Collection of Composite Ceramics(202409)
Owing to the high strength/toughness and excellent anti-oxidation ability, continuous fiber reinforced ceramic matrix composites have become the preferred candidates for high temperature structural materials in aerospace field. Reactive melt infiltration can achieve the large-scale, short-cycle and low-cost production of ceramic matrix composites, which has been widely considered to be one of the most promising technologies from a commercial perspective. However, the mechanical and anti-oxidation/ablation properties of obtained composites prepared by conventional reactive melt infiltration are not satisfactory due to the existence of residual carbon and corroded fibers. In order to address the problems, relevant researchers constructed porous carbon matrix to replace conventional densified structure to promote its ceramic transformation and the consumption of reactive melt, thus achieving the improved performance of ceramic matrix composites. This paper reviewed the research progress about the preparation of SiC ceramics, SiC/SiC composites, C/SiC composites, and ultra-high temperature ceramic matrix composites by porous carbon ceramization strategy. Besides, the superiority of the method was verified compared to conventional reactive melt infiltration. The development of preparation methods for porous carbon matrix was also summarized. Finally, in term of the requirements of basic theory and technology for advanced ceramic matrix composites, the prospect for the future development of improved reactive melt infiltration to prepared ceramic matrix composites was discussed.
Continuous SiC fiber-reinforced SiC (SiCf/SiC) composites possess high specific strength, high specific modulus, high-temperature resistance, and radiation resistance, making them suitable for applications in hot-end parts of advanced aero-engines and claddings of nuclear reactors. SiCf/SiC composites are composed of fibers, interfaces and matrix, endowing them with complex multi-scale structural characteristics. These composites are designed to serve in harsh environment, and their damage and failure process are complex. A profound understanding and accurate analysis of damage and failure mechanisms of SiCf/SiC composites under service environments are of great significance for the optimized design of materials and the reliable service of components. Traditional “post-mortem analysis” methods are incapable of acquiring data during the damage and failure process of materials under complex service environments. Therefore, there is an urgent need to develop in-situ characterization techniques for composites under high-temperature service environments. This paper reviewed the principles, advantages, and limitations of in-situ monitoring methods based on scanning electron microscopy, digital image correlation, micro computational tomography, acoustic emission, and electrical resistance. It focused on the latest research progress in the high-temperature mechanical characterization of SiCf/SiC composites using various in-situ monitoring methods and combinations thereof. It summarized the challenges in the in-situ monitoring technologies of SiCf/SiC composites under high-temperature environments and provided a preliminary outlook on the future development directions, such as the combined use of multiple in-situ monitoring techniques, new detection technologies like terahertz radiation, and in-situ damage monitoring methods for complex components.
High-entropy boride ceramics (HEBs) consisting of four or more principle metallic elements rapidly develop in recent years due to their outstanding unique physical properties and excellent elevated temperature properties, showing extraordinary promise as potential thermal protection materials applied in extreme environments. However, on the basis of unclear role of each element on their oxidation reaction, HEBs are generally difficult to densify because of their low self-diffusion coefficients and possible sluggish diffusion effect, resulting in limited mechanical properties and low oxidation resistance. In this work, a novel type of HEBs, (Ti0.25Zr0.25Hf0.25Ta0.25)B2-B4C composites, were prepared by boro/carbothermal reduction method combined with hot-pressing sintering at 1900 ℃. The effect of B4C at the volume fractions ranging from 10% to 30% on the mechanical properties and oxidation resistance of the composites was systematically investigated. Microstructure analyses indicate that homogenously distributed B4C can suppress grain growth of the HEBs matrix and promote toughening mechanisms such as crack deflection and crack branching, consequently resulting in strengthening and toughening composites. When the volume fraction of B4C is 20%, the as-prepared composite shows a high relative density (96.1%) and good mechanical properties with Vickers hardness of (24.6±1.1) GPa, flexural strength of (570.0±27.6) MPa and fracture toughness of (5.58±0.36) MPa·m1/2. In addition, exploration on the oxidation resistance of (Ti0.25Zr0.25Hf0.25Ta0.25)B2-B4C composites at temperatures ranging from 800 ℃ to 1400 ℃ shows that excellent oxidation resistance occurs at the chosen temperatures due to the formation of a dense and continuous oxidation scale, which acts as a barrier layer preventing oxygen inward diffusion. The main compositions of the oxide scale are TiOx, (Zr, Hf)O2 oxides and B2O3 at 800 ℃, while multicomponent oxidation products of (Zr, Hf, Ta)Ox, (Zr, Hf)O2 and TiTaO4 are formed in the oxide scale at 1100 ℃. As the temperature increased to 1400 ℃, thickness of the oxide layer significantly increases due to their volatilization of B2O3, while continuous B2O3 glassy phase plays a crucial role in the oxidation process of HEBs. When the B4C volume fraction not less than 20%, TiTa2O7 and TiO2 which were embedded in B2O3 glass, could effectively insulate inward oxygen and interfacial oxide thickness and enhance oxidation resistance of the composites. In summary, the primary work can be used as a reference to the researches relating to optimizing mechanical properties and oxidation resistance for HEBs.
In response to the urgent demand for ultra-high temperature ceramic matrix composites with integrated thermal protection and load-bearing capabilities for high-speed aircrafts, this study prepared stable ceramic slurry from submicron HfC ceramic powder, and utilized the slurry pressure impregnation-assisted precursor infiltration pyrolysis (PIP) process to fabricate C/HfC-SiC composites with uniformly distributed HfC matrix to overcome the shortcomings of the existing reaction-derived HfC precursor, such as high cost, low efficiency, and poor densification effect. The influence of HfC content on the microstructure, mechanical properties, and ablation resistance of composites was investigated. Results showed that the composites had density of 2.20-2.58 g·cm-3 and open porosity of approximately 5% when the actual volume fraction of HfC was in range of 13.1%-20.3%. Utilizing a single layer of carbon cloth to impregnate the ceramic slurry with pressure, HfC particles were able to disperse into the interior of the fiber bundle and distributed relatively evenly in the composites. Increasing the HfC content resulted in reducted fiber content, and decreased mechanical properties of composites. Specifically, when HfC volume fraction was 20.3%, the composites exhibited density, tensile strength and fracture toughness of 2.58 g·cm-3, 147 MPa and 9.3 MPa·m1/2, respectively. Following 60 s of ablation under an oxygen acetylene flame, the composites demonstrated linear ablation rate of 0.0062 mm/s and mass ablation rate of 0.005 g/s. The molten phase HfxSiyOz formed during the ablation process could effectively cover the composites surface and provide protection.
High-performance structural material components are widely researched because of their curcial applications in aerospace, transportation and automotive, electronic information, metallurgy, and other fields. Traditional methods for enhancing the overall performance of structural material components mainly include improving intrinsic material properties and optimizing structural composite design. However, research on enhancing the intrinsic mechanical properties of single structural materials is reaching its limits. This study aims to explore a new paradigm for the development of high-performance structural composites by proposing the concept of periodic ordered structural materials and preparing the structural composites with improved overall properties through integrated sintering. The TiB-Ti functional unit with high hardness of ceramics and strong toughness of metal was structured through periodical sequencing, and high-performance TiB-Ti structural composites with different periodical sequencing modes were designed and prepared. On this basis, the mechanical properties of these structures were investigated, and their fracture modes were analyzed to understand how different ordering modes affect the overall properties of the materials. The results show that periodic ordered structure can improve the overall performance of materials by altering their macroscopic fracture modes and stress distribution properties. This new paradigm of research provides valuable insights and guidance for the structural design and performance breakthrough of other structural composites. Future research may focus on the exploration of the complexity of the periodic ordered structure modes, identifying potential application scenarios for these materials, and conducting additional performance testing studies.
Continuous fiber-reinforced ceramic matrix composites are widely used for high temperature components like aerospace engines due to their superior performance at elevated temperature. However, these materials are susceptible to damage from foreign object debris during service, which has become a significant concern. To investigate the impact damage characteristics of 2D-SiC/SiC composites, this study utilized a light gas gun to subject specimens prepared using chemical vapor infiltration (CVI) technology to ballistic impact. The impact processes were recorded with a high-speed camera, while the surface and internal structures of foreign object damage (FOD) were examined by optical microscopy and computed tomography (CT). This investigation revealed that conical cracks, interlaminar delamination, fiber fracture, and matrix collapse were the primary manifestations of high-speed impact damage. Damage characterization indicated that backside damage and edge delamination damage were caused by reflected tensile waves. As the impact velocity increased, the combined action of the projectile and tensile waves resulted in specimen penetration and weakening of edge delamination damage. Quasi-static tensile tests on high-speed impact specimens elucidated the relationship between residual mechanical properties and impact velocity, as well as projectile diameter. The results showed that residual tensile strength was a crucial parameter indicative of the severity of impact damage. Additionally, digital image correlation (DIC) was employed to determine strain distribution during tensile processes. By integrating residual tensile strength after impact with different projectile diameters and impact velocities, the study further explored the effect of varied parameters on impact damage. The research findings highlighted that projectile diameter as the primary factor influencing the extent of high-speed impact damage.
MAX phase ceramics, with their mixed covalent-metallic-ionic atomic bonds, can uniquely combine the advantages of both metals and ceramics, offering a series of distinctive characteristics. The particular layered atomic structure further endows them with decent fracture toughness, good damping capacity, and self-lubricating property. As such, MAX phase ceramics are more appealing to serve as reinforcements for metal matrix composites (MMCs) than conventional ceramic materials. Here, we foused on the development. To date, fabrication of MMCs reinforced by MAX phase ceramics still involves the use of stir casting, powder metallurgy, and melt infiltration techniques. The obtained composites made by different methods may display distinct differences in their structural characteristics, show notable enhancement in strength, hardness, and stiffness as compared to their metal matrices, and exhibit good wear resistance, high electrical conductivity and remarkable arc erosion resistance. Moreover, ultrafine MAX phase platelets can be preferentially oriented and aligned, e.g., by using vacuum filtration or ice templating techniques. By infiltrating metal melt into partially sintered porous ceramic scaffolds, bioinspired composites with nacre-like architectures can be obtained, thereby affording further improvement in strength and fracture toughness. Sufficient combinations of mechanical and functional properties enable the MMCs reinforced by MAX phase ceramics promising for a variety of applications, such as load-bearing structures, electrical contact materials. These composites can offer enhanced strength, stiffness, and wear resistance, making them ideal candidates for these applications.
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 (KIC value) peaked at 5.67 MPa·m1/2, 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·m1/2. 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 SiCf/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 (SiCf/SiC) is a key material for the advanced aero-engines. It is required to possess excellent high-temperature creep resistance for SiCf/SiC to meet the long-term service lifetime of the aero-engines. Here, tensile creep behaviors of a plain woven Cansas-II SiCf/SiC (2D-SiCf/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-SiCf/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 (σPLS). 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 σPLS. 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 (SiCf/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 SiCf/SiC composites for nuclear application, and numerous important achievements have been made. This paper summarized and analysed some critical directions of SiCf/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 SiCf/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·m1/2 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.