Carbon Inclusions in Sintered Silicon Carbide

The carbon additions in the pressureless sintering of SiC are commonly used for the removal of SiO₂ layers on the starting powders. In practice, it is common to add more C than is necessary for stoichiometric removal to ensure a complete deoxidation. As a result, inclusions of excess free C are a general feature of the microstructure of sintered SiC. This phenomenon was studied by high-resolution Auger electron spectroscopy on ultra-high-vacuum-exposed fracture surfaces as well as by high-resolution transmission electron microscopy of B- and C-doped materials.     

碳化硼材料的制备技术

本文综述了国内外碳化硼粉末和碳化硼陶瓷制备技术的研究现状与进展情况,重点介绍了碳管炉、电弧炉碳热还原法、自蔓延高温合成法、激光诱导化学气相沉积法、溶胶凝胶碳热还原法合成碳化硼粉末以及热压、热等静压、无压烧结、放电等离子烧结和反应烧结制备碳化硼陶瓷的研究进展。     

Pressureless Sintering of Boron Carbide

B₄C powder compacts were sintered using a graphite dilatometer in flowing He under constant heating rates. Densification started at 1800°C. The rate of densification increased rapidly in the range 1870°–2010°C, which was attributed to direct B₄C–B₄C contact between particles permitted via volatilization of B₂O₃ particle coatings. Limited particle coarsening, attributed to the presence or evolution of the oxide coatings, occurred in the range 1870°–1950°C. In the temperature range 2010°–2140°C, densification continued at a slower rate while particles simultaneously coarsened by evaporation–condensation of B₄C. Above 2140°C, rapid densification ensued, which was interpreted to be the result of the formation of a eutectic grain boundary liquid, or activated sintering facilitated by nonstoichiometric volatilization of B₄C, leaving carbon behind. Rapid heating through temperature ranges in which coarsening occurred fostered increased densities. Carbon doping (3 wt%) in the form of phenolic resin resulted in more dense sintered compacts. Carbon reacted with B₂O₃ to form B₄C and CO gas, thereby extracting the B₂O₃ coatings, permitting sintering to start at ∼1350°C.     

质量法测定铝基碳化硼材料中碳化硼的含量

采用HCl溶解质量法测定铝基碳化硼材料中碳化硼的含量。对称样量、水浴温度等实验条件进行了优化,并验证了方法的精密度和准确度。结果表明,质量法测定铝基碳化硼材料中碳化硼的含量,简单、快速,精密度高,结果准确可靠,可用于批量样品测试。     

Synthesis of Calcium Hexaboride Powder via the Reaction of Calcium Carbonate with Boron Carbide and Carbon

The synthesis of calcium hexaboride (CaB₆) powder via the reaction of calcium carbonate (CaCO₃) with boron carbide (B₄C) and carbon has been investigated systematically in the present study. The influences of heating temperature and holding time on the reaction products have been studied using X-ray diffractometry, and the morphologies of CaB₆ obtained at various temperatures and holding times have been investigated via scanning electron microscopy. The interaction in the CaCO₃–B₄C–carbon system by which CaB₆ is formed is a solid-phase process that passes through the transition phases Ca₃B₂O₆ and CaB₂C₂. The optimal conditions for CaB₆ synthesis are a holding time of 2.5 h at a temperature of 1673 K, under vacuum (a pressure of 10⁻² Pa). CaB₆ powder has the same morphology as B₄C, and the properties and the shape of CaB₆ powders can be improved by choosing good-quality raw materials.     

Sintering of Boron Carbide Heat-Treated with Hydrogen

Hydrogen gas (H₂) was used to extract B₂O₃ coatings from boron carbide (B₄C) particles, permitting a lower temperature onset of sintering and restricting coarsening via solution and precipitation of B₄C in B₂O₃ liquid. Remnant H₂ had to be removed from the furnace before specimens were heated through temperature ranges in which evaporation-condensation coarsening competed with sintering (2010°–2140°C), because the presence of H₂ increased the B₄C vapor pressure. Heat treatment of B₄C compacts in a 50:50 H₂-He mixture at 1350°C, followed by a purge of the H₂ gas and then rapid heating to 2230°C, resulted in a percentage of theoretical density of 94.7%. This is higher than the value of 92.8%, which was the highest achieved without the use of H₂.     

硼酐碳热法合成碳化硼的研究

用Ｘ－射线衍射分析方法，研究了温度及反应物中Ｂ２Ｏ３摩尔量对碳热合成Ｂ４Ｃ粉末中残留含量的影响，同时探讨其反应机理，实验表明，碳化硼粉末中的残留碳含量不仅取决于反应而且与Ｂ２Ｏ３的摩尔量有关。在碳管炉内合成Ｂ４Ｃ的反应以液固反应为主，在反应时间一定条件下Ｂ２Ｏ３摩尔量为５－６ｍｏｌ的反应物料，在１６５０℃Ａｒ气氛中反应，其残留碳含量最低，其相对量为１０％左右。     

Quick Analysis of Free Carbon in Metal Carbide by Plasma Oxidation

Plasma oxidation combined with emission monitoring was applied to the quantitative analysis of free carbon in ZrC powder. The emission was monitored with an optical color analyzer and was calibrated with standard samples of ZrO₂+ C mixtures. Oxidation rates of the free and the combined carbons are so different that it is possible to estimate the amount of the former from the emission. No chemical treatment is necessary.     

Boron Carbide Reinforced Alumina Composites

The mechanical properties of alumina have been successfully improved by adding isolated boron carbide particles of two different shapes. A K _Ic of 7.26 ± 0.20 MPa · m^1/2 for alumina—boron carbide whiskerlike composites and of 5.27 ± 0.12 MPa · m^1/2 alumina—boron carbide shardlike particle composites has been achieved. The fracture toughness of these composites is dependent on the volume fraction of the boron carbide particles as well as their size and shape. The flexural strength is also appreciably enhanced to a constant value with from 5 to 20 vol% boron carbide additions. The whiskerlike particles increase the flexural strength by 25% and the shardlike particles produce a 47% improvement.     

碳化硼合成工艺及其产业化

介绍了碳化硼合成工艺的三种合成方法,重点论述了工业上生产碳化硼主要采用的碳热还原法。展示了碳化硼行业产业化的可喜前景。     

Microstructural Coarsening During Sintering of Boron Carbide

The sintering behavior of boron carbide was investigated with particular attention given to microstructure development at various stages in the sintering process. Hot-pressing and pressureless sintering techniques were employed and the effects of heating rate, firing atmosphere, and composition were used to characterize the sintering behavior. Pressureless sintering at temperatures up to 2300°C produces only limited densification. Microstructural coarsening is responsible for this since it leads to conditions where densification is slow. Hot-pressing and carbon additions suppressed coarsening and permitted densification to >95% of theoretical density.     

Hot Isostatic Pressing of Silicon Nitride with Boron Nitride, Boron Carbide, and Carbon Additions

Si₃ N₄ test bars containing additions of BN, B₄C, and C, were hot isostatically pressed in Ta cladding at 1900° and 2050°C to 98.9% to 99.5% theoretical density. Room-temperature strength data on specimens containing 2 wt% BN and 0.5 wt% C were comparable to data obtained for Si₃ N₄ sintered with Y₂O₃, Y₂O₃ and Al₂O₃, or ZrO₂. The 1370°C strengths were less than those obtained for additions of Y₂O₃ or ZrO₂ but greater than those obtained from a combination of Y₂O₃ and Al₂O₃. Scanning electron microscope fractography indicated that, as with other types of Si₃N₄, roomtemperature strength was controlled by processing flaws. The decrease in strength at 1370°C was typical of Si₃N₄ having an amorphous grainboundary phase. The primary advantage of non-oxide additions appears to be in facilitating specimen removal from the Ta cladding.     

Microstructural Characterization of a Commercial Hot‐Pressed Boron Carbide Armor Plate

Detailed microstructural characterization was carried out on a commercial‐grade hot‐pressed boron carbide armor plate. The boron carbide grains have close to B₄C stoichiometry, and most of them have no planar defects. The most prominent second phase is intergranular graphite inclusions that are surrounded by multiple boron carbide grains. Submicrometer intragranular and intergranular BN and AlN precipitates were also observed. In addition, fine dispersions of AlN nanoprecipitates were observed in some but not all grains. No intergranular films were found. These microstructural characteristics are compared with the lab‐consolidated boron carbide and their effects on the mechanical properties of boron carbide are addressed.     

碳化硼的研究进展

碳化硼是高性能陶瓷材料中的一种重要原料,包含诸多的优良性能,除了高硬度、低密度等性能外,它还具备高化学稳定性和中子吸收截面及热电性能等特性,在国防军事设备、功能陶瓷、热电元件等诸多领域具有十分广泛的应用。本文重点介绍了碳化硼的相关性质、研究进展和应用现状。详细地介绍了碳化硼的制备方法,如电弧炉碳热还原法、自蔓延高温法、化学气相沉积法、溶胶-凝胶法等方法,并分析了它们的优缺点。     

纳米碳颗粒/碳化硅陶瓷基复合材料的氧化行为

以微米硅(Si)和纳米碳黑(Cp)粉体为主要原料,采用经机械化学法合成的碳化硅(SiC)和15%和25%的纳米碳颗粒与碳化硅(Cp-SiC)的复合粉体,并经无压烧结得到了Cp/SiC陶瓷基复合材料,分析了在不同温度条件下Cp/SiC烧结体的氧化行为。结果表明:当温度小于700℃时,Cp/SiC复合陶瓷在空气中的氧化受C—O2反应控制,致使其为均匀氧化;700℃时,氧化后的复合材料显气孔率最大,弯曲强度达极小值;大于700℃,氧化过程受O2的气相扩散控制,呈非均匀氧化;700~900℃之间,O2通过微裂纹的扩散控制着Cp/SiC的氧化过程;900~1 100℃之间,O2通过SiC缺陷的扩散控制着Cp/SiC的氧化过程,并在1 000℃时的最初的2 h内,复合材料弯曲强度增大,且达到了极大值。同时表明,纳米碳含量是影响复合材料强度及氧化行为的关键因素,添加纳米碳质量分数为15%的Cp/SiC复合陶瓷可以作为一种抗氧化性能优良的玻璃夹具材料。     

Effect of Amount of Boron Doping on Compression Deformation of Fine-Grained Silicon Carbide at Elevated Temperature

The effect of the amount of boron doping in the range of 0 to 1.0 wt% on the high-temperature deformation of fine-grained β-silicon carbide (SiC) was investigated by compression testing. Flow stress at the same grain size increased as the amount of boron doping decreased. The stress exponent increased from 1.3 to 3.4 as the amount of boron doping decreased. The strain rates of undoped SiC were ∼2 orders of magnitude lower than those of 1.0-wt%-boron-doped SiC of the same grain size. The apparent activation energies of SiC doped with 1.0 wt% boron and of undoped SiC were 771 ± 12 and 884 ± 80 kJ/mol, respectively. These results suggest that the actual contribution of grain-boundary diffusion to the accommodation process of grain-boundary sliding decreased as the amount of boron doping decreased. Consequently, the apparent contribution of the dislocation glide increased.     

Anisotropy of Mechanical Properties in a Hot‐Pressed Boron Carbide

Effects of microstructure and material properties on the mechanical behavior of hot‐pressed boron carbide are presented. The microstructure and intrinsic microstructural inhomogeneities have been characterized using scanning electron microscopy characterization techniques (SEM/EDS/EBSD). In situ mechanical characterizations of the boron carbide microstructure and its larger inhomogeneities have been performed by nanoindentation. Macroscopic dynamic and quasi‐static compressive responses have been studied in two characteristic orientations (parallel and perpendicular to the hot‐pressing direction) using a modified compression Kolsky bar setup (strain rates of /s) and standard MTS test machine (strain rates of /s). The microstructure characterization showed that boron carbide has a fine‐grained microstructure with a complex superposition of nonmetallic inclusions, such as free carbon, AlN, and BN. Nanoindentation tests conducted in three principal planes of the plate revealed an anisotropy of the mechanical properties. The compression tests revealed that the strength of this hot‐pressed boron carbide is orientation dependent. Detailed SEM analysis indicated transgranular fracture and microcracking originating at large carbon inclusions. Influences of microstructural anisotropy on the mechanical response of the material are discussed.     

Rate‐Dependent Mechanical Behavior and Amorphization of Ultrafine‐Grained Boron Carbide

An investigation into mechanical properties and amorphization behavior of ultrafine‐grained (0.3 μm) boron carbide (B₄C) is conducted and compared to a baseline coarse‐grained (10 μm) boron carbide. Static and dynamic uniaxial compressive strength, and static and dynamic Vickers indentation hardness were determined, and Raman spectroscopy was then conducted on indented regions to quantify and compare the intensity of amorphization. In relation to coarse‐grained B₄C the ultrafine‐grained material exhibited, on average, a 33% higher static compressive strength, 20% higher dynamic compressive strength, 10% higher static Vickers hardness, and 23% higher dynamic Vickers hardness. In addition, there was an 18% reduction in indentation‐induced radial crack length in ultrafine‐grained B₄C, which corresponded to an increase in estimated fracture toughness. Although traditional coarse‐grained B₄C exhibits an 8.6% decrease in hardness from the static to dynamic regimes, ultrafine‐grained B₄C showed only negligible change under similar conditions, suggesting a reduced propensity for amorphization. Raman spectroscopic analysis confirmed this result by revealing significantly lower amorphization intensity in ultrafine‐B₄C compared to coarse‐grained B₄C. These results may have significant positive implications in the implementation of ultrafine‐grained boron carbide as a material for improved performance in impact and other high‐pressure applications.     

In Situ Reaction Synthesis of Silicon Carbide–Boron Nitride Composite from Silicon Nitride–Boron Oxide–Carbon

This work proposes a new approach, based on the reaction Si₃N₄+ 2B₂O₃+ 9C → 3SiC + 4BN + 6CO, to synthesize an SiC–BN composite. The composite was prepared by reactive hot pressing (RHP), at 2000°C for 60 min at 30 MPa under an argon atmosphere, following a 60 min hold at 1700°C without applied pressure before reaching the RHP temperature. TG-DTA results showed that a nitrogen atmosphere inhibited denitrification somewhat and retarded the reaction rate. The chemical composition of the obtained material was consistent with theoretical values. FE-SEM observation showed that in situ -formed SiC and BN phases were of spherical morphology with very fine particle size of ∼100 nm.     

Hardness and Fracture Toughness of Pressureless-Sintered Boron Carbide (B4C)

This is a companion work to our previous study on the pressureless sintering of boron carbide (B₄C). The Vickers hardness and indentation fracture toughness of B₄C compacts were measured after various sintering heat treatments. Increases in hardness and decreases in indentation fracture toughness as the grain size decreased in sintered B₄C were attributed to the effects of more rapid strain hardening associated with dislocation pileups at grain boundaries.