Physicomechanical and thermophysical properties of SiC-based ceramics

Fine-grain SiC-based ceramics have been produced via infiltration of molten silicon into preforms fabricated from SiC and graphite powders, with a phenol-formaldehyde resin as a binder. The materials thus prepared have a density of 2.70–3.15 g/cm³, dynamic modulus of elasticity from 200 to 400 GPa, compressive strength from 800 to 1900 MPa, bending strength from 150 to 315 MPa, thermal expansion coefficient (KTE) of 4.1 × 10⁻⁶ K⁻¹, and thermal conductivity of 140–150 W/(m K). Their properties are compared to those of known silicon carbide materials fabricated by other processes. The results indicate that the density and physicomechanical properties of the silicon carbide ceramics depend little on the fabrication process and are determined primarily by the SiC content. Increasing the SiC content from 20 to 99.5 wt % increases the density of the ceramics from 2.2 to 3.15 g/cm³ and leads to an exponential rise in their physicomechanical parameters: an increase in modulus of elasticity from 95 to 430 GPa, in compressive strength from 120 to 4200 MPa, and in bending strength from 70 to 410 MPa. The thermal conductivity of the ceramics depends very little on the fabrication process, falling in the range 100–150 W/(m K) over the entire range of SiC concentrations. Their KTE decreases slightly, from 4.3 × 10⁻⁶ to 2.4 × 10⁻⁶ K⁻¹, as the SiC content increases to 99–100 wt %.     

Microstructure and mechanical properties of RB-SiC/MoSi2 composite

Microstructure, high temperature strength and oxidation behaviour of reaction bonded silicon carbide, RB-SiC/17 wt% MoSi₂ composite prepared by infiltrating a porous RB-SiC bulk (after removal of free silicon) with molten MoSi₂ were investigated. There was good bonding between the SiC and MoSi₂ particle, without a significant reaction zone and microcracking caused by the thermal mismatch stresses. A thin (2 nm) layer, however, was observed at the SiC/MoSi₂ interfaces. At room temperature, the composite exhibited a bending strength of 410 MPa, which is 20% loss in comparison to that of RB-SiC alone (containing 10 wt% free silicon). However, the composite strength increased to a maximum of 590 MPa in the temperature range 1100 and 1200° C and dropped to 460 MPa between 1200 to 1400° C, after which the strength remained constant. The passive oxidation of the composite in dry air in the temperature range 1300 to 1400° C was found to follow the parabolic rate law with the formation of a protective layer of cristobalite on the surface.     

二氧化硅/木材复合材料的微观结构与物理性能

为了明确二氧化硅/ 木材复合材料的微观结构与物理性能, 通过EDAX 及XRD 的测定分析了溶胶2凝胶法制备的二氧化硅/ 木材复合材料的微观结构, 通过应力松弛、介电性质、热重、表面显微硬度的测定分析了该材料的物理性能。结果表明: 生成的二氧化硅与增重率呈正相关, 在纤维饱和点以下, 生成的二氧化硅存在于细胞壁中。XRD 衍射峰位置没有改变, 增重率增大, 峰强减弱, 结晶度减小。应力松弛量变小, 材料内部的结合力增强, 分子间产生了交联结合。介电常数值增大, 介电损耗随着频率的增加呈先增加后减小的趋势, 室温下, 出现最大峰值的频率均在log f = 6. 5 Hz 附近。热失重过程中, 快速失重的起始温度提高, 残余质量增加, 表面显微硬度提高。      

Microstructure and mechanical properties of an Al–Ni–Co intermetallics reinforced Al matrix composite

A new intermetallic particle reinforced metal matrix composite was produced from pure Al and 15 wt% Al₇₂Ni₁₂Co₁₆ quasicrystalline particles by stir-casting method, followed by hot-extrusion. Microstructural analysis of the as-cast composite shows that the Al₇₂Ni₁₂Co₁₆ quasicrystalline phase has transformed to the crystalline phase Al₉(Co, Ni)₂ and an eutectic structure has formed in the Al matrix during the casting process. The particle size of the Al₉(Co, Ni)₂ phase is much smaller than that of the original quasicrystalline particles. After extrusion, the composite has a more uniform distribution of the reinforcement particles and eutectic structure as well as a reduced porosity. Tensile tests indicate that the mechanical properties of the as-cast composite are improved over the matrix properties remarkably, except for the ductility. The strength and ductility of the composite can be improved by the hot-extrusion, while the elastic modulus can be slightly decreased.     

Fibre and interfacial properties measured in situ for a 3D woven SiC/SiC-based composite with glass sealant

In situ fibre strength Weibull parameters and fibre pullout length distributions were investigated for a 3D woven SiC/SiC-based composite with glass sealant oxidation protection system after tensile testing at 1000°C and 1200°C in air. Results were similar to those previously observed for unsealed specimens tested in vacuum (1200–1380°C), but significantly different from the case of unsealed specimens tested in air (1100–1200°C). Overall, it was concluded that the glass sealant provided excellent oxidation protection for the composite at the test conditions employed, i.e. short-term exposure up to 1200°C in air.     

Microstructure and mechanical properties of silicon carbide fibre-reinforced aluminium nitride composite

SiC continuous fibre (15 vol%)/AlN composite was fabricated using a sintering additive of 4Ca(OH)₂ · Al₂O₃ by hot-pressing at 1650 °C and 17.6 MPa in vacuum. Analytical transmission electron microscopy and scanning electron microscopy were used to investigate the microstructure of as-fabricated and crept SiC fibre/AlN composites. The room-temperature mechanical and high-temperature creep properties of the composite were investigated by four-point bending. The incorporation of SiC fibre into AlN matrix improved significantly the room-temperature mechanical properties. This improvement could result from the crack deflections around the SiC fibres. However, the incorporation degraded severely the high-temperature creep properties under oxidizing atmosphere. This could be attributed to the development of the pores and various oxides at the matrix grain boundary and matrix/fibre interface during creep test.     

Microstructure and mechanical properties of graphite fiber-reinforced high-purity aluminum matrix composite

Industrial pure aluminum (0.5 wt% impurity elements) was utilized in many investigations of aluminum matrix composites at home and abroad. However, impurity elements in industrial pure aluminum may influence the interface during fabrication of composite at high temperature. Thereby, it is necessary to use high-purity aluminum (impurity elements less than 0.01%) as matrix to enable study the interface reaction between reinforcement and matrix. In this study, stretches of brittle Al₄C₃ at the fiber/matrix interfaces in Gr_f/Al composite were observed. The fracture surface of the composite after tensile and bending tests was flat with no fiber pull-out, which revealed characteristic of brittle fracture. This was related to Al₄C₃, as this brittle phase may break before the fiber during loading and become a crack initiation point, while the corresponding crack may propagate in the fiber and the surrounding aluminum matrix, finally resulting in low stress fracture of composites.     

Microstructure and mechanical properties of tungsten composite reinforced by fibre network

In this paper the tungsten-fibre-net-reinforced tungsten composites were produced by spark plasma sintering (SPS) using fine W powders and commercial tungsten fibres. The relative density of the samples is above 95%. It was found that the recrystallization area in the fibres became bigger with increasing sintering temperature and pressure. The tungsten grains of fibres kept stable when sintered at 1350°C/16 kN while grown up when sintered at 1800°C/16 kN. The composite sintered at 1350°C/16 kN have a Vickers-hardness of ~610 HV, about 2 times that of the 1800°C/16 kN sintered one. Tensile tests imply that the temperature at which the composites (1350°C/16 kN) begin to exhibit plastic deformation is about 200°C-250°C, which is 400°C lower than that of SPSed pure W. The tensile fracture surfaces show that the increasing fracture ductility comes from pull-out, interface debonding and fracture of fibres.     

Microstructure and properties of squeeze cast Cu-carbon fibre metal matrix composite

There have been reported attempts of producing Cu based MMCs employing solid phase routes. In this work, copper was reinforced with short carbon fibres by pressure infiltration (squeeze casting) of molten metal through dry-separated carbon fibres. The resulting MMC's microstructure revealed uniform distribution of fibres with minimum amount of clustering. Hardness values are considerably higher than that for the unreinforced matrix. Addition of carbon fibres has brought in strain in the crystal lattice of the matrix, resulting in higher microhardness of MMCs and improved wear resistance. Tensile strength values of MMCs at elevated temperatures are considerably higher than that of the unreinforced matrix processed under identical conditions.     

Microstructure and bonding properties of diffusion–bonded composite comprising an Fe–Al alloy and carbon steel

This study discusses microstructure evolution, diffusion behavior and bonding strength of a couple comprising of an iron aluminium alloy (Fe–Al) and high carbon-steel (FeCMn) during diffusion bonding. A columnar microstructure evolves from the joint interface toward FeCMn and disappears in couples bonded for a long period. Aluminium diffusion from Fe–Al to FeCMn and columnar microstructure evolution are retarded as compared to a couple consisting of an Fe–Al and ferrite steel. The carbide in the FeCMn impedes the aluminium diffusion and retards the columnar grain evolution. When the carbide is dissolved in the ferrite during the aluminium diffsuion from Fe–Al, coarse grains evolve due to the coalescence of the columnar grains and a high-bonding strength is obtained. The hardness variation is minimum in the FeCMn of a couple bonded for a short period, which is explained by the microstructural changes in the columnar grain evolution and carbide dissociation.     

纳米陶瓷微米高温合金复合涂层组织与性能

采用溶胶-凝胶化学包覆法制备纳米陶瓷微米高温合金复合粉末,用HVOF喷涂技术制备了复合涂层,采用SEM观察和摩擦磨损实验分析了复合粉末和复合涂层的组织和性能.研究表明:复合粉末是以纳米陶瓷为外壳包覆微米级高温合金颗粒核心的核壳式结构;陶瓷壳在喷涂过程中形成液相与高温合金液相熔合,烧结成致密陶瓷相,部分陶瓷在冷却过程中析出结晶体;复合涂层与基体的结合强度为59.2 MPa,摩擦系数为0.766,磨损率比纯高温合金涂层降低了32%.     

Microstructure effect on the mechanical properties of heterogeneous composite materials

Statistical continuum theory is a powerful tool for predicting the effective properties of heterogeneous materials, where the shape of the fillers is random over the representative volume element (RVE). Due to this geometrical complexity of the shape of fillers, the heterogeneous material might present some anisotropy, which can be difficult to measure experimentally. In these cases the statistical continuum theory can be used as a tool to predict the degree of anisotropy. The aim of the present work is to present an implementation method based on analytical probability functions that can be easily integrated numerically to predict the effective properties of heterogeneous materials. In this regard, the strong-contrast version of the statistical continuum theory is used to predict the effective mechanical properties of heterogeneous materials. For validation, the effective mechanical properties of porous P-311 glass are predicted using the strong-contrast approach, and compared to experimental results and to ones calculated using a differential scheme (DS) model, which is based on Eshelby’s theory of inclusion embedded in an equivalent continuum matrix. Further, to demonstrate the effectiveness of the strong-contrast approach in dealing with anisotropic materials, the effective mechanical properties of a macroscopically anisotropic heterogeneous material are predicted and compared to ones calculated using the DS model. Finally, remarks on the implementation of the strong-contrast approach are highlighted through calculations using fillers with different sizes.     

Microstructure and mechanical properties ofin-situ network composite fibres of PBZT with nylon

A method for preparing composite fibres by infiltrating nylon into swollen poly(p-phenylene benzobisthiazole) (PBZT) fibre is described. PBZT fibre forms a microfibrillar network structure during the coagulation process.In-situ network composite (IC) fibres may be prepared by exchanging the coagulant with a solution containing the desired matrix material. These new composite fibres exhibit nearly identical mechanical properties and similar thermomechanical properties to those of so-called molecular composite (MC) fibres prepared from isotropic solutions of PBZT and nylon in methane sulphonic acid (MSA), The mechanical properties of these fibres were determined before and after heat treatment under tension. The structure of pure PBZT and its composite fibres (ICs' and MCs') were characterized using nitrogen adsorption (Brunauer-Emmett-Teller (BET) experiments), small-angle X-ray scattering, and scanning and transmission electron microscopy (SEM and TEM, respectively). The structure of both composite fibres was found to be a microfibrillar network of PBZT in a matrix of amorphous nylon. The average diameters of the PBZT microfibrils were in the range of 10 to 20 nm forin-situ network composites and approximately 4 nm for molecular composites.     

Microstructure and properties of in-situ prepared cellulosic biomass carbon based diatomite composite

Cellulosic biomass carbon based diatomite composite powders are synthesised in situ via the hypoxia pyrolysis method, and the micro-structure, surface morphology and performance of the as-prepared samples are characterised. When the concentration of the diatomite increases, the carbonaceous structure of the composites is highly disordered, the surface functional groups increase, and the typical porous structure of cellulosic biomass carbon based porous diatomite composite is measured. The adsorption capacity of organic dyes onto biomass carbon composite is enhanced. As the composite has many pore channels in the mesoporous region (especially for 2～10?nm), porous diatomite with excellent hydraulic conductivity favours a conducting MB solution and improves the chemisorption performance (153.2?mg/g) of biomass carbon in the composite.     

Microstructural characterization and fracture properties of SiC-based fibers annealed at elevated temperatures

Ceramic matrix composites (CMCs) have been proposed as potential structural materials for application of high temperature technologies. Excellent high temperature performance of CMCs requires that fibers must have high enough thermal stability and sufficient mechanical properties throughout the service life. In order to clarify the correlation between the mechanical properties and the microstructure of SiC-based fibers, SiC-based fibers were annealed at elevated temperatures in Ar for 1 h. After annealing, the fracture strengths on these fibers were evaluated at room temperature by tensile test; the microstructural features were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). Furthermore, the fracture mechanics was applied to estimate the fracture toughness and the critical fracture energy of these fibers. As a result, excellent microstructure and mechanical stabilities were observed for SiC fibers with near-stoichiometric composition and high-crystallite structure. Combining the microstructure examination with tensile test indicates that the thermal and mechanical stabilities of SiC fibers at high temperatures were mainly controlled by their crystallization and composition as well as other factors.     

3D打印琼脂糖和海藻酸钠复合水凝胶组织与性能研究

组织工程支架要求材料具有良好的生物相容性、相匹配的力学性能,以及利于细胞生长繁殖的形貌和结构.尽管人们已经开发出了大量生物材料用于制备组织工程支架,然而,组织工程支架的成形困难和力学性能差等问题仍然严重限制着其发展.以海藻酸钠为原材料,通过添加琼脂糖增强其力学性能,研究不同比例海藻酸钠/琼脂糖复合凝胶的结构和形貌变化,测试其力学性能.利用直写打印成形复合水凝胶支架,观察复合凝胶中微观孔隙的大小.结果表明:不同比例的海藻酸钠/琼脂糖复合凝胶含水量差异较小,均在90％附近.除了纯琼脂糖凝胶和体积比为1:2的复合凝胶外,其他比例的复合凝胶表面和断面均比较粗糙.琼脂糖能在一定程度上增强复合凝胶,海藻酸钠与琼脂糖的体积比2:1的复合凝胶压缩模量最高,可达0.353 MPa.碳酸钙的分解在复合凝胶中产生了亚微米级的孔隙,因此制备出的复合凝胶具有适合细胞生长繁殖的粗糙表面和微观孔隙.     

Magneto-thermo-elasticity of an electroconductive circular cylindrical shell featuring nonlinear deformations

The magneto-thermo-elastic interactions of a circular cylindrical shell immersed in applied magnetic and thermal fields are modeled and the shell’ nonlinear response and stability are investigated. The shell is made of a transversely isotropic, perfectly electroconductive non-ferromagnetic material. The shell’s deformation is characterized by small strains and moderately small rotations. A geometrically nonlinear, first-order transverse shear deformation shell model is developed and Galerkin’s method is used for spatial semi-discretization. The close connection between the static nonlinear response and the temperature-frequency interaction is explored to interpret the multi-solutions of the nonlinear response. Influence of prebuckling deformations on the buckling results is addressed and the significance of the applied magnetic fields on the response and stability is evaluated.     

Microstructure and properties of an infra-red transmitting chalcogenide glass-ceramic

A chalcogenide glass-ceramic (0.3 PbSe-0.7 Ge_1.5 As_0.5 Se₃) which transmits in the infra-red region between 8 and 12 m was produced from a phase separated parent glass. The glass transition temperature (T _g) was increased from 280 to 340°C by crystallizing the phase with the lowerT _g. Further heat-treatment produced a glass-ceramic that was up to 60% crystalline and contained PbSe, PbSe₂ and GeSe₂ crystals with a gran size of 0.5 m. The infra-red transmission of the glass-ceramic decreased with increased crystallinity. The glass-ceramic modulus of rupture (38 MN m^–2) was increased to as much as twice that of the glass and the Vickers hardness increased by 30% to 280 kg mm^–2.