Polysilane-derived porous SiC preforms for the preparation of SiC-glass composites

Three different types of SiC preforms, with open porosities between 60 and 70%, were prepared by pyrolysis of polysilanes and polycarbosilanes at 1670 K. some samples of each type of SiC structure were oxidized in air at 1070 K to promote wetting by the glass melt. Preforms were then applied for gas pressure infiltration with an alumosilicate glass melt at 1670 K in 3 MPa argon, and the influence of pore-size distribution and surface composition on infiltration time was investigated. The composition of the precursors was determined by chemical analysis, and pyrolysed SiC was analysed by thermal decomposition. The porosity and pore-size distribution of the SiC-preforms were measured on photographs of sample cross-sections. The SiC-glass composites were examined by light microscopy, scanning electron microscopy and electron scanning microanalysis.     

The effect of particle size distribution on the rheology of an industrial suspension

The viscosity of a proprietary dental composite material, consisting of suspensions of crushed glass in a polymeric liquid of a 50/50 w/w urethane dimethacrylate and triethylene glycol dimethacrylate mixture has been measured using a tube viscometer. Narrow-sized fine (ultimate particle size of 0.2 m, which agglomerate to form particles with a mean diameter of 0.05 m), medium (d ₅₀= 1.7 m) and coarse (25.5 m) particle fractions were used as well as bimodal and trimodal mixtures. Total solids concentrations from 17% to 76(77)% by volume were covered. The results were analysed using extensions of the Farris theory for mixtures and reduced to the viscosity functions, h _i ( _i), for the three monomodal fractions. They were fitted to the Mooney, Krieger-Dougherty or the three-parameter Cheng equation. The effect of particle size distribution on the Krieger-Dougherty parameters is discussed. The viscosity functions summarize the experimental results and allow the viscosities of bimodal and trimodal mixtures not measured to be predicted. The use of the predictions for the formulation of the dental material is discussed. The methodology described can be used in the design of other suspension products.     

Particulate silicon nitride-based composites

In an attempt to optimize the structure and properties of silicon nitride ceramics, a variety of novel processing techniques and materials compositions have evolved over the last 15 years. Among the most important, was the development of various silicon nitride-based composites. A review of particulate, silicon nitride-based composites other than whisker- or platelets-reinforced, is presented. Materials based on silicon nitride and SiAlONs, with additions of carbides, nitrides and borides of transition metals are described. Special emphasis is placed on TiN- and TiC-containing ceramics. The manufacture of composites by hot pressing, reaction sintering, pressureless and gas-pressure sintering is discussed. The data on properties, including conductivity, density, Young's modulus, strength, fracture toughness, hardness, thermal expansion, wear, creep and oxidation resistance are presented. Analysis of actual and potential uses of the selected composites demonstrates that the particulate composites are very promising as tool, structural and electronic materials.     

Anisotropic rheology of continuous fibre thermoplastic composites

The anisotropic rheology of continuous fibre thermoplastic composites has been resolved into along fibre and transverse components using a balanced pair of off-axis specimens in a commercial rotational rheometer with a parallel disc geometry. Existing theory has been used to demonstrate a small but consistent difference between the components of shear modulus and viscosity obtained from dynamic viscoelastic measurements. Transverse viscosity and yield stress are higher. The use of maximum shear rate (product of maximum shear strain and angular frequency), used here with apparent Maxwell viscosity, eliminates the strain dependence of the rates and correlates with steady shear viscosity and shear rate data. Differences in yield stress and viscosity are associated with the fibre-polymer system and the fibre volume fractions.     

Grain size effect on the strengthening behavior of aluminum-based composites containing multi-walled carbon nanotubes

Strengthening efficiency of multi-walled carbon nanotubes (MWCNTs) is investigated for aluminum-based composites with grain sizes ranging from ∼250 to ∼65 nm. The strength of composites is significantly enhanced proportional to an increase of the MWCNT volume. However, the increment differs depending on deformation mode of the matrix. The strengthening efficiency of MWCNTs in ultrafine-grained composites is comparable with that predicted by the discontinuous fiber model, whereas the efficiency becomes half of the theoretical prediction as grain size is reduced below ∼70 nm. For nano-grained aluminum, activities of forest dislocations diminish and dislocations emitted from grain boundaries are dynamically annihilated during the recovery process, providing a weak plastic strain field around MWCNTs. The observation may provide a basic understanding of the strengthening behavior of nano-grained metal matrix composites.     

Effect of temperature on interfacial shear strengths of SiC-glass interfaces

The in situ temperature dependencies of both the debonding, _d, and frictional, _f, shear stresses of a C-coated 140 m SiC monofilament (Textron SCS-6 SiC fibre) were measured using the single fibre pullout-test. Two matrices, a borosilicate (7740 Corning Glass) and a soda-lime (Thomas Scientific) with different thermal expansion coefficients, were tested. At lower temperatures both _d and _f were found to decrease linearly with increasing temperature as a result of the relaxation of the residual stresses developed during processing, which were compressive in both cases. The stress free debonding shear stress for the borosilicate matrix was found to be 3.5 ± 1 M Pa and the friction coefficient between that matrix and the fibres was calculated to be 0.18. Fibre oxidation are believed to be responsible for enhanced bonding between the fibres and the borosilicate matrix at higher temperatures which results in an increase in both _d and _f. The large thermal expansion mismatch between the soda-lime matrix and the SiC fibres resulted in radial cracking of the former during processing. A technique is described where the whole temperature dependence of the interfacial shear stresses can be measured by a single specimen.     

Specimen size effect on the residual properties of engineered cementitious composites subjected to high temperatures

In this study, size effect on the residual properties of Engineered Cementitious Composites (ECC) was investigated on the specimens exposed to high temperatures up to 800 °C. Cylindrical specimens having different sizes were produced with a standard ECC mixture. Changes in pore structure, residual compressive strength and stress–strain curves due to high temperatures were determined after air cooling. Experimental results indicate that despite the increase of specimen size, no explosive spalling occurred in any of the specimens during the high temperature exposure. Increasing the specimen size and exposure temperature decreased the compressive strength and stiffness. Percent reduction in compressive strength and stiffness due to high temperature was similar for all specimen sizes.     

Particulate dental composites under sliding wear conditions

The wear of behaviour of dental composites, consisting of glass particles in a PMMA-matrix, is investigated under dry sliding conditions against a smooth hardened steel surface. Wear mechanisms were identified, such as plowing and patch formation; their individual contribution to the total wear of the composite is a function of particle size, filler content and filler/matrix adhesion. In addition, the inter-particle-distance determines the transition from one dominating mechanism to the other.     

Influence of dispersant size on rheology of non-aqueous ceramic particle suspensions

The surface of alumina, which is hydrophilic in general, was made hydrophobic either by adsorption of polymer (phys-adsorption) or by an alkylation reaction with alcohol (chem-adsorption) to enable dispersion in dodecane. Hypermer A70 (8.2 nm) was used as the polymer and 1-octanol (1.2 nm), 1-decanol (1.5 nm) and 1-hexadecanol (2.5 nm) were used as the alcohol (values in brackets are the approximate thickness of the steric barrier). Rheological measurements of ceramic suspensions indicate that it is possible to achieve a high solid loading (50 vol.%) with relatively low viscosity (0.25 Pa s at 100 s^?1, the typical shear rate for pumping of liquids in pipes) as long as the stabilising molecule is large enough. The observed rheological behaviour fitted the Quemada viscosity model quite well when excluded volume effects were taken into account. Addition of 2.8 wt.% of Hypermer A70 with respect to weight of alumina was enough to stabilise the particles.     

An asymptotic approach to size effect on fracture toughness and fracture energy of composites

Size effect on fracture toughness and fracture energy of composites is investigated by a simple asymptotic approach. This asymptotic analysis based on the elastic/plastic fracture transition of a large plate with a small edge crack is extended to study fracture of composite. A reference crack length, a^*, is used in the model, which indicates an ideal elastic/plastic fracture transition defined by the yield strength and plane strain fracture toughness criteria. Experimental results of cementitious materials available in literature are analyzed and compared. It is shown that the common K_R-curves can also be obtained by the current asymptotic model. Furthermore, a local fracture energy distribution concept is also discussed and compared with the present asymptotic approach.     

麦秸纤维增强聚丙烯复合材料的熔融流变性

用挤塑模压的方法制备麦秸纤维增强聚丙烯复合材料, 研究了麦秸纤维添加量、尺寸及马来酸酐接枝聚丙烯(MAPP) 添加量、温度对麦秸纤维增强聚丙烯复合材料熔融流变性的影响。麦秸纤维添加量从10 wt%增加至30 wt%时, 复合材料的熔融黏度增加。马来酸酐接枝聚丙烯的加入提高了体系的流动性, 熔融黏度降低。麦秸纤维以长纤维和纤维束作为增强材料时, 复合材料的熔融黏度降低, 以细小纤维作为增强材料时, 复合材料的熔融黏度增加。温度由170 ℃升高至190 ℃, 剪切速率由0. 01 s -1增加至0. 1 s -1 时, 麦秸纤维增强聚丙烯复合材料的黏度降低。      

The joint effect of shape and size of reinforcement on progressive damage of random composites

The properties of composites made by placing inclusions in a matrix are often controlled by the shape and size of the particles. In order to study the joint effect of shape and size of inclusions, we characterize the random shape of particles in composite mathematically by applying Fourier series, then generating random mesostructure of composite for cases of inclusions with (1) same size and different shape, (2) different size and same shape, or (3) random size and shape. Crack paths and effective stress–strain curves of these cases are predicted using spring network method which is given in detail. The study shows we need more elaborate statistical evaluation due to the random nature of composites. This paper outlines an approach to study effect of inclusion geometry on the elastic properties and crack of random composites.     

Particulate reinforced metal matrix composites — a review

The physical and mechanical properties that can be obtained with metal matrix composites (MMCs) have made them attractive candidate materials for aerospace, automotive and numerous other applications. More recently, particulate reinforced MMCs have attracted considerable attention as a result of their relatively low costs and characteristic isotropic properties. Reinforcement materials include carbides, nitrides and oxides. In an effort to optimize the structure and properties of particulate reinforced MMCs various processing techniques have evolved over the last 20 years. The processing methods utilized to manufacture particulate reinforced MMCs can be grouped depending on the temperature of the metallic matrix during processing. Accordingly, the processes can be classified into three categories: (a) liquid phase processes, (b) solid state processes, and (c) two phase (solid-liquid) processes. Regarding physical properties, strengthening in metal matrix composites has been related to dislocations of a very high density in the matrix originating from differential thermal contraction, geometrical constraints and plastic deformation during processing.     

Strain gradients and continuum modeling of size effect in metal matrix composites

Summary Constitutive modeling for the particle size effect on the strength of particulate-reinforced metal matrix composites is investigated. The approach is based on a gradient-dependent theory of plasticity that incorporates strain gradients into the expression of the flow stress of matrix materials, and a finite unit cell technique that is used to calculate the overall flow properties of composites. It is shown that the strain gradient term introduces a spatial length scale in the constitutive equations for composites, and the dependence of the flow stress on the particle size/spacing can be obtained. Moreover, a nondimensional analysis along with the numerical result yields an explicit relation for the strain gradient coefficient in terms of particle size, strain, and yield stress. Typical results for aluminum matrix composites with ellipsoidal particles are calculated and compare well with data measured experimentally.     

Investigating the effect of mixing time on the crystallite size and lattice strain of the AA7075/TiC composites

With numerous reinforcements, aluminum and its alloys are finding growing applications in every sector of industry. Titanium carbide (TiC) is regarded as an outstanding reinforcing material as compared to widely used carbide particles because of its excellent physical and mechanical characteristics, as well as its especially good interfacial bonding (wetting) capacity with aluminum. In the present research work, the effect of the mixing time of the matrix and reinforcement powders has been investigated on the crystallite size and lattice strain of the AA7075–5 wt.% TiC composites. The mechanical properties of the developed composites were also investigated in terms of microhardness values. X-ray diffraction and scanning electron microscopy (SEM), transmission electron microscopy (TEM), particles size distribution analysis and x-ray energy dispersive spectroscopy (EDS) of the synthesized powder samples were done to see the effect of mixing time on their microstructures. The increase in mixing time led to a homogeneous distribution of 5 wt.% of TiC particles, a decrease in particles clustering. The considerable grain refining was confirmed, which reflected a reduction in particle size originating from a prolonged mixing time. The significant improvement in the crystallite size and microhardness of the produced composites were achieved with increasing mixing time.     

Characterization of rheology of fresh fiber reinforced cementitious composites through ram extrusion

Rheology behavior of fresh short fiber reinforced cementitious composites was investigated through ram extrusion. The experimental results were interpreted by a six-parameter Benbow-Bridgwater model. The effects of water contents, matrix composition, fiber volume and fiber types on the die entry and die land pressure were investigated and related to the compositions of the fresh short fiber reinforced cementitious composites. The six rheological parameters, together with the dynamic bulk stress and dynamic surface shear stress, were derived and compared for various composites. It was found that fresh short fiber reinforced cementitious composites had an obvious pseudo-plasticity and shear-thinning behavior. The water content, fiber concentration, fiber type and cement matrix influenced the rheology of the fresh composites in different mechanisms. The quantitative rheology parameters, such as bulk yield stress, dynamic bulk stress, wall shear yield stress and dynamic surface shear stress, provided promising interpretations for experimental results.

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Résumé Le comportement rhéologique de composites cimentaires, frais, renforcés de fibres courtes a été identifié à l'aide d'une extrudeuse à piston. Les résultats expérimentaux sont interprétés en utilisant le modèle de Benbow-Bridgwater. L'influence du dosage en eau, de la composition du ciment, du type de fibres et de leur pourcentage sur le diamètre d'entrée de la filière ainsi que le champ de pression dans la filière est étudiée et reliée à la composition de ces composites cimentaires. Six paramètres rhéologiques dont les seuils de cisaillement, interne et aux interfaces, sont identifiés et comparés pour les différentes compositions. Les composites cimentaires, renforcés de fibres courtes, étudiés présentent une pseudo-plasticité et un comportement rhéofluidifiant. Le dosage en eau, le pourcentage de fibre ainsi que le type de fibre et la composition du ciment, influencent le comportement rhéologique de ces composites cimentaires de différentes manières. Les valeurs numériques obtenues pour les paramètres rhéologiques et tribologiques, tels que les seuils et les contraintes de cisaillement, internes et aux interfaces, sont très encourageantes et permettent de conclure sur la fiabilité de ces résultats expérimentaux.