Nanogrinding Process Research Articles

Understanding the machining mechanism in ultrasonic vibration-assisted nanogrinding of GaN

As a typical difficult-to-machine material, gallium nitride (GaN) crystals have the merits of excellent chemical stability and large bandwidth, which can be used in fields of space optical components, photoelectric devices, and electronic communication. The smooth surface of GaN is the basis of these applications. Grinding of GaN has thus drawn increasing attention. Furthermore, vibration-assisted nanogrinding has the potential to reduce grinding forces compared with conventional nanogrinding. In this study, ultrasonic-vibration-assisted nanogrinding of GaN with a single grit is conducted based on molecular dynamic (MD) simulations. The plastic deformation mechanism of GaN single crystals as well as the influence of grinding parameters, such as vibration period and amplitude, on the grinding outcomes are investigated. The MD simulations reveal that plastic deformation of GaN in the vibration-assisted nanogrinding process is dominated by amorphous transformation, dislocation, stacking faults, lattice distortion, and formation of nanocrystals. Furthermore, amorphous GaN atoms are mainly induced in front of the grit and at either side of the grinded groove. In the grinding process, the periodic fluctuation of grinding force components, which include the normal, tangential, and horizontal forces, are stable. A smaller grinding force is obtained when grinding with a small vibration period. Moreover, a small vibration period and large vibration amplitude lead to improved grinding efficiency, suppressing the grinding force and subsurface damage, and improving the grinding quality. The findings in this study facilitate an understanding of the plastic deformation mechanism of GaN single crystals and also provide guidance for parameter optimization during vibration-assisted nanogrinding.

Wear behaviors and plastic deformation mechanisms induced by nano-grinding of indium-doped gallium nitride single crystal

The wear behaviors and plastic deformation mechanisms of indium (In)-doped gallium nitride (GaN) induced by nano-grinding are investigated in this work, which is of great significance for improving the quality and precision of the machined surface of the nitride semiconductor, as well as for elucidating their mechanical properties during the nano-grinding process. A numerical simulation approach based on molecular dynamics (MD) is used to carry out a comparative analysis of the nano-grinding process for the undoped and In-doped GaN by analyzing the machined surface morphology, formation mechanisms of the subsurface damage (SSD) layer, dislocation behaviors, and residual stress. In addition, the effects of material temperature and grinding speed on the nano-grinding process of the In-doped GaN are investigated. Compared to the undoped GaN, indium doping increases the pile-ups and chips on both sides of the abrasive, the roughness of the machined surface, and the coefficient of friction. Moreover, it reduces the thickness of the SSD layer and restricts the occurrence of phase transformations. Indium doping also weakens the dislocation interactions and reduces the generation of dislocation entanglements as well as wall-like dislocations. Furthermore, it decreases the maximum value of the residual stress and increases the area of high residual stress regions. The wear behaviors and plastic deformation mechanisms of In-doped GaN are quite sensitive to material temperature, but not much influenced by grinding speed.

The effects of polycrystalline 3 C-SiC with different roughness coefficients on the crystal structure of nano-grin ding based on molecular dynamics

To investigate the effect of different roughness subsurface on the crystal structure and grain boundary changes during polycrystalline 3 C-SiC nanocrystalline grinding. Polycrystalline 3 C-SiC with different roughness subsurface is constructed by Voronoi method, the stress, crystal structure change and grain boundary crossing mechanism of polycrystalline 3 C-SiC are analyzed by molecular dynamics method. Three different roughness surfaces (Ry=0.02 um, Ry=0.04 um and Ry=0.045 um) are selected for analysis, before the nano-grinding process, the polycrystalline 3 C-SiC is first annealed to make the atoms closely arranged and eliminate the vacancy defects inside the grains. Different roughness affects the friction force of polycrystalline 3 C-SiC and changes the deformation of its subsurface. Consequently, affecting the elastoplastic properties of polycrystalline 3 C-SiC. Different roughness will affect the shear force on the polycrystalline 3 C-SiC subsurface, the smaller the roughness, the greater the shear stress on the subsurface. The polycrystalline 3 C-SiC crystal structure with a roughness of Ry=0.04um is most obviously affected, the length of the dislocations it produces is relatively small, and the situation is more obvious at the boundary, fewer internal dislocations indicating internal dislocation slip phenomenon is less and more favorable for processing.

Investigation of Gallium Arsenide Deformation Anisotropy during Nanopolishing via Molecular Dynamics Simulation.

Crystal orientation significantly influences deformation during nanopolishing due to crystal anisotropy. In this work, molecular dynamics (MD) simulations were employed to examine the process of surface generation and subsurface damage. We conducted analyses of surface morphology, mechanical response, and amorphization in various crystal orientations to elucidate the impact of crystal orientation on deformation and amorphization severity. Additionally, we investigated the concentration of residual stress and temperature. This work unveils the underlying deformation mechanism and enhances our comprehension of the anisotropic deformation in gallium arsenide during the nanogrinding process.

Effect of diamond grain shape on gallium nitride nano-grinding process

In the grinding process, the quality of the product in the process is affected by many factors. In order to improve the processing quality and take into account the processing efficiency, the selection of a reasonable abrasive grain shape can be considered. This is a low-cost and highly feasible approach. In this article, the molecular dynamics method was employed to simulate the process of grinding GaN with four common synthetic diamond of abrasive grains (spherical, octahedron, cubic and cub-octahedron), and the surface morphology, material removal rate, temperature, potential energy, grinding force, substrate structure deformation, subsurface damage and dislocation were investigated. The results show that the cubic abrasive grain has the highest material removal rate with the number of chip atoms per unit area about 1.68 times that of the cub-octahedron abrasive grain. The lowest subsurface temperature of the octahedron abrasive grain during processing is 498 K. In contrast, the other three grits have similar subsurface temperatures of 540 K, 535 K and 545 K, respectively. The tangential force per unit area of octahedron abrasive grain is about 0.68761 (spherical: 0.23987, cubic: 0.4476, cubo-octahedral: 0.4314), but its potential energy is the most stable during processing. The shape of the abrasive grain has no significant effect on the coefficient of friction, while it has a significant effect on the subsurface damage. The depth of subsurface damage is the smallest for both cubic and cub-octahedron abrasive grain (2.4 nm), while cub-octahedron abrasive grain is more capable of promoting the generation of dislocations within the substrate. In addition, the performance of cubic and cub-octahedron abrasive grain at different grinding depths was further explored. Number of chip atoms, temperature, grinding force and subsurface damage all increase with the grinding depth. Nevertheless, the machining performance of cubic abrasive grain is still superior to that of cub-octahedron abrasive grain when the grinding depth is more than 1 nm. The outcome of this study may provide a micro-scale insight into the selection of abrasive grain shapes for processing GaN substrates.

Abrasive machining induced surface layer damage behavior and formation mechanism of monocrystalline β-Ga2O3: A comparative study of nanoindentation and nanogrinding

Beta-phase gallium oxide (β-Ga2O3) has garnered significant attention in recent years as an ultra-wide bandgap semiconductor material. However, its surface layer damage behavior and machining deformation mechanism remain poorly understood. In this work, a systematic comparative study was conducted through nanoindentation and nanogrinding for monocrystalline β-Ga2O3 (−201) crystal plane. Transmission electron microscopy is employed as the primary characterization method. The results show that the microstructural evolution patterns of monocrystalline β-Ga2O3 can be categorized into four stages, i.e. stacking faults, twins, slip bands, and cleavage cracks are observed sequentially. According to the tests results, subsurface damage diagram models based on nanoindentation and nanogrinding are established, both of which exhibit the above evolution patterns as the indentation depth and grit depth of cut increases. However, there are some differences. In the nanogrinding process, nanocrystals and amorphous layers were found near the ground surface, which are related to the high strain rate grinding conditions and the unique material properties of monocrystalline β-Ga2O3 with low stacking fault energy. The results in this work contribute to an enhanced understanding of the deformation characteristics of monocrystalline β-Ga2O3 at macro and micro scales, while offering valuable insights for achieving damage control in ultra-precision machining processes.

Numerical Investigation on the Effects of Grain Size and Grinding Depth on Nano-Grinding of Cadmium Telluride Using Molecular Dynamics Simulation.

Cadmium telluride (CdTe) is known as an important semiconductor material with favorable physical properties. However, as a soft-brittle material, the fabrication of high-quality surfaces on CdTe is quite challenging. To improve the fundamental understanding of the nanoscale deformation mechanisms of CdTe, in this paper, MD simulation was performed to explore the nano-grinding process of CdTe with consideration of the effects of grain size and grinding depth. The simulation results indicate that during nano-grinding, the dominant grinding mechanism could switch from elastic deformation to ploughing, and then cutting as the grinding depth increases. It was observed that the critical relative grain sharpness (RGS) for the transition from ploughing to cutting is greatly influenced by the grain size. Furthermore, as the grinding depth increases, the dominant subsurface damage mechanism could switch from surface friction into slip motion along the <110> directions. Meanwhile, as the grain size increases, less friction-induced damage is generated in the subsurface workpiece, and more dislocations are formed near the machined groove. Moreover, regardless of the grain size, it was observed that the generation of dislocation is more apparent as the dominant grinding mechanism becomes ploughing and cutting.

Analysis of crystal structure transition of polycrystalline 3C-SiC in nanocrystalline grinding based on molecular dynamics simulation

In order to study the grain/grain boundary crossing mechanism of polycrystalline 3C-SiC materials under nano-grinding, molecular dynamics(MD) method was used to simulate the grinding process of polycrystalline 3C-SiC materials under diamond abrasive particles. Through the coupling method of Voronoi construction of polycrystalline 3C-SiC workpiece model and MD construction process, the nano-grinding process of polycrystalline 3C-SiC material was established, the potential function field parameters were optimized, and the residual stress in polycrystalline 3C-SiC was eliminated. The energy transition equation is modified to reduce the time for the system to enter dynamic equilibrium. The crystal structure and dislocation generation of polycrystalline 3C-SiC were analyzed by post-processing methods such as defect analysis and crystal structure recognition. The results found that: there are cubic lattice, simple cubic lattice, hexagonal lattice and graphene mechanism type atoms at the grain boundary. During the machining process, the cubic lattice type atoms are converted to other types atoms, and other types structure atoms are converted to each other. The internal structure atoms of grains are converted to the grain boundary structure atoms during the machining process, and the crystal atoms near the grain boundary have low structural stability. The grinding force mainly acts on the forward direction and just below the grinding grain, and also has a small amount of distribution at the grain boundary, but it is relatively weak. The dislocation caused by grinding mainly occurs at grain boundaries. The resulting debris was monolithic, and a few were granular. The interconversion between grains and different structural atoms at the grain boundary is mainly from the crystal lattice atoms in grains to other atoms at the grain boundary. The force is transferred to the grain through the grain boundary, which hinders the transboundary transfer of force and protects the grains. The shearing force plays a dominant role in the grinding process.

Thermal effects on removal mechanism of monocrystal SiC during micro-laser assisted nanogrinding process

As a hard-brittle material, research into efficient processing methods of monocrystalline SiC with low damage has attracted a lot of attention. In this paper, removal behavior of monocrystalline SiC using micro-laser assisted method is analyzed using molecular dynamics method. This enables an in-depth theoretical analysis of thermal effects on material removal mechanism for 3C–SiC during nanogrinding process. Taking average laser pulse intensity as a variable, the micro deformation mechanism and machinability under the micro-laser assisted method are analyzed in detail. The results show that laser irradiation can directly affect material removal rate and subsurface damage depth of 3C–SiC. The results of this work significantly improve the understanding of the feasibility of micro-laser assisted method and its optimization for 3C–SiC.

Revealing nanoscale material deformation mechanism and surface/subsurface characteristics in vibration-assisted nano-grinding of single-crystal iron

Ultrasonic vibration-assisted grinding (UVAG) has attracted extensive attention as it can significantly improve the surface integrity of the machined workpiece. However, the atom-scale material deformation mechanism with the presence of ultrasonic vibration is still unclear, which hider the application of the UVAG to the ultra-precision machining process. To fill this gap, we present a molecular dynamics (MD) model for the vibration-assisted nano-grinding process (VANG) of single-crystal iron. The characteristics of the material deformation process induced by ultrasonic vibration are studied, including material accumulation, temperature, dislocation, subsurface damage (SSD), contact area, and grinding force. The results show that vibration changes the dislocation distribution in the plastic zone, forms the phenomena of expansion and contraction and causes the fluctuation of SSD depth. Due to the different distribution of stress and temperature during VANG, the details of plastic deformation, such as dislocation movement, dislocation nucleation and phase transformation, change with the varying vibration frequency. In addition, the reduction of the projected contact area is the main reason for the reduction of grinding force. However, dislocation pileup in the grinding zone and chips further increase the grinding hardness. This research could enrich the understanding of nano-scale deformation mechanisms in vibration-assisted processing of metallic materials.

Molecular Dynamics Simulation of Chip Morphology in Nanogrinding of Monocrystalline Nickel

In this study, the nanogrinding process for single-crystal nickel was investigated using a molecular dynamics simulation. A series of simulations were conducted with different tool radii and grinding methods to explore the effects of chip morphology, friction forces, subsurface damage, and defect evolution on the nanogrinding process. The results demonstrate that the workpiece atoms at the back of the tool were affected by the forward stretching and upward elastic recovery when no chips were produced. Although the machining depth was the smallest, the normal force was the largest, and dislocation entanglement was formed. The small number of defect atoms indicates that the extent of subsurface damage was minimal. Moreover, when spherical chips were produced, a typical columnar defect was generated. The displacement vector of the chip atoms aligned with the machining direction and as the chips were removed by extrusion, the crystal structure of the chip atoms disintegrated, resulting in severe subsurface damage. By contrast, when strip chips were produced, the displacement vector of the chip atoms deviated from the substrate, dislocation blocks were formed at the initial stage of machining, and the rebound-to-depth ratio of the machined surface was the smallest.

Effects of grain size and protrusion height on the surface integrity generation in the nanogrinding of 6H-SiC

The size and protrusion height of abrasive grains of a grinding wheel are not uniform, which can greatly influence the surface integrity generation of a ground component. The purpose of this paper is to explore such effects on the deformation and material removal mechanisms of 6H-SiC subjected to nanogrinding with the aid of molecular dynamics simulations. It was found that a nanogrinding process can fall into one or more of the following regimes with respect to material removal, i.e., no-wear, adhering, ploughing and cutting regimes, depending on both the depth of grain cut and size of abrasive grains. The plastic deformation in 6H-SiC starts with surface amorphization under the grinding stresses, followed by the activation of partial and perfect dislocations in the shuffle set of the material’s basal plane.

Influence of the nanogrinding process on the performance of lithium iron phosphate

Wet grinding is one of the most common forms of technology in powder production and preparation. Sand mill is the latest technology of wet grinding, which can effectively reduce the grinding particle size and improve the powder reactivity. LiFePO4 is one of the widely used cathode materials for lithium batteries at present, and the grinding process will affect the product performance. In this paper, parallel type and series type grinding processes were simulated by the single-cylinder system and the double cylinder system, which were studied and compared through the preparation of LiFePO4. It was discovered that the series type grinding process can not only increase the grinding efficiency but also improve the consistency of particle size distribution. LiFePO4 prepared by the series type grinding process has better rate performance and a higher discharge voltage platform. Attributing to the uniform particle size of LiFePO4, the lithium-ion diffusion coefficient was raised, which was proved by the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). To sum up, based on the results of this paper, the properties of lithium iron phosphate will be affected by the particle size distribution, which is related to the grinding process. Therefore, in order to obtain the best performance of LiFePO4, it is necessary to choose the appropriate production process. This research work provides references and the basis for the research of powder grinding production technology.

Nano-grinding process of single-crystal silicon using molecular dynamics simulation: Nano-grinding parameters effect

Investigation of the surface & subsurface generation process of the single-crystal silicon during the ultra-precision process is significant in improving the machined surface quality and durability of the machined wafer. In this work, the molecular dynamics (MD) simulation method is used as the theoretical basis to study the surface generation, mechanical response, phase transformation, and residual stress of the single-crystal silicon during the nano-grinding process. The nano-grinding process is studied by adjusting the nano-grinding parameters in the MD simulations. The simulation results prove that the nano-grinding speed, ambient temperature, and nano-grinding depth can affect the atomic movement process in the surface & subsurface of the workpiece to a large extent, thereby affecting the processing force and machined surface quality of the workpiece. In addition, adjusting the nano-grinding speed, ambient temperature, and nano-grinding depth can effectively inhibit the generation of the subsurface damage layer (SDL), the phase transformation, and the residual stress concentration in the SDL, improving the machined subsurface quality of the workpiece. This work clarifies the surface generation mechanism of single-crystal silicon from the atomic perspective and has a guiding role in realizing surface & subsurface ultra-precision machining.

Molecular dynamics study of crystal orientation effect on surface generation mechanism of single-crystal silicon during the nano-grinding process

Crystal orientation of the crystalline materials surfaces significantly affects the surface generation and subsurface damage of the materials during the ultra-precision machining. This work is based on the molecular dynamics (MD) simulation method to study the nano-grinding process of the single-crystal silicon workpiece with three different types of surface crystal orientations. And the MD simulation results are compared with the experimental results. The results show that the machined surface quality of workpieces with different surface crystal orientations is similar, but the degree of subsurface damage, processing force, phase transition, and internal residual stress is quite different. By comparing all the three types of the workpiece, the surface and subsurface quality of the single-crystal silicon workpiece with {110} surface crystal orientation has a better processing effect. This work reveals the evolution of the crystal structure of single-crystal silicon with different surface crystal orientations and the mechanism of subsurface damage from the atomic point of view.

Effects of grinding speeds on the subsurface damage of single crystal silicon based on molecular dynamics simulations

Understanding the damage mechanism of silicon under interfacial shear is vital as it can provide insights for the ultra-precision low damage machining. In this work, the nano-grinding process of single crystal silicon was studied by molecular dynamics (MD) simulations, the damage mechanism of single crystal silicon were analyzed in details under different grinding speeds. The results show that the maximum height of the grinding chip does not always increase with the increase of the grinding speed. When the speed exceeds 150 m/s, more atoms will flow to both sides of the groove. During grinding, the workpiece changes from cubic diamond structure to non-diamond structure and a small amount of hexagonal diamond structure. The Si-II phase was found in the subsurface damage layer. Residual stresses are mainly distributed in the subsurface damage layer (SDL) and do not always show compressive or tensile stresses as the depth increases. This investigation may shed light on the damage mechanism of silicon from an atomic perspective.

A molecular dynamic study of nano-grinding of a monocrystalline copper-silicon substrate

We performed molecular dynamics simulations to study the nano-grinding process of copper-silicon with a single diamond abrasive grain. The Cu-Si model was based on the modified embedded-atom method. The effects of grinding depth, speed, and Cu thickness on material removal, defects, grinding forces, and temperature were analyzed based on dislocations and phase transitions. Our results show that effects of the interface emerge at a 4.3-nm Cu layer. Shockley dislocations extend from the ground surface to the Cu-Si interface, accompanied by formation of hexagonal-close-packed (HCP) Cu. Shockley dislocations, which lead to the HCP transition, clearly increase at a grinding depth of 4.3 nm. The phase transitions from the face-centered cubic to body-centered cubic structures are accompanied by an increase in the atomic kinetic energy. Kinetic energy is released as material recovers from elastic deformation. At high absorbed energies of 7.3 eV/atom potential and 4.4 eV/atom kinetic, the HCP structure is formed between the body-centered cubic and face-centered cubic phases. For the thinner Cu layer (2.2 nm), as the grinding depth gets closer to the Cu-Si interface, more chips form, causing the tangential forces to increase sharply, whereas the normal forces remain almost unchanged. Within a grinding speed range of 40 and 120 m/s, there is a linear positive correlation between the grinding temperature and the speed. The workpiece temperature rises by approximately 30 K for every 20 m/s increase in the speed.

Mechanical properties of silicon in subsurface damage layer from nano-grinding studied by atomistic simulation

Ultra-thin silicon wafer is highly demanded by semi-conductor industry. During wafer thinning process, the grinding technology will inevitably induce damage to the surface and subsurface of silicon wafer. To understand the mechanism of subsurface damage (SSD) layer formation and mechanical properties of SSD layer, atomistic simulation is the effective tool to perform the study, since the SSD layer is in the scale of nanometer and hardly to be separated from underneath undamaged silicon. This paper is devoted to understand the formation of SSD layer, and the difference between mechanical properties of damaged silicon in SSD layer and ideal silicon. With the atomistic model, the nano-grinding process could be performed between a silicon workpiece and diamond tool under different grinding speed. To reach a thinnest SSD layer, nano-grinding speed will be optimized in the range of 50-400 m/s. Mechanical properties of six damaged silicon workpieces with different depths of cut will be studied. The SSD layer from each workpiece will be isolated, and a quasi-static tensile test is simulated to perform on the isolated SSD layer. The obtained stress-strain curve is an illustration of overall mechanical properties of SSD layer. By comparing the stress-strain curves of damaged silicon and ideal silicon, a degradation of Young’s modulus, ultimate tensile strength (UTS), and strain at fracture is observed.

Molecular dynamics simulation of multi-pass nano-grinding process

Grinding involves the use of a large number of micrometric abrasive grains in order to remove material from workpiece surface efficiently and finally render a high quality surface. More specifically, grinding in the nano-metric level serves for attaining nano-level surface quality by removing several layers of atoms from the workpiece surface. The abrasive grains act as individual cutting tools, performing primarily material removal but also induce alterations in the subsurface regions. In order to study the nano-grinding process, Molecular Dynamics (MD) method is particularly capable to provide comprehensive observations of the process and its outcome. In this study, MD simulations of multi-pass grinding for copper substrates, using two abrasive grains, are performed. After the simulations are carried out, results concerning grinding forces and temperatures are presented and discussed.

Mechanical fabrication of high-strength and redispersible wood nanofibers from unbleached groundwood pulp

In the past, the direct production of lignin-containing nanofibers from wood materials has been very limited, and nanoscale fibers (nanocelluloses) have been mainly isolated from chemically delignified, bleached cellulose pulp. In this study, we have introduced a newly adapted, heat-intensified disc nanogrinding process for the enhanced nanofibrillation of wood nanofibers (WNF) with a high lignin content (27.4 wt%). The WNF produced this way have many unique and intriguing properties in their naturally occurring form, for example, being able to be dispersed in ethanol and having ethanol solution viscosities higher than water solution viscosities. When WNF nanopapers were formed with ethanol, the properties of the nanofibers were recoverable without a notable decrease in the viscosity or mechanical strength after redispersing them in water. The preservation of lignin in the WNF was noticed as an increase in the water contact angles (89°), the rapid removal of water in the fabrication of the nanopapers, and the enhanced strength of the nanopapers when subjected to high pressure and heat. The nanopapers fabricated from the WNF were mechanically stable, having an elastic modulus of 6.2 GPa, a maximum stress of 103.4 MPa, and a maximum strain of 3.5%. Throughout the study, characteristics of the WNF were compared to those of the delignified and bleached reference cellulose nanofibers. We envision that the exciting characteristics of the WNF and their lower cost of production compared to that of bleached cellulose nanofibers may offer new opportunities for nanocellulose and biocomposite research.