), which shows the toughening effect and low thermal conductivity design.

Silicon carbide nanowires (SiCNWs) possess excellent electromagnetic absorption performance and a three-dimensional (3D) network structure is beneficial to the multiple reflection and absorption of electromagnetic waves (EMWs). The 3D staggered SiCNWs network preforms with a volume fraction of 20% was realized by vacuum filtration method. And then the PyC interphase and SiC matrix were prepared through chemical vapor infiltration (CVI) process, and the densified SiCNWs/SiC ceramic matrix composites were obtained through CVI and precursor impregnation pyrolysis (PIP) process. Methane (CH₄) and trichloromethylsilane (MTS) were selected as gaseous precursor of the PyC and SiC, respectively. With increase of deposited PyC from 0 to 29.5%, the electromagnetic interference (EMI) shielding efficiency (SE) of the porous SiCNWs increases from 9.2 dB to 64.1 dB in 8-12 GHz (X-band). The densified SiCNWs/SiC ceramic matrix composites with a mass gain of about 13% of PyC interphase present an average EMI SE of 37.8 dB in X-band. The achieved EMI shielding properties suggested that the potential application of the SiCNWs/SiC ceramic matrix composites may be a promising new-generation EMI shielding material.

Ultra-high temperature composite ceramic matrix composites ZrC-SiC, ZrB₂-ZrC-SiC and HfB₂-HfC-SiC were fabricated by precursor infiltration and pyrolysis method. The ultra-high temperature ceramic phases in the materials were characterized by submicron/ nanometer uniform dispersion distribution. Ablation behaviors of ZrC-SiC, ZrB₂-ZrC-SiC and HfB₂-HfC-SiC matrix composites under atmospheric plasma and on-ground arc-jet wind tunnel were investigated comparatively. The main factors that affect design for ultra-high temperature composite ceramic matrix composites were summarized. The result shows that, compared with traditional SiC-based composites, ultra-high temperature composite ceramic matrix composites have a solid-liquid two-phase dense oxide film formed in situ on the surface of the composites after ablation. Synergistic effect of the two phases has achieved effects of erosion resistance and oxidation resistance, which plays a very important role in hindering the loss of liquid SiO₂ and greatly improves the ultra-high temperature ablation performance of the materials. On this basis, the important factors that should be considered in the matrix design of ultra-high temperature composite ceramic matrix composites are obtained. The above results have instructional significance for the ultra-high temperature and the limited life application of ceramic matrix composites.

The resistivity characteristics of 2D SiC/SiC composites were studied experimentally. In the oxygen-free environment, the resistivity increases when temperature decreases. With curve fitting, the mapping relationship between resistivity and temperature is established. After oxidation at 1300 ℃ in the air for 20 and 60 h, the conductivity of composites is greatly reduced due to the oxidation of the PyC interface and the SiC matrix. The degree of oxidation was characterized by the content of SiO₂, and the quantitative relationship between resistivity and oxidative damage was obtained. The changes in resistivity and stress with strain are similar. In the linear segment of stress-strain curve, with few matrix cracks the stiffness is almost unchanged, and the resistivity increases slowly. In the non-linear section, the resistivity rate and stiffness increase quickly because crack increases rapidly. They eventually stabilize when the increase in cracks slows down.

Si₃N₄-BN-SiC composites present desirable potential for engineering applications because of their improved mechanical properties and oxidation resistance. In present work, Si₃N₄-BN-SiC composites were successfully fabricated by combustion synthesis using Si, Si₃N₄ diluent, B₄C, and Y₂O₃ as initial powders. BN and SiC were in situ introduced into Si₃N₄ ceramics by the reaction between Si, B₄C, and N₂ gas. The obtained Si₃N₄-BN-SiC composites were composed of elongated β-Si₃N₄ matrix and hollow spherical composites. The formation mechanism of the hollow spherical microstructure was investigated. The results show that the generated SiC and BN particles and glass phase cover on the raw materials, and hollow spherical microstructure is formed when raw particles are depleted. Furthermore, the impacts of B₄C content on the mechanical properties of Si₃N₄-BN-SiC composites were investigated in detail. The in-situ introduction of BN and SiC is beneficial to improving mechanical properties of the composites to some extent. Finally, Si₃N₄-BN-SiC composites with bending strength of 28-144 MPa, fracture toughness of 0.6-2.3 MPa·m ^1/2, Young's modulus of 17.4-54.5 GPa, and porosity of 37.7%-51.8% were obtained for the samples with 0-20% (in mass) B₄C addition.

Effect of particle size of boron carbide raw material on the phase composition, microstructure and properties of reaction bonded boron carbide composites was investigated. It was found that particle gradation can make the powder packing more compact and effectively improve the volume density of green body, decreasing the content of free Si in the composites. Addition of coarse particles can reduce the reaction between B₄C and Si, which can generate SiC phase. When the weight ratio of B₄C powders with different particle sizes (3.5, 14, 28, 45 μm) is 1.5 : 4 : 1.5 : 3, the Vickers hardness, flexure strength, fracture toughness and volume density of the composites are (29±5) GPa, (320±32) MPa, (3.9±0.2) MPa·m^1/2 and 2.51g/cm³, respectively. The retard of reaction between B₄C and Si, and the decrease of free Si content along with the shrinkage of size of Si zone in the composites, are the main reasons for the improvement of the composite mechanical properties.

SiC fibers reinforced SiC ceramic matrix composites were brazed to Hastelloy N alloy using Cu-2.67Ni (mass percentage) alloy. The obtained joints were corroded in FLiNaK molten salt at 800 ℃for 100 h. Microstructure evolution and corrosion behavior of joints were characterized. Alloy elements, e.g. Ni, Cr and Mo, diffuse from Hastelloy N alloy into Cu-Ni joint seam, while the Si element in SiC/SiC composites diffuses into joint seam even the Hastelloy N alloy. Cr element enriches near the interface between SiC composites and joint alloy to form discontinuity interlayer, which acts as the active metal instead of Ni. Higher temperature contributes to both the diffusion process and the erosion of SiC by Ni, and lower temperature would lead to the incomplete fusion of brazing fillers. The diffusion of elements during the brazing changes the composition of the joint seam and Hastelloy N alloy, which caused the deterioration of corrosion resistance of alloy. The selective corrosion of Cr and Si, supported by thermodynamic calculation, results in the corrosion of both joint seam and alloy.

Oxide fiber has good thermostability and oxidation resistance at high temperature, is one of the important candidates for the reinforcements of composites for aerospace field. The tensile property at high temperature is one of the critical properties of oxide fiber used in harsh environment, but the related research about domestic 550-grade fiber is rarely reported. Here the tensile property of domestic 550-grade continuous alumina fiber at high temperature and its room temperature tensile property after heat treatment were studied. Relationships between the tensile strength and the phase transition, the microstructures, as well as their internal mechanism were investegated. The results showed that the fiber multifilament and filaments had relatively high tensile strength with strength retention rate up to 1100 ℃. The poor thermostability of amorphous SiO₂ had obviously adverse effect on the tensile property of the fiber at temperature above 1200 ℃. Importantly, at the critical phase transition temperature (1300 ℃) mullite plase was formed, which could improve the fiber tensile strength at 1250-1400 ℃. This work demonstrated that the tensile strength of SIC550 fiber at 1100 ℃ was close to that of Nextel 720 and CeraFib by considering the effect of different gauge length.