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

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

Direct bonding of silicon carbide ceramics sintered with yttria

SiC ceramics sintered with yttria were successfully joined without an interlayer by conventional hot pressing at lower temperatures (2000–2050 °C) compared to those of the sintering temperatures (2050–2200 °C). The joined SiC ceramics sintered with 2000 ppm Y₂O₃ showed almost the same thermal conductivity (˜198 Wm⁻¹ K⁻¹), fracture toughness (3.7 ± 0.2 MPa m^1/2), and hardness (23.4 ± 0.8 GPa) as those of the base material, as well as excellent flexural strength (449 MPa). In contrast, the joined SiC ceramics sintered with 4 wt% Y₂O₃ showed very high thermal conductivity (˜204 Wm⁻¹ K⁻¹) and excellent flexural strength (˜505 MPa). Approximately 16–22% decreases in strength compared to those of the base SC materials were observed in both joined ceramics, due to the segregation of liquid phase at the interface. This issue might be overcome by preparing well-polished and highly flat surfaces before joining.     

Process-tolerant pressureless-sintered silicon carbide ceramics with alumina-yttria-calcia-strontia

Process-tolerant SiC ceramics were prepared by pressureless sintering at 1850–1950 °C for 2 h in an argon atmosphere with a new quaternary additive (Al₂O₃-Y₂O₃-CaO-SrO). The SiC ceramics can be sintered to a > 94% theoretical density at 1800–1950 °C by pressureless sintering. Toughened microstructures consisting of relatively large platelet grains and small equiaxed grains were obtained when SiC ceramics were sintered at 1850–1950 °C. The presently fabricated SiC ceramics showed little variability of the microstructure and mechanical properties with sintering within the temperature range of 1850–1950 °C, demonstrating process-tolerant behavior. The thermal conductivity of the SiC ceramics increased with increasing sintering temperature from 1800 °C to 1900 °C due to decreases of the lattice oxygen content of the SiC grains and residual porosity. The flexural strength, fracture toughness, and thermal conductivity of the SiC ceramics sintered at 1850–1950 °C were in the ranges of 444–457 MPa, 4.9–5.0 MPa m^1/2, and 76–82 Wm^?1 K^?1, respectively.     

Fabrication of carbon-coated boron carbide particle and its role in the reaction bonding of boron carbide by silicon infiltration

To tackle the dissolution problem of boron carbide particles in silicon infiltration process, carbon-coated boron carbide particles were fabricated for the preparation of the reaction-bonded boron carbide composites. The carbon coating can effectively protect the boron carbide from reacting with liquid Si and their dissolution, thus maintaining the irregular shape of boron carbide particles and preventing the growth of boron carbide particles and reaction formed SiC regions. Furthermore, the nano-SiC particles, originated from the reaction of the carbon coating and the infiltrated Si, uniformly coated on the surfaces of boron carbide particles, thus forming a ceramic skeleton of the nano-SiC particles-coated and -bonded boron carbide particles. The Vickers hardness, flexural strength and fracture toughness of the composites can be increased by 26 %, 45 %, and 37 % respectively, by using carbon-coated boron carbide particles as raw materials.     

反应烧结SiC材料的高温氧化行为研究

研究了反应烧结SiC材料在 110 0℃空气中的高温氧化行为。结果表明 :反应烧结SiC在110 0℃的氧化动力学曲线符合抛物线规律 ;材料的氧化受O2 和CO在玻璃态硅酸盐中的扩散所控制 ;材料中的杂质元素降低了SiO2 氧化膜的粘度 ,促进了O2 和CO在氧化膜中的扩散     

Microstructure and mechanical properties of tantalum carbide ceramics: Effects of Si3N4 as sintering aid

Stoichiometric Tantalum carbide (TaC) ceramics were prepared by reaction spark plasma sintering using 0.333–2.50 mol% Si₃N₄ as sintering aid. Effects of the Si₃N₄ addition on densification, microstructure and mechanical properties of the TaC ceramics were investigated. Si₃N₄ reacted with TaC and tantalum oxides such as Ta₂O₅ to form a small concentration of tantalum silicides, SiC and SiO₂, with significant decrease in oxygen content in the consolidated TaC ceramics. Dense TaC ceramics having relative densities >97% could be obtained at 0.667% Si₃N₄ addition and above. Average grain size in the consolidated TaC ceramics decreased from 11 µm at 0.333 mol% Si₃N₄ to 4 µm at 2.50 mol% Si₃N₄ addition. The Young's modulus, Vickers hardness and flexural strength at room temperature of the TaC ceramic with 2.50 mol% Si₃N₄ addition was 508 GPa, 15.5 GPa and 605 MPa, respectively. A slight decrease in bending strength was observed at 1200 °C due to oxidation of the samples.     

Corrosion behaviors of porous reaction-bonded silicon carbide ceramics incorporated with CaO

Reaction-bonded SiC (RBSC) porous ceramics were fabricated at 1450?°C in air by incorporating CaO using ZrO₂ as sintering aids, activated carbon as pore-forming agent, and mullite fibers as reinforcing agent. The effects of CaO content on the properties of the porous RBSC ceramics were studied. Corrosion behaviors of the prepared RBSC porous ceramics in different environments were also investigated. The optimal open porosity, bending strength, average pore size and gas permeability of the ceramics with 0.5% CaO were 40%, 22.5?MPa, 42.9?µm, and 2100?m³/m² h?kPa, respectively. A well-developed neck reaction-bonded by calcium zirconium silicate (Ca₃ZrSi₂O₉) was identified. The porous RBSC ceramics exhibited excellent corrosion resistance in acid and basic solutions. The anti-oxidation temperature of the porous RBSC ceramics could reach 1200?°C in air. The RBSC ceramics maintained the bending strength of 17.5?MPa after 60 cold-hot cycles in air (0–800?°C). The porous RBSC ceramics also exhibited relatively good corrosion resistance in molten salts (NaCl, Na₂SO₄ and CaCl₂). Melten NaOH can aggravate the reaction by breaking the SiO₂ layers on the SiC surface. Overall, these findings offer significant insights into expanding the applications porous RBSC ceramics incorporated with CaO.     

Electrochemical corrosion of silicon carbide ceramics in sodium hydroxide

Sintered silicon carbide ceramics have found widespread use due to their high corrosion stability. This corrosion stability can be affected by electrochemical processes. Electrochemical corrosion experiments conducted on an SSiC material in NaOH at different voltages and subsequent detailed investigation of the formed surfaces were carried out. Systematic local measurement of the corrosion rate was carried out using the AFM technique. The results revealed the recession of the SiC grain surfaces under anodic electrochemical loading, with the extents differing strongly from grain to grain. The recession rates were not found to correlate with the SiC grain orientations or polytypes. Rather, the data and the observed microstructure indicated that the behaviour was caused by variations in the resistivities of the grain boundaries.     

高强度刚玉碳化硅浇注料的研制

以棕刚玉和碳化硅为主要原料、铝酸钙水泥为结合剂,制备出了一种高强度的刚玉碳化硅浇注料。通过优化颗粒级配,调整水泥和复合微粉的加入量,测定试样在各种因素影响下的常温和高温强度。结果表明,研制出的刚玉碳化硅浇注料既具有较高的常温强度,又具有很高的高温强度。     

锆刚玉莫来石－碳化硅复合材料的显微结构

用热压的方法制备了Ａｌ２Ｏ３／莫来石（３／１）＋１５％ＡｒＯ２十１０％ＳｉＣ四元复合材料。采用ＡＥＭ和ＡＥＭ－ＥＤＳ等研究了材料的显微结构以及品界两侧金属元素的分布，确定了本材料晶界直接结合的三种形式。     

Pressureless sintered silicon carbide matrix with a new quaternary additive for fully ceramic microencapsulated fuels

This study suggests a new additive composition based on AlN–Y₂O₃–Sc₂O₃–MgO to achieve successful densification of SiC without applied pressure at a temperature as low as 1850 °C. The typical sintered density, flexural strength, fracture toughness, and hardness of the SiC ceramics sintered at 1850 °C without applied pressure were determined as 98.3%, 510 MPa, 6.9 MPa·m^1/2, and 24.7 GPa, respectively.Fully ceramic microencapsulated (FCM) fuels containing 37 vol% tristructural isotropic (TRISO) particles could be successfully sintered at 1850 °C using the above matrix without applied pressure. The residual porosity of the SiC matrix in the FCM fuels was only 1.6%. TRISO particles were not damaged during processing, which included cold isostatic pressing under 204 MPa and sintering at 1850 °C for 2 h in an argon atmosphere. The thermal conductivity of the pressureless sintered FCM pellet with 37 vol% TRISO particles was 44.4 Wm^?1 K^?1 at room temperature.     

Electrochemical corrosion of silicon carbide ceramics in H2SO4

Sintered silicon carbide materials have found widespread use due to their high corrosion stability. This corrosion stability can be affected by electrochemical processes. Electrochemical corrosion experiments conducted on a SSiC material in H₂SO₄ at different voltages and subsequent detailed investigation of the formed surfaces was carried out. The first time a systematic local measurement of the thickness of the oxide layers was carried out. The measurements revealed the formation of SiO₂ surface layers with thickness up to 125 μm. The measured values also showed a strong deviation from grain to grain. The thickness of the layers does not correlate with the crystallographic orientation of the grains or the SiC-polytypes. The data indicate that the behaviour is caused by the variation of the resistivity of the grain boundaries. The measured thicknesses as a function of the electrical charge transferred indicate that the electrochemical oxidation results in the SiO₂ and carbon dioxide.     

Microstructures and properties of solid-state-sintered silicon carbide membrane supports

Porous solid-state-sintered SiC (S–SiC) membrane supports were successfully fabricated by pressureless sintering at 2150 °C in argon, using fine and coarse graded SiC powders as the main starting material. There were uniformly distributed and fully interconnected pores in as-acquired S–SiC membrane supports, accompanied with similar apparent porosities for all of them. When increasing the size of coarse SiC powder, their average pore sizes were distinctly enlarged from ∼1.6 μm to ∼2.3 μm, which significantly enhanced their nitrogen permeability from 0.9 × 10⁻¹³ m² to 2.6 × 10⁻¹³ m². Moreover, S–SiC membrane supports possessed outstanding flexure strengths of 134.1 ± 21.3 MPa at room temperature and 88.7 ± 8.4 MPa at 1000 °C owing to strong interface bonding between SiC grains. Compared with the traditional SiO₂ -bonded and mullite-bonded SiC supports, S–SiC membrane supports presented their great superiority in high-temperature flexure strength as well as acid and alkali corrosion resistance, which permitted them to be potentially applied in high-temperature and strongly corrosive environments.     

Synthesis of monolithic SiC aerogels with high mechanical strength and low thermal conductivity

The monolithic silicon carbide (SiC) aerogels were converted from catechol-formaldehyde/silicon composite (CF/SiO₂) aerogels through carbothermal reduction and calcination. In the process of preparing the CF/SiO₂ aerogel, a new method was proposed to produce more silicon carbide and enhanced the mechanical properties of the SiC aerogel. This method was realized by adding an alkaline silica sol as supplemental silicon source. The principle process of CF/SiO₂ aerogels converting to SiC aerogels was discussed based on experiment and results analysis, while the microstructure, mechanical properties, and thermal properties of the prepared SiC aerogels were investigated. The results show that the as-synthesized SiC aerogels consist of β-SiC and a small amount of α-SiC nanocrystalline. It possessed a mesoporous structure and a low thermal conductivity 0.049 W/(m∙K), a relatively high compressive strength 1.32 MPa, and a relatively high specific surface area 162 m²/g. Due to their outstanding thermal and mechanical properties, the prepared SiC aerogels present potential applications in thermal insulation field, such as space shuttles and aerospace carrier thermal protection materials.     

Corrosion resistance of silicon-infiltrated silicon carbide (SiSiC)

Silicon carbide ceramics have found widespread use due to their high corrosion stability. Both solid state-sintered silicon carbide, which has an extremely high corrosion resistance, and silicon-infiltrated silicon carbide are used for various applications. The latter material contains SiC as well as free silicon, which is less stable. Hence, in the present work, the corrosion behavior of silicon-infiltrated silicon carbide ceramics was investigated in NaOH solutions. Long-term corrosion experiments were conducted, and a method for analyzing the corrosion behavior in short-term experiments was developed. The short-term method is based on the accurate measurement of the corrosion depth by laser scanning microscopy on polished surfaces. The results of both methods were in good agreement. The advantage of the short-term method is that it provides information on changes in corrosion mechanisms and corrosion rates in the initial period and as a function of the impurities present. Preferential corrosion of Si at the interface to SiC was observed. TEM investigations revealed that this enhanced corrosion was caused by the segregation of impurities.     

Surface preparation of silicon carbide for improved adhesive bond strength in armour applications

Surface treatments of silicon carbide have been investigated with the aim of improving the strength of the bond between the ceramic and an epoxy adhesive. Three surface conditions have been characterised; as-fired, air re-fired and KrF laser processed. A number of characterisation techniques have been used to determine the morphological and chemical changes that have occurred to the surface. Scanning electron microscopy of the re-fired and laser processed samples showed surfaces that appeared glassy, with the laser processed surface showing a different morphology. X-ray photoelectron spectroscopy indicated both treatments had oxidised the surface and the laser processed surface also had a greater concentration of hydroxyl groups. The wettability of both surfaces had improved and the laser processed surface was found to be highly hydrophilic. Mechanical testing of joints prepared with this technique showed them to have the highest strength in tension, with the locus of failure being cohesive.     

Evaporation-condensation derived silicon carbide membrane from silicon carbide particles with different sizes

Silicon carbide ceramic is a promising membrane material because of the high corrosive and high temperature resistance, and the excellent hydrophility. Here, a silicon carbide ceramic membrane with both substrate layer and separate layer composed of pure silicon carbide phase was successfully prepared. The effect of particle size on the microstructure and properties was investigated. The substrates were prepared from three silicon carbide particles at 2200 ℃. With the content increase of fine particle, the average pore size increased from 5.6 μm to 14.1 μm; meanwhile, the flexural strength of the substrate increased from 14.1 MPa to 24.6 MPa. The separation layers were made from particles of 3.0 μm and 0.5 μm. When sintered at 1900 ℃, the separation layer formed pore network with homogeneous structure. Such silicon ceramic membrane can be used in harsh conditions, including high temperature wastewater and strongly corrosive wastewater.     

Microstructure control and mechanical properties of porous silicon nitride ceramics

Porous silicon nitride ceramics with a fibrous interlocking microstructure were synthesized by carbothermal nitridation of silicon dioxide. The influences of different starting powders on microstructure and mechanical properties of the samples were studied. The results showed that the microstructure and mechanical properties of porous silicon nitride ceramics depended mostly on the size of starting powders. The formation of single-phase β-Si₃N₄ and the microstructure of the samples were demonstrated by XRD and SEM, respectively. The resultant porous Si₃N₄ ceramics with a porosity of 71% showed a relative higher flexural strength of 24 MPa.     

Relationship between bending strength of bulk porous silicon carbide ceramics and grain boundary strength measured using microcantilever beam specimens

The relationship between the bending strength of bulk porous SiC ceramics and the grain boundary strength measured using microcantilever beam specimens of SiC bicrystals was investigated. The average value of the grain boundary strength was 39.2 GPa, and its higher value was roughly equal to that derived using an ab-initio calculation. The strengths of the specimens having only one neck were estimated by analyzing the effect of the specimen size on the strength of bulk porous SiC ceramics and by also analyzing the grain boundary strength and the stress concentration at the neck surface. The estimated strengths were generally consistent of the order of several hundred MPa, meaning that the strength of porous SiC ceramics should be dependent on the stress concentration at the neck and the grain boundary strength. Furthermore, they were in a better agreement using smaller neck curvature, smaller neck diameter, and lower grain boundary strength.     

酚醛树脂对反应烧结碳化硅显微结构与性能的影响

以碳化硅(w(SiC)=99%,d50=5μm)、炭黑(w(C)=99.8%,d50=0.38μm)和单质硅(w(Si)=98.6%)为原料,无水乙醇(w(乙醇)=99.6%)、热塑性酚醛树脂(0.074μm、工业级)为结合剂,乌洛脱品为固化剂,以50 MPa的单向压力,分别将采用干混工艺、湿混工艺混合、干燥后的物料压制成型为50 mm×5 mm的生坯,在N2保护下经800℃焙烧、炭化处理,有机物热降解后得到陶瓷素坯。研究了酚醛树脂在不同加入量(其质量分数分别为4%、8%、10%、12%、16%)及混练工艺(干混、湿混)对反应烧结碳化硅素坯强度和烧结体显微结构的影响,并采用SEM和光学显微镜分析了试样的显微结构和断面形貌。结果表明:当酚醛树脂加入量为12%时,采用湿混工艺可以制备出具有良好可浸渗性且抗折强度高达45 MPa的素坯,完全可以满足复杂异型件在烧成以前进行机械加工的要求,烧结体的抗折强度最高可达455 MPa。与干混工艺相比,用湿混工艺制成烧结体的显微结构更加均匀,晶粒更加细小,裂纹扩展更加曲折。