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Interfacial adhesion between polymer matrix and reinforcing silica nanoparticles plays an important role in strengthening polypropylene (PP) composite. To improve the adhesion strength, the surface of silica nanoparticles can be modified by grafted functional molecules. Using atomistic simulations, we examined the effect of functionalization of silica nanoparticles by hexamethyldisilazane (HMDS) and octyltriethoxysilane (OTES) molecules on the deformation and failure of silica-reinforced PP composite. We found that the ultimate tensile strength (UTS) of PP composite functionalized by OTES (28 MPa) is higher than that of HMDS (25 MPa), which is in turn higher than that passivated only by hydrogen (22 MPa). To understand the underlying mechanistic origin, we calculated the adhesive energy and interfacial strength of the interphase region, and found that both the adhesive energy and interfacial strength are the highest for the silica nanoparticles functionalized by OTES molecules, while both are the lowest by hydrogen. The ultimate failure of the polymer composite is initiated by the cavitation in the interphase region with the lowest mass density, and this cavitation failure mode is common for all the examined PP composites, but the cavitation position is dependent on the tail length of the functional molecules. The present work provides interesting insights into the deformation and cavitation failure mechanisms of the silica-reinforced PP composites, and the findings can be used as useful guidelines in selecting chemical agents for surface treatment of silica nanoparticles.

Graphene-based layered materials have attracted particular interests recently owing to their excellent mechanical and thermal properties, which are attributed to their layer-by-layer hierarchical structures with intralayer strong sp2 bonds combined with interlayer crosslinks for efficient load and heat transfer. They have many potential applications such as high thermal conductive films in flexible electronics. In this paper, we firstly develop an analytical model to calculate the thermal conductivity of graphene layered materials. Then, extensive molecular dynamics simulations are performed to validate the analytical model. A good agreement is obtained between the analytical and simulation results. Besides, the effects of geometry factors such as graphene sheet size and crosslink density on the temperature distribution and thermal conductivity of graphene layered materials are investigated. It is found that the thermal conductivity of graphene layered materials can be effectively tuned by the graphene sheet size and crosslink density. Our works are of practical importance for the design and application of graphene layered materials as thermal film materials.

We investigate the mechanical behavior of polypropylene/silica composites under mode I and II loading by using molecular dynamics simulations, focusing on the effects of two typical surface coupling agents of HMDZ and APTES on the interface deformation and failure. It is found that under mode I loading, the silica surface functionalized with APTES can enhance the tensile strength of polypropylene/silica composites effectively. In contrast, the silica surface functionalized with HMDZ has little effect on the tensile strength. The simulation results are supported by experiments. Under mode II loading, it is found that both HMDZ and APTES can enhance the shear strength of polypropylene/silica composites with APTES being more effective. The underlying mechanisms for the enhancement of interface strength are analyzed. Our work provides a deep understanding of the interface structure, deformation and failure and will be useful for selecting suitable surface coupling agents in enhancing the interface strength of polymer composites.

Pentadiamond is a newly predicted 3D carbon allotrope by first-principles calculations. Using molecular dynamics simulations, we show that pentadiamond can keep structure stable at high temperatures up to 1100 K, but its fracture strength and fracture strain are very sensitive to temperature. As the temperature increases from 100 K to 900 K, the fracture strength and fracture strain decreases by 68.4% and 69.1%, respectively. We also find that the mono-vacancy and di-vacancy related to C2 position lead to stronger reduction in the fracture strength and fracture strain than the atomic vacancies at other positions. To be surprising, the mono-vacancy at C2position has stronger effect than the di-vacancy at C2-C3positions. Besides, we find that hydrogen functionalization has strong effect on the mechanical properties of pentadiamond. As the H-coverage increases from 0.0 to 1.0, the fracture strength and fracture strain drop by 75.7% and 76.6%, respectively.

Three-dimensional hexagonal boron nitride aerogels (hBNAGs) are novel porous materials with many promising applications such as energy storage, thermal insulation and sensing. However, the structure–property relationships of hBNAGs in complicated thermo-mechanical coupled environments are still not clear. In this study, we employed a binary phase-field crystal (PFC) model to construct the atomic structures of hBNAGs, upon which the mechanical and thermal behaviors of hBNAGs were systematically investigated using large-scale atomistic simulations. It is found that the hBNAG geometry and topological defects strongly affect the mechanical and thermal properties. For example, the Young's modulus and tensile strength follow the scaling laws of mass density with a power factor of about 1.4 and 1.2, respectively, indicating that the stretching and bending combine toward tensile deformation. In addition, cracks nucleate around the octagon defects, indicating that the tensile strength is also influenced by the topological defects. Under compression, complicated crumpled deformations and ridges in the entire region are observed and the compression strength follows the scaling law of mass density with a power factor above 2.0, which means that a large portion of the hBNAGs do not contribute to the compression load bearing. We find that hBNAGs have a very low thermal conductivity of about two orders of magnitude lower than that of a hBN sheet. Also, the thermal conductivity of hBNAGs increases with increasing mass density, which also follows a scaling law. The power of the scaling law is about 0.5, indicating that the thermal conductivity has a strong nonlinear dependence on the mass density. Our work provides a deep understanding of the structure–property relationships of hBNAGs, which is useful for the engineering applications of hBNAGs.

In recent years, short fiber reinforced thermoplastic composites (SFRTPCs) have attracted significant interest due to their low density, high strength and toughness, environmental resistance and low cost. Currently, an important task is to optimize their internal structures so as to maximize their mechanical properties. Interestingly, a previous experimental study showed that by adding a small amount of a stronger material in the matrix to link up short fibers, the strength of SFRTPCs was enhanced. We note, however, that a micromechanical study on the effect of fiber linkage on the mechanical properties is still lacking. In this work, we have developed a finite element model to conduct such micromechanical analysis. In this model, the polymer matrix is taken as elastic-plastic material, the fiber as a brittle material following a brittle damage model, while the interphase and linkage as the same polymer following a progressive damage model. Our simulations show that for a SFRTPC with a given short fiber weight fraction, by increasing the weight fraction of the linkage material, the Young's modulus, yielding strength, ultimate tensile strength, and tensile toughness of the SFRTPCs are all increased. By increasing the strength of the linkage material, the ultimate tensile strength and tensile toughness of the SFRTPCs are increased. Modified rules of mixing have been proposed to consider the fiber crosslinking effect. The present study provides a useful reference for the optimal design of SFRTPCs to maximize their mechanical performance.

Atomic defects can be generated relatively easily in phosphorene due to their low formation energies. How these defects affect the buckling behavior of phosphorene nanotubes (PNTs) remains unexplored. Using molecular dynamics simulations, we investigate the effect of vacancies on the buckling properties of PNTs. We show that compared to a pristine PNT, the defective one exhibits a much lower buckling strength and strain. Remarkably, 1% concentration of vacancies in a PNT is able to cause a 30% reduction in buckling strength. Interestingly, for long PNTs, the buckling occurs via column or global buckling. As a result, the buckling strength decreases significantly with the increase (decrease) in the tube length (diameter) for both the pristine and defective PNTs, consistent with the Euler buckling theory. For short PNTs with small slenderness ratio (L/D), however, buckling occurs via shell or local buckling. As a result, the buckling strength increases with decreasing the tube diameter, consistent with shell buckling theory. Finally, with the increase in temperature, the buckling strength and strain can be reduced significantly for both the pristine and defective PNTs. These findings may provide important guidelines for the design and applications of PNTs-based nanodevices.

The potential application of metallic glasses (MGs) as structural materials demands accurate measurements of their toughness and the understanding of the underlying factors affecting it. Currently, it is challenging to precisely measure the toughness of MGs, especially ductile MGs. Measured toughness values are widely scattered, even for MGs with identical compositions. That is attributed to the combined effect of intrinsic and extrinsic factors including processing, sample geometries, and loading conditions. A fundamental understanding of the influences of these elements is thus of great significance. In the present study, molecular dynamics simulations are performed to investigate the influence of intrinsic and extrinsic effects on the fracture toughness of CuZr MGs. In particular, focused is placed on the effects of cooling rate, notch shape and size, strain rate, and temperature. The results indicate that the fracture toughness of a MG scales with the cooling rate used to prepare it. This is attributed to increased free volume content generated at high cooling rates, which enhances plastic deformation and amplifies the associated energy dissipation during plastic deformation events. Fracture toughness also increases with the strain rate, arguably due to strain rate-induced crack extension delay. Overall, the results demonstrate that the largest fracture toughness are achieved when MG samples are fabricated at high cooling rates and subjected to high strain rate deformation. In addition, results suggest that the fracture toughness decreases with increasing temperature, due to the significant decrease in strength. The correlations revealed between these crucial intrinsic and extrinsic parameters and the calculated MG fracture toughness support the development of a framework to understand the root of the discrepancies in the measurement of the toughness of MGs and provide insights into the design of tough MGs for structural applications.

Pentadiamond is a newly predicted 3D carbon allotrope by first-principles calculations. Using molecular dynamics simulations, we show that pentadiamond can keep structure stable at high temperatures up to 1100 K, but its fracture strength and fracture strain are very sensitive to temperature. As the temperature increases from 100 K to 900 K, the fracture strength and fracture strain decreases by 68.4% and 69.1%, respectively. We also find that the mono-vacancy and di-vacancy related to C2 position lead to stronger reduction in the fracture strength and fracture strain than the atomic vacancies at other positions. To be surprising, the mono-vacancy at C2 position has stronger effect than the di-vacancy at C2-C3 positions. Besides, we find that hydrogen functionalization has strong effect on the mechanical properties of pentadiamond. As the H-coverage increases from 0.0 to 1.0, the fracture strength and fracture strain drop by 75.7% and 76.6%, respectively. Our work provides valuable insights into the mechanical behaviors of this new material.

Metallic glasses (MGs) possess large elastic limit and high strength, but unfortunately they are of limited commercial utility due to their macroscopic brittle nature. Here, we report that a chiral nanolattice can be used to design large-scale MGs with negative Poisson’s ratio, large elastic deformation capability, extensive hardening, and large ductility. Furthermore, the mechanical behaviors of the metallic glass chiral nanolattice (MGCN) can be significantly altered through changing the thickness and length of the ligaments in the nanolattice. An exceptional combination of high strength and ductility is observed for MGCN with thin and long ligaments, wherein the sample density is only twice that of water. The deformation mechanism that characterizes the chiral topology is a combination of rotation of the nodes with bending and extension of the ligaments, leading to the extraordinary mechanical behaviors. The present study not only offers a potential solution to mitigating the brittleness of MGs, but also provides some guidelines in designing large-scale MG meta-materials for prospective applications in the fields of acoustics and energy absorption.

Two-dimensional (2D) materials have attracted a great deal of attention recently owing to their fascinating structural, mechanical, and electronic properties. The failure phenomena in 2D materials can be diverse and manifested in different forms due to the presence of defects. Here, we review the structural features of seven types of defects, including vacancies, dislocations, Stone-Wales (S-W) defects, chemical functionalization, grain boundary, holes, and cracks in 2D materials, as well as their diverse mechanical failure mechanisms. It is shown that in general, the failure behaviors of 2D materials are highly sensitive to the presence of defects, and their size, shape, and orientation also matter. It is also shown that the failure behaviors originated from these defects can be captured by the maximum bond-stretching criterion, where structural mechanics is suitable to describe the deformation and failure of 2D materials. While for a well-established crack, fracture mechanics-based failure criteria are still valid. It is expected that these findings may also hold for other nanomaterials. This overview presents a useful reference for the defect manipulation and design of 2D materials toward engineering applications.

A hexagonal graphene-like boron-carbon-nitrogen (h-BCN) monolayer, a new two-dimensional (2D) material, has been synthesized recently. Herein we investigate for the first time the thermal conductivity of this novel 2D material. Using molecular dynamics simulations based on the optimized Tersoff potential, we found that the h-BCN monolayers are isotropic in the basal plane with close thermal conductivity magnitudes. Though h-BCN has the same hexagonal lattice as graphene and hexagonal boron nitride (h-BN), it exhibits a much lower thermal conductivity than the latter two materials. In addition, the thermal conductivity of h-BCN monolayers is found to be size-dependent but less temperature-dependent. Modulation of the thermal conductivity of h-BCN monolayers can also be realized by strain engineering. Compressive strain leads to a monotonic decrease in the thermal conductivity while the tensile strain induces an up-then-down trend in the thermal conductivity. Surprisingly, the small tensile strain can facilitate the heat transport of the h-BCN monolayers.

Depending on fiber size and volume fraction, polymer matrix type, processing temperature, injection speed, etc., short fiber reinforced composites produced by injection molding may exhibit complex fiber concentration, orientation and length distributions at different locations, causing large variations in their mechanical properties. In this work, a representative volume element (RVE) scheme is developed to describe the elastic properties of short glass fiber reinforced PA6,6 composites produced by injection molding. In the RVE formulation, fiber length is assumed to follow a two-parameter Weibull distribution, and a new concept, namely preferential angle, is introduced to account for the preferential alignment of fibers at different locations of injection molded composites. A random sequential insertion algorithm is developed to implement the fiber length distribution and fiber preferential alignment, and finite element method (FEM) is employed to analyze the relations of the elastic properties of RVE with three parameters, that is, the mean length, volume fraction and preferential angle of the fibers. Based on the FEM results, quantitative formulas for these relations are obtained. It is found that the variations in the mean length, volume fraction, and preferential angle of the fibers can cause large variations in elastic moduli and elastic anisotropy of the RVE.

Cellular metallic glasses (MGs) can be good candidates for structural and functional applications due to their light weight, enhanced ductility and excellent energy absorption performance. Motivated by the size-dependent mechanical properties of MGs, we perform molecular dynamics simulations of uniaxial compression testing on MGs with nanoscale cellular structures, focusing on the influence of cell size, shape, pattern, and ratio of the cell size to the spacing on the deformation behaviors. With increasing cell size, a clear transition in failure mode from localized plastic deformation to homogeneous plastic deformation is observed. Furthermore, we show that MGs with circular cellular structures exhibit higher strength and energy absorption capacity than those with hexagonal cellular structures due to the reduction of the stress concentration in the honeycomb's joints, in agreement with recent experimental studies. Finally, with decreasing the cell size to spacing ratio, shear band (SB) formation is observed when the shortest distance between the neighbouring cells in the diagonal direction reaches a value comparable to the SB thickness. Our present results not only provide an in-depth atomic understanding of the deformation and failure mechanisms of MGs with nanoscale cellular structures, but also propose an effective way to attain optimized material properties through controlling the cell features.

We investigate the thermal conductivity of interlayer-bonded bilayer graphene, trilayer graphene, and pyrolytic graphite using molecular dynamics simulations. We find that interlayer sp 3 bonding greatly reduces the thermal conductivity, with the reduction up to 80 %, depending on the distribution and coverage of sp 3 bonds. Besides, we find that tensile strain reduces the thermal conductivity of interlayer-bonded graphene further by up to 50 %. Our findings suggest the possibility of using interlayer sp 3 bonds and tensile strain to tune and manipulate the thermal conductivity of multi-layer graphene, which may be useful in thermal management of graphene-based nanodevices and in thermoelectric applications of graphene.

Monolayer molybdenum disulfide (MoS2), a new two-dimensional material beyond graphene, has attracted tremendous attention recently. Its applications in nanoelectronic and thermoelectric devices usually require manipulating the thermal transport properties. Using nonequilibrium molecular dynamics simulations, we investigated the effects of lattice defects and mechanical strain on the thermal conductivity of MoS2. We found that the thermal conductivity of monolayer MoS2 can be effectively tuned by introducing even a small amount of lattice defects. For example, a 0.5% concentration of mono-Mo vacancies is able to reduce the thermal conductivity by about 60%. Remarkably, the thermal conductivity of the defected sample can further be tuned by mechanical strain. For example, a 12% tensile strain is able to reduce the thermal conductivity by another 60%. We also found that the tensile strain exerts almost the same impact on the thermal conductivity of both pristine and defective MoS2, which signifies that there...

We investigate the mechanical properties of polycrystalline graphene deforming under uniaxial tension by using molecular dynamics simulations, focusing on the effects of grain size, temperature and strain rate. It is found that polycrystalline graphene with smaller grain size shows a larger drop of Young’s modulus and fracture strength. For the polycrystalline graphene with grain size of 7.5 nm, the Young’s modulus and fracture strength is about 80% and 40% that of single-crystalline graphene, respectively. Our simulation results also reveal that the Young’s modulus and fracture strength of polycrystalline graphene are more sensitive to the changes of temperature and strain rate than that of single-crystalline graphene. When temperature increases from 100 to 1200 K, the fracture strength of polycrystalline graphene reduces by around 45%. At room temperature (300 K), the fracture strength of polycrystalline graphene increases by 10% as the strain rate increases from 5 × 10−5 to 5 × 10−3 ps−1. Furthermore, the strain rate has a stronger influence on the fracture strength of polycrystalline graphene at a higher temperature than at a lower temperature; while the temperature has a stronger influence at a lower strain rate than at a higher strain rate. Our study thus provides a comprehensive understanding of the mechanical properties of polycrystalline graphene.

The study of structural flaws is of paramount importance in engineering applications. However, the effect of flaws on both the macroscale and nanoscale metallic glasses (MGs) is a debatable topic, drawing from conflicting reports on notch strengthening, notch weakening, and notch insensitivity for MGs. In the present study, molecular dynamics simulations have been performed on nanoscale notched MGs to systematically investigate the influence of notch geometrical features on their fracture strengths and failure mechanisms. It is found that the symmetric double-edge notched MGs always induce a notch strengthening effect, which becomes more pronounced with increasing notch depth or sharpness. It is reasoned that the constrained growth of the plastic zone is responsible for the notch strengthening behavior. Accompanied by this notch strengthening, the deformation mode shifts from shear banding to homogeneous deformation within the un-notched ligament, when the un-notched ligament length is less than twice the shear band width. Furthermore, our simulations reveal that notch strengthening effect vanishes for asymmetric double-edge notched MGs and single-edge notched MGs. The present study provides significant insights into the deformation and failure mechanisms of nano-sized notched MGs, as well as useful guidelines for the design of MGs-based nanostructures.

The strength-ductility tradeoff has been a common long-standing dilemma in materials science. For example, superplasticity with a tradeoff in strength has been reported for Cu50Zr50 nanoglass (NG) with grain sizes below 5 nm. Here we report an improvement in strength without sacrificing superplasticity in Cu50Zr50 NG by using a bimodal grain size distribution. Our results reveal that large grains impart high strength, which is in striking contrast to the physical origin of the improvement in strength reported in the traditional nanostructured metals/alloys. Furthermore, the mechanical properties of NG with a bimodal nanostructure depend critically upon the fraction of large grains. By increasing the fraction of the large grains, a transition from superplastic flow to failure by shear banding is clearly observed. We expect that these results will be useful in the development of a novel strong and superplastic NG.

Graphene has great potential to be used in flexible electronics owing to its novel physical and electronic properties, and super-high mechanical strength. However, its small mechanical strain is unable to meet the needs for most flexible and stretchable device applications where large deformation occurs. Kirigami is one of the useful techniques to increase the flexibility of graphene under deformation. In this study, we investigate the shear flexibility of kirigami graphene under shear loading by using molecular dynamics simulations. We found that kirigami is very efficient in modulating the shear properties of graphene. With the trade-off between strength and flexibility, the shear flexibility of kirigami graphene can be improved significantly. The flexibility can be further improved by using longer interior cuts. Our results indicate that kirigami can be a promising technique to manipulate the shear properties of graphene and other two-dimensional nanomaterials.

To examine the effect of aspect ratio, atomistic simulations are performed on the tensile failure of Cu 50 Zr 50 metallic glasses (MGs) with aspect ratios from 2:1 to 12:1. All samples fail via a single shear band (SB), and the Young’s modulus and the tensile strength are independent of the aspect ratio. For long MGs, however, an abrupt unstable shear banding failure is observed, while a combined abrupt unstable shear banding and slow stable SB sliding is observed for short MGs.

The mechanical properties of a sandwich architecture composed of Cu 50 Zr 50 nanoglass (NG)/Cu 50 Zr 50 metallic glass (MG)/Cu 50 Zr 50 NG are investigated using molecular dynamics simulation. The plastic deformation along the glass–glass interfaces in NG layers and the compressive residual stress in the MG layer provide the enhanced ductility, manifested as improved strain localization and delayed shear band formation; meanwhile the strength remains high in the MG layer. We provide routes to obtain strong, ductile MGs and to protect MGs by NG surface coating.

In the growth of polycrystalline graphene via chemical vapor deposition, grain boundaries (GBs) were shown to be energetically favorable sites for hydrogenation. Thus, it is of both scientific interest and technological significance to understand how hydrogenation on GBs affects the mechanical properties of polycrystalline graphene. Here we perform molecular dynamics simulations to investigate the mechanical properties of hydrogenated polycrystalline graphene. Our simulations reveal that the fracture strength of hydrogenated polycrystalline graphene is significantly reduced by the combined weakening effect of bond prestraining in highly defective GBs and sp3 hybridization of hydrogenated GB atoms. In addition, this reduction in fracture strength due to GB hydrogenation is observed in polycrystalline graphene samples of different grain sizes ranging from 2.5 to 10 nm. Our findings show that the loss of mechanical strength due to GB hydrogenation must be taken into account in the application of polycrystall...

We perform molecular dynamics simulations on large-area polycrystalline graphene containing a pre-existing circular notch with focus on the notch effect on tensile strength and failure pattern. Our results show that the failure of polycrystalline graphene becomes notch-sensitive if there is an overlapping of stress concentration zones induced by the notch and grain boundaries (GBs); otherwise, the failure becomes notch-insensitive. More specifically, when the notch diameter is larger than the average grain size, the failure is generally notch-sensitive. However, if the notch size is smaller than the grain size, whether the failure is notch-sensitive or not depends on the notch location. These observations can be well explained by following attributes: (1) Both GBs and circular notch can create stress concentration; (2) The stress concentration created by the notch is generally weaker than that by GBs; and (3) The strength of GBs is weaker than that of the grain interior. Our work provides useful guideline for designing polycrystalline graphene for structure and device applications.

Borophene, a new two-dimensional (2D) material with metallic characteristic, is promising for use as electrodes and interconnects in flexible nanodevices. Here, we study the mechanical properties of single-layer borophene using molecular dynamics simulations based on a newly developed interatomic potential. It is found that the hexagonal borophene exhibits highly anisotropic mechanical properties. The Young’s modulus and fracture strength in the zigzag direction are much lower than those in the armchair direction. The simulated fracture strength and fracture strain of borophene at 10 K are in good agreement with those predicted by first-principles calculations. We further reveal that the fracture properties of borophene are very sensitive to temperature. When the loading temperature increases from 10 to 700 K, the fracture strength and fracture strain decreases by 50% and 60%, respectively. In contrast, the fracture properties show a relatively weak sensitivity to the strain-rate. When the strain rate changes from 0.00001 to 0.01 ps−1, the fracture strength and fracture strain increase only by 6.8–8.6% and by 11–15%, respectively. Our observations on the temperature and strain-rate dependent fracture strength can be rationalized by the kinetic theory of fracture. The present study provides valuable insights into the deformation and failure behavior of borophene, which are of importance for the design and application of borophene-based nanodevices. ASTAR (Agency for Sci., Tech. and Research, S’pore)

Due to low formation energies, it is very easy to create atomic defects in phosphorene during its fabrication process. How these atomic defects affect its mechanical behavior, however, remain unknown. Here, we report on a systematic study of the effect of atomic vacancies on the mechanical properties and failure behavior of phosphorene using molecular dynamics simulations. It is found that atomic vacancies induce local stress concentration and cause early bond-breaking, leading to a significant degradation of the mechanical properties of the material. More specifically, a 2% concentration of randomly distributed mono-vacancies is able to reduce the fracture strength by ∼40%. An increase in temperature from 10 to 400 K can further deteriorate the fracture strength by ∼60%. The fracture strength of defective phosphorene is also found to be affected by defect distribution. When the defects are patterned in a line, the reduction in fracture strength greatly depends on the tilt angle and the loading direction. Furthermore, we find that di-vacancies cause an even larger reduction in fracture strength than mono-vacancies when the loading is in an armchair direction. These findings provide important guidelines for the structural design of phosphorene in future applications.

Molecular dynamics simulations of ultrananocrystalline diamond (UNCD), with random distribution of grain sizes and grain boundaries (GBs), are performed to investigate the effect of grain size and temperature on the mechanical properties and failure mechanisms under tensile loading. Results show that when the grain size of UNCDs decreases from 4.1 nm to 2.26 nm, the Young's modulus decreases from 891 GPa to 840 GPa, while the obtained intrinsic fracture strength, 113 GPa, is insensitive to the grain size. Elastic softening is attributed to the increased volume fraction of amorphous-like atoms. Our analysis reveals that at room temperature, UNCD fails via sliding along a grain boundary with a large shear stress. Such sliding triggers crack initiation at an adjacent triple junction and subsequent propagation along an adjacent grain boundary with a large normal stress. With increasing temperature, a crossover from grain sliding to a direct intergranular fracture is observed. The crossover is caused by a different dependence of GB shear and tensile strength on temperature. The present work provides information that may be useful to the design and optimization of the mechanical properties and failure behavior of UNCDs.

Molecular dynamics simulations have been performed to investigate the mechanical properties of hydrogen functionalized graphene for H-coverages spanning the entire range from graphene (H-0%) to graphane (H-100%). We find that the Young’s modulus, tensile strength, and fracture strain of the functionalized graphene deteriorate drastically with increasing H-coverage up to about 30%. Beyond this limit the mechanical properties remain insensitive to H-coverage. While the Young’s modulus of graphane is smaller than that of graphene by 30%, the tensile strength and fracture strain show a much larger drop of about 65%. We show that this drastic deterioration in mechanical strength arises both from the conversion of sp 2 to sp 3 bonding and due to easy-rotation of unsupported sp 3 bonds. Our results suggest that the coverage-dependent deterioration of the mechanical properties must be taken into account when analyzing the performance characteristics of nanodevices fabricated from functionalized graphene sheets.

A new method on examining the micro cracks of monocrystalline silicon during nano indentation is proposed. It is established based on a study of the increasing rate of absorbed energy in nano indentation. This method provides a simple approach in understanding whether cracks on the silicon surfaces occur, while it is tedious in conventional method.

Nanometric machining involves removal of materials at the order of a few nanometers or less. At such a small length scale, molecular dynamics (MD) simulation is an important tool in studying the nanometric machining mechanism and process. In this study, a series of large scale MD simulations with the model size of more than four-million atoms have been performed to study the nanometric machining of copper. The dislocations at finite temperature during the cutting processes are identified and their nucleation and movement are studied. The effects of cutting depth, cutting speed, crystal orientation and cutting direction on the material deformation, lattice defects and cutting forces are investigated. The simulation results show that a smaller cutting depth results in less plastic deformation and fewer dislocations in the workpiece and thus result in a smoother machined surface. It is found that as the cutting depth decreases, the specific cutting force increases rapidly, which shows that the “size effect” exists in nanometric machining. It is observed that a higher cutting speed results in more lattice defects at the cutting region and higher cutting forces. It is revealed that the crystal orientation and cutting direction have a strong effect on material deformation, dislocation movement and cutting forces.

Molecular dynamics simulations using the embedded atom method (EAM) potential are carried out to study the nanoimprint of copper single crystal. The effects of mould taper angle, mould tooth spacing and crystal orientation on the deformation behaviour, dislocation movement and imprint force are investigated. A four-stage deformation is observed for the mould with a small taper angle, while it cannot be clearly identified for the mould with a large taper angle. The four-stage deformation is: (1) a purely elastic deformation with a rapid increase of imprint force, (2) dislocation nucleation with a small drop of imprint force, (3) local pile-up of dislocations with a second rapid increase of imprint force, and (4) active dislocation movement with almost a plateau of imprint force. Among the three imprint surfaces of (0 0 1), (1 1 0) and (1 1 1), it is found that the (0 0 1) surface results in the lowest imprint force and the (1 1 1) surface results in the highest one. It is also observed that the pattern transfer on the (1 1 1) surface is less accurate than on the (0 0 1) and (1 1 0) surfaces. It is shown that the smaller the mould tooth spacing, the stronger is the interaction of the stress fields in the substrate, which results in more obvious influence on the material deformation, the dislocation movement and the imprint force. (Some figures in this article are in colour only in the electronic version)

Severe plastic deformation (SPD) is an efficient approach for producing ultrafine or nanostructured bulk materials. Equal channel angular extrusion (ECAE) is the most effective SPD solution for material nanostructuring, as material billet undergoes severe and large deformation and the grains are efficiently broken up in the process. To improve material nanostructuring, the ECAE die design and process configuration are critical. The deformation behavior study through FE simulation in ECAE process provides basic and useful information for optimizing die design and process determination. In this research, the deformation behavior for three different die design scenarios is studied and the related deformation mechanisms and nanostructuring performance are investigated via FE simulation. Through multi-pass simulation, the optimal design scenario is then identified. The simulation results reveal deformation phenomena, and nanostructuring performance of the designs and the corresponding process can be recommende...

Thermo-mechanical finite element analysis was carried out to study the deformation behavior and temperature distribution during equal channel angular pressing (ECAP). The material model used is the Johnson-Cook constitution model that can consider the multiplication effect of strain, strain rate, and temperature on the flow stress. The effects of pressing speed, pressing temperature, workpiece material and die geometry on the temperature rise and flow behavior during ECAP process were investigated. The simulated temperature rise due to deformation heating was compared with published experimental results and a good agreement was obtained. Among the various die geometries studied, the two-turn die with 0° round corner generates the highest and most uniform plastic strain in the workpiece.

We investigate the in-plane and cross-plane thermal conductivities of molybdenum disulfide (MoS2) using non-equilibrium molecular dynamics simulations. We find that the in-plane thermal conductivity of monolayer MoS2 is about 19.76 W mK(-1). Interestingly, the in-plane thermal conductivity of multilayer MoS2 is insensitive to the number of layers, which is in strong contrast to the in-plane thermal conductivity of graphene where the interlayer interaction strongly affects the in-plane thermal conductivity. This layer number insensitivity is attributable to the finite energy gap in the phonon spectrum of MoS2, which makes the phonon-phonon scattering channel almost unchanged with increasing layer number. For the cross-plane thermal transport, we find that the cross-plane thermal conductivity of multilayer MoS2 can be effectively tuned by applying cross-plane strain. More specifically, a 10% cross-plane compressive strain can enhance the thermal conductivity by a factor of 10, while a 5% cross-plane tensile strain can reduce the thermal conductivity by 90%. Our findings are important for thermal management in MoS2 based nanodevices and for thermoelectric applications of MoS2.

This work investigates the nanoscale friction between diamond-structure silicon (Si) and diamond via molecular dynamics simulation. The interaction between the interfaces is considered as strong covalent bonds. The effects of load, sliding velocity, temperature and lattice orientation are investigated. Results show that the friction can be divided into two stages: the static friction and the kinetic friction. During the static friction stage, the load, lattice orientation and temperature dramatically affects the friction by changing the elastic limit of Si. Large elastic deformation is induced in the Si block, which eventually leads to the formation of a thin layer of amorphous Si near the Si-diamond interface and thus the beginning of the kinetic friction stage. During the kinetic friction stage, only temperature and velocity have an effect on the friction. The investigation of the microstructural evolution of Si demonstrated that the kinetic friction can be categorized into two modes (stick-slip and smooth sliding) depending on the temperature of the fracture region. ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore)

The easy formation of vacancy defects and the asymmetry in the two sublayers of phosphorene nanotubes (PNTs) may result in brand new mechanical properties and failure behaviour. Herein, we investigate the mechanical properties and fracture behaviour of defective PNTs under uniaxial tension using molecular dynamics simulations. Our simulation results show that atomic vacancies cause local stress concentration and thus significantly reduce the fracture strength and fracture strain of PNTs. More specifically, a 1% defect concentration is able to reduce the fracture strength and fracture strain by as much as 50% and 66%, respectively. Interestingly, the reduction in the mechanical properties is found to depend on the defect location: a defect located in the outer sublayer has a stronger effect than one located in the inner layer, especially for PNTs with a small diameter. Temperature is also found to strongly influence the mechanical properties of both defect-free and defective PNTs. When the temperature is increased from 0 K to 400 K, the fracture strength and fracture strain of defective PNTs with a defect concentration of 1% are reduced further by 71% and 61%, respectively. These findings are of great importance for the structural design of PNTs as building blocks in nanodevices.

Because of the complex interactions among the energy beam, the powder bed, and the material phase transformations, powder-bed fusion additive manufacturing is very sensitive to process parameters, such as beam power and scan speed. As a result, the process window to produce fully-dense, ASTM-grade components is narrow. In such scenario, envisioning further control of mechanical properties is very challenging. As a departure from traditional attempts to control microstructure by changing the process parameters, the authors propose the introduction of a thermoelectric module (TEM) as an active device inside the build chamber. Using process modeling, the authors show that by altering the heat flow into the material through the TEM device, the volume fraction of martensite can be controlled in its entire range. In particular, the authors show that modern TEM modules can deliver sufficient thermal power to block the formation of martensite. As a result, microstructure can be controlled locally while retaining the beam power and scan speed optimal for part density and surface finish. While the results are demonstrated for Ti6Al4V and the electron beam melting process, the concept is general and, in principle, applicable to other materials and machines systems such as IN781 and selective laser melting.

Notched metallic glasses (MGs) have received much attention recently due to their intriguing mechanical properties compared to their unnotched counterparts, but so far no fundamental understanding of the correlation between failure behavior and notch depth/sharpness exists. Using molecular dynamics simulations, we report necking and large notch strengthening in MGs with symmetric sharp-and-deep notches. Our work reveals that the failure mode and strength of notched MGs are strongly dependent on the notch depth and notch sharpness. By increasing the notch depth and the notch sharpness, we observe a failure mode transition from shear banding to necking and also a large notch strengthening. This necking is found to be caused by the combined effects of large stress gradient at the notch roots and the impingement and subsequent arrest of shear bands emanating from the notch roots. The present study not only shows the failure mode transition and the large notch strengthening in notched MGs, but also provides significant insights into the deformation and failure mechanisms of notched MGs that may offer new strategies for the design and engineering of MGs.

Nanoindentation has been recently used to measure the mechanical properties of polycrystalline graphene. However, the measured failure loads are found to be scattered widely and vary from lab to lab. We perform molecular dynamics simulations of nanoindentation on polycrystalline graphene at different sites including grain center, grain boundary (GB), GB triple junction, and holes. Depending on the relative position between the indenter tip and defects, significant scattering in failure load is observed. This scattering is found to arise from a combination of the non-uniform stress state, varied and weakened strengths of different defects, and the relative location between the indenter tip and the defects in polycrystalline graphene. Consequently, the failure behavior of polycrystalline graphene by nanoindentation is critically dependent on the indentation site, and is thus distinct from uniaxial tensile loading. Our work highlights the importance of the interaction between the indentation tip and defects, and the need to explicitly consider the defect characteristics at and near the indentation site in polycrystalline graphene during nanoindentation.

We perform molecular dynamics simulations to investigate the nanoscale frictional behavior of a perfluoropolyether (PFPE) film sandwiched between two diamond-like-carbon (DLC) coatings. We show that the PFPE films behave like a solid and can perform either a motion-station movement or a continuous motion with fluctuating velocities. The former movement is caused by the alternating stick and slip at the two individual interfaces, while the latter is due to the dynamic sliding motions simultaneously occurring at both interfaces. We reveal that these motion characteristics are governed by the competition between the two interfacial adhesion energies, which are strongly affected by the thermal vibrations and interface roughness fluctuations. We also find that the Amonton's law modified by incorporating the adhesion effect can be used to describe the mean friction traction vs normal pressure relation, but large fluctuations are present at low contact pressures. The magnitude of atomic level friction forces at the interface is found to be highly nonuniform. The directions of atomic level friction forces can even be opposite. With increasing the normal pressure, the nonuniformity of atomic level friction forces decreases first and then increases again. This change can be explained by the concurrent effects from the large difference in material stiffness and the changes in surface roughness under normal pressure. The present work reveals interesting insights into the sliding mechanisms in sandwiched structures and provides useful guidelines for the design of nanoscale lubricant systems.

Molecular dynamics simulations have been performed to study the mechanical properties of methyl (CH(3)) functionalized graphene. It is found that the mechanical properties of functionalized graphene greatly depend on the location, distribution and coverage of CH(3) radicals on graphene. Surface functionalization exhibits a much stronger influence on the mechanical properties than edge functionalization. For patterned functionalization on graphene surfaces, the radicals arranged in lines perpendicular to the tensile direction lead to larger strength deterioration than those parallel to the tensile direction. For random functionalization, the elastic modulus of graphene decreases gradually with increasing CH(3) coverage, while both the strength and fracture strain show a sharp drop at low coverage. When CH(3) coverage reaches saturation, the elastic modulus, strength and fracture strain of graphene drop by as much as 18%, 43% and 47%, respectively.

The thermal transport behavior of polycrystalline graphene is studied using molecular dynamics simulations, with focus on the effects of grain size, tensile strain, and temperature on the thermal conductivity. All the simulation samples have the same overall dimensions of 30 × 30 nm with average grain sizes ranging from 2.5 to 12.5 nm. It is found that polycrystalline graphene exhibits a significant reduction in thermal conductivity compared to single-crystalline graphene, and the smaller the grain size is, the more the thermal conductivity drops. The thermal conductivity of polycrystalline graphene with average grain size of 2.5 nm is only about 20% of single-crystalline graphene. However, the thermal conductivity of polycrystalline graphene is less sensitive to both the applied strain and temperature than that of single-crystalline graphene. The underlying mechanisms for the differences in thermal behavior are examined and discussed. These findings are important for the thermal management of graphene-based devices. Published version

Graphyne is an allotrope of graphene. The mechanical properties of graphynes (α-, β-, γ- and 6,6,12-graphynes) under uniaxial tension deformation at different temperatures and strain rates are studied using molecular dynamics simulations. It is found that graphynes are more sensitive to temperature changes than graphene in terms of fracture strength and Young's modulus. The temperature sensitivity of the different graphynes is proportionally related to the percentage of acetylenic linkages in their structures, with the α-graphyne (having 100% of acetylenic linkages) being most sensitive to temperature. For the same graphyne, temperature exerts a more pronounced effect on the Young's modulus than fracture strength, which is different from that of graphene. The mechanical properties of graphynes are also sensitive to strain rate, in particular at higher temperatures.

Recently, nanoglass (NG) was found to exhibit a surprising homogeneous superplastic deformation behavior. However, how the presence of notch affects its mechanical properties remains unexplored. Here, we perform molecular dynamics simulations on a superplastic Cu50Zr50 NG containing a pre-existing notch under tensile loading, with focus on the notch sensitivity. Our results show that when the notch size is smaller than or comparable to the average grain size (d), the NG still exhibits a superplastic deformation and shows notch-insensitivity. When the notch size is larger than d, however, the NG fails by localized shear banding emanating from the notch root and shows notch-sensitivity. The origin of this transition arises from the competition between the shear band nucleation induced by the stress concentration at the notch root and the growth of shear transformation zones distributed in the glass-glass interfaces. Our results provide useful guidelines for the design and engineering of NG with notch for st...

Silicene, a graphene-like two-dimensional silicon, has attracted great attention due to its fascinating electronic properties similar to graphene and its compatibility with existing semiconducting technology. So far, the effects of temperature and strain rate on its mechanical properties remain unexplored. We investigate the mechanical properties of silicene under uniaxial tensile deformation by using molecular dynamics simulations. We find that the fracture strength and fracture strain of silicene are much higher than those of bulk silicon, though the Young's modulus of silicene is lower than that of bulk silicon. An increase in temperature decreases the fracture strength and fracture strain of silicene significantly, while an increase in strain rate enhances them slightly. The fracture process of silicene is also studied and brittle fracture behavior is observed in the simulations.

Graphyne, a new type of carbon allotropes, has attracted considerable attention in recent years. Using molecular dynamics simulations, we investigate the mechanical properties of four different graphynes (α-, β-, γ-, and 6,6,12-graphynes) functionalized with hydrogen. The simulations results show that hydrogenation can greatly deteriorate the mechanical properties of the graphynes. For the different graphynes with 100% H-coverage, the reduction in fracture stress depends on the percentage of acetylenic linkages in the graphyne structures: The more the acetylenic linkages, the larger the reduction. For the same graphyne, the reduction in fracture stress depends on the hydrogenation location, distribution, and coverage. Hydrogenation on the acetylenic linkages causes a larger reduction in fracture stress than that on the hexagonal rings. A line hydrogenation perpendicular to the tensile direction leads to a larger reduction in fracture stress than that when the line hydrogenation is parallel to the tensile direction. For random hydrogenation, the fracture stress and Young's modulus decrease rapidly at low H-coverage (

Atomistic simulations are performed to study the effects of size and shape of a superficial or internal notch on the strength and failure mechanism of CuZr metallic glass (MG) under tensile loading. Our results show that plastic deformation originating at the notch root reduces the stress concentration there and leads to a notch-insensitive normalized tensile strength. The notch, however, dictates the failure location as the plastic zone at the notch root serves as a nucleation site for shear band (SB) formation. It is shown that when the plastic zone size reaches a critical value, a SB starts to propagate from the notch root across the entire sample, causing the material failure. These results provide useful guidelines for the design, testing, and engineering of MG for structural applications.

We perform large-scale molecular dynamics simulations on diamond-like carbon to study wear mechanism and law at the nanoscale. Our simulations show that material loss during sliding varies linearly with normal load and sliding distance, consistent with Archard's law. Our simulations also show that the number of chemical bonds across the contact interface during sliding correlates well with friction force, but not with material loss, indicating that friction and wear follow different mechanisms. Our analysis reveals the following wear mechanism: the shear traction causes mass accumulation at the trailing end of contact, which is then lost by a cluster detachment process.