Increasing Particle Volume Fraction Research Articles

Stress analysis in hydroxyapatite/poly- l-lactide composite biomaterials

A three-dimensional (3D) finite element (FE) analysis of the stress concentration factor (SCF) in biocomposite model cell has been performed. The model composite consisted of a hydroxyapatite hard particle (HAp) embedded in poly- l-lactide soft matrix (PLLA). Two cases were considered, namely the shape of the particle was held constant while the volume fraction of HAp was varied and the particle shape was changed whilst the volume fraction was constant. For block shaped embedded particles, it was found that the SCF decreases with an increase of the HAp particle volume fraction. It was also found that the shape of reinforcing particles had little effect on the mechanical behaviour of material.

Dimensioanl stability of TiC particle reinforced ZA43 matrix composite

TiC particulate reinforced ZA43 composites were produced successfully by XD and stirring-casting techniques. Their thermal-expansion coefficients (CTEs) were measured in the range of 25–250°C with a dilatometer. Kerner and Turner models were employed to calculate the CTE of TiC/ZA43 in the same temperature range. It was shown that TiC particles could improve the microstructure and thermal expansion property of ZA43 alloy, and the CTE of the composite decreased with increasing particle volume fraction. The calculated CTE values were very close to the experimental values. Thermal stress developed as a result of the CTE mismatch between the reinforcement and the matrix. Interfacial thermal stress of TiC/ZA43 increased rapidly with increasing temperature and a little with the increment of TiC content. When t≥200°C, plastic deformation occurred in the matrix. The effect of the stress state on CTE values depended on temperature range.

Thermal stability and its effects on the mechanical properties of rapidly solidified Al-Ti alloys

Microstructure and mechanical properties of the rapidly solidified (RS) Al-Ti alloy after thermal exposure treatment have been investigated. It is revealed that the Al 3Ti particle, which disperses homogeneously within the Al matrix, is coarsened via a volume diffusion controlled growth mechanism described by the LSW equation. The coarsening rate increases with increasing volume fraction of the particle. The Young’s modulus of the Al-Ti alloy increases with increasing particle volume fraction, which can be predicted by the Halpin–Tsai equation and the Eshelby method. The modified shear lag model used to estimate Young’s modulus of the particulate reinforced composite is hereby re-modified to give a more precise evaluation. The yield strength of the RS Al-Ti alloy is found to increase with increasing volume fraction and decreasing particle size of the reinforcement. It is indicated that the continuum strengthening mechanism (the modified shear lag model and the Eshelby model) fails to explain the increased yield strength. Therefore, strengthening mechanisms based on dislocation movement have been taken into account. Considering microstructure changes in the matrix due to the presence of the reinforcement, the Eshelby type model is believed to give the yield strength of the RS Al-Ti alloy quite accurately.

Contributions of the particulate reinforcement to dry sliding wear resistance of rapidly solidified Al-Ti alloys

Dry sliding wear and single-point scratch tests have been performed to study contributions of the fine Al 3Ti particulate reinforcement to the dry sliding wear resistance of rapidly solidified (RS) Al-Ti alloys. It is indicated that the dry sliding wear resistance of the alloy increases with increasing particle volume fraction and decreasing particle size. The homogeneously dispersed spherical Al 3Ti particles with sizes of 0.4–2.6 μm in diameter are believed to contribute to the wear resistance through strengthening the Al matrix both at ambient and elevated temperatures. However, the beneficial effect of the reinforcement decreases with increasing normal load. The possible causes were provided based mainly on results revealed by the single-point scratch test results.

Fabrication and optical/thermal properties of glass particle–epoxy optically transparent composites

The possibility of fabricating a new type of optically transparent glass particle dispersed epoxy matrix composite by tailoring the refractive index of the particle and matrix was studied. Glass particles with an average diameter of ≈8μm were incorporated into a clear epoxy matrix and the particle volume fraction ranged from 0.01 to 0.17. The refractive index difference after fabrication was less than 10−3. The light transmittance of the composite in the UV–visible region and the coefficient of thermal expansion (CTE) of the composite were measured. The results demonstrated that the optically transparent composite was obtained with a lower CTE than the pure epoxy matrix. The light transmittance of the composite decreased with increase in particle volume fraction, while the CTE of the composite showed an opposite tendency. The light transmittance of the composite was explained theoretically by a refractive index matching in the composite, which was different from that of the raw materials owing to thermal stress.

Incremental damage theory and its application to glass-particle-reinforced nylon 66 composites

This paper deals with influence of particle volume fraction and debonding damage between particles and matrix on the stress-strain response in particle-reinforced ductile matrix composites. Tensile tests are carried out on seven kinds of glass-particle-reinforced nylon 66 composites, which are different in a particle volume fraction and treatment of interface between the particles and matrix. The stress-strain response of the composites depends on both the particle volume fraction and the interface treatment. Young's modulus and Poisson's ratio are characterized by only the particle volume fraction, while tensile strength depends on both the particle volume fraction and interface treatment. With increasing particle volume fraction, the tensile strength increases first and then becomes constant in the interface-treated composites, and decreases in the interface-untreated composites. Numerical analyses of the stress-strain response and damage behavior of the composites are carried out based on an incremental damage theory which describes the plasticity of the matrix and the debonding damage. The stress-strain relations of the interface treated composites are characterized only by influence of particle volume fraction while those of the interface-untreated composites are explained by considering the particle volume fraction and interfacial debonding.

Microstructure evolution during multidirectional solidification of Al-Cu-SiC composites

The present investigation is aimed at understanding the solidification behaviour of Al-Cu-SiCp composites under multidirectional solidification conditions. Macro- and microstructures of composites solidified using small and large size moulds have been studied. It was observed that an increase in particle volume fraction and/or an increase in cooling rate reduced the extent of macrosegregation of reinforcements in the composites. In the case of composites solidified in large size permanent moulds, the differential cooling rate along the transverse direction led to the formation of a conical-shaped particle-depleted zone in the bottom part of the casting. Columnar grains were observed in the particle-depleted zone, while the particle-containing region exhibited equiaxed grain morphology. SiC particles did not facilitate heterogeneous nucleation of matrix phases. In the absence of convection, SiC particles promoted grain refinement in the matrix. However, when there was a significant amount of convection, the presence of SiC particles led to a coarser grain structure. Matrix dendrite arm spacing (DAS) was not significantly altered by the presence of SiC particles.

Aggregate formation in colloidal dispersions

The formation of aggregates in colloidal dispersions has been investigated using conductivity-counting and microscopy. Monodispersed sterically stabilized polystyrene, latex particles with diameters of 0.92 and 1.9 μm, concentrated oil-in-water emulsion with an average droplet size 2.5 μm and human red blood cells (∼ 5.7 μm in diameter) were used for investigating the effect of particle size and volume fraction on the aggregate formation. It was found that the aggregate size increased with increase in particle volume fraction. At high volume fraction the orifice could be blocked by the large aggregate. Internal labeling (using standard calibration latex particles) showed that about 0.1% of the dispersion was in aggregate form. Theory suggested this was due to adhesion between the particles with an estimated adhesion energy around 3 kT.

Three-dimensional intrinsic convection in dilute and dense dispersions of settling spheres

The three-dimensional intrinsic convection in a monodisperse dispersion of spheres settling in a vertical container of arbitrary cross section is calculated using the simple model of point forces with excluded volume near the walls, proposed by Bruneau et al. [Phys. Fluids 8, 2236 (1996)]. An exact solution of the model equations for a container with rectangular cross-section shows that corners have no significant influence on the convection. A dense suspension is modeled by assuming an equilibrium particle distribution in the near wall region. It is predicted that as a result of near wall ordering, the intrinsic convection decreases with increasing particle volume fraction.

Improvement of wear and adherence properties of composite coatings by a gradual increase in particle volume fraction

Electrodeposited composites (ECs) are thin composite deposits made of a base metal or alloy which is reinforced by second phase particles of ceramic, polymer or graphite. ECs are produced rather inexpensively at temperatures below 60 C, where no strong interfacial reaction may occur and residual thermal stresses are negligible. Electrodeposited composites play an important role in tailoring the surface properties of bulk materials. Indeed, ECs containing particle volume fractions (PVF) up to 30% of hard ceramic particles exhibit superior hardness and wear resistance compared with pure metallic deposits and are therefore used as wear-resistant coatings. However, as the inert particles are not adhesive to the substrates, an increase in PVF is often accompanied by a decrease in adherence of the coating to the substrate. Furthermore, a high PVF is often accompanied by a dramatic loss of functional property, gradient composite coatings having a lower PVF at the substrate side and a higher PVF at the surface side of ECs is produced. In this work the authors present the wear and adhesion behavior of such gradient coatings. The matrix was either Cu or Ni and the second phase particles were alumina and silicon carbide.

Particle‐Inhibited Grain Growth in AI2O3‐SiC: I, Experimental Results

An extensive experimental study has been conducted to examine the influence of ultraflne SiC particles on grain growth in A12O3. Grain growth has been studied as a function of annealing time, particle volume fraction, and annealing temperature. The SiC particles reduce the grain‐growth rate of A12O3 by >3 orders of magnitude, resulting in final grain sizes that decrease with increasing particle volume fraction. The fraction of SiC particles on grain boundaries, φ, has been measured as a function of annealing time, particle volume fraction, and annealing temperature. φ decreases during grain‐growth anneals, implying that grain boundaries are able to break free from particles and this is true to a greater extent for smaller volume fractions of particles. Further, it has been discovered that φ and average grain size, G, are strongly correlated, such that any increase in G results in a predictable decrease in pH. This new finding has revealed a path for microstructure development in this system that proves to be critical in the equilibrium and kinetic analyses described in a companion paper.

Compressive and tribological properties of Al2O3 fibre and hexagonal BN particle hybrid reinforced Al-Si alloys

Al2O3 fibre-hexagonal BN particle hybrid reinforced aluminium-silicon alloys were fabricated by centrifugal force infiltration route. Hardness test and ultimate compressive test results are reported. The wear and friction properties of hybrid MMCs was investigated by means of a block-on-ring (bearing steel) type wear rig in a dry sliding condition. It is shown that the hardness and ultimate compressive strength of hybrid MMCs was evidently decreased with the addition of hexagonal BN particles, however, the wear rate and coefficient of friction of hybrid MMCs was improved simultaneously with increase of BN particle volume fraction, especially for the higher applied loads in the test.

Numerical simulations of quasistatic remagnetization processes in fine magnetic particle systems

Various numerical methods used for simulation of quasistatic remagnetization processes in interacting systems of fine single-domain magnetic particles are analyzed. Among them methods based on the sequential alignment of magnetic moments along the effective field are found to be the fastest, especially in the most interesting case of a small single particle anisotropy when cooperative remagnetization modes play a dominant role. Further it is shown, that the random field approximation (RFA) which is often used to take the interparticle interaction effects into account is valid for large particle anisotropies only. The distribution density of the interaction field is found to transform from the Lorentzian to the Gaussian form with the increasing particle volume fraction. Finally hysteresis loops for a wide range of single-particle anisotropy values and particle concentration are calculated and the crossover from the single particle to the collective behavior with decreasing single particle anisotropy is investigated.

The deformation characteristics of SiC particulate-reinforced aluminium alloy 6061

The deformation characteristics of aluminium alloy 6061 reinforced with particulate SiC have been investigated using experimental and numerical methods. The materials investigated contained 0, 5, 10, 20% by volume and 3, 10, 30 μm by size of SiC particulates. The matrix microstructure was systematically heat treated to natural aged (NA), peak aged (PA) and over-aged (OA) conditions. In order to set up a comparative materials database, the microstructural characteristics of the unreinforced matrix material with identical fabricating and processing history were examined. The tensile strength and stiffness of the composites increased with increasing particle volume fraction for heat treatments provided that it was over a limiting value. The PA specimens had the highest tensile strength, but the NA specimens had the greatest elongation to failure and the largest ratio of tensile to yield strength. Good agreement was obtained between experimental results and model predictions of mechanical properties.

On the spurious increase in particle volume fraction during coarsening simulations

Abstract The problem of spurious increase in volume fraction of second-phase particles during computer simulations of coarsening is examined. The origin of this problem is traced to the use of too long a time step (used for numerical integration of growth rates with respect to time) which leads to small particles with large negative growth rates shrinking to negative radii at the end of the time step. Such a shrinkage to negative sizes has the effect of pumping solute into the system. It is therefore suggested that the length of the time step be chosen in accordance with the size of the smallest particle present in the system. It is shown that spurious increase in particle volume has a significant effect on the particle size distributions in the scaling regime (making them broader and more skewed in the Lifshitz-Slyozov-Wagner model). Its effect on coarsening kinetics, however, is found to be small.

Modelling friction and wear of scratching ceramic particle-reinforced metal composites

The friction and wear behaviour of 6061 aluminium matrix composites reinforced with SiC and Al 2O 3 particles has been investigated at relatively high loads and speeds on a conventional scratch machine using a pyramidal indenter. It is identified that ploughing, adhesion and particle fracture all contribute to the friction and wear. The friction coefficient increases with particle volume fraction but is independent of the range of normal loads (1–20 N) and sliding speeds (1–10 mm s − 1) tested. The wear volume generally increases with normal load and sliding distance (or pass number), and decreases with increasing particle volume fraction. A comparison of the measured scratch hardness and Vickers hardness shows that the former is not simply proportional to the latter for composites reinforced with large particles. A new friction and wear model is then established based on the theories of adhesion and ploughing and the effect of particle fracture. The validity of the model is confirmed by experiments and microscopic observations on the scratch topography of the metal matrix composites.

Film formation and sintering of colloidal monoclinic zirconia

Hydrothermal processing was used to produce colloidal monoclinic zirconia suspensions from zirconium carboxylate solutions. The colloidal suspensions exhibit high stability with respect to aggregation and films can be obtained by solvent evaporation. The as-prepared films yield dense monoclinic zirconia ceramics with average grain sizes of 60 nm after firing at a temperature of 1100°C. The distribution of colloidal zirconia particles in aqueous suspension was investigated by small-angle X-ray scattering. The scattering curves showed a broad peak, the position of which shifted to higher angles with increasing particle volume fraction. The average interparticle distance was estimated using this interference peak position. Sintering of the colloidal films was monitored by dilatometry, scanning electron microscopy and small-angle X-ray scattering.

Viscoelastic properties of polyacrylonitrile particles stabilised by a poly-2-vinylpyridine/polytert butylstyrene block copolymer

The viscoelastic properties of a dispersion of polyacrylonitrile particles stabilised by a block copolymer poly-2-vinylpyridine/polytert butylstyrene dispersed in solvesso have been measured as a function of particle concentration and frequency at ambient temperatures. At low volume fraction of particles it was found that the loss modulus of the dispersions was larger than the storage modulus, whilst at volume fractions > 0.40 the storage modulus dominates the rheology. This is attributable to there being a steric repulsion between the particles as a result of an increasing concentration of particles and the resultant reduction in interparticle separation in the dispersion. In addition the observed exponential increase of the storage modulus with increasing particle volume fraction mirrors the exponential increase in force with decreasing surface separation of the same type of polymers adsorbed to mica.

Particle concentration effects on shear coagulation in the presence of repulsive interparticle forces

The shear stability of monodisperse polystyrene latices, both in the presence and absence of surfactants, was observed as a function of particle concentration in the range of particle volume fractions from 4.29 × 10 −5 to 6.86 × 10 −4. Two different latices were tested; one containing ionizable groups chemically attached to the particle surface and one without such groups. All latices were tested in a Couette device capable of generating shear rates as high as 29000 s −1. Both the Smoluchowski theory and the more recent trajectory analysis approach predict that the coagulation rate should increase linearly with the particle volume fraction. This behavior was observed for latex particles which possessed only weak repulsive forces. For latex particles with significant repulsive forces, whether arising from adsorbed surfactants or chemically bound surface groups, the shear coagulation rate was observed to decrease with increasing particle volume fraction. This decrease in rate must be due to a decrease in the collisional efficiency with increasing particle volume fraction. A more definitive explanation of the effect must await future improvements in the theoretical description of the shear coagulation process.

Radiative Transfer With Dependent Scattering by Particles: Part 2—Experimental Investigation

Dependent radiative scattering by particles is experimentally investigated using plane-parallel cells containing latex spheres of 11, 2, and 0.08 μm diameter dispersed in an air or water matrix. The dependent scattering efficiencies and the bidirectional transmittance and reflectance were measured and compared with analytical results. The close-packed 2-μm spheres, which were expected to show dependent scattering from the previous criterion, gave results identical to independent scattering. Measured dependent scattering efficiencies of the small particles tested decrease with increasing particle volume fraction and were compared with those predicted by the theoretical investigation. The bidirectional transmittance and reflectance of dependent scattering were compared with those of independent scattering with the same number of spheres within the test cells. Several different patterns of dependent transmittance and reflectance appeared depending on the optical thickness. Finally, a newly proposed regime map bounding independent and dependent scattering is compared with the present and previous experimental data.