In situ hot-pressed ZrB2-based composite with polycarbosilane-derived SiC: Microstructure and mechanical behavior

In this study, a homogenously dispersed finer SiC particles-containing ZrB₂ composite was prepared using nanosized polycarbosilane (PCS) particles-containing ZrB₂ mixture powder, followed by hot pressing. The microstructure of the resulting composite was characterized by field-emission scanning electron microscopy and transmission electron microscopy. The composite microstructure comprised finer equiaxed ZrB₂ and SiC grains. The mechanical behavior of the composite was evaluated using four-point bending test at different temperatures between room temperature (RT) and 1600°C. The results show that the composite exhibited only linear deformation behavior prior fracture at or below 1500°C. However, a trace quantity of nonlinear deformation was observed at 1600°C. In addition, the flexural strength of the composite decreased as the temperature increased from RT to 1200°C, then the strength increased as the temperature raised to 1400°C. Subsequently, the flexural strength remained almost the constant between 1400°C and 1600°C, with a strength of ~760 MPa.     

High-temperature flexural strength of SiC ceramics prepared by additive manufacturing

Silicon carbide (SiC) ceramics, as a kind of candidate material for aero-engine, its high-temperature performance is a critical factor to determine its applicability. This investigation focuses on studying the high-temperature properties of SiC ceramics fabricated by using additive manufacturing technology. In this paper, SiC ceramics were prepared by combining selective laser sintering (SLS) with precursor infiltration and pyrolysis (PIP) technique. The microstructure, phase evolution, and failure mechanism after high-temperature tests were explored. SiC ceramic samples tested at room temperature (RT), 800°C, 1200°C, 1400°C, and 1600°C demonstrated bending strengths of 220.0, 226.1, 234.9, 215.5, and 203.7 MPa, respectively. The RT strength of this material can be maintained at 1400°C, but it decreased at 1600°C. The strength retention at 1400°C and 1600°C were 98% and 92%, respectively. The results indicate that the mechanical properties of SiC ceramics prepared using this method have excellent stability. As the temperature increases, the bending strength of the specimens increased slightly and reached the peak value at 1200°C, and dropped to 203.7 MPa at 1600°C. Such an evolution could be mainly due to the crack healing, and the softening of the glassy phase.     

Ni-SiC复合电镀工艺的优化及镀层结构表征

利用电沉积法制备出Ni-SiC复合镀层,研究了阴极电流密度、温度、pH、搅拌速率、表面活性剂等工艺参数对镀层显微硬度和沉积速率的影响,通过正交试验得出了最佳工艺参数:阴极电流密度4A/dm2,SiC微粒悬浮量60g/L,温度40℃,pH 2.5,搅拌速率300 r/min.用SEM、XRD和TEM分析了镀层的表面形貌、组织结构及镀层中粒子的分布,结果表明:SiC微粒均匀分布于复合镀层中,镀层表面平整光滑,显微组织均匀、致密,其显微硬度也较纯镍镀层有显著提高.     

Microstructural,electrical, and mechanical properties of conductive SiC ceramics fabricated by spark plasma sintering

Nitrogen (N)-doped conductive silicon carbide (SiC) of various electrical resistivity grades can satisfy diverse requirements in engineering applications. To understand the mechanisms that determine the electrical resistivity of N-doped conductive SiC ceramics during the fast spark plasma sintering (SPS) process, SiC ceramics were synthesized using SPS in an N₂ atmosphere with SiC powder and traditional Al₂O₃–Y₂O₃ additive as raw materials at a sintering temperature of 1850–2000°C for 1–10 min. The electrical resistivity was successfully varied over a wide range of 10⁻³–10¹ Ω cm by modifying the sintering conditions. The SPS-SiC ceramics consisted of mainly Y–Al–Si–O–C–N glass phase and N-doped SiC. The Y–Al–Si–O–C–N glass phase decomposed to an Si-rich phase and N-doped Y_xSi_yC_z at 2000°C. The Vickers hardness, elastic modulus, and fracture toughness of the SPS-SiC ceramics varied within the ranges of 14.35–25.12 GPa, 310.97–400.12 GPa, and 2.46–5.39 MPa m^1/2, respectively. The electrical resistivity of the obtained SPS-SiC ceramics was primarily determined by their carrier mobility.     

碳化硅材料中游离硅及游离碳对性能的影响

研究了全碳粉反应渗硅碳化硅（PCRBSC）材料的结构与力学性能的关系。分析了渗硅碳化硅材料中游离硅（fsi),游离碳（fc)含量对抗折强度的影响。结果表明：参硅碳化硅材料中随游离硅（fsi)含量的增加,其抗折强度下降,并且二者呈直线关系,符合线性复合规划,另一方面,游离碳（fc)含量较高的渗硅碳化硅材料,尽管游离硅（fsi)含量低,但其抗折强度低于等量或较多游离硅（fsi)含量的渗硅碳化硅材料的抗折强度。     

Experimental investigation of mechanical,thermal, and ablation performance of ZrO2/SiC modified C-Ph composites

In this study, an effort has been made to improve the mechanical, thermal, and ablation performance of carbon-phenolic (C-Ph) composites. The ZrO₂, SiC, and ZrO₂/SiC hybrid fillers were synthesized using sol-gel method followed by individual incorporation into C-Ph composites. The thermal stability and flexural strength of these C-Ph composites were analyzed using thermogravimetry analysis and three-point bending test, respectively. A significant improvement in the flexural strength and modulus of the reinforced C-Ph composites was observed and also exhibited the higher thermal stability. The oxyacetylene flame test was conducted to measure the ablation behavior of these filler reinforced C-Ph composites under a heat flux of 4.0 MW/m² for 60 seconds. ZrO₂/SiC0.5 reinforcement in the C-Ph composite decreased the linear and mass ablation rates by 46% and 22%, respectively when compared with pure C-Ph composite. The surface morphology analysis revealed that the burnt composite covered with the ZrC ceramic phase and SiO₂ bubble-like structure, which could have improved the ablation resistance of composites. These results were found well within the acceptable range when using the surface energy dispersive spectroscopy and X-ray diffraction analysis.     

Activated combustion of a silicon—carbon mixture in nitrogen and SHS of Si3N4—SiC composite ceramic powders and silicon carbide

It is established that Si₃N₄—SiC composites with a mass content of SiC 5–60% and a dominating content of the β-modification of silicon nitride can be produced by interaction of the components in the Si—C—N₂ system in the combustion regime. It is found that the fraction of α-Si₃N₄ can be increased by diluting the starting mixture with the end products, but this leads to the occurrence of a certain amount of unreacted silicon in the products. It is shown that the use of chemical activation allows one to perform a single-stage synthesis of Si₃N₄—SiC composites with any contents of the individual components (from 0 to 100%), including pure carbide silicon. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 5, pp. 56–62, September–October, 2006.     

Effect of BN nanoparticle content in SiC matrix on microstructure and mechanical properties of SiC/SiC composites

BN-nanoparticle-containing SiC-matrix-based composites comprising SiC fibers and lacking a fiber/matrix interface (SiC/BN + SiC composites) were fabricated by spark plasma sintering (SPS) at 1800°C for 10 min under 50 MPa in Ar. The content of added BN nanoparticles was varied from 0 to 50 vol.%. The mechanical properties of the SiC/BN + SiC composites were investigated thoroughly. The SiC/BN + SiC composites with a BN nanoparticle content of 50 vol.%, which had a bulk density of 2.73 g/cm³ and an open porosity of 5.8%, exhibited quasiductile fracture behavior, as indicated by a short nonlinear region and significantly shorter fiber pullouts owing to the relatively high modulus. The composites also exhibited high strength as well as bending, proportional limit stress, and ultimate tensile strength values of 496 ± 13, 251 ± 30, and 301 MPa ± 56 MPa, respectively, under ambient conditions. The SiC fibers with contents of BN nanoparticles above 30 vol.% were not severely damaged during SPS and adhered to the matrix to form a relatively weak fiber/matrix interface.     

Microstructural Evolution in Liquid-Phase-Sintered SiC: Part II, Effects of Planar Defects and Seeds in the Starting Powder

The effects of planar-defect density in a β-SiC starting powder and the addition of α-SiC seeds to that powder on microstructural evolution in liquid-phase-sintered (LPS) SiC have been studied separately. Planar-defect density is altered by appropriate heat treatment of an as-received β-SiC starting powder. It was found that a decrease in the planar-defect density in the powder retards the β→α phase transformation rate. It is proposed that, because nucleation of α-SiC occurs on the planar defects present in the β-SiC starting powders, the nucleation rate and the attendant rate of transformation decrease with a reduction in planar-defect density. Consequently, this reduces the frequency of formation of elongated β/α composite grains, resulting in lower average aspect ratios, as the initial untransformed β-SiC grains coarsen in an equiaxed manner. In contrast, addition of external α-SiC seeds has no effect on the β→α phase transformation rate, although a significant reduction in the average aspect ratio occurs. It is proposed that preferential equiaxed coarsening of the α-SiC seeds over elongated coarsening of β/α composite grains occurs, resulting in a reduction of overall coarsening anisotropy.     

Improved mechanical strength and thermal resistance of porous SiC ceramics with gradient pore sizes

Thermal insulation applications of porous SiC ceramics require low thermal conductivity and high mechanical strength. However, low thermal conductivity and high mechanical strength possess a trade-off relationship, because improving the mechanical strength requires decreasing the porosity, which increases the thermal conductivity. In this study, we established a new strategy for improving both the mechanical strengths and thermal resistances of porous SiC ceramics with micron-sized pores by applying a double-layer coating with successively decreasing pore sizes (submicron- and nano-sized pores). This resulted in a unique gradient pore structure. The double-layer coating increased the flexural strengths and decreased the thermal conductivities of the porous SiC ceramics by 24–70 % and 29–49 % depending on the porosity (48–62 %), improving both their mechanical strengths and thermal resistances. This strategy may be applicable to other porous ceramics for thermal insulation applications.     

Synthesis and characterization of submicron silicon carbide powders with silicon and phenolic resin

A novel three-step process is used to fabricate submicron silicon carbide powders in this paper. The commercially available silicon powders and phenolic resin are used as raw materials. In the first step, precursor powders are produced by coating each silicon powder with phenolic resin shell. Then, precursor powders are converted into carbonized powders by decomposing the phenolic resin shell. The submicron silicon carbide powders are formed in the reaction of silicon with carbon during the third step of thermal treatment. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and thermogravimetric (TG) analyses are employed to characterize the microstructure, phase composition and free carbon content. It is found that the sintered powders consist of β-SiC with less than 0.2 wt.% of free carbon. The particle size of the obtained silicon carbide powders varies from 0.1 to 0.4 μm and the mean particle size is 0.2 μm. The silicon carbide formation mechanism of this method is based on the liquid-solid reaction between liquid silicon and carbon derived from phenolic resin. The heat generated during the reaction leads to great thermal stress in silicon carbide shell, which plays an important role in its fragmenting into submicron powders.     

Ni-Fe-P/碳化硅纳米复合粉体的制备及微观结构研究

研究了化学镀制备Ni-Fe-P/碳化硅纳米复合粉体的方法,比较了复配络合剂与单一络合剂的络合性能及其对镀液稳定性的影响.实验表明,以乳酸、酒石酸钾钠为复配络合剂的碱性镀液中可获得较好的Ni-Fe-P/碳化硅纳米复合粒子;以硼酸-NaOH为缓冲体系可避免因NH3挥发而引起镀液pH变化,镀液的稳定性好.用SEM、XRD及EDS等手段分析了镀层的组成和结构.     

碳化硅纳米纤维薄膜的制备及表征

通过溶胶-凝胶碳热还原法制备了超长碳化硅纳米纤维(SiCNF),并采用丙酮辅助滚压法制备了新型自支撑SiCNF薄膜。通过X射线衍射、场发射扫描电镜、红外光谱等测试手段,分析了SiCNF的形貌、晶相和成分,表明产物为直径约50nm的超长立方相β-SiCNF。自支撑纳米薄膜作为NF的宏观体材料,在柔性电子器件、复合材料、能源材料、高温过滤、高温催化载体、环境治理、生物工程等领域具有广阔的应用前景。     

重力沉降原理在微粉和超细粉粒度分析中的应用

探讨了利用重力沉降原理对粉体颗粒粒度分布进行测试时 ,颗粒的布朗运动对测定精度的影响。并推导出常见的耐火材料粉体最小的测试颗粒粒径范围应为 0 .6～ 0 .95 μm。同时介绍了采用国产KCT - 1型沉降天平进行微粉与超细粉的粒度分布测定时的操作要点与技巧。并对测试过程中出现的异常现象和解决的方法进行了阐述     

Liquid phase sintering and characterization of SiC ceramics

The present study focuses on the sintering of silicon carbide-based ceramics (SiC) by liquid phase sintering (LPS) followed by characterization of the produced ceramics. AlN/Re₂O₃ mixtures were used as additives in the LPS process. In the first step, the LPS-SiC materials were produced in a graphite resistance furnace in the form of discs at different temperatures. The conditions with the best results regarding real density and relative density were taken as reference for sintering in the form of prismatic bars. In the second step, these samples were evaluated regarding fracture toughness (K_IC), by the Single Edge V Notch Beam – SEVNB – method, and flexural strength. K_IC behavior was evaluated according to the depth and curvature radius of the notches. Reliable K_IC values were presented when the ceramic displayed a small curvature radius at the notch tip. When the radius was large, it did not maintain the square root singularity of the notch tip. Tests were carried out to determine K_IC values in atmospheric air and water. K_IC results were lower in water than air, with a decrease ranging between 2.56% and 11.26%. The observations indicated a direct grain size correlation between K_IC values and fracture strength of the SiC ceramics.     

环氧树脂/改性SiC复合材料的制备及性能

采用硅烷偶联剂KH-560和丙烯酰胺对SiC进行表面改性,将其添加到环氧树脂中制备环氧树脂/改性SiC复合材料.采用傅里叶变换红外光谱仪、X射线衍射仪以及接触角测试仪探究改性SiC的性能,并对复合材料的性能进行测试.结果表明:SiC表面带有憎水基团,与环氧树脂相容性提高;SiC用量为环氧树脂质量的20％时,拉伸强度和弯...     

锆莫来石-碳化硅材料反应烧结过程的研究

研究了用锆英石、氧化铝和炭黑的混合物制备原位SiC颗料复合锆莫来石材料的反应烧结进程和显微结构特征。结果表明：反应烧过程中,SiC、莫来石的生成反应滞后于锆莫来石的分解反应;反应前期,SiC的生成占主导作用。反应后期,莫来石的形成及致密化进程占主导作用;材料中存在大量气孔,ZrO3以均匀分布和聚集体两种形式分布于莫来石、SiC及玻璃相构成的基质中。     

Wetting behavior and brazing of titanium-coated SiC ceramics using Sn0.3Ag0.7Cu filler

By coating active titanium, Sn0.3Ag0.7Cu (SAC) filler wetted SiC effectively, as the contact angle decreased significantly from ~145° to ~10°. Ti₃SiC₂ and TiO_x (x ≤ 1) reaction layers were formed at the droplet/SiC interface, leading to the reduction of contact angle. Reliable brazing of SiC was achieved using titanium deposition at 900°C for 10 minutes, and the typical interfacial microstructure of Ti-coated SiC/SAC was SiC/TiO_x + Ti₃SiC₂/Sn(s,s). Comparing to direct brazing, Ti–Sn compounds in the brazing seam were effectively reduced and the mechanical property of joints was dramatically improved by titanium coating. The optimal average shear strength of SiC joints reached 25.3 MPa using titanium coating- assisted brazing, which was ∼62% higher than that of SiC brazed joints using SAC-Ti filler directly.     

Intermediate temperature oxidative strength degradation of a SiC/SiNC composite with a polymer‐derived matrix

The article describes an experimental investigation of oxidative degradation in mechanical performance of a SiC fiber‐reinforced composite with a SiCN matrix produced by polymer infiltration and pyrolysis. Tensile stress rupture and retained strength tests were performed at 800°C in dry air and in water vapor. Fracture surfaces were examined to determine the degree of fiber pull‐out and constituent oxidation and to measure radii of representative fiber fracture mirrors. The results indicate that degradation in tows adjacent to cut surfaces occurs equally rapidly in water vapor with or without application of stress; regions in the composite interior and near as‐processed (uncut) surfaces appear far less affected. Similar effects are evident but less pronounced in dry air. Although oxidation of fiber coatings is observed in some cases, collectively the results suggest that fiber degradation is the main mechanism leading to reduced composite strength.