Particle-matrix Interface Research Articles

Wavevector and polarization resolved analysis of phonon scattering from embedded nanoparticles

Embedded nanocrystals are capable of dramatically reducing the thermal conductivity of alloy semiconductors through increased phonon scattering, but many aspects of thermal wavelength phonon interactions with embedded nanoparticles remain understudied. Here, the wavevector- and polarization-resolved capabilities of the Frequency Domain Perfectly Matched Layer (FDPML) computational technique are exploited to study several fundamentally important phonon scattering problems across the entire Brillouin zone. We compare the atomistic predictions of FDPML against continuum mechanics approaches for spherically embedded particles. For long to mid-wavelength phonons, reasonable agreement is found with continuum theories that consider the Mie regime accurately, while commonly used “patching” theories which empirically connect the Rayleigh scattering to the geometric limit are shown to have poor agreement with more rigorous approaches. Next, the scattering cross section of optical phonons from nanoparticles is explored for the first time. We show that the scattering behavior of optical phonons is fundamentally different than their acoustic counterparts in that long wavelength optical phonons exhibit a scattering cross section nearly independent of wavelength, which we interpret as being due to zone folding of short wavelength acoustic modes from the geometric scattering regime into long wavelength optical modes. Finally, we study the scattering cross section of nanoparticles exhibiting atomic interdiffusion at the matrix-particle interface, where we find that interdiffusion both suppresses Mie oscillations and substantially increases the scattering cross section at short wavelength, compared to a solid nanoparticle with the same number of impurity atoms.

Thermal distribution analysis and suppression mechanism of carbonized tracking and erosion in silicone rubber/SiO2 nanocomposites

For high voltage outdoor silicone rubber insulators, carbonized tracking growth and erosion of the material are the major failure modes and it can adversely influence the reliability of power delivery networks. Thermal accumulation in the dry band arcing region is a key factor for temperature build-up and it causes thermal depolymerization and promotes tracking on the insulator surface. This work investigates the effect of nano–SiO2 doping on the thermal accumulation induced tracking growth and erosion of silicone rubber. A tracking-erosion model is presented to explain how doping directly affects such mechanisms. Experimental results show excellent enhancement in the performance of silicone rubber with higher doping contents in terms of physical tracking growth, erosion, leakage current and thermal accumulation in the arcing region. These favorable findings could be attributed to the better thermal conductivity which allows the high energy thermal flux to dissipate away more efficiently. Moreover, thermogravimetric analysis results indicate that nano doping enhances the physical and chemical cross-linking points which restrict the segmental motion at the particle matrix interface. It can promote thermal stability of the nanocomposites which is clearly witnessed in the physical results. The proposed tracking-erosion model suggests that colliding scattering of nano–SiO2 particles reduces the secondary electron collapse which impedes the release of high energy and restricts thermal degradation.

Analytical Model of Cutting Force in Micromilling of Particle-Reinforced Metal Matrix Composites Considering Interface Failure

During the micromachining processes of particle-reinforced metal matrix composites (PMMCs), matrix-particle interface failure plays an important role in the cutting mechanism. This paper presents a novel analytical model to predict the cutting forces in micromilling of this material considering particle debonding caused by interface failure. The particle debonding is observed not only in the processed surface but also in the chip. A new algorithm is proposed to estimate the particles debonding force caused by interface failure with the aid of Nardin–Schultz model. Then, several aspects of the cutting force generation mechanism are considered in this paper, including particles debonding force in the shear zone and build-up region, particles cracking force in the build-up region, shearing and ploughing forces of metal matrix, and varying sliding friction coefficients due to the reinforced particles in the chip-tool interface. The micro-slot milling experiments are carried out on a self-made three-axis high-precision machine tool, and the comparison between the predicted cutting forces and measured values shows that the proposed model can provide accurate prediction. Finally, the effects of interface failure, reinforced particles, and tool edge radius on cutting forces are investigated by the proposed model and some conclusions are given as follows: the particles debonding force caused by interface failure is significant and takes averagely about 23% of the cutting forces under the given cutting conditions; reinforced particles and edge radius can greatly affect the micromilling process of PMMCs.

On the origin and contribution of extended kinks and jogs and stacking fault ribbons to deformation behavior in an ultrahigh strength cobalt-free maraging steel with high density of low lattice misfit precipitates

We elucidate here the deformation mechanisms and underlying reasons that contributed to high ductility (10.2%) and high static toughness (112.5 MJ m−3) in an ultrahigh strength (1860 MPa) cobalt-free 19Ni3Mo1.5Ti maraging steel characterized by high density (2.3 × 1024 m−3) of η-Ni3(Ti,Mo) and B2-Ni(Mo,Fe) nanoscale precipitates with low lattice misfit of ＜1% with the martensite matrix. Multiple deformation processes occurred during plastic deformation. Lath-morphology of martensite was dramatically segmented with angles of 30°, 60° or 120° with large pile-up of dislocations at the segmented boundaries. This occurred because of the interactive ability of edge and screw dislocations along the martensite habit planes, which led to kinks and jogs. The low lattice misfit (0.6% ~ 0.9%) precipitates interacted with dislocations leaving stacking fault ribbons within precipitates that build a large long range of back stress producing a high strain-hardening response. Additionally, nanoscale twinning occurred. The above contributions to ductility are envisaged to be in addition to the significantly reduced elastic interaction between the low lattice misfit nanoscale precipitates and dislocations that reduces the ability for crack initiation at the particle-matrix interface.

Morphology, Mechanical Properties and Dimensional Stability of Biomass Particles/High Density Polyethylene Composites: Effect of Species and Composition.

The utilization of four types of biomass particles, including hardwood (poplar), softwood (radiata pine), crop (wheat straw) and bamboo (moso bamboo), as reinforcing fillers in preparing high density polyethylene (HDPE) based composites was studied. To improve interfacial compatibility, maleic anhydride grafted polyethylene (MAPE) was applied as the coupling agent. The effects of the biomass species on the mechanical and water absorption properties of the resulting composites were evaluated based on chemical composition analysis. A creep-recovery test was conducted in single cantilever mode using a dynamic mechanical analyzer. Results show that the four types of biomass particles had similar chemical compositions but different composition contents. Poplar particles with high cellulose content loading in the HDPE matrix exhibited higher tensile and flexure properties and creep resistance. Fracture morphology analysis indicated a weak particle-matrix interface in wheat straw based composites. Given the high crystallinity and minimum hemicellulose content, the moso bamboo reinforced composite showed high impact strength and better water resistance.

Stress analysis on the reinforcement particles of the metal matrix composite by Raman spectroscopy

In this research, the stress state of the reinforcing SiC particles in a hybrid MMC is investigated by micro Raman spectroscopy. The experiment was carried out in situ in the Raman spectroscopy. Experimental results show that cracks due to monotonic loading propagates by the debonding of the particle/matrix interface and particle fracture. Moreover, secondary cracks form in front of the main crack tip coalesce with the main crack in subsequent loading and final failure occurs. A high decrease in stress (several hundred in MPa) is observed with the interfacial debonding at the interface and with the particle fracture on the particle. The critical tensile stresses for particle-matrix interface debonding and particle fracture develop in hybrid MMC are also estimated during the crack propagation.

Dynamic fracture and hot-spot modeling in energetic composites

Defects such as cracks, pores, and particle-matrix interface debonding affect the sensitivity of energetic materials by reducing the time-to-ignition and the threshold pressure to initiate an explosion. Frictional sliding of preexisting cracks is considered to be one of the most important causes of localized heating. Therefore, understanding the dynamic fracture of crystalline energetic materials is of extreme importance to assess the reliability and safety of polymer-bonded explosives. Phase field damage model simulations, based on the regularization of the crack surface as a diffuse delta function, are used to describe crack propagation in cyclotetramethylene-tetranitramine crystals embedded in a Sylgard matrix. A thermal transport model that includes heat generation by friction at crack interfaces is coupled to the solution of crack propagation. 2D and 3D dynamic compression simulations are performed with different boundary velocities and initial distributions of cracks and interface defects to understand their effect on crack propagation and heat generation. It is found that, at an impact velocity of 400 m/s, localized damage at the particle-binder interface is of key importance and that the sample reaches temperatures high enough to create a hot-spot that will lead to ignition. At an impact velocity of 10 m/s, preexisting cracks advanced inside the particle, but the increase of temperature will not cause ignition.

Microstructure and mechanical properties of CP-Ti fabricated via powder metallurgy with non-uniformly dispersed impurity solutes

Oxygen and nitrogen have been both known to have a strong hardening effect on Ti and its alloys, while also imposing a serious embrittlement effect. In the present work, Ti samples with non-uniformly dispersed oxygen and nitrogen solid solutions were produced using high purity Ti via powder metallurgy (PM) methods. The experimental results suggested that, when the solutes are non-uniformly distributed, the high solute content region can serve as strengthening particles as that in metal matrix composites. In light of this, we propose a design of novel structure in high oxygen/nitrogen Ti materials to achieve both improved strengths and ductility. The highlight of the structure is a design of strain gradient at the matrix-particle interface, which can mitigate the strain compatibility that commonly reported in general MMCs as the crack initiator. Instead of using ceramics as reinforcements, here we propose to use the matrix phase itself that hardened by high oxygen/nitrogen solutes as reinforcing particles.

Impact of nanoparticle surface modification on the mechanical properties of polystyrene-based nanocomposites.

Nanocomposites consisting of metal oxide nanoparticles in a polymeric matrix enable the improvement of material properties and have become highly relevant for numerous applications, such as in lightweight structures with an enhanced Young's modulus for automotive and aircraft applications. The mechanical properties can be adjusted by controlling the amount of particles, their degree of agglomeration and their direct interaction with the matrix. Whilst the latter aspect is particularly promising to achieve high reinforcement at low filler contents, the mechanisms behind this effect are still not fully understood, preventing the rational design of a particle–polymer system with customized properties. In this work, a two-step modification strategy is used to tailor the particle–matrix interface via chemical groups bound to the surface of zirconia nanoparticles. Two modifications featuring terminal vinyl functions as potentially polymerizable groups are compared. Moreover, an inert reference modification is used to determine the influence of the terminal vinylic groups. In contrast to previous studies, all groups are covalently linked to the particle surface, thereby excluding effects such as detachment or weak coordination and ensuring that changes in the mechanical properties can be correlated to chemical groups on the particle surface. After embedding modified particles in polystyrene, the mechanical properties as well as the cross-linkage between the particles and the matrix are characterized, clearly showing the significant impact of a covalent particle–matrix linkage, with an increase of the Young's modulus by up to 28% with only 3 wt% filler content.

Spall damage of a Ta particle-reinforced metallic glass matrix composite under high strain rate loading

We investigate deformation and damage of a Zr-based bulk metallic glass (BMG) and its Ta particle-reinforced composite (MGMC) under impact loading, as well as quasi-static tension for comparison. Yield strength, spall strength, and damage accumulation rate are obtained from free-surface velocity histories, and MGMC appears to be more damage-resistant. Scanning electron microscopy, electron back scattering diffraction and x-ray computed tomography, are utilized for characterizing microstructures, which show features consistent with macroscopic measurements. Different damage and fracture modes are observed for BMG and MGMC. Multiple well-defined spall planes are observed in BMG, while isolated and scattered cracking around reinforced particles dominates fracture of MGMC. Particle–matrix interface serves as the source and barrier to crack nucleation and propagation under both quasi-static and impact loading. Deformation twinning and grain refinement play a key role in plastic deformation during shock loading but not in quasi-static loading. In addition, 3D cup-cone structures are resolved in BMG, but not in MGMC due to its heterogeneous stress field.

Monomer Derived Poly(Furfuryl)/BaTiO3 0-3 Nanocomposite Capacitors: Maximization of the Effective Permittivity Through Control at the Interface.

Frequency stable, high permittivity nanocomposite capacitors produced under mild processing conditions offer an attractive replacement to MLCCs derived from conventional ceramic firing. Here, 0-3 nanocomposites were prepared using gel-collection derived barium titanate nanocrystals, suspended in a poly(furfuryl alcohol) matrix, resulting in a stable, high effective permittivity, low loss dielectric. The nanocrystals are produced at 60 °C, emerging as fully crystallized cubic BTO, 8 nm, paraelectric with a highly functional surface that enables both suspension and chemical reaction in organic solvents. The nanocrystals were suspended in furfuryl alcohol inside a uniquely prepared mold, in which volume fraction of nanocrystal filler (νf) could be varied. Polymerization of the matrix in situ at 70-90 °C resulted in a nanocomposite with a higher than anticipated effective permittivity (up to 50, with νf only 0.41, 0.5-2000 kHz), exceptional stability as a function of frequency, and very favorable dissipation factors (tan δ < 0.01, νf < 0.41; tan δ < 0.05, νf < 0.5). The increased permittivity is attributed to the covalent attachment of the poly(furfuryl alcohol) matrix to the surface of the nanocrystals, homogenizing the particle-matrix interface, limiting undercoordinated surface sites and reducing void space. XPS and FTIR confirmed strong interfacial interaction between matrix and nanocrystal surface. Effective medium approximations were used to compare this with similar nanocomposite systems. It was found that the high effective permittivity could not be attributed to the combination of two components alone, rather the creation of a hybrid nanocomposite possessing its own dielectric behavior. A nondispersive medium was selected to focus on the frequency dependent permittivity of the 8 nm barium titanate nanocrystals. Experimental corroboration with known theory is evident until a specific volume fraction (νf ≈ 0.3) where, due to a sharp increase in the effective permittivity, approximations fail to adequately describe the nanocomposite medium.

Simultaneous improvement of mechanical strength, ductility and corrosion resistance of stir cast Al7075-2% SiC micro- and nanocomposites by friction stir processing

Present work investigates the mechanisms of simultaneous improvement of tensile properties, wear properties and corrosion resistance of stir cast Al7075–2wt.% SiC micro- and nanocomposites through microstructural refinement by friction stir processing (FSP). Optical, scanning and transmission electron microscopy were used to investigate the microstructural evolution. After the FSP, the nanoparticles reinforced composite showed better mechanical properties than that of the microparticles reinforced composite. Tensile strength ( >3 times) and wear resistance were found to increase significantly with simultaneous enhancement of the ductility (10 times). The improvement is ascribed to the grain size reduction, distribution of SiC nanoparticles uniformly within the matrix, increase particle-matrix interface characteristics and elimination of casting defects such as porosity after the FSP. The corrosion potentials of the as-cast composites were found to shift towards noble direction after the FSP. Enhancement of corrosion resistance after the FSP is attributed to the decrease in the heterogeneity on the surface and uniform dispersion of the reinforced particles, which reduced the effective active surface area exposed to the corrosive solution. All such beneficial effects were found to be superior for the nanoparticles reinforced composite due to the improved particle/matrix interface characteristics and dispersion strengthening.

Zirconia based feedstocks: Influence of particle surface modification on the rheological properties

Controlling the particle-matrix interface of filled systems is generally an essential challenge for reaching target characteristics. Herein, a simple method is described to characterize and optimize the filler surface chemistry in the case of zirconia feedstocks using stearic acid as a surfactant. Our technique involves contact angle measurements by the sessile drop method between water and thermally treated zirconia powders mixed with stearic acid. It is shown that this is a powerful way to determine the optimum additive concentration and processing parameters. In addition, the effect of coated particles on the dynamic rheological behavior of the molten feedstock is investigated. Furthermore, the kinetics of the hydrophobization of the powder surface induced by esterification between the fatty acid and the adsorbed hydroxyls at the powder surface has been accurately followed.

Effect of particle shape and size distribution on the dissolution behavior of Al2Cu particles during homogenization in aluminum casting alloy Al-Si-Cu-Mg

Finite element and finite difference modeling approaches were used in this study. Planar, cylindrical, spherical and elliptical particle shapes were considered. A log-normal distribution function was utilized to evaluate the effect of the particle size distribution on the dissolution behavior of the Al2Cu particles. The models were validated by conducting homogenization experiments, and using microscopy and image analysis. The dissolution kinetic is not affected by the particle-matrix interface. The assumption of spherical shape results in the best agreement between the modeling predictions and the experimental data provided that the effect of the particle size distribution instead of an average particle size is taken into account.

Investigation of cleavage fracture under dynamic loading conditions: Part II numerical analysis

Part I of this study was an extensive fractographic investigation that covered the topics local crack arrest, and cleavage fracture-inducing mechanisms under dynamic loading situations. Also, it produced data regarding the origin of cleavage fracture. Now, this data is used for the numerical part of this study. First, the development of temperature and strain rate increase at the origin of cleavage fracture is conducted, and linked to discrepancies regarding experiments and the Master Curve concept in a phenomenological way. Then, cleavage fracture controlling mechanical field variables at the origin of fracture are analyzed, whereas very similar conditions regarding crack initiation and propagation are found when compared to quasi-static data. The influence of wave phenomena is examined as well. Finally, micromechanical simulations showed that a local temperature increase at the particle–matrix interface does not influence fracture behavior either, and that conclusively, the actual physical mechanism of cleavage fracture initiation (crack initiation and instability) takes place under the same conditions at elevated loading rates as under quasi-static conditions. Ultimately, the mechanisms responsible for the shortcomings of the Master Curve concept under dynamic loading conditions are identified, and current local approach concepts need to be adjusted to consider local crack arrest to be reliable.

Facile fabrication of epoxy-TiO2 nanocomposites: A critical analysis of TiO2 impact on mechanical properties and toughening mechanisms

Optimized ultrasonic assisted dispersion of un-functionalized titanium dioxide (TiO2) nanoparticles (0.5–20wt%) into epoxy resin is reported. The investigation shows that there is a direct relation among nanoparticles content, inter-particle spacing and cluster size of the particles on the glass transition temperature (Tg) and tensile properties of the prepared nanocomposites. A significant improvement in tensile strength and modulus with minimal detrimental effect on the toughness was observed for the prepared composites, where compared to pristine epoxy resins, about 26% and 18% improvement in tensile strength and strain-to-break %, respectively, was observed for 10wt% particles loading, whereas a maximum improvement of about 54% for tensile toughness was observed for 5wt% particles loaded resins. The investigations found that a strong particle–matrix interface results in the enhancement of the mechanical properties due to leading toughening mechanisms such as crack deflection, particle pull out and plastic deformation.

Overall properties of particulate composites with periodic microstructure in second strain gradient theory of elasticity

In the context of Mindlin’s second strain gradient theory, a homogenization scheme for the determination of the effective overall properties of particulate composites with periodic microstructure is developed in this paper. This homogenization method is based on the direct application of an equivalent inclusion method adapted to second strain gradient theory in such a manner that the consistency conditions, in addition to the stress field, are applied to the double stress and triple stress fields of the representative unit cell of composite and its corresponding inclusion problem. Subsequently, by equating the potential energy of the composite material with that of an equivalent homogeneous material, the effective overall elastic moduli of the composite are determined. Moreover, it is shown that the energy expression of the equivalent homogeneous material includes two additional parameters, namely, the cohesion-mismatch-induced initial stress and the cohesion-mismatch-induced surface tension, both arising from the mismatch in the cohesion moduli of the matrix and the reinforcing particles of the composite. The effects of particle size and particle–matrix interface manifest themselves in the formulations and the obtained results of the paper.

Numerical analysis of the mechanical behavior of ZTAp/Fe composites

Composites are complex and inhomogeneous. Various factors of microstructure will affect their mechanical behaviors. Based on a systematic numerical analysis, we present a coupling effect of the volume fraction of reinforced particles and the particle-matrix interface properties on the mechanical properties of ZTAp/Fe composites. We find that for yield limit and peak stress, the particle-matrix interface properties show a dominant impact while for the critical strain the particle volume fraction is the main factor. A combined effect of the particle volume fraction and the interface properties is found on the elastic modulus. The von Mises stress distributions are investigated and the results show that the particles clustered in the direction perpendicular to tensile load have larger debonding potential. In addition, the probability of particle cracking and the number of interface debonding are investigated to analyze the damage mechanism of the composites. We find that strong bonded interface can withstand more extra stress and protect the particle from cracking. The interface debonding rate becomes smooth after some critical value as the strain increases continuously due to a steady crack propagation after crack initiation along the interface line. This paper provides a guidance towards the design of advanced ceramic particle reinforced metal matrix composite materials.

Fracture behavior of silica nanoparticles reinforced rubber/epoxy composite

Hybrid composites consisting of soft rubber (carboxyl-terminated butadiene acrylonitrile, CTBN) and silica nanoparticles with average particle size of 20 nm were studied for improving toughness of epoxy resins (diglycidyl ether of bisphenol A, DGEBA). The hybrid carboxyl-terminated butadiene acrylonitrile /silica nanoparticles epoxy systems exhibited improvements in the Young’s modulus, and more importantly, fracture toughness (KIC), which can be explained by synergistic impact from the inherent characteristics of each filler. In this study, the highest KIC was reached with addition of small amounts of silica nanoparticles (5 vol.%) to the epoxy containing 15 vol.% carboxyl-terminated butadiene acrylonitrile, where the KIC was distinctly higher than with the epoxy containing carboxyl-terminated butadiene acrylonitrile or silica nanoparticles alone. Cavitation of rubber particles with matrix dilation and particle debonding with subsequent void growth were determined as the toughening mechanisms responsible for the toughness improvements for epoxy. The evidence indicates that debonding of the silica nanoparticles causes a weakening of the matrix–particle interface. The toughness enhancement in hybrid nanocomposites involves an increase in silica nanoparticles particle debonding an increase in plastic zone size, which allows the epoxy matrix to dissipate more fracture energy.

Modeling homogenization behavior of Al-Si-Cu-Mg aluminum alloy

The effect of the as-cast secondary dendrite arm spacing and homogenization temperature on the secondary phase dissolution and solute redistribution during homogenization of cast Al-Si-Cu-Mg aluminum alloy was investigated. Particularly, the concurrent dissolution of θ-Al2Cu and Q-Al5Cu2Mg8Si6 phases was studied for the first time. Two modeling approaches were developed using finite-difference analysis to predict the dissolution kinetics; (i) a stationary interface modeling approach in which the interface is considered to be stationary and (ii) a moving boundary modeling approach in which the particle-matrix interface is moving. The dissolving particle shape was considered to be spherical and the dissolution process to be diffusion controlled. The two model predictions were compared against the experimental data obtained from microstructural characterizations. It was shown that although the stationary interface approach provides reasonable results in predicting the dissolution behavior at higher volume fractions of the secondary phase, the results deviate from the experimental measurements at lower volume fractions. On the other hand, the moving boundary approach was capable of predicting the final dissolution time of the secondary phase. It was also shown that the Q-phase does not dissolve completely as opposed to the θ-phase.