MCMB–SiC composites; new class high-temperature structural materials for aerospace applications

Graphite–silicon carbide (G–SiC), carbon/carbon–silicon carbide (C/C–SiC) and mesocarbon microbeads–silicon carbide (MCMB–SiC) composites were produced using liquid silicon infiltration (LSI) method and their physical and mechanical properties, including density, porosity, flexural strength and ablation resistance were investigated. In comparison with G–SiC and C/C–SiC composites, MCMB–SiC composites have the highest bending strength (210 MPa) and ablation resistance (9.1%). Moreover, scanning electron microscopy (SEM) and optical microscopy (OM) are used to analyze the reacted microstructure, pore morphology and pore distribution of carbon-based matrices. As a result, SiC network reinforcement was formed in situ via a reaction between liquid silicon and carbon. The unreacted carbon and solidified silicon are two phases present in the final microstructure and are characterized by X-ray diffraction (XRD). Based on the results obtained and the low-cost processing of pitch-based materials, the MCMB–SiC composite is a promising candidate for aerospace applications.     

Core–shell spherical graphite@SiC attenuating agent for AlN-based microwave attenuating ceramics with high–efficiency thermal conduction and microwave absorption abilities

A core–shell structured spherical graphite (SG)@SiC attenuating agent with a tunable silicon carbide (SiC) shell thickness was synthesized via in-situ solid-liquid reaction of SG and Si. Then, fully dense 10 wt%SG@SiC/AlN microwave attenuating composite ceramics were prepared through hot-pressing sintering, and the morphology of SG@SiC particle was well maintained. By moderately modulating the thickness of the SiC shell with relatively low complex permittivity and thermal conductivity, an effectively inhibited solid solution of SiC into AlN, weakened dipole and electron polarization, enhanced conduction loss, and an improved impedance matching, thermal conductivity and microwave loss capacity were simultaneously achieved. Thus, the SG@SiC/AlN composite exhibit excellent and impressive thermal conductivity of 63.92 W m⁻¹·K⁻¹ and minimum reflection loss of −34.2 dB. The outstanding performance of SG@SiC/AlN composite indicates that the composite is promising microwave attenuating ceramic with excellent thermal conduction and microwave absorption ability. This work opens up a new core–shell structure strategy for designing and developing a high-efficiency attenuating agent and microwave attenuating ceramic.     

Microstructural characterization of C-SiC-carbon nanotube composite flakes

Composite flakes of carbon, silicon carbide (SiC) and carbon nanotubes were synthesized by spray pyrolysis employing a slurry of ferrocene (Fe(C₅H₅)₂) and SiC powders in xylene at 1000 ± 50 °C. These flakes were characterized for their microstructure and composition by transmission electron microscopy, scanning electron microscopy and high-resolution electron energy loss spectroscopy. These studies suggest the in situ formation of nanotubes and sintering of the SiC nanoparticles to form the freestanding composite flakes.     

Structure and performance of Si3N4/SiC/CNT composite fibres

Herein, a homogeneously distributed and well-orientated ceramic-CNT composite fibre (Si₃N₄/SiC/CNTs) has been prepared using carbon nanotube fibres (CNTFs) premixed with silicon powder, followed by the reaction-bonded sintering process. The SiC layers around the CNT bundles interspersed in the composite are formed during the silicon reaction stage through the contact of silicon and CNTs, and the densification of the ceramic through the further reaction-bonded silicon carbide and nitride. Due to strong interface bonding, the composite fibres exhibit the potential for CNT-based damage sensing with a tensile strength upto 225 Mpa. Furthermore, the high-volume distribution of CNT sresults in a significant enhancement of the electrical and thermal conductivities as well as photoluminescence properties. Our work provides a useful approach for thefabrication of multifunctional fibres for imaging, engineering, and other complex applications.     

Development of multi-phase B–Si–C ceramic composite by reaction sintering

A dense ceramic composite in the system B–C–Si has been synthesized by the reaction sintering technique based on infiltration of silicon melt at 1550 °C under vacuum into a porous compact made of boron carbide and petroleum coke powder. The final material is around 99% dense and microstructurally contains B₄C, SiC and Si as the major phases. The B₄C-phase reacted at its interface with Si-phase, which is explained in terms of dissolution of Si in the carbide phase.     

New Approach for Macro Porous RB‐SiC Derived from SiC/Novolac‐type Phenolic Composite

A novel fabrication route to make macroporous silicon carbide (SiC) has been proposed in this study. The route is composed of the following two steps: the fabrication of porous α‐SiC/novolac‐type phenolic composite using hexamethylenetetramine (HMT) as a curing/blowing agent for the novolac monomer and a conventional reaction‐bonded (RB) sintering of the composite. The α‐SiC/novolac‐type phenolic composite was carbonized at 800°C for 2 h in N₂ gas and then reacted with the molten silicon at 1450°C for 30 min under vacuum, resulting in the macroporous RB‐SiC with an open porosity of 48% and relatively large pore size of ~110 μm. The compressive strength of the macroporous RB‐SiC was 113 MPa, which is relatively high compared to those reported for macroporous SiC of equivalent porosities and pore sizes.     

Multidimensional nano-graphite constructed TiB2–SiC–B4C composite ceramics with the integration of mechanical and electromagnetic properties

The goal of this study is to create structure-functional integrated ceramic matrix composites with high structural strength and electromagnetic absorbing properties. The multidimensional nano-graphite (1-Dimensional rod-like nano-graphite, 0-Dimensional dispersive nano-graphite, and 2-Dimensional lamellar nano-graphite) were employed to construct TiB₂–SiC–B₄C composites via high-energy ball milling, vacuum filtration, and reactive SPS sintering. The microstructure of multidimensional nano-graphite was investigated using XRD and HRTEM and determined to be a crystal-amorphous coexisting. Furthermore, solid solution reaction and interfacial evolution are confirmed as the primary influence on the microstructure of TiB₂–SiC–B₄C composite. A significant improvement occurs on the flexural strength (647.6 MPa) and bending toughness (5.1 MPa m^1/2). Meanwhile, the multi-dimensional nano-graphite gives the TiB₂–SiC–B₄C composite the loss ability of electromagnetic waves, and the matching thickness of the 10 vol% sample is 2.4 mm and the absorption range is 10.4–11.3 GHz.     

Microstructural development during spark plasma sintering of ZrB2–SiC–Ti composite

The present study investigates the effect of Ti addition on the microstructure development and phase evolution during spark plasma sintering of ZrB₂–SiC ceramic composite. A ZrB₂–20?vol% SiC sample with 15?wt% Ti was prepared by high-energy milling and spark plasma sintering at 2000?°C for 7?min under 50?MPa. The X-ray diffraction test, microstructural studies and thermodynamic assessments indicated the in-situ formation of several compounds due to the chemical reactions of Ti with ZrB₂ and SiC. The Ti additive was completely consumed during the sintering process and converted to the ceramic compounds of TiC, TiB and TiSi₂. In addition, another refractory phase of ZrC was also formed as a result of sidelong reaction of ZrB₂ and SiC with the Ti additive.     

Improving the sinterability of ZrC–SiC composite powders by Mg addition

High-density ZrC–SiC composite ceramics are typically sintered under demanding conditions, specifically, high sintering pressures and high temperatures. However, the need for such conditions can be alleviated by the use of ZrC–SiC composite nanoparticles with a high sintering activity. In the present study, core-shell-structured hybrid ZrC–SiC composite nanoparticles were synthesised with the addition of Mg by using a sol-gel process combined with in-situ carbothermal reduction reactions. The synthesis route, characterisation, and sintering mechanism were investigated in detail. It was found that the addition of MgCl₂ to the precursors of ZrC–SiC can not only strengthen the network structure of ZrC–SiC gel but also lead to the formation of an amorphous Mg–Si–O oxide coating on the nanoparticle surfaces, which enhances the sinterability of ZrC–SiC nanoparticles. As a result, a compact ZrC–SiC composite ceramic with a higher relative density (up to 91.3%) than the contrast sample was successfully prepared by pressure-free sintering at 1700 °C.     

Combination of direct ink writing and reaction bonded for rapid fabrication of SiCw/SiC composites

Silicon carbide ceramic matrix composites are widely used in aerospace field due to their advantages of high temperature resistance, high strength and corrosion resistance. However, its application is greatly limited because of the difficulty in preparing complex shape structures by traditional machining methods. Here, a new strategy for preparing SiC_w/SiC complex structure by combining direct ink writing with reaction bonding is proposed. A water-based slurry consisting of silicon carbide, carbon powder and silicon carbide whisker was developed. The influence laws of C content and SiC_w content in slurry on sintering properties of direct-written samples were studied. The reaction bonding mechanism and whisker reinforcing and toughening mechanism were analyzed by means of microstructure and phase composition. The results show that the slurry exhibits shear thinning behavior with stress yield point, and its flow behavior and plasticity meet the requirements of direct writing. When the carbon content is 6.4 wt%, the maximum flexural strength is 239.3 MPa. When 15 wt% SiC_w was added, the flexural strength of the composite reached 301.6 MPa, and when 20 wt% SiC_w was added, the fracture toughness of the composite reached 4.02 MPa m^1/2, which was increased by 26% and 18% compared with single-phase SiC, respectively. The reinforcing and toughening mechanisms of the whiskers mainly include whisker pullout, crack deflection and whisker bridging. After direct ink writing and reaction bonded, the whole process shows good near net forming ability. 3D printed SiC_w/SiC composites have great application prospects in aerospace field.     

Sintering behavior of ZrB2–SiC composites doped with Si3N4: A fractographical approach

ZrB₂–SiC composite ceramics were doped with 0, 1, 3 and 5 wt% Si₃N₄ plus 1.6 wt% carbon (pyrolized phenolic resin) as sintering aids and fabricated by hot pressing process under a relatively low pressure of 10 MPa at 1900 °C for 2 h. For a comparative study, similar ceramic compositions were also prepared by pressureless sintering route in the same processing conditions, with no applied external pressure. The effect of silicon nitride dopant on the microstructural evolution and sintering process of such ceramic composites was investigated by a fractographical approach as well as a thermodynamical analysis. The relative density increased by the addition of Si₃N₄ in hot pressed samples as a fully dense composite was achieved by adding 5 wt% silicon nitride. A reverse trend was observed in pressureless sintered composites and the relative density values decreased by further addition of Si₃N₄, due to the formation of gaseous products which resulted in the entrapment of more porosities in the final structure. The formation of ZrC phases in pressureless sintered samples and layered BN structures in hot pressed ceramics was detected by HRXRD method and discussed by fractographical SEM-EDS as well as thermodynamical analyses.     

Pressureless sintering of silicon carbide ceramics containing zirconium diboride

SiC-5 wt.% ZrB₂ composite ceramics with 10 wt.% Al₂O₃ and Y₂O₃ as sintering aids were prepared by presureless liquid-phase sintering at temperature ranging from 1850 to 1950 °C. The effect of sintering temperature on phase composition, sintering behavior, microstructure and mechanical properties of SiC/ZrB₂ ceramic was investigated. Main phases of SiC/ZrB₂ composite ceramics are all 6H-SiC, 4H-SiC, ZrB₂ and YAG. The grain size, densification and mechanical properties of the composite ceramic all increase with the increase of sintering temperatures. The values of flexural strength, hardness and fracture toughness were 565.70 MPa, 19.94 GPa and 6.68 MPa m^1/2 at 1950 °C, respectively. The addition of ZrB₂ proves to enhance the properties of SiC ceramic by crack deflection and bridging.     

SiC复相陶瓷内加热器套管的制备及性能

用SiC粉作内加热器套管基本原料,采用反应烧结及常压烧结工艺制备具有复层结构的SiC复相陶瓷套管。套管内层为高致密SiC/Si复合材料,外层为SiC/C/SiC复相陶瓷。通过实验模拟内加热器服役环境,将套管内置热源在750℃熔融铝液中加热360 h测试其使用性能,研究外部复杂熔液腐蚀环境及内部强氧化气氛对SiC复相陶瓷套管的密度、抗弯强度、物相组成及显微形貌的影响。结果表明：复层结构SiC套管在模拟实验中展现出良好的抗热震及抗熔液浸蚀能力;内层材料由于高致密性具有良好的抗氧化性能;外层材料浸入熔体部分强度损伤小于10%,液面以上部分残余强度达到48MPa。SiC复相陶瓷套管在有色金属熔炼行业展现出良好的应用前景。     

Additive manufacturing of high mechanical strength continuous Cf/SiC composites using a 3D extrusion technique and polycarbosilane-coated carbon fibers

A novel additive manufacturing approach is herein reported for manufacturing high mechanical strength continuous carbon fiber-reinforced silicon carbide (C_f/SiC) composite materials. Continuous carbon fibers were coated with polycarbosilane (PCS) using a colloidal evaporative deposition process and then coextruded with high solid content SiC ink. The zeta potential of the SiC ink was adjusted to optimize the printing ability of the suspension. During sintering, small SiC grains and whiskers were generated in the gaps in and around the PCS-coated carbon fibers, which led to the improved flexural strength and density of the composites. Meanwhile, the PCS coating on the surface of the carbon fibers prevented the carbon fibers from reacting with SiO gas generated by reactions between the SiC matrix and SiO₂ and sintering additives (Al₂O₃ and Y₂O₃), effectively preserving the structural integrity of the carbon fibers. Compared to the SiC specimens containing uncoated carbon fibers, the density of the specimens fabricated with coated carbon fibers was increased from 2.51 to 2.85 g/cm³, and the strength was increased from 190 to 232 MPa.     

High fracture toughness of HfC through nano-scale templating and novel sintering aids

An approach to improve the sintering ability and the fracture toughness of hafnium carbide (HfC) ceramic by designing a unique composite structure is reported. The uniform and ultra-fine HfC nanoparticles (~300 nm) are synthesized at 1450°C by vacuum carbonization reaction with the glucose-derived hydrothermal precursor as a carbon source and template. HfC ceramic sintered with a SiCN sintering aid of 15 vol. % possesses a beneficial three-dimensional network microstructure composed of inter-penetrating phases of carbon, SiC and HfC with varied stoichiometry at multi-length scales. The obtained HfC exhibits a higher fracture toughness of 5.5 MPa m^1/2, which can be attributed to the unique composite structure able to promote stress releases in the crack tip and enhance the resistance to crack propagation.     

Preparation of silicon carbide–silicon nitride composite foams from pre-ceramic polymers

A new method of forming silicon carbide–silicon nitride composite foams is presented. These are prepared by immersing a polyurethane foam in a polysilane precursor solution mixed with Si₃N₄ powder to form a pre-foam followed by heating it in nitrogen at >900°C. X-ray diffraction patterns indicate that a SiC–Si₃N₄ composite was formed after sintering the ceramic foam at >1500°C. Micrographs show that most of these foams have well-defined open-cell structures and macro-defect free struts. The shrinkage is reduced considerably due to the addition of Si₃N₄ particles.     

Preparation and mechanical behaviors of SiOC-modified carbon-bonded carbon fiber composite with in-situ growth of three-dimensional SiC nanowires

Carbon-bonded carbon fiber (CBCF) composites are novel and important high-temperature insulation materials owing to their light weight, low thermal conductivity and high fracture tolerance. To further improve the mechanical property of CBCF composite, we propose a three-dimensional (3D) SiC nanowires structure, which is in situ grown on a CBCF matrix via directly annealing silicon oxycarbide (SiOC) ceramic precursor. The synthesized multiscale reinforcements including microscale SiOC ceramics and nanoscale SiC nanowires are mainly attributed to the initial phase separation of SiOC phase and subsequent solid-phase reaction of SiO and C phases. Compared to SiOC/CBCF composite, the resulting 3D SiC nanowires/SiOC/CBCF hybrid structure exhibited high flexural/tensile strength and fractured strain due to the pull-out and bridging behavior of SiC nanowires. This one-step process supplied a feasible way to synthesize 3D SiC nanowires to reinforce and toughen SiOC-modified CBCF composite.     

SiC-based ceramics with remarkable electrical conductivity prepared by ultrafast high-temperature sintering

SiC-based ceramics with high electrical conductivity are applied widely as electrode materials and semiconductor materials. In this study, a SiC-based ceramic with relative density of 96% was prepared by ultrafast high-temperature sintering (UHS) at 2000 ℃ (with a heating rate of 1000 ℃/min) for 40 s. The resistivity of as UHS-ed SiC-based ceramic was 1/15 of that prepared by the pressureless sintering. We found that the components of as-sintered body (SiC, Si and Y₃Si₅) by UHS were different from those (SiC and YAG) prepared by the pressureless sintering. The reason for the remarkable increase of the electrical conductivity of UHS-ed body was that the Si with higher electrical conductivity than SiC had emerged. Besides, the reaction mechanism was proposed and the unusual composition of the SiC-based ceramic sintered by UHS may also provide new reference for the application of SiC in specific fields.     

Boron-modified phenol formaldehyde resin-based self-healing matrix for Cf/SiBOC composites

C_f/SiBOC was fabricated from 2D carbon fabric as reinforcement and slurry-containing boron-modified phenol formaldehyde (BPF) resin with silicon as matrix resin using reaction-bonded silicon carbide method. The processing involves synthesis of (BPF) resin by reacting various amount of boric acid with phenol formaldehyde resin, polymer to ceramic transformation at 1450°C under argon atmosphere, with and without silicon, thermal transformation of the polymer matrix composite into a ceramic matrix composite and evaluation of isothermal oxidation for ceramics and its composites at 1000, 1250 and 1500°C. The ceramic studies, confirmed the formation of B₄C, SiC and SiB₄ (SiBOC) mixed phase and the role of boron as a catalyst for graphitisation of free carbon present in the ceramic. Oxidation of C_f/SiBOC composite at various temperatures leads to the formation of borosilicate glass which heals the cracks, hindering the inwards diffusion of oxygen.     

Densification,solid solution formation,and microstructural investigation of reactive pressureless sintered HfB2-TiB2-SiC-MoSi2 quadruplet composite

Dense HfB₂-TiB₂-SiC-MoSi₂ quadruplet composite was produced by a reactive pressureless sintering method at 2050 °C for 5 h. The relative density was improved and reached 98% by in situ formation of SiC and MoSi₂ phases. Microstructural studies proved that SiC and MoSi₂ second phases were mostly formed during the sintering process. Moreover, the Sintering mechanism of the composite was investigated by HSC software. TiB₂ co-matrix was improved the sinterability of the composite by the formation of (Hf,Ti,Mo)–B and (Hf,Ti,Mo)–C solid solutions.Mechanical properties such as Vickers hardness (23.2 GPa), fracture toughness (5.4 MPa m^1/2), and elastic modulus (430 GPa) were effectively enhanced by tailoring the composite.