High Particle Volume Fractions Research Articles

Deposition of ordered two-phase materials using microfluidic print nozzles with acoustic focusing

The deposition of multiphase materials with microstructural control would enable new fabrication modalities, such as spatial variation in composition and the integration of microstructure and structural features. In this work, acoustically-excited microfluidic print nozzles were used to tailor the microstructure of printed composite filaments consisting of SiC fibers, solid BaTiO3 spheres, or hollow SiO2 spheres in an epoxy matrix. The results demonstrate that acoustic focusing is a promising technique to control microparticles and deposit two-phase ordered structures using a single nozzle. In addition to tuning the microstructure within a single print line, the overall concentration of particles can be increased by focusing the particle stream in a branched geometry, enabling the deposition of material with higher particle volume fractions than the initial ink composition. We show that even modest volume fractions of acoustically-focused and concentrated SiC fibers can produce printed composite filaments with unprecedented control over microstructural ordering that exhibit strengths rivaling polymer-matrix composites with higher volume fractions of stiffer fibers.

Friction performance of WC-Co coating with lithium grease and water lubrication

WC-Co coatings were sprayed by high-velocity oxygen fuel (HVOF) on bearing steels. The coating consisted of a high volume fraction of WC particles in cobalt matrix. The hardness of WC-Co coating was 60% higher than that of the bearing steel. The friction coefficients of WC-Co coating and bearing steel sliding against mild steel were tested with lithium grease and water lubrication. The friction results indicated that the friction coefficient increased with increasing loads. The friction coefficient of WC-Co coating was lower than that of bearing steels no matter with grease lubrication or with water lubrication.

Startup of electrophoresis in a suspension of colloidal spheres.

The transient electrophoretic response of a homogeneous suspension of spherical particles to the step application of an electric field is analyzed. The electric double layer encompassing each particle is assumed to be thin but finite, and the effect of dynamic electroosmosis within it is incorporated. The momentum equation for the fluid outside the double layers is solved through the use of a unit cell model. Closed-form formulas for the time-evolving electrophoretic and settling velocities of the particles in the Laplace transform are obtained in terms of the electrokinetic radius, relative mass density, and volume fraction of the particles. The time scale for the development of electrophoresis and sedimentation is significantly smaller for a suspension with a higher particle volume fraction or a smaller particle-to-fluid density ratio, and the electrophoretic mobility at any instant increases with an increase in the electrokinetic particle radius. The transient electrophoretic mobility is a decreasing function of the particle volume fraction if the particle-to-fluid density ratio is relatively small, but it may increase with an increase in the particle volume fraction if this density ratio is relatively large. The particle interaction effect in a suspension on the transient electrophoresis is much weaker than that on the transient sedimentation of the particles.

Mechanical and electrical properties of an ultrafine grained Al–8.5 wt. % RE (RE = 5.4 wt.% Ce, 3.1 wt.% La) alloy processed by severe plastic deformation

This work focuses on the effect of high pressure torsion (HPT) on the thermostability, microstructure, mechanical properties and electrical conductivity of an Al–8.5wt.% RE (RE stands for rare earth Ce and La in the present case) alloy with respect to its potential application in electrical engineering. HPT processing leads to the formation of a very homogeneous ultra-fine grained microstructure resulting from the fragmentation of RE-rich intermetallic particles down to the nanoscale. Deformation induced supersaturated solid solution of RE atoms in the Al matrix is demonstrated for the first time. The HPT processed material shows an extraordinary high mechanical strength attributed to the high volume fraction of nanoscaled intermetallic particles and the ultrafine grained (UFG) microstructure. The various strengthening contributions were analyzed, and it was shown that the increase of strength during short time annealing could be attributed to the clustering of RE atoms in solid solution. The HPT processing induces a significant reduction of the electrical conductivity, but it was partly restored by annealing thanks to the concomitant clustering of RE atoms, reduction of dislocation density and grain growth. The potential applications of UFG Al–RE alloys in electrical engineering are finally discussed.

Breakup of high solid volume fraction oil–particle cluster in simple shear flow

Abstract Experiments were conducted of the low-Reynolds number breakup and separation of oil–particle clusters characterized by a high volume fraction of relatively large solid particles, under simple shear in an aqueous solution. The breakup of such oil–particle clusters, and the separation of particles from the clusters, are evaluated as a function of viscosity ratio of the oil phase to the aqueous solution, interfacial tension, and shearing time. When the viscosity ratio is high, clusters are more difficult to breakup, a longer shear time is needed, and few particles detach. At low viscosity ratios, clusters are easily deformed and break up into smaller clusters, and more clean particles detach. Lowering the interfacial tension also facilitates the liberation of oil droplets from particles. The results show that the breakup of oil–particle clusters with a high volume fraction of large particles can be characterized as simple droplet breakup, meaning that the viscosity of the oil governs the dynamics of the cluster, rather than an effective viscosity that is a function of the solid volume fraction. The degree of cluster breakup and the separation of particles from the primary cluster is related to a cluster-based capillary number and the viscosity ratio. There is a critical capillary number required to obtain clean particles.

Friction stir processing of a D2 tool steel layer fabricated by laser cladding

Abstract Friction stir processing of a D2 tool steel layer fabricated by laser cladding was carried out to modify its as-cast microstructure. The microstructural evolution in the stir zone was observed and micro-Vickers hardness was measured. A maximum hardness of 857 HV was obtained by friction stir processing at 200 rpm. This value is higher than that of the clad layer when it is subjected to austenitizing heat treatment without friction stir processing. This superior property is attributed to the decrease of volume fraction of retained austenite after friction stir processing. The friction stir processing condition with low heat input led to high volume fractions of fine M7C3 carbide particles and martensite.

Characterization of Al-17wt.%Si Using Centrifugal Casting

Present work is focused on Horizontal centrifugal casting process, by which a wear resistant material with considerable strength can be designed. A material of heterogeneous microstructure designed in which a high volume fraction of hard particles is dispersed at the inner surface, where better wear properties are needed, and a gradually decreasing lower volume fraction of hard particles at the outer surface. By this, a better strength and ductility will be achieved at the desired surfaces. The present work is focused on the preparation of cylinder liners using centrifugal casting for Al-17wt%Si. These cylinder liners are characterized for Microstructure, Hardness, Tensile strength and Wear.

Effect Of SiC Particles On Sinterability Of Al-Zn-Mg-Cu P/M Alloy

Abstract Premix Al-5.5Zn-2.5Mg-0.5Cu alloy powder was analyzed as matrix in this research. Gas atomized powder Al-9Si with 20% volume fraction of SiC particles was used as reinforcement and added into the alloy with varied concentration. Mix powders were compacted by dual action press with compaction pressure of 700 MPa. High volume fraction of SiC particles gave lower green density due to resistance of SiC particles to plastic deformation during compaction process and resulted voids between particles and this might reduce sinterability of this mix powder. Sintering was carried out under ultra high purity nitrogen gas from 565°-580°C for 1 hour. High content of premix Al-5.5Zn-2.5Mg-0.5Cu alloy powder gave better sintering density and reached up to 98% relative. Void between particles, oxide layer on aluminum powder and lower wettability between matrix and reinforcement particles lead to uncompleted liquid phase sintering, and resulted on lower sintering density and mechanical properties on powder with high content of SiC particles. Mix powder with wt90% of Alumix 431D and wt10% of Al-9Si-vf20SiC powder gave higher tensile strength compare to another mix powder for 270 MPa. From chemical compositions, sintering precipitates might form after sintering such as MgZn2, CuAl2 and Mg2Si. X-ray diffraction, DSC-TGA, and SEM were used to characterize these materials.

The Compressive Response of Idealized Cermetlike Materials

Metals reinforced with a high volume fraction of hard particles, e.g., cermets, have properties that are more akin to those of granular media than conventional composites. Here, the mechanical properties and deformation mechanisms of this class of materials are investigated through the fabrication and testing of idealized cermets, comprising steel spheres in a Sn/Pb solder matrix. These materials have a similar contrast in the properties of constituent phases compared to commercial cermets; however, the simpler microstructure allows an easier interpretation of their properties. A combination of X-ray tomography and multiaxial strain measurements revealed that deformation at large strains occurs by the development of shear bands similar to granular media, with the material dilating under hydrostatic pressure within these shear bands. Predictions of finite element models with a random arrangement of inclusions were in excellent agreement with the experimental results of idealized cermets. These calculations showed that at large inclusion volume fractions, composites with a random arrangement of inclusions are significantly stronger compared to their periodic counterparts, due to the development of a network of force chains through the percolated particles.

Rheological effect of particle clustering in gelled dispersions

A theoretical model is presented to describe the effect of particle clustering on the elastic modulus of composite gelled systems. In this model, particle clusters are described as regions with an increased volume fraction of the dispersed particles and with a firmness that is determined by the volume fraction of the particles in the cluster. The firmness of the composite gel is then calculated on the basis of the volume fraction and firmness of these clusters, which are treated as cluster particles. In this way, the Kerner equation (including compressibility, but neglecting the particle surface) and the Palierne equation (including the particle surface, but neglecting compressibility), both corrected for particle crowding at high volume fractions of the dispersed particles by the method of Lewis and Nielsen, are extended to describe the effect of particle clustering. It is demonstrated that, even in the absence of discrete bonds between the particles, clustering considerably amplifies the effect of the dispersed phase on the elastic modulus of the composite gel. This amplifying effect increases for higher volume fractions of the dispersed particles.

On small deformation interfacial debonding in composite materials containing multi-coated particles

This paper presents an integrated theoretical and computational investigation into the macroscopic behavior of composite materials containing multi-phase reinforcing particles with simultaneous nonlinear debonding along the microconstituent interfaces. The interfacial debonding is characterized by the nonlinear Park–Paulino–Roesler potential-based cohesive zone model. The extended Mori–Tanaka method is employed as the basis for the theoretical model, which enables micromechanical formulations for composite materials with high particle volume fractions. The computational analysis is performed using a three-dimensional finite element-based cohesive zone model with intrinsic cohesive elements. To place the generality and robustness of the proposed technique in perspective, we consider several examples of composite materials with single or double separation along the interfaces of coated particles. The effects of many microstructural parameters, such as the geometry of the microstructure, the location of debonding, the material properties of the coating layer (i.e. homogenous and functionally graded coatings), and the fracture parameters, are comprehensively investigated by both theoretical and numerical approaches. We verify that both theoretical and numerical results agree well with one another in estimating the macroscopic constitutive relationship of corresponding composite materials. The strong dependence of the overall response of composite materials on their microstructure is well recognized for all hardening, snap-back, and softening stages.

Investigation of Theoretical Models for the Elastic Stiffness of Nanoparticle‐Modified Polymer Composites

Mathematical models are investigated and suggested for the calculation of the elastic stiffness of polymer nanocomposites. Particular emphasis is placed on the effect on the elastic stiffness from agglomerates and the particle interphase properties. The multiphase Mori‐Tanaka model and an interphase model are considered as two relevant models. These models only include and require the designation of a few system independent parameters with a clear physical meaning. Extensions of the models are also presented. The model calculations are compared to results from other models, as well as experimental data for different nanocomposites. For nanocomposites with spherical particles and with fiber‐like particles, the suggested models are found to be the most flexible ones and are applicable to estimate the stiffness increase of nanocomposites for both low and high particle volume fractions. The suggested theoretical models can hence be considered as a general multiscale “model toolbox” for analysis of various nanocomposites.

Multiscale Validation of the Applicability of Micromechanical Models for Asphalt Mixture

Asphalt mixture is more complicated than other composite materials in terms of the higher volume fraction of aggregate particles and the viscoelastic property of asphalt matrix, which obviously affect the applicabilities of the micromechanical models. The applicabilities of five micromechanical models were validated based on the shear modulus of the multiscale asphalt materials in this paper, including the asphalt mastic, mortar, and mixture scales. It is found that all of the five models are applicable for the mastic scale, but the prediction accuracies for mortar and mixture scales are poorer. For the mixture scale, all models tend to overestimate at the intermediate frequencies but show good agreement at low and high frequencies except for the Self-Consistent (SC) model. The Three-Phase Sphere (TPS) model is relatively better than others for the mortar scale. The applicability of all the existing micromechanical models is challenged due to the high particle volume fraction in the multiscale asphalt materials as well as the modulus mismatch between particles and matrix, especially at the lower frequencies (or higher temperatures). The particle interaction contributes more to the stiffening effect within higher fraction than 30%, and the prediction accuracy is then deteriorated. The higher the frequency (or the lower the temperature) is, the better the model applicability will be.

Suppression of grain growth in nanocrystalline Bi2Te3 through oxide particle dispersions

The strategy of suppressing grain growth by dispersing nanoscale particles that pin the grain boundaries is demonstrated in a nanocrystalline thermoelectric compound. Yttria nanoparticles that were incorporated by mechanical alloying enabled nanocrystalline (i.e., d < 100 nm) Bi2Te3 to be retained up to a homologous temperature of 0.94 Tm for durations over which the grain size of the unreinforced compound grew to several microns. The nanostructure appeared to saturate at a grain size that depended on volume fraction (f) according to an f −1/3 relationship, in accordance with theoretical models in the limit of high volume fractions of particles. Interestingly, at low temperatures, the particles stimulate enhanced grain growth over the unreinforced compound, due to particle-stimulated nucleation of recrystallization. To help prevent this effect, in-situ composites formed by internal oxidation of yttrium are compared with those made ex-situ by incorporation of yttria nanoparticles, with the result that the in-situ dispersion eliminates recrystallization at low temperatures and therefore improves nanostructure stabilization. These developments offer a pathway to thermally stabilized bulk nanocrystalline thermoelectrics processed via a powder route.

Rheology and internal dynamics of colloidal gels from the dilute to the concentrated regime

We explore the relations between dynamical, rheological and structural properties of colloidal gels extending the range previously considered towards higher particle volume fractions . Up to the spectrum of the time-dependent mean-square displacement is shown to be fully described by models accounting for the fractal character of gels. Beyond that, the direct relations between structural and stress-bearing properties used in fractal descriptions do not hold anymore. However, by considering the concepts of passive tracer rheology we show that the macroscopic shear modulus G0 can be derived from the dynamical properties up to even higher volume fractions.

Homogeneous viscous flow behavior of a Cu–Zr based bulk metallic glass composites

Abstract In this paper, Cu40Zr44Ag8Al8 bulk metallic glass composites (BMGCs) consisting of various volume fraction of nanocrystals embedded in the amorphous matrix was synthesized by controlled annealing treatment of an as-cast BMGCs. The high temperature compression behaviors of the BMGCs were characterized in the supercooled liquid region. Results show that the flow stresses keep increasing after an initial decrease with extension of the annealing time. With annealing the values of activation volume Vact is determined to be increasing from 283.6216 Ǻ3 to 305.553 Ǻ3, suggesting that the jump of atoms is a cooperative process during the high-temperature deformation. Flow behavior of the BMGCs annealed for less than 8 min transform from Newtonian to non-Newtonian dependant on the stain rate and can be successively fitted by the visco-plasticity model. Fitting results indicate that deformation behaviors of these samples are governed by homogeneous flow of the amorphous matrix and indeed determined by the viscosities in the Newtonian flow stage. However, the BMGCs annealed for 8 min exhibit a non-Newtonian flow over the entire compression process and fail to be fitted by the visco-plasticity model. Micrographs of the sample reflect an impinged structure, indicating that high temperature deformation behavior of the BMGCs with high volume fractions of particles is indeed controlled by that of a backbone of particles.

The thermal expansion behaviour of SiCp/Al–20Si composites solidified under high pressures

Al–20Si matrix composites reinforced with SiC particles have been fabricated by high pressure solidification and the effects of solidification pressure and SiC volume fraction on the thermal expansion behaviour have been systematically investigated. A peak of the coefficient of thermal expansion (CTE) is observed during the first heating process due to the precipitation of Si from the supersaturated Al(Si) solid solution, whereas the CTE increases monotonically during the second heating process. The Al–20Si alloy solidified at 3GPa shows the lowest CTE value during the second heating as a result of the modification of the eutectic silicon and of the small size and high volume fraction of the precipitated Si particles. The CTE of the composite decreases with increasing the SiC content in both heating processes. The theoretical model agrees well with the experimental results and our study indicates that high pressure solidification is a promising route for the development of SiCp/Al–Si composites for packaging applications.

Sustaining the compact shape during the quick spontaneous infiltration process with Al-Ti-B4C-CuO powder mixtures

Aluminum matrix composites with a high volume fraction of reinforcements are fabricated using a quick spontaneous infiltration process through a combustion reaction of an Al-Ti-B4C-CuO powder mixture in molten aluminum. A cold-compacted powder mixture in a cylindrical shape was used as a preform. The effects of the composition of the initial powder mixture on the sustaining of the compact shape were systematically examined and thereby an optimal composition for fabricating sound aluminum matrix composites was suggested. The compact shape was greatly affected by the initial composition of the powder mixture. A sufficiently high relative volume fraction of the solid particles in the compact is critical for sustaining the compact shape during the combustion reaction. The volume fraction of Al and Ti powders in the initial mixture affects whether crumbling of the compact occurs during the ignition delay time. This study can be beneficial to utilizing the Al-Ti-B4C system as to fabricate components with any desired shape.

High-Speed Tribological and Mechanical Properties of Layered Fe/SiC Composites

In this work, we studied the high-speed tribological and mechanical properties of layered SiC particulate reinforced iron matrix composites. The layered composites consisted of a surface layer with high volume fraction of the reinforcement particles and a layer with low volume fraction in the bulk. The layered composites are a form of functionally graded materials with high wear resistance near the surface and high thermal conductivity in the bulk. The composites were prepared by standard powder metallurgy techniques. The tribological behavior of the composites was evaluated at 25 to 35 m/s sliding speeds using a sub-scale dynamometer disk brake testing system. The properties of the layered composites were compared to those of uniform composites. The results showed that the layered composites have better wear resistance and braking effectiveness in the range of braking speeds considered. The layered composites also showed higher bending strength than the monolayer composites due to the presence of the interfaces between the layers.

Pinning force from multiple second-phase particles in grain growth

A factor that can reduce particle pinning force significantly in grain growth is found when the grain-boundary is pinned by multiple particles. The pinning force, in this case, is a function of particle radius over inter-particle distance. A previously proposed phase-field model for particle pinning is used to validate this predicted pinning force reduction in two and three dimensions. When applied to coherent pinning particles, the same effect is observed in simulations. It is shown that, at application relevant high particle volume fraction, the average grain size is affected by this reduction of pinning force.