Microstructural effects on the sliding-wear resistance of pressureless liquid-phase-sintered SiC under diesel fuel

The sliding wear of pressureless liquid-phase-sintered (PLPS) SiC ceramics under diesel fuel lubrication was investigated. It was found that the sliding-wear behaviour under diesel fuel is similar to what is observed under lubrication with the chemically similar paraffin oil: initial mild, plasticity-controlled wear followed by severe, fracture-controlled wear, with a well-defined wear transition. Compared to paraffin oil, however, diesel fuel lubrication decreases the extent of the mild-wear regime and results in damage that is more severe due to its lower viscosity. Also, it was found that the wear resistance under diesel fuel decreases with increasing contact load and sliding speed. Subsequent investigation of the effects of the intergranular phase content, grain size, and grain shape demonstrates nevertheless that the wear performance of PLPS SiC under diesel lubrication can be improved via microstructural control by (i) decreasing the intergranular phase content, and (ii) decreasing the SiC grain size or increasing its aspect ratio. The results of this work are likely to have relevant implications for the design of wear resistant PLPS SiC ceramic components for diesel engines.     

Anomalous oxidation behaviour of pressureless liquid-phase-sintered SiC

The oxidation behaviour of pressureless liquid-phase-sintered (PLPS) SiC, an important non-oxide engineering ceramic, was investigated, and was found to be universally anomalous. Thermogravimetry oxidation tests performed in oxygen in the temperature range 1000-1225 °C on three PLPS SiC ceramics fabricated with different combinations of Al₂O₃-RE₂O₃ (RE = Gd, Sc, or Sm) as sintering aids indicated that the oxidation is in all cases passive and protective, but unexpectedly anomalous in the sense that the oxidation resistance does not scale inversely with temperature. In particular, in all cases it was observed that there is less oxidation above 1100 °C than below, in clear contradiction to the expectation for a diffusional process. Exhaustive characterization of the oxide scales by scanning electron microscopy, X-ray energy dispersive spectrometry, and X-ray diffractometry, together with detailed modeling of the oxidation curves, showed that the origin of this universal anomalous oxidation behaviour lies in the marked crystallization within the oxide scale of rare-earth silicates that act as effective barriers against the inward diffusion of oxygen thus improving notably the oxidation resistance. A strategy is proposed to provide PLPS SiC, and probably other SiO₂-scale-forming ceramics that are sintered using rare-earth oxides, with the superior oxidation resistance at moderate temperatures (i.e., <1100 °C) that they do not currently have.     

Sliding-wear resistance of liquid-phase-sintered SiC containing graphite nanodispersoids

The influence was investigated of a graphite nanodispersoid addition on the lubricated sliding-wear behaviour of liquid-phase-sintered (LPS) SiC ceramics fabricated by spark-plasma sintering (SPS). The graphite nanodispersoids, introduced into the microstructure of the LPS SiC ceramic to act as self-lubricating phase, were obtained by graphitization of diamond nanoparticles during the SPS. It was found that the graphite nanodispersoid addition results in a lower resistance to mild wear, attributable to the lower hardness of graphite and the null lubrication it provides. Moreover, the graphite nanodispersoids, which mostly locate at grain boundaries/faces, worsen the cohesion of SiC grains, contributing together with the higher mild-wear rate to an early transition to the severe-wear stage. On the contrary, the graphite nanodispersoids were effective at improving the resistance to severe wear because they increase the fracture toughness while providing some external lubrication. Relevant implications for the microstructural design of advanced triboceramics are discussed.     

Hardness degradation in liquid-phase-sintered SiC with prolonged sintering

The effect of the sintering time on the Vickers hardness of liquid-phase-sintered SiC with 10 wt% YAG additives was studied for materials fabricated in Ar and N₂ atmospheres. The hardness of the Ar-sintered materials was found to decrease markedly with increasing sintering time, whereas, in contrast, the decrease was only marginal for the N₂-sintered materials. Berkovich nanoindentation tests showed that the softening could not be attributed in either case to degradation of the SiC grains, thereby pointing to the intergranular YAG phase as the responsible. That the cause was degradation of the YAG phase was confirmed by X-ray energy-dispersive spectrometry and Hertzian indentation tests. The far more pronounced softening observed for the Ar-sintered materials reflects the more severe degradation that the YAG phase undergoes during sintering in this atmosphere.     

Microstructural development of liquid-phase-sintered silicon carbide during annealing with uniaxial pressure

β-SiC powders containing 1.1 wt.% α-SiC particles as seeds were hot-pressed at 1800 °C and then annealed at 2000 °C under 25 MPa uniaxial pressure to enhance grain growth. Microstructural development during annealing with pressure was investigated quantitatively and statistically using image analysis. The bimodal grain-thickness distribution in samples annealed with pressure was obtained due to abnormal grain growth of some grains. In situ-toughened microstructure has been developed after 3-h annealing. The grain-thickness and aspect ratio of large grains increase with annealing time, but grain growth comes mainly from increases in thickness after 3-h annealing, owing to the impingement of large gains. Typical flexural strength and fracture toughness of 4-h annealed sample were ∼500 MPa and ∼7.5 MPa m^1/2, respectively.     

非规则框架结构设计概述

随着建筑行业的快速发展,以及城镇化进程的加快,越来越多的非规则框架结构建筑出现,传统的框架结构设计已经不能满足结构设计的需要,非规则框架结构的设计优点在传统框架结构设计中显得尤为突出.笔者从框架结构在结构设计中存在的主要问题入手,对其中非规则形状的框架结构的重要设计要点和特殊部位的具体设计进行说明.     

空间望远镜用C/SiC镜面研制综述

综述了空间望远镜的主镜用高强度、高表面精度、低热膨胀系数的低温（约4K）用镜面的制备和检测过程.日本将Φ710mm的高强度反应烧结SiC材料已用于红外望远镜镜面.在短切炭纤维增强C/C复合材料毛坯的基础上进行液相硅渗透（LSI）而制备的C/SiC复合材料在光学镜面方面具有更大的优势.通过提高C/C复合材料毛坯中沥青基炭纤维体积分数及控制硅化速度,可有效地提高LSI-C/SiC复合材料的机械性能和表面光学精度;通过不同规格的炭纤维的混杂化,可使C/SiC复合材料热膨胀系数的各向异性降低至小于4％的差异.SiC、Si-SiC浆料涂层处理可有效地提高表面精度至2 nm rms的极高要求.     

Microstructural evolution of SiC powder in molten silicon

SiC_powder/Si_matrix composites represent a new class of microstructurally toughened materials. The interactions between molten silicon and submicronic SiC powder have been considered since it could originate some limitations on the final properties of the material. Experiments putting in interaction a SiC powder and molten Si were performed while heating up to final values ranging between 1450 and 1600?°C for duration up to 8?h. The volume ratio of SiC and silicon was equal to one and SiC particles were freely dispersed within the liquid. X-ray diffraction analyses demonstrated that the apparent crystallites size increase of SiC powder followed a ripening law corresponding to a limitation either by volume diffusion or by dissolution into the liquid. Depending on the relevant mechanism, the activation energy of the crystallites’ growth has been found equal to 357?±?50?kJ?mol^?1 or 441?±?57?kJ?mol^?1. An agglomeration-coarsening process of SiC particles was also identified which promoted a quick formation of larger particles.     

Effect of the sintering additive content on the protective passive oxidation behaviour of pressureless liquid-phase-sintered SiC

The long-term oxidation behaviour in air of pressureless liquid-phase-sintered SiC was investigated as a function of the sintering-additive content (a mixture of Y₂O₃ and Al₂O₃ in the 3:5 molar ratio) at oxidizing temperatures in the interval 1100–1300 °C. It is shown that oxidation under these mild conditions is always passive, and with formation of protective oxide scales. However, the oxidation kinetics cannot be described appropriately by the parabolic-rate law. Instead, due to the gradual crystallization of the oxide scales during oxidation, it is more complex, exhibiting two different stretches given respectively by the arctan- and parabolic-rate laws. Furthermore, it was found that the rate-limiting mechanism of the initial arctan oxidation is the outward diffusion of metal cations from the secondary intergranular phase into the oxide scale, with the activation energy of the oxidation being very high and decreasing from 545 to 432 kJ/mol with increasing sintering-additive content from 5 to 20 wt%. The rate-limiting mechanism of the subsequent parabolic oxidation is however the inward diffusion of oxygen through the multicomponent oxide scale, with the activation energy being lower than before and also decreasing from 345 to 205 kJ/mol as the sintering-additive content increases from 5 to 20 wt%. It is also shown that the oxidation resistance decreases with increasing sintering-additive content, but that while the decrease is moderate up to 10 wt%, it is very marked for greater contents.     

Effects of sintering atmosphere on grain morphology of liquid-phase-sintered SiC with Al2O3 additions

The effects of sintering atmospheres of Ar and N₂ on grain morphology were investigated for pressureless liquid-phase-sintered (LPS) SiC with Al₂O₃ additions. When increasing the sintering temperature, the SiC grain size and its aspect ratio increased in both sintering atmospheres. With a 2 mass% Al₂O₃ addition, no distinct difference was observed between the grain morphology of SiC sintered in the Ar atmosphere and that sintered in the N₂ atmosphere. With a 15 mass% Al₂O₃ addition, sintering in a N₂ atmosphere led to retarded grain growth and this resulted in a fine homogeneous microstructure, whereas sintering in an Ar atmosphere enhanced the grain growth compared with that in 2 mass% Al₂O₃. The effects of atmosphere on the grain morphology depend on the amount of Al₂O₃ addition, and this also affects the grain growth process of solution-reprecipitation. The mechanical properties of the SiC are also considered.     

浅谈胶鞋设计新概念

<正>胶鞋是贴近人们日常生活、工农业劳动和体育锻炼的橡胶制品。长期以来,通过性能改进和外形变化不断出现新的品种和款式,以满足人们参与各种活动的需要。特别从20世纪七、八十年代以来,     

Toward rational design of hierarchical beta zeolites: An overview and beyond

The ever-growing need to increase the efficiency of crude oil refining raises the prospects for utilizing advanced catalytic materials to supply increasing global demands for fine chemicals and petrochemicals. Zeolites have been used as indispensable catalysts in many commercial refining processes. Among them, zeolite beta is one of the most widely produced zeolite materials with industrial significance due to its large micropores and three-dimensional pore structures. This article discusses recent progress on hierarchical beta zeolites from various synthetic strategies. Using zeolite beta as a representative case, we provide an overview on key aspects that are applicable to different zeolites via a variety of topics, such as selection of different template materials, tailoring of mesopore sizes by base leaching, organotemplate-free synthesis of hierarchical zeolites, and selective desilication. This article concludes with a perspective on the design of scalable hierarchical zeolites that are more relevant as industrial catalysts in commercial processes.     

Microstructural design of phlogopite glass-ceramics

Fluorophlogopite glass-ceramics in the MgO–Al₂O₃–SiO₂ glass systems were crystallized by two stages thermal treatments. Obtained glass-ceramics were characterized by DTA, X-ray diffraction and scanning electron microscopy. The heat treated samples at 705°C/3 h + 1000°C/15 min showed 2570.32 ± 98 MPa microhardness. The effect of heat treatment and preparation methods on microhardness and microstructure were studied. The microstructure of samples showed phlogopite and forsterite phases and solved or soft edge in the semi-rod like crystals was recorded which can explain the small value of microhardness. The various microhardness were determined in the range between 1591.61 to 9610 ± 147 MPa.     

Microstructural Evolution in Liquid-Phase-Sintered SiC: Part I, Effect of Starting Powder

The effect of starting SiC powder (β-SiC or α-SiC), with simultaneous additions of Al₂O₃ and Y₂O₃, on the microstructural evolution of liquid-phase-sintered (LPS) SiC has been studied. When using α-SiC starting powder, the resulting microstructures contain hexagonal platelike α-SiC grains with an average aspect ratio of 1.4. This anisotropic coarsening is consistent with interface energy anisotropy in α-SiC. When using β-SiC starting powder, the β→α phase transformation induces additional anisotropy in the coarsening of platelike SiC grains. A strong correlation between the extent of β→α phase transformation, as determined using quantitative XRD analysis, and the average grain aspect ratio is observed, with the maximum average aspect ratio reaching 3.8. Based on these observations and additional SEM and TEM characterizations of the microstructures, a model for the growth of these high-aspect-ratio SiC grains is proposed.     

Mechanical, Thermal, and Microstructural Properties of Neutron-Irradiated SiC

The thermal and electrical conductivities, fracture strength, swelling, and microstructure of sintered alpha-phase Sic and siliconized reaction-bonded Sic materials were studied before and after irradiation with reactor neutrons. The effects of helium gas on the properties were studied using boron enriched in '^OB to produce helium from ¹⁰B(n, α) ⁷Li. The decrease in thermal conductivity after irradiation is due to enhanced phonon scattering by radiation-induced defects. Electrical, mechanical, and thermal properties exhibit partial recovery after heat treatment to 1000 to 1500 K. An overall consistency is observed in radiation-produced property changes and annealing and a model is presented to explain the changes.     

Microstructural Characterization of SiC–SiC Joint by Raman Spectroscopy

Analysis of the SiC–SiC joint and its brazing mixture has been performed using a Raman spectroscopy microprobe technique. A careful mapping of the sample clearly shows the spatial distribution of the chemical species close to and within the joint. A different distribution of the 4H and 6H α-SiC polytypes, grown during the brazing process, was observed inside the joint. Furthermore, identification of the bands related to the Nowotny phase was also possible.     

Microstructural Evolution in Liquid-Phase-Sintered SiC: Part III, Effect of Nitrogen-Gas Sintering Atmosphere

Effects of N₂ sintering atmosphere and the starting SiC powder on the microstructural evolution of liquid-phase-sintered (LPS) SiC were studied. It was found that, for the β-SiC starting powder case, there was complete suppression of the β→α phase transformation, which otherwise goes to completion in Ar atmosphere. It was also found that the microstructures were equiaxed and that the coarsening was severely retarded, which was in contrast with the Ar-atmosphere case. Chemical analyses of the specimens sintered in N₂ atmosphere revealed the presence of significant amounts of nitrogen, which was believed to reside mostly in the intergranular phase. It was argued that the presence of nitrogen in the LPS SiC helped stabilize the β-SiC phase, thereby preventing the β→α phase transformation and the attendant formation of elongated grains. To investigate the coarsening retardation, internal friction measurements were performed on LPS SiC specimens sintered in either Ar or N₂ atmosphere. For specimens sintered in N₂ atmosphere, a remarkable shift of the grain-boundary sliding relaxation peak toward higher temperatures and very high activation energy values were observed, possibly due to the incorporation of nitrogen into the structure of the intergranular liquid phase. The highly refractory and viscous nature of the intergranular phase was deemed responsible for retarding the solution–reprecipitation coarsening in these materials. Parallel experiments with specimens sintered using α-SiC starting powders further reinforce these arguments. Thus, processing of LPS SiC in N₂ atmosphere open the possibility of tailoring their microstructures for room-temperature mechanical properties and for making high-temperature materials that are highly resistant to coarsening and creep.     

Microstructural and mechanical characteristics of hybrid SiC/Cu composites with nano- and micro-sized SiC particles

Hybrid Cu-SiC composites have been highly considered in order to achieve a combination of electrical and thermal properties along with high strength and wear resistance. However, limited investigations have ever been conducted over the effects of using hybrid (combination of nano and micro size) particles on the wear resistance behavior of these composites. Hence, in the present study, Cu-SiC nanocomposite with 4?vol% nanosize and 4?vol% microsize SiC, and Cu-SiC microcomposite with 8?vol% micro- SiC were fabricated through mechanical milling and hot pressing process. Results revealed the homogeneous dispersion of SiC particles in the matrix, high densification, and ultrafine-grain matrix for the samples. The hybrid nanocomposite showed higher wear resistance, lower friction coefficient and enhanced compressive strength in comparison to the microcomposite. The presence of hybrid particles caused a significant decrease of 61% in the matrix grain size, 53% decrease in the width of wear track, and 35% increase in the compressive strength compared to the nanostructured Cu sample. Investigation of the worn surfaces showed that delamination is the predominant wear mechanism in the Cu-SiC composites. Using hybrid SiC led to decreasing the formation of cracks and pits, and plastic deformation in the worn surfaces.     

SHS extrusion: An overview

Overviewed is the state-of-art in the science and technology for extrusion of still hot SHS products (also termed SHS extrusion). Addressed are the experimental and theoretical studies on rheological behavior of materials with a limited duration of their ductility. The structures/properties of extruded materials and items are also presented and discussed. Presented are the examples of practical implementation of the process to production of rod-like electrodes for electrospark alloying from new multifunctional materials, such as nanograined cermets, MAX compounds, and intermetallics.     

Green composites: An overview

The use of natural fibers to reinforce polymers is a well‐established practice, and biocomposites are increasingly used in sectors such as automotive and construction. Green composites are a specific class of biocomposites, where a bio‐based polymer matrix is reinforced by natural fibers, and they represent an emerging area in polymer science. This work discusses the environmental benefits deriving from the use of natural fibers in polymer composites and from substitution of oil‐derived polymers by bio‐based polymers as matrix material. New trends in the selection of natural fibers, that is, from waste rather than from valuable crops are described. Recently developed thermoplastic and thermosetting bio‐based polymers are reviewed, and commercially available green composites obtained thereof are discussed. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers