Abstract
Molecular dynamics is applied to explore the deformation mechanism and crystal structure development of the AlCoCrFeNi high-entropy alloys under nanoimprinting. The influences of crystal ...structure, alloy composition, grain size, and twin boundary distance on the mechanical properties are carefully analyzed. The imprinting load indicates that the highest loading force is in ascending order with polycrystalline, nano-twinned (NT) polycrystalline, and monocrystalline. The change in alloy composition suggests that the imprinting force increases as the Al content in the alloy increases. The reverse Hall–Petch relation found for the polycrystalline structure, while the Hall–Petch and reverse Hall–Petch relations are discovered in the NT-polycrystalline, which is due to the interactions between the dislocations and grain/twin boundaries (GBs/TBs). The deformation behavior shows that shear strain and local stress are concentrated not only around the punch but also on GBs and adjacent to GBs. The slide and twist of the GBs play a major in controlling the deformation mechanism of polycrystalline structure. The twin boundary migrations are detected during the nanoimprinting of the NT-polycrystalline. Furthermore, the elastic recovery of material is insensitive to changes in alloy composition and grain size, and the formability of the pattern is higher with a decrease in TB distance.
Cross-section with atomic strain and total kinetic energy of three mechanisms: (a) sliding, (b) rolling, and (c) oscillating of removing silicon substrate atoms by diamond abrasive at a moving ...velocity of 1 Å/ps.
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•In sliding, the probability of atoms removed from the pathway is calculated.•Rolling achieves the highest number of atoms removed.•Oscillating removes some atoms from the substrate.•Combining sliding, rolling and oscillating mechanisms gains high-quality surface.
Molecular dynamics simulation is employed to analyze the effect of sliding, rolling and oscillating movements on nanotribology properties of a diamond abrasive on a silicon substrate. The abrasive oscillating mechanism is achieved by simulating megasonic vibration-assisted on the planarization process. In this paper, the effects of abrasive size, sliding velocity, depths of polishing, rolling velocity, rolling direction, oscillating amplitude and oscillating frequency on material removal are considered. The results showed that the rolling mechanism reaches the highest number of atoms removed while the suitable oscillating mechanism gains the lowest height of asperity. The oscillating movement has remarkable results in wiping out the asperity atoms, although at a high amplitude and a low frequency causing some atoms from the flat substrate stuck to the abrasive and left some surface defects. Moreover, a multi-asperities model is set up to simulate the global-scale ability of sliding, oscillating and rolling mechanisms on polishing. This model indicates the saturated behavior of the rolling mechanism, while the lowest value of the surface roughness attained by the sliding mechanism. Combining three types of removing mechanisms could create a smooth surface with a high effective removal rate on both local and global-scales.
Evaluating the effect of porosity and ambient temperature on mechanical characteristics and thermal conductivity is vital for practical application and fundamental material property. Here we report ...that ambient temperature and porosity greatly influence fracture behavior and material properties. With the existence of the pore, the most significant stresses will be concentrated around the pore position during the uniaxial and biaxial processes, making fracture easier to occur than when tensing the perfect sheet. Ultimate strength and Young's modulus degrade as porosity increases. The ultimate strength and Young's modulus in the zigzag direction is lower than the armchair one, proving that the borophene membrane has anisotropy characteristics. The deformation behavior of borophene sheets when stretching biaxial is more complicated and rough than that of uniaxial tension. In addition, the results show that the ultimate strength, failure strain, and Young's modulus degrade with growing temperature. Besides the tensile test, this paper also uses the non-equilibrium molecular dynamics (NEMD) approach to investigate the effects of length size, porosity, and temperature on the thermal conductivity (κ) of borophene membranes. The result points out that κ increases as the length increases. As the ambient temperature increases, κ decreases. Interestingly, the more porosity increases, the more κ decreases. Moreover, the results also show that the borophene membrane is anisotropic in heat transfer.
Abstract
This report explores the effects of machining depth, velocity, temperature, multi-machining, and grain size on the tribological properties of a diamond substrate. The results show that the ...appearance of graphite atoms can assist the machining process as it reduces the force. Moreover, the number of graphite atoms relies on the machining speed and substrate temperature improvement caused by the friction force. Besides, machining in a machined surface for multi-time is affected by its rough, amorphous, and deformed surface. Therefore, machining in the vertical direction for multi-time leads to a higher rate of deformation but a reduction in the rate of graphite atoms generation. Increasing the grain size could produce a larger graphite cluster, a higher elastic recovery rate, and a higher temperature but a lower force and pile-up height. Because the existence of the grain boundaries hinders the force transformation process, and the reduction in the grain size can soften the diamond substrate material.
The effect of the sliding and rolling motions of the abrasive particle on the silicon carbide substrate covered by a thin film of silica atoms is investigated in this paper using molecular dynamics ...simulation. The influence of the sliding depth, sliding speed, rolling depth and rolling speed on the material removal and wear mechanism are surveyed. The results represent that increasing the sliding depth gives rise to higher forces, a higher temperature, a deeper groove, deeper subsurface damage (SSD) layer, and a higher amount of atoms erased. Improving the sliding velocity results in a higher fraction of atoms removed out of the pathway and a larger number of atoms erased. In the rolling mechanism, improving the depth and the rolling speed leads to the same effect as in the sliding mechanism but with a lower rate. Notably, the number of atoms erased in the rolling motion is greatly lower than the sliding one. In the ploughing regime, the silica film is effectively removed out of the SiC substrate, especially at a high sliding speed. The ploughing regime in the rolling motion requires a deeper depth to achieve, and the ploughing indications are weaker than the sliding motion. This report also considers both mechanical and chemical effects as some models with pure SiC workpiece or with different silica thickness are constructed and investigated. With the same removing conditions, the pure SiC sample suffers higher forces, a higher level of deformation and a larger number of atoms erased as compared to the sample covered by a silica thin film. The ploughing regime does not only require a sufficient penetration depth but also needs a suitable silica thickness to happen.
In this paper, molecular dynamics simulation is applied to inspect the effect of sliding, rolling, vibrating and rolling plus vibrating movements of a diamond abrasive on nanotribology properties of ...a 4H–SiC substrate. The effect of the abrasive size, depth of polishing, sliding speed, rolling, and vibrating movements on material removal are surveyed. For the first time in literature, the rolling in x-axis, vibrating in horizontal direction, and rolling combined with vibrating movements are considered. The results present that a bigger abrasive mostly gains a lower asperity height and a higher number of atoms removed. The rolling motion in the x-axis achieves the highest number of atoms removed while the rolling motion in the reverse z-axis achieves the lowest asperity height. In the vibrating movements, the horizontal vibrating movement creates a lower asperity and a higher number of atoms removed than the vertical and the rolling plus vibrating motions. The number of atoms eliminated does not sensitive to the vibrating amplitude and the vibrating frequency. Moreover, a multi-asperities model is constructed to estimate the global-scale surface of the substrate after removing the asperities. The asperity height is removed effectively by the rolling plus vibrating motion. The surface roughness produced by rolling in x-axis movement and vibrating motions are better than other motions. This model also points out the saturated behavior of all types of movements, except the rolling plus vibrating motion. The interactions between the abrasive and the asperity could create a lengthened asperity, amorphization, new C–C bonding, clusters of atoms adhered on the abrasive surface, a high temperature at the interface, atomic strain, cleavage event and fly away atoms.
The figure shows the microstructure evolution of AlCoCrFeNi high-entropy alloy with different crystallographic orientations (a), twin boundary spacings (b), twin boundary inclination angles (c).
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•Microstructure evolution reveals the formation of Lomer-Cottrell and Hirth dislocation locks.•Both the Hall–Petch and inverse Hall–Petch relationships are observed with the change of twin boundary spacing.•Microstructure evolution and atomic flow are greatly dependent on the spacing and inclination angle of the twin boundary.•Surface morphology and wear volume depend on the microstructure of the material.
Surface nanotribological properties and subsurface damage of Al0.4CoCrFeNi high-entropy alloy during the nano-scratching processes are investigated using molecular dynamics. The results show that the surface wear characteristics and scratching-caused surface damage significantly depend on the crystallographic orientation, spacing and inclination angle of the twin boundary. For the variation of crystallographic orientation, the largest friction coefficient belongs to the crystallographic orientation 001, indicating that the movement of the indenter in this substrate is most restricted. The microstructure evolution reveals the formation of Lomer-Cottrell and Hirth dislocation locks because of the distinctness of angle between various slip systems. Both the Hall–Petch and inverse Hall–Petch relationships are observed for the difference of twin boundary spacing, and the maximum indentation force is achieved with a tilt angle of 0° resulting from the various interactions among the dislocations and twin boundaries. The microstructure evolution and the atomic flow are greatly dependent on the spacing and the inclination angle of twin boundary, where the twin boundary migration is the significant factor. Furthermore, the surface morphology is distinct between workpieces due to the elastic recovery at the surface, nucleation and slipping of dislocation, which implies that the wear volume depends on the material microstructure.
Abstract
For practical application, determining the thermal and mechanical characterization of nanoporous two-dimensional MoS
2
membranes is critical. To understand the influences of the temperature ...and porosity on the mechanical properties of single-layer MoS
2
membrane, uniaxial and biaxial tensions were conducted using molecular dynamics simulations. It was found that Young’s modulus, ultimate strength, and fracture strain reduce with the temperature increases. At the same time, porosity effects were found to cause a decrease in the ultimate strength, fracture strain, and Young’s modulus of MoS
2
membranes. Because the pore exists, the most considerable stresses will be concentrated around the pore site throughout uniaxial and biaxial tensile tests, increasing the possibility of fracture compared to tensing the pristine membrane. Moreover, this article investigates the impacts of temperature, porosity, and length size on the thermal conductivity of MoS
2
membrane using the non-equilibrium molecular dynamics (NEMD) method. The results show that the thermal conductivity of the MoS
2
membrane is strongly dependent on the temperature, porosity, and length size. Specifically, the thermal conductivity decreases as the temperature increases, and the thermal conductivity reduces as the porosity density increases. Interestingly, the thermal and mechanical properties of the pristine MoS
2
membrane are similar in armchair and zigzag directions.
Hexagonal boron nitride (h-BN) is a promising 2D material due to its outstanding mechanical and thermal properties. In the present study, we use molecular dynamics simulations to investigate the ...influence of porosity and temperature on the mechanical characteristics of h-BN based on uniaxial and biaxial tensions. Meanwhile, the progression of the microstructure of h-BN up to fracture is studied in order to clarify its fractures mechanism during the tension process. Our results reveal that depending on the porosity and tensile direction, the phase transition occurs more or less. The strength, and Young's modulus of h-BN membranes reduce as increasing porosity. Due to the presence of the pores, the most substantial stresses will be centred around the pores site in the tensile test. Then the fracture starts on the pore edge and spreads preferentially along the zigzag direction of h-BN. Furthermore, fracture strain, strength, and Young's modulus decrease when the temperature rises. In addition, the non-equilibrium molecular dynamics (NEMD) simulations are performed to investigate the influence of various porosities and temperatures on the thermal conductivity of h-BN membranes. The results reveal that the thermal conductivity is greatly reduced by nanoporous. The higher the porosity, the lower the thermal conductivity. The vibration density of states of h-BN membranes is calculated; the result suggests that the defects might reduce the phonon mean free path because of the high collision of the phonons. These alterations represent the scattering influence of defects on phonons, which reduces phonon life and considerably lowers thermal conductivity. Moreover, the findings also proved that as temperature increases, the intrinsic thermal conductivity of h-BN decreases. The thermal conductivity and mechanical properties of the pristine h-BN thin film are interestingly equivalent in the zigzag and armchair orientations.
In this paper, the polishing process of a silicon carbide (SiC) substrate covered with a thin amorphous SiC (a-SiC) film is investigated by molecular dynamics simulation method. A diamond abrasive ...particle slides or rolls over the SiC workpiece surface with different depths and speeds. The results indicated that in the sliding motion, increasing the sliding depth leads to a higher force, stress, temperature, and a higher number of atoms removed. The best surface quality gains by sliding at 10 Å depth or at the depth that equal to the a-SiC layer thickness. At this polishing depth, the a-SiC plays a role as a padding layer that prevents the SiC substrate from subsurface damage. At a deeper depth, the crystalline SiC atoms at the interfacial area are transformed into the amorphous state. In sliding motion, the ploughing and cutting regimes dominate the atomic removal mechanism. Similar to the sliding motion, improving the rolling depth results in a higher force, stress, temperature, and the number of atoms removed. However, the rolling movement generates higher stress, deeper subsurface damage (SSD) zone, rougher surface and lower number of atoms removed. In rolling motion, the removal mechanism is mainly adhering to which the workpiece atoms are adhered by the abrasive surface. This report also investigates the effect of the abrasive size and the a-SiC thickness to the removal process. The bigger abrasive the higher force, stress, and the number of atoms removed. While increasing the a-SiC film thickness leads to a lower force and shallower SSD zone. Despite the differences in the abrasive size and the thickness, the sliding motion always performs a dramatically better ability to remove the workpiece atoms than the rolling one. A SiC model with the ripple structure is also constructed and analyzed. The bigger ripple mostly generates a higher force, temperature, strain, and a higher number of atoms removed. The high-temperature and high-value atomic strain areas mainly exist in the a-SiC film, at the upper side of the interface. The sliding motion again wipes out more atoms and creates a smoother groove than the rolling motion. Additionally, in the rolling motion, the large ripple can be lengthened and stuck to the substrate.
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•In sliding, the atomic removal mechanisms are ploughing and cutting, depending on the polishing depth.•In rolling, when the abrasive reaches or passes the interfacial area, the von Mises stress grows dramatically.•The a-SiC film act as a padding layer that protects the pure SiC substrate from the SSD formation.•The large ripple structure can be lengthened by the rolling movement.
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