Damage evolution and removal behaviors of GaN crystals involved in double-grits grinding

Elucidating the complex interactions between the work material and abrasives during grinding of gallium nitride(GaN)single crystals is an active and challenging research area.In this study,molecular dynamics simulations were performed on double-grits interacted grinding of GaN crystals;and the grinding force,coefficient of friction,stress distribution,plastic damage behaviors,and abrasive damage were systematically investigated.The results demonstrated that the interacted distance in both radial and transverse directions achieved better grinding quality than that in only one direction.The grinding force,grinding induced stress,subsurface damage depth,and abrasive wear increase as the transverse interacted distance increases.However,there was no clear correlation between the interaction distance and the number of atoms in the phase transition and dislocation length.Appropriate interacted distances between abrasives can decrease grinding force,coefficient of friction,grinding induced stress,subsurface damage depth,and abrasive wear during the grinding process.The results of grinding tests combined with cross-sectional transmission electron micrographs validated the simulated damage results,i.e.amorphous atoms,high-pressure phase transition,dislocations,stacking faults,and lattice distortions.The results of this study will deepen our understanding of damage accumulation and material removal resulting from coupling between abrasives during grinding and can be used to develop a feasible approach to the wheel design of ordered abrasives.

Fabrication of Periodic Nanostructures Using AFM Tip-Based Nanomachining:Combining Groove and Material Pile-Up Topographies

This paper presents an atomic force microscopy(AFM)tip-based nanomachining method to fabricate periodic nanostructures.This method relies on combining the topography generated by machined grooves with the topography resulting from accumulated pile-up material on the side of these grooves.It is shown that controlling the distance between adjacent and parallel grooves is the key factor in ensuring the quality of the resulting nanostructures.The presented experimental data show that periodic patterns with good quality can be achieved when the feed value between adjacent scratching paths is equal to the width between the two peaks of material pile-up on the sides of a single groove.The quality of the periodicity of the obtained nanostructures is evaluated by applying one-and two-dimensional fast Fourier transform(FFT)algorithms.The ratio of the area of the peak part to the total area in the normalized amplitude–frequency characteristic diagram of the cross-section of the measured AFM image is employed to quantitatively analyze the periodic nanostructures.Finally,the optical effect induced by the use of machined periodic nanostructures for surface colorization is investigated for potential applications in the fields of anti-counterfeiting and metal sensing.

Study of machining indentations over the entire surface of a target ball using the force modulation approach

A modified five-axis cutting system using a force control cutting strategy was to machine indentations in different annuli on the entire surface of a target ball.The relationship between the cutting depths and the applied load as well as the microsphere rotation speed were studied experimentally to reveal the micromachining mechanism.In particular,aligning the rotating center of the high precision spindle with the microsphere center is essential for guaranteeing the machining accuracy of indentations.The distance between adjacent indentations on the same annulus and the vertical distance between adjacent annuli were determined by the rotating speed of the micro-ball and the controllable movement of the high-precision stage,respectively.In order to verify the feasibility and effect of the proposed cutting strategy,indentations with constant and expected depths were conducted on the entire surface of a hollow thin-walled micro-ball with a diameter of 1 mm.The results imply that this machining methodology has the potential to provide the target ball with desired modulated defects for simulating the inertial confinement fusion implosion experiment.

Fabrication of Ordered Micro/Nanostructures Using Probe‑Based Force‑Controlled Micromachining System

This paper presents a probe-based force-controlled nanoindentation method to fabricate ordered micro/nanostructures.Both the experimental and finite element simulation approaches are employed to investigate the influence of the interval between the adjacent indentations and the rotation angle of the probe on the formed micro/nanostructures.The non-contacting part between indenter and the sample material and the height of the material pile-up are two competing factors to determine the depth relationship between the adjacent indentations.For the one array indentations,nanostructures with good depth consistency and periodicity can be formed after the depth of the indentation becoming stable,and the variation of the rotation angle results in the large difference between the morphology of the formed nanostructures at the bottom of the one array indentation.In addition,for the indentation arrays,the nanostructures with good consistency and periodicity of the shape and depth can be generated with the spacing greater than 1μm.Finally,Raman tests are also carried out based on the obtained ordered micro/nanostructures with Rhodamine probe molecule.The indentation arrays with a smaller spacing lead to better the enhancement effect of the substrate,which has the potential applications in the fields of biological or chemical molecular detection.

Towards understanding the surface rippling process by periodic reciprocal nanoscratching

The bundle structure formed perpendicular to the scratching direction is a type of wear-induced structure for thermoplastics.In this study,the formation mechanism of bundle structures on polycarbonate(PC)surfaces is investigated by reciprocal scratching experiments.Based on the analysis of the morphologies,friction forces,and height signals,the formation of the bundle structure is reproduced.The influence of scratching parameters,including the feed value and scratching direction,on the formation of the bundle structure is also studied.It is found that the bundle structure is accumulated by the continuous stacking of the sample materials plowed by the tip in stick–slip motion,and that the stick–slip behavior is enhanced with increased scratching times.This work reproduces the formation process of bundle structure in experiments for the first time and demonstrates that the stick–slip enhancement mechanism exists in the reciprocal scratching process,providing further insight into the friction behavior of polymers.

A Simulated Investigation of Ductile Response of GaAs in Single-Point Diamond Turning and Experimental Validation

In this paper,molecular dynamic(MD)simulation was adopted to study the ductile response of single-crystal GaAs during single-point diamond turning(SPDT).The variations of cutting temperature,coordination number,and cutting forces were revealed through MD simulations.SPDT experiment was also carried out to qualitatively validate MD simulation model from the aspects of normal cutting force.The simulation results show that the fundamental reason for ductile response of GaAs during SPDT is phase transition from a perfect zinc blende structure(GaAs-I)to a rock-salt structure(GaAs-II)under high pressure.Finally,a strong anisotropic machinability of GaAs was also found through MD simulations.

Insight into Atomic-Scale Adhesion at the C-Cu Interface During theInitial Stage of Nanoindentation

Adhesion is a common phenomenon in nanomachining which affects processing accuracy and repeatability.As material removal approaches the atomic or close-to-atomic scale,quantum mechanics becomes the dominant principle behind the atomic-level interaction.However,atomic-scale effects cannot be properly described by empirical potential function-based molecular dynamics simulations.This study uses a first-principles method to reveal the atomic-scale adhesion between a diamond tip and a copper slab during initial-stage nanoindentation.Using a simplified tip and slab model,adhesion energy,electronic distribution,and density of states are analyzed based on quantum chemistry calculation.Results show that atomic adhesion is primarily due to the covalent bonding interaction between C and Cu atoms,which can induce structural changes to the diamond tip and copper slab.The effects of tip position and angles on adhesion are further studied through a series of simulations.The results show that adhesion between the tip and slab is sensitive to the lattice structure and a variant in angstroms is enough to cause different adhesion and structural changes.The actual determinants of adhesion can only be the atomic and electronic structures at the tip-slab interface.Bond rotation and breakage are observed during simulation and their effects on adhesion are further discussed.To conclude,the first-principles method is important for the analysis of an atomic-scale interaction system,even if only as an aid to describing adhesion at atomic and electronic scales.

Tip-Based Nanomachining on Thin Films:A Mini Review

As one of the most widely used nanofabrication methods,the atomic force microscopy(AFM)tip-based nanomachining technique offers important advantages,including nanoscale manipulation accuracy,low maintenance cost,and flexible experimental operation.This technique has been applied to one-,two-,and even three-dimensional nanomachining patterns on thin films made of polymers,metals,and two-dimensional materials.These structures are widely used in the fields of nanooptics,nanoelectronics,data storage,super lubrication,and so forth.Moreover,they are believed to have a wide application in other fields,and their possible industrialization may be realized in the future.In this work,the current state of the research into the use of the AFM tip-based nanomachining method in thin-film machining is presented.First,the state of the structures machined on thin films is reviewed according to the type of thin-film materials(i.e.,polymers,metals,and two-dimensional materials).Second,the related applications of tip-based nanomachining to film machining are presented.Finally,the current situation of this area and its potential development direction are discussed.This review is expected to enrich the understanding of the research status of the use of the tip-based nanomachining method in thin-film machining and ultimately broaden its application.