Transition metal diboride-silicon carbide-boron carbide ceramics with super-high hardness and strength

Three phase boride and carbide ceramics were found to have remarkably high hardness values. Six different compositions were produced by hot pressing ternary mixtures of Group IVB transition metal diborides, SiC, and B₄C. Vickers’ hardness at 9.8 N was ~31 GPa for a ceramic containing 70 vol% TiB₂, 15 vol% SiC, and 15 vol% B₄C, increasing to ~33 GPa for a ceramic containing equal volume fractions of the three constituents. Hardness values for the ceramics containing ZrB₂ and HfB₂ were ~30% and 20% lower than the corresponding TiB₂ containing ceramics, respectively. Hardness values also increased as indentation load decreased due to the indentation size effect. At an indentation load of 0.49 N, the hardness of the previously reported ceramic containing equal volume fractions of TiB₂, SiC and B₄C was ~54 GPa, the highest of the ceramics in the present study and higher than the hardness values reported for so-called “superhard” ceramics at comparable indentation loads. The previously reported ceramic containing 70 vol% TiB₂, 15 vol% SiC, and 15 vol% B₄C also displayed the highest flexural strength of ~1.3 GPa and fracture toughness of 5.7 MPa·m^1/2, decreasing to ~0.9 GPa and 4.5 MPa·m^1/2 for a ceramic containing equal volume fractions of the constituents.     

Recent progress in the pore size control of silicon carbide ceramic membranes

The demand for separation and purification applications under harsh conditions has grown strongly in recent years. Silicon carbide (SiC) ceramic membranes have broad prospects in this aspect due to their unique characteristics, but its pore size control is a crucial problem. Therefore, it is of great significance to develop simple and feasible methods for precise control of the pore size of SiC membranes to improve membrane selectivity and expand their application range. This review describes the pore formation process in the preparation of SiC membranes, focusing on the selection of SiC particles, sintering temperature, sacrificial template, sintering aids, oxidation process and other factors affecting the pore size and analysis. Finally, the control of SiC membrane pore size is summarized and the outlook is proposed.     

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.     

Improving specific stiffness of silicon carbide ceramics by adding boron carbide

A strategy for improving the specific stiffness of silicon carbide (SiC) ceramics by adding B₄C was developed. The addition of B₄C is effective because (1) the mass density of B₄C is lower than that of SiC, (2) its Young’s modulus is higher than that of SiC, and (3) B₄C is an effective additive for sintering SiC ceramics. Specifically, the specific stiffness of SiC ceramics increased from ~142 × 10⁶ m²?s^?2 to ~153 × 10⁶ m²?s^?2 when the B₄C content was increased from 0.7 wt% to 25 wt%. The strength of the SiC ceramics was maximal with the incorporation of 10 wt% B₄C (755 MPa), and the thermal conductivity decreased linearly from ~183 to ~81 W?m^?1?K^?1 when the B₄C content was increased from 0.7 to 30 wt%. The flexural strength and thermal conductivity of the developed SiC ceramic containing 25 wt% B₄C were ~690 MPa and ~95 W?m^?1?K^?1, respectively.     

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.     

Processing,microstructure, and mechanical properties of zirconium diboride-boron carbide ceramics

The processing, microstructure, and mechanical properties of zirconium diboride-boron carbide (ZrB₂-B₄C) ceramics were characterized. Ceramics containing nominally 5, 10, 20, 30, and 40 vol% B₄C were hot-pressed to full density at 1900 °C. The ZrB₂ grain size decreased from 4 to 2 µm and B₄C inclusion size increased from 3 to 5 µm for B₄C additions of 5 and 40 vol% B₄C, respectively. Elastic modulus decreased from 525 to 515 GPa and Vickers hardness increased from 15 to 21 GPa as the B₄C content increased from 5 to 40 vol%, respectively, following trends predicted using linear rules of mixtures. Flexure strength and fracture toughness both increased with increasing B₄C content. Fracture toughness increased from 4.1 MPa m^½ at 5 vol% B₄C to 5.3 MPa m^½ at 40 vol% B₄C additions. Flexure strength was 450 MPa with a 5 vol% B₄C addition, increasing to 590 MPa for a 40 vol% addition. The critical flaw size was calculated to be ~30 µm for all compositions, and analysis of the fracture surfaces indicated that strength was controlled by edge flaws generated by machining induced sub-surface damage. Increasing amounts of B₄C added to ZrB₂ led to increasing hardness due to the higher hardness of B₄C compared to ZrB₂ and increased crack deflection. Additions of B₄C also lead to increases in fracture toughness due to increased crack deflection and intergranular fracture.     

Microstructure and high-temperature strength of silicon carbide with 2000 ppm yttria

A dense silicon carbide (SiC) ceramic with a very high flexural strength at 2000 °C (981 ± 128 MPa) was obtained by conventional hot-pressing with extremely low additive content (2000 ppm Y₂O₃). Observations using high-resolution transmission electron microscopy (HRTEM) showed that (1) homophase (SiC/SiC) boundaries were clean without an intergranular glassy phase and (2) junction pockets consisted of nanocrystalline Y-containing phase embedded in an amorphous Y-Si-O-C-N phase. The excellent strength at 2000 °C was attributed to the clean SiC/SiC boundary and the strengthening effect of plastic deformation.     

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。     

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.     

Self-healing behavior and strength recovery of ytterbium disilicate ceramic reinforced with silicon carbide nanofillers

Environmental barrier coatings (EBCs) are necessary to protect SiC/SiC ceramic components against oxidation and hot corrosion in high-temperature applications. The volatilization of SiO₂ in SiC-reinforced materials is a major obstacle for the implementation of these self-crack-healing ceramics. The Yb₂Si₂O₇-Yb₂SiO₅-SiC composite is known as a self-healing material that can help to avoid this SiC recession. In this research, the crack-healing behavior of this composite is investigated by using pre-cracking followed by annealing in an oxidizing environment. The crack-healing mechanism is explored and elucidated as a function of the filler morphology, crack size, annealing time, and annealing temperature. The two main crack-healing mechanisms are the filling of cracks with SiO₂ glass and the volume expansion of Yb₂Si₂O₇ induced by the reaction between SiO₂ and Yb₂SiO₅. Full crack recovery is achieved with only 10 vol% SiC, with evidence from XRD and EDS analyses. SiC nanoparticulates are more efficient fillers than nanofibers and nanowhiskers.     

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.     

Uniaxial tensile strength and fracture analysis of a hot-pressed boron carbide

Tensile specimens with three different gage dimensions were used to determine the tensile strength of a hot-pressed boron carbide. The strength-limiting flaw population and information on the fracture behavior under an applied tensile stress were also determined. The material follows traditional strength size-scaling for advanced ceramics, and flexure strength data for the same boron carbide was an excellent fit with the uniaxial tensile strength data. The strength-limiting flaw in all specimens was a carbon-based inclusion or agglomerate. The size of the fracture mirror around the origin and the fracture toughness value estimated from the measured strength-limiting flaw size are in excellent agreement with previously reported data. This information shows that standard four-point flexure strength tests can be used to confidently predict the uniaxial tensile strength of this hot-pressed boron carbide.     

Oxidation kinetics of boron carbide ceramic under high gamma irradiation dose in the high temperature

In the presented work, high purity boron carbide (purity of 99.9%, and bulk density 1.8 g/cm³ at 20 °C) compounds were gamma-irradiated at dose rate of D = 30 rd/sec using Co⁶⁰ gamma source. The irradiation of the samples at different gamma doses (5·10⁹; 1·10¹⁰; 1.5·10¹⁰ and 2·10¹⁰ Mrd) was carried out at room temperature and atmospheric pressure. The Differential Scanning Calorimetry (DSC) was used to study the solvothermal chemical reaction mechanisms of boron carbide from room temperature up to 900 K and the oxidation kinetics, rate of oxidation diffusion (plots of Jander), depth of oxidation and activation energy from 900 ≤ T ≤ 1250 K after gamma irradiation at different gamma absorption doses. The depth of the gamma irradiation dose on the surface determined the “critical limit” for the thin oxide layer for boron carbide. Also, depths of gamma irradiation dose is calculated activation energy by the Arrhenius plots. The activation energy was found to increase from 99.59 J mol⁻¹ to 102.93 J mol⁻¹ as the gamma irradiation dose increased from 5·10⁹ to 2·10¹⁰ Mrad.     

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.     

氮化硅结合碳化硅耐火材料的氧化

氮化硅结合碳化硅耐火材料高温氧化后，其抗折强度有所提高，但经扫描电镜观察，材料断面结构已发生了明显的变化。该材料长时间在氧化气氛中使用，可靠性将下降。     

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。     

Preparation of novel carborane-containing boron carbide precursor and its derived ceramic hollow microsphere

High melting point and hardness of boron carbide make it extremely difficult to be directly prepared as hollow microsphere. However, precursor derived method is an effective approach to prepare ceramic materials with complex shape. Therefore, in this work a novel boron carbide precursor, poly[1,7-bis(4-chlorophenyl)-m-carborane] (P4CB), was synthesized. The ceramic yield of the precursor P4CB reached as high as 90.25% at 900 °C in nitrogen. Oxidation of P4CB in air was barely observed below 500 °C, and a passive oxidation was exhibited beyond 700 °C. The P4CB/PAN slurry was prepared and coated on a polyoxymethylene (POM) ball substrate. After air crosslinking, substrate decomposition and heat-treatment at 1100 °C in Ar atmosphere, boron carbide hollow microsphere with diameter of approximate 1.34 mm and average shell thickness of 30 μm was finally obtained. The novel precursor could be also utilized to fabricate boron carbide ceramics with different shapes due to its high ceramic yield.     

Fabrication and characterization of hot-pressed B-rich boron carbide

Boron carbide has a wide solubility range owing to the substitution of B and C atoms in the crystal. In this study, boron carbides with different stoichiometric ratios were prepared using a hot-pressing sintering method, and the influences of the B/C atomic ratio on the microstructures and properties were explored in detail. X-ray diffraction analysis showed that excessive B atoms caused lattice expansion. Raman spectroscopy analysis showed disordered substitution of B atoms in the chains and icosahedra. Analysis of the densification process and microstructure evolution revealed that the addition of B promoted densification, and more stacking faults and twins occurred in B-rich boron carbide, and result in the densification mechanism gradually changes from atomic diffusion mechanism driven by thermal energy to plastic deformation mechanism dominated by the proliferation of dislocation and substructures. The introduction of chemical composition changes by dissolving excessive B into boron carbide further affected the microstructure and consequently the mechanical properties. The Vickers hardness, modulus, and sound velocity all decreased with the increase in B content. Moreover, the fracture toughness improved with increased B content. The flexural strength of the samples was optimised at the B/C stoichiometric ratio of 6.1.     

Processing and microstructure-permeation properties of silica bonded silicon carbide ceramic membrane

Porous SiC is a valuable membrane material for the microfiltration and ultrafiltration of various wastewaters because of its high hydrophilicity and low fouling tendency, but its preparation has been limited by the high sintering temperature. Here, a silica bonded silicon carbide membrane was prepared by the oxidation of SiC at low temperature. When the SiC substrates were sintered in the temperature range from 1200 to 1400℃, their porosity decreased from 45 % to 37 % while the flexural strength increased from 45 to 59 MPa. For the selective layers made from SiC particles with 0.5 μm in diameter, the average pore sizes that sintered at 1050 and 1150 ℃ were 0.34 and 0.26 μm, respectively, corresponding to the water fluxes of 1080 and 1240 L/(m² h, respectively. Thus, this technique provided a cost-effective path to prepare ceramic membrane at low sintering temperature.     

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.