Synthesis of Si3N4/SiC reaction-bonded SiC refractories: The effects of Si/C molar ratio on microstructure and properties

Si₃N₄/SiC reaction-bonded SiC refractories have been fabricated on the basis of the microstructure design concept by introducing a binary-phases binding system. The influence of Si/C molar ratio on phase transformation, microstructure and mechanical properties was studied systematically. Thermodynamic analysis result proved the microstructure design was feasible under 0.03 MPa pressure of N₂ and the selected sintering temperature. In-situ grown SiC nano-whiskers/granule and lamellar Si₃N₄ were both observed in the matrix. The specimen with 2:1 Si/C molar ratio possessed highest cold modulus of rupture (28.27 MPa) but showed low toughness. The strength and toughness of such materials were controlled by two main factors, such as SiC grain boundary binding morphology and in situ grown of SiC in the matrix. The different mechanisms occurred predominantly to meet diverse practical cases and caused to various mechanical properties of final products. The corresponding strengthening and toughening mechanisms were explained in this paper.     

Nickel-catalyzed preparation of self-bonded SiC refractories with improved microstructure and properties

3C-SiC was synthesized after 3?h firing of Si and expanded graphite in flowing Ar at 1300?°C with in-situ formed Ni nanoparticle (NP) catalyst. First-principles calculations suggest that Ni catalyst accelerated the formation of SiC via weakening the bonds in adsorbed CC bond, and CO and SiO molecules. Apart from this, Ni NP catalyst facilitated the epitaxial growth of SiC nanowires. Based on these findings, self-bonded SiC refractories were prepared by using black SiC grain, expanded graphite and Si powders as raw materials and Ni NP as a catalyst. Large amounts of SiC nanowires were catalytically formed in the fired refractories specimens, which resulted in significant improvements in both mechanical strength (MOR of 32.2?MPa at 1400?°C) and thermal shock resistance. The catalytic formation method investigated in this work could be readily modified and extended to develop many other types of high performance refractories.     

Electrical properties of biomorphic SiC ceramics and SiC/Si composites fabricated from medium density fiberboard

A study has been made of the dependences of the electrical resistivity and the Hall coefficient on the temperature in the range 1.8-1300 K and on magnetic fields of up to 28 kOe for the biomorphic SiC/Si (MDF-SiC/Si) composite and biomorphic porous SiC (MDF-SiC) based upon artificial cellulosic precursor (MDF - medium density fiberboards). It has been shown that electric transport in MDF-SiC is effected by carriers of n-type with a high concentration of ∼10²⁰ cm⁻³ and a low mobility of ∼0.4 cm² V⁻¹ s⁻¹. The specific features in the conductivity of MDF-SiC are explained by quantum effects arising in disordered systems and requiring quantum corrections to conductivity. The TEM studies confirmed the presence of disordering structural features (nanocrystalline regions) in MDF-SiC. The conductivity of MDF-SiC/Si composite originates primarily from Si component in the temperature range 1.8-500 K and since ∼500 to 600 K the contribution of MDF-SiC matrix becomes dominant.     

离心场强化晶硅切割废料Si/SiC分离过程油水分相

对切割料中Si和SiC的高效分离进行了研究,利用晶硅切割废料中Si和SiC表面性质的差异,向浆料中加入柴油并充分乳化,使SiC吸附在油滴上实现Si/SiC分离,对乳化后的浆料施加离心力强化油水分相,调节浆料pH值改变颗粒表面Zeta电位,调控乳化后的油滴大小,研究了Si/SiC分离效果、分相时间与浆料pH的关系及附有SiC的油滴表观密度与油滴直径的关系,对乳化后的浆料分别施加超重力系数为10, 50, 100, 150和200的离心力,考察了离心时间2 min时的分相效果和Si/SiC分离效果。结果表明,常重力场中,油滴尺寸越小,分相时间越长,但SiC去除效果变好,pH=7时,水相SiC含量为4.23wt%。油滴直径小于64 ?m时,油滴在浆料中不可上浮。离心场中,超重力系数为100, pH=7时,水相中SiC含量为5.47wt%,分相时间由460 min缩短为2 min。通过对离心场中SiC的受力分析解析了离心场中SiC在油滴表面的赋存状态,证实离心场作用下,SiC沿油滴表面向离心力方向移动使油滴对SiC的吸附力减小。     

Mechanical properties and microstructure evolution of KD-SA SiCf/BN/SiC CMCs oxidized at different temperatures

Silicon carbide fiber-reinforced SiC ceramic matrix composites (SiC_f/SiC CMCs) based on a domestic KD-SA SiC fiber were exposed to a wet oxygen atmosphere for 135 h at 800, 1100, and 1300°C. The evolution of the microstructure and mechanical properties of SiC_f/SiC CMCs have been systematically investigated following oxidation. For weight change, CMC-1300 showed the greatest gain (0.394%), followed by CMC-1100 (0.356%) and CMC-800 (0.149%). The volatilization of boron oxide (B₂O₃) combined with the slight oxidation of the SiC matrix at 800°C caused crack deflection and fiber pull-out. The complete dissipation of the interphase could be found when the oxidation temperature increases to 1100°C, generated a fracture surface with brittle fracture characteristics. At 1300°C, crystalline SiO₂ hindered oxygen diffusion, with evidence of fiber pull-out. Based on thermodynamic calculations and microscopic observations, we propose a mechanism to explain the thermal degradation of SiC_f/SiC CMCs. This work offers valuable guidance for the fabrication of SiC_f/SiC CMCs that are suitable for high-temperature applications.     

Si/SiC coated Cf/SiC composites via tape casting and reaction bonding: The effect of carbon content

In this paper, a novel surface modification method for C_f/SiC composites is proposed. Si/SiC coating on C_f/SiC composites is prepared by tape casting and reaction bonding method. The effects of carbon content on the rheological property of the slurries along with the microstructure of the sintered coatings are investigated. The best result has been obtained by infiltrating liquid silicon into a porous green tape with a carbon density of 0.84 g/cm³. In addition, the effect of sintering parameters on the phase composition of the coatings is studied. Dense Si/SiC coating with high density as well as strong bonding onto the substrate is obtained. This Si/SiC coating exhibits an excellent mechanical property with HV hardness of 16.29±0.53 GPa and fracture toughness of 3.01±0.32 MPa m^1/2. Fine surface with roughness (RMS) as low as 2.164 nm is achieved after precision grinding and polishing. This study inspires a novel and effective surface modification method for C_f/SiC composites.     

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.     

Effect of solid loading and carbon additive on microstructure and mechanical properties of 3D-printed SiC ceramic

3D-printed SiC ceramics were prepared by direct ink writing and solid-phase sintering. The effects of sintering temperature, solid loading, and carbon additive on microstructures and mechanical properties of 3D-printed SiC parts were investigated. It was found that the sintering temperature affected the evolution of the microstructure and mechanical properties of the sintered SiC parts. A high solid loading promoted the densification and mechanical properties of the sintered SiC parts. However, the solid loading exceeded 40 vol.%, which decreased the density and mechanical properties of the samples. The carbon additives could improve the densification of the SiC parts and enhance their mechanical properties. When the sucrose content increased from 0 to 8 wt.%, the open porosity of the SiC part decreased from 26.12% to 3.15%, whereas the flexural strength increased 2.19 times. Using the optimized components and process parameters, the high-performance 3D-printed SiC parts were achieved.     

Environment-oriented low-cost Al2O3 ceramics with hierarchical pore structure fabricated from SiC solid waste

Environment-oriented low-cost Al₂O₃ reticulated porous ceramics with hierarchical pore structure were fabricated by the polymer sponge replica method combined with vacuum infiltration methods, using Al₂O₃ powders and SiC solid waste (SCSW) as raw material and a pore-forming agent. The effects of SCSW addition amount on mechanical properties, microstructure and pore size of Ceramics were investigated. The results showed that the thermal shock resistance of specimens increased gradually with addition of SCSW, however, the median pore diameter increased firstly and then decreased, due to the generation of mullite and liquid phase. After calcination, the residual stress was generated within the coating layer because of the difference in the thermal expansion coefficients of ceramic matrix and coating layer, which could improve the properties of Ceramics by deflecting and bifurcating crack growth path. The results showed that the best dosage of SCSW was 30 wt%.     

Synthesis of Al2O3-SiC powder from electroceramics waste and its application in low-carbon MgO-C refractories

Composite additives are an efficient means to improve the high-temperature stability and slag resistance of low-carbon MgO-C refractories. In this work, Al₂O₃-SiC powder was firstly synthesized from electroceramics waste by carbon embedded method at 1500°C, 1550°C, and 1600°C for 4 h, and then the as-synthesized Al₂O₃-SiC powder was used as an additive to low-carbon MgO-C refractories. The effects of its addition amounts of 0, 2.5 wt.%, 5.0 wt.%, and 7.5 wt.% on the properties of the refractories were investigated in detail. It was found that increasing the heat treatment temperature is beneficial to the phase conversion of mullite and quartz to alumina and silicon carbide in the electroceramics waste. Furthermore, the addition of Al₂O₃-SiC powder effectively improves the performance of low-carbon MgO-C samples, and the formation of spinel dense layer and high-viscosity isolation layer is the internal reason for the improvement of the oxidation resistance and slag resistance of low-carbon MgO-C samples. This work provides ideas for the reuse of electroceramics waste and presents an alternative strategy for the performance optimization of low-carbon MgO-C refractories.     

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.     

Effect of Ti–Si–Fe alloys on microstructure and properties of nitride/oxynitride bonded SiC ceramics sintered under CO/N2 atmosphere

Ti–Si–Fe alloys extracted from high-titanium blast furnace slag were utilized to replace part of the silicon powders, and then nitride/oxynitride bonded SiC ceramics were prepared by reactive sintering in graphite bed. Ti–Si–Fe alloys could react with CO/N₂ at a low temperature (1200 °C), and the addition of Ti–Si–Fe alloys could reduce the nitriding temperature of Si. Density functional theory calculations suggested that Ti–Si–Fe alloys enhanced reaction activity via weakening the strength of CO and NN bonds. The regional equilibrium phase diagrams of Si–C–N–O and Ti–Si–C–N–O under CO/N₂ atmosphere were calculated by thermodynamics. The change of whiskers morphology was observed by scanning electron microscope. Furthermore, the bulk density, the cold modulus of rupture, and the cold compressive strength improved with Ti–Si–Fe alloys content. The results showed that the addition of Ti–Si–Fe alloys not only significantly promoted nitriding of Si and formation of Si₃N₄ whiskers, but also improved the mechanical properties of the samples.     

Microstructure and properties of dense Tyranno-ZMI SiC/SiC containing Ti3Si(Al)C2 with plastic deformation toughening mechanism

In this paper, Ti₃Si(Al)C₂ was introduced into dense SiC/SiC to improve the mechanical and electromagnetic interference (EMI) shielding properties. In order to reveal the effect of Ti₃Si(Al)C₂, dense SiC/SiC-Ti₃Si(Al)C₂ and dense SiC/SiC without Ti₃Si(Al)C₂ were fabricated. Owing to the plastic deformation toughening mechanism of Ti₃Si(Al)C₂, SiC/SiC-Ti₃Si(Al)C₂ performs a new damage mode characterized by matrix/matrix (m/m) debonding. High interfacial shear strength (IFSS) due to large thermal residual stress (TRS) is weakened by m/m debonding. This new mode also brings high effective volume fraction of loading fibers and long path of crack propagation. Hence SiC/SiC-Ti₃Si(Al)C₂ exhibits higher flexural strength (503 MPa) and fracture toughness (23.7 MPa · m^1/2) than the dense SiC/SiC without Ti₃Si(Al)C₂. In addition, dense SiC/SiC-Ti₃Si(Al)C₂ shows excellent electromagnetic interference shielding effectiveness (EMI SE, 43.0 dB) in X-band, revealing great potential as thermo-structural and functional material.     

Effects of Fe addition on the mechanical and thermo-mechanical properties of SiC/FeSi2/Si composites produced via reactive infiltration

Remaining silicon in SiC-based materials produced via reactive infiltration limits their use in high-temperature applications due to the poor mechanical properties of silicon: low fracture toughness, extreme fragility and creep phenomena above 1000 °C. In this paper SiC–FeSi₂ composites are fabricated by reactive infiltration of Si–Fe alloys into porous Cf/C preforms. The resulting materials are SiC/FeSi2 composites, in which remaining silicon is reduced by formation of FeSi₂. For the richest Fe alloys (35 wt% Fe) a nominal residual silicon content below 1% has been observed. However this, the relatively poor mechanical properties (bending strength) measured for those resulting materials can be explained by the thermal mismatch of FeSi2 and SiC, which weakens the interface and does even generate new porosity, associated with a debonding phenomenon between the two phases.     

The fabrication and mechanical properties of SiC/SiC composites prepared by SLS combined with PIP

Traditionally, SiC components with complex shapes are very difficult or even impossible to fabricate. This paper aims to develop a new manufacturing process, combining selective laser sintering (SLS), cold isostatic pressing (CIP) and polymer infiltration pyrolysis (PIP), to manufacture complex silicon carbide parts and improve the mechanical properties of silicon carbide ceramic parts. The density and porosity of SiC/SiC composites were measured. Furthermore, the mechanical properties of the specimens with cold isostatic pressing and the specimens without cold isostatic pressing were compared. The bending strength of the specimens with cold isostatic pressing was 201?MPa, and the elastic modulus was 1.27?GPa. And, the bending strength of the specimens without cold isostatic pressing was 142?MPa, and the elastic modulus was 0.88?GPa. Increasing the density of SiC/SiC can enhance the mechanical properties of SiC/SiC composites.     

Thermal plasma synthesis of Si/SiC nanoparticles from silicon and activated carbon powders

In this study, Si/SiC nanocomposites were synthesized by non-transferred arc thermal plasma processing of micron-sized SiC powder. First, micron-sized SiC was synthesized by solid-state method where waste silicon (Si) and activated carbon (C) powder were used as precursor materials. The effect of Si/C mole ratio and solid-state synthesis temperature on structural and phase formation of SiC was investigated. Diffraction pattern confirmed the formation of SiC at 1300 °C. High C content was required for the synthesis of pure SiC as Si remained unreacted when Si/C mole ratio was below 1/1.5. Highly agglomerated micron-size (0.6–10 µm) SiC particles were formed after solid-state synthesis. Thermal plasma processing of solid-state synthesized micron-sized SiC resulted into the formation of uniformly dispersed (20–60 nm) Si/SiC nanoparticles. It was proposed that Si/SiC nanocomposites were formed due to partial decomposition of SiC during high temperature plasma processing. The formation of Si/SiC nanoparticles from micron-sized SiC was resulted from dissociation of grains from their grain boundary during plasma processing.     

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.     

Improved properties of low-carbon MgO-C refractories with the addition of multilayer graphene/MgAl2O4 composite powders

The paper investigated the effects of different amounts (0%, 3%, 6%, 9%) of in situ multilayer graphene/MgAl₂O₄ composite powders on the slag resistance, thermal shock resistance, and oxidation resistance of low-carbon MgO-C refractories. Comparing with commercial MgAl₂O₄, the MgAl₂O₄ in in situ multilayer graphene/MgAl₂O₄ composite powders has higher lattice strain of crystal, which can trap more Mn and Fe ions, resulting in the better slag resistance. The oxidation decarbonation layer of MgO-C specimen with 3% composite powders is 9.71 mm, which is lower than not only the specimen with other contents but also specimen containing carbon black/MgAl₂O₄ powders. Moreover, the residual strength ratio of the specimen C/MA-3 was 47.47%, which is 28.5% and 8.08% higher than specimens with no additive and with carbon black/MgAl₂O₄ powders, respectively. Both improving thermal shock and oxidation resistance properties are related with the unique nano structure, multilayer graphene in situ formed between MgAl₂O₄ grains, of added composite powders. The former is due to higher strain energy consumed by multi-deflection of cracks inside the multilayer graphene/MgAl₂O₄ composite powders. And the latter is due to the higher energy of oxidation activation of multilayer graphene/MgAl₂O₄ composite powders due to effective protection of multilayer graphene by MgAl₂O₄.     

Effects of channel modification on microstructure and mechanical properties of C/SiC composites prepared by LA-CVI process

Carbon fiber reinforced SiC matrix composites (C/SiC) with four different deposition channel sizes were fabricated via a novel laser-assisted chemical vapor infiltration (LA-CVI) method. Effects of infiltration channel sizes on microstructure and mechanical properties of C/SiC composites were investigated. The results showed that increasing the size of channels could expand infiltration passages and densification bands, which was consistent with theoretical calculations. Due to the presence of channels, the flexural strength of C/SiC composite increased by 14.47% when the channel diameter was 0.3?mm, compared to C/SiC composites prepared via conventional CVI process. Characteristics of matrix cracking and crack propagation on fracture surface were analyzed by using scanning electron microscopy. LA-CVI C/SiC composites displayed significantly improved damage-tolerant fracture behavior. Thus, findings of this work demonstrate that LA-CVI fabricated C/SiC composites are promising for a wide range of applications, particularly for enclosed-structure and thick-section C/SiC composites.     

Micromechanical properties and microstructural evolution of Amosic-3 SiC/SiC composites irradiated by silicon ions

In this work, Amosic-3 SiC/SiC composites were irradiated to 10 dpa and 115 dpa with 300 keV Si ions at 300 °C. To evaluate its irradiation behaviour and investigate the underlying mechanism, nanoindentation, AFM, Raman and electron microscopy were utilized. Nanoindentation showed that although micromechanical properties declined after irradiation, hardness and Young’s modulus were maintained better under 115 dpa. AFM manifested differential swelling among PyC interface, fiber and matrix and SEM showed irradiation-induced partial interface debonding, which are both more obvious under 115 dpa. TEM revealed the generation and proliferation of amorphous regions, which is according with the decline and broadening of peaks in Raman spectra. The material was almost completely amorphous after irradiated to 10 dpa while recrystallization occurred under 115 dpa. All results mentioned above contribute to the decline of hardness and Young’s modulus and may explain why the micromechanical degradation was more significant under 10 dpa.