Thickness Of Damaged Layer Research Articles

Molecular dynamics study on grinding mechanism of polycrystalline silicon carbide

In this paper, stress, temperature, dislocation, surface morphology and crystal structure of polycrystalline silicon carbide under different grinding speeds and grain sizes are studied by molecular dynamics simulation. Influence of silicon carbide with different grain sizes on formation and emission of dislocations and relationship between formation and emission of dislocations under different grinding speeds are revealed. By comparing number of amorphous atoms and generation of dislocations between monocrystalline silicon carbide and polycrystalline silicon carbide at different grinding speeds, it is observed that thickness of subsurface damage layer of monocrystalline silicon carbide correspondingly increases with grinding speed. In contrast, damage layer of polycrystalline silicon carbide is determined by the joint action of grain size and grinding speed, and it does not decrease with increase in grinding speed, unlike monocrystalline silicon carbide. Accumulation of dislocations at the grain boundary causes strengthening between the grain boundaries, and alters the cutting force. In the grinding process, although grinding speed is not the primary factor affecting surface topography, it is the main factor affecting workpiece temperature.

Synergistic effect of metakaolin and silica fume on hydrochloric acid resistance of concrete

The issue caused by acid corrosion of concrete merits continued scientific attention. In this paper, the acid resistance of concrete incorporated different dosages of metakaolin (5 and 10% by weight of total cementitious materials) and silica fume (5, 10, and 15% by weight of total cementitious materials) was studied. Herein, special attention was paid to compressive strength, mass loss, dynamic elastic modulus, thickness of damage layer, and stress-strain response (including ductility and energy dissipation capacity). In addition, the stress-strain behavior was described by the damage constitutive model with the initial damage based on Weibull theory. Moreover, the morphology and characteristic of microstructure and erosion products formed in the specimen during the process of hydrochloric acid attack were studied via X-ray diffraction (XRD), thermo-gravimetric (TG) analysis, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The result revealed that the acid resistance of concrete was improved by the synergistic effect of MK and SF, in which the effect was notable when the dosage of both is 5%. The result of the study is of practical significance in improving the acid resistance of concrete.

Mechanical Properties and Damage Layer Thickness of Green Concrete under a Low-Temperature Environment.

To study the influence of mineral admixtures on concrete's mechanical properties after a low-temperature exposure, green concrete was prepared by mixing fly ash and slag at different replacement rates. By analysing the changes to concrete's mechanical properties and the damage layer thickness under different ambient temperatures (20, -10, -20, -30, and -40 °C), the change rule of concrete at low temperatures was explored. The results revealed that the compressive strength of concrete, containing either fly ash or slag, peaked at 30 °C; moreover, the slag concrete's flexural and splitting tensile strength peaked at -40 °C. The best mechanical properties were observed for a fly ash-to-slag ratio of 1:2 (F10S20; i.e., 10% fly ash and 20% slag) and its compressive strength at different temperatures was higher than that of concrete, containing 30% fly ash (F30) or 30% slag (S30), but the flexural and splitting tensile strength was lower than S30. Further, as the temperature decreased, the fly ash concrete's damaged layer thickness gradually increased. When the content of fly ash and slag were both 15% (F15S15), the damaged layer thickness was minimal at different low temperatures, especially at -30 °C, where the thickness was only 8.31 mm.

Experimental Study on Surface Integrity of Solar Cell Silicon Wafers Sliced by Electrochemical Multi-Wire Saw.

Electrochemical multi-wire sawing (EMWS) is a hybrid machining method based on a traditional multi-wire sawing (MWS) system. In this new method, a silicon ingot is connected to a positive electrode; the slicing wire is connected to a negative electrode. Material is removed by the interaction of mechanical grinding and an electrochemical reaction. In this paper, contrast experiments of EMWS and MWS were conducted based on industrialized equipment to verify the beneficial effects of the hybrid method. The experimental statistical results show that the composite processing method improved the processing qualification rate by 1.28%, and the Bow of silicon wafers was reduced by about 2.74 microns. Further testing on the surface of the silicon wafer after electrochemical action showed that obvious holes were present on the surface, and the surface hardness of the wafer decreased significantly. Therefore, the scratches on the surface of wafer sliced by EMWS were reduced; in addition, the thickness of the surface damage layer was reduced by about 9 microns. After standard texturing, the average reflectivity of the wafers sliced by EMWS was about 2–10% lower than that of the wafers sliced by MWS in the wavelength of 300–1100 nm. In this paper, the voltage parameter of the composite machining is set to 48 V; the amount of electrolyte added in each experiment is 2 L; and a good machining effect is obtained. In the future, the electric parameters and cutting fluid components will be further studied to improve the electrochemical effect.

Effects of Cutting Force on Formation of Subsurface Damage During Nano-Cutting of Single-Crystal Tungsten

Abstract Single-crystal tungsten is widely utilized in various fields, benefiting from its outstanding properties. Nano-cutting, as an ultra-precision machining method, can realize high efficiency and low damage. However, from the atomic perspective, the formation mechanism of subsurface damage during the nano-cutting of tungsten is still unclear. Herein, the molecular dynamics (MD) simulation of nano-cutting single-crystal tungsten was established to elucidate the evolution of subsurface damage and the effects of cutting force on subsurface damage. The corresponding results showed the existence of damage including atomic cluster, vacancy defect, “V-shaped” dislocation, stair-rod dislocation, and dislocation ring on the subsurface during the cutting. There were dislocation lines in 1/2<111>, <100>, <110>, and other directions due to plastic deformation dominated by dislocation slip, and the 1/2<111> dislocation lines could be merged into stable <100> dislocation lines under certain circumstances during the cutting. The variation of cutting force and cutting force fluctuation induced by changing cutting parameters had a great influence on the subsurface damage of tungsten, including the number of surface defect atoms, dislocation density, and thickness of the subsurface damage layer. In nano-cutting of single-crystal tungsten, a smaller cutting depth and appropriate cutting speed should be selected to reduce subsurface damage. This study provides an insight into the evolution mechanism of subsurface damage of tungsten and is high of significance for achieving low-damage machining of tungsten components.

Effects of tool geometry on tungsten removal behavior during nano-cutting

Although the researches in effects of tool geometry on the removal behavior of many metals have been studied, the deformation and removal mechanisms of tungsten have changed greatly due to the higher brittleness of tungsten compared with other metals and the research on the removal behavior of tungsten involved changing of tool geometry during nano-cutting has not been reported, which limits the development of high precision manufacturing of tungsten components. Herein, molecular dynamics (MD) simulation model of nano-cutting tungsten was established, and the material removal behavior induced by changing of tool geometry was comprehensively studied. The results showed that a larger positive rake angle or larger clearance angle or smaller edge radius of the tool was beneficial to reduce the surface roughness, elastic recovery, thickness of subsurface damage layer, cutting force, cutting temperature, friction coefficient, and maximum stress while the surface presented less residual compressive stress or even tensile stress. In the plastic deformation dominated by phase transition and dislocation slip, the region of 1/2<111> dislocation line presented compressive stress state whereas the region of <100> dislocation line presented tensile stress state during nano-cutting tungsten. Meanwhile, there was no phase transition from BCC structure to other crystal structures, but rather from ordered structure to disordered structure (amorphous structure). The correlation between amorphous phase transition and dislocation slip was affected by the tool geometry owing to the fact that the temperature and stress were different under different tool geometries. The research findings provide a comprehensive theoretical basis for the micro removal behavior of tungsten and technical reference for the design of tools in nano-cutting.

Deformation and removal mechanism of single crystal gallium nitride in nanoscratching

Understanding the deformation mechanism of a brittle material under mechanical loading is of value for unveiling the material removal in an abrasive machining process. In this work, instrumented nanoscratching was performed on the (0001) plane of single crystal gallium nitride (GaN) using a conical diamond indenter and the scratch-induced surface/subsurface deformation patterns were characterized by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A finite element model of scratching was established to relate stress distribution and deformation patterns. The stress threshold for the elastic-plastic deformation transition of the crystal was found being relatively high in comparison to single crystal silicon and gallium oxide. The plastic deformation in the GaN sublayer was dominated by slip, dislocation and lattice distortion under relatively low normal loads, but those defects were accompanied with grain rotation when the load was sufficiently high. The thickness of the damage layer was directly related to the ratio of lateral force to normal force during scratching. Such a force ratio can be used as an indicator for controlling the damage layer thickness during abrasive machining, thus providing a valuable guidance for developing the optimal parameters of an ultraprecision abrasive machining process.

Deterioration mechanism of coal gangue concrete under the coupling action of bending load and freeze–thaw

Coal gangue, as a solid waste produced in the process of coal mining, has caused great harm to the environment and society. It is the most practical means to make concrete as coarse aggregate. In practical engineering, concrete materials are often subjected to the combined action of external load and environmental factors. Simply considering the freeze–thaw effect cannot reflect the deterioration characteristics of coal gangue concrete, so that the popularization and application of coal gangue cannot be realized. Based on this, through bending load and freeze–thaw coupling test (stress level 0, 0.2, 0.4, 0.6, 0.8), the mass loss rate, relative dynamic elastic modulus, damage layer thickness, and compressive strength of coal gangue concrete with a replacement rate of 40% were analyzed. The degradation characteristics of pore structure and interface transition zone of coal gangue concrete were studied by nuclear magnetic resonance test and microhardness test. The effects of different stress levels on pore size distribution, pore grade ratio, pore fluid characteristics, and interface zone characteristics were analyzed. The results show that the larger the bending stress ratio is, the faster the degradation of compressive strength and relative dynamic elastic modulus of coal gangue concrete is, the faster the thickness of damage layer increases, the larger the proportion of large pores in the specimen is, and the pore connectivity is enhanced. In addition, the interface area between coal gangue aggregate and mortar is the weak position of coal gangue concrete, and its thickness and microhardness are widened and decreased with the increase of stress level. The deterioration of the interface area makes the specimen close to failure.

High surface integrity fabrication of silicon wafers using a newly developed nonwoven structured grind-polishing wheel

Silicon wafer is a predominant substrate material in integrated circuits (IC) manufacturing. Currently, grinding is employed as a major machining method for back-thinning and flattening of the wafers. To obtain high surface/subsurface quality of ground silicon wafer, a novel nonwoven structured grind-polishing wheel with ultrafine abrasives is developed in this work. The developed wheel uses zirconia (ZrO2) with a grit diameter of 200 nm as abrasives, where the nonwoven fibers are employed as structures to support the abrasives. By measuring the surface and subsurface quality of ground silicon, grinding performance of the developed grind-polishing wheel is investigated and compared to that of #8000 diamond wheel, which is a commercial grinding wheel with the smallest grit size. The grinding experimental results show that the surface produced by the developed wheel has a surface roughness Ra of 0.45 nm, and a thickness of subsurface damage layer about 67 nm. In addition, the material removal rate for silicon wafers machined by the developed wheel is 0.86 μm/min. Compared with the #8000 diamond wheel, the developed wheel is more suitable for the semi-fine finishing process to produce a smooth surface at reduced time and cost.

Atomistic understanding of the subsurface damage mechanism of silicon (100) during the secondary nano-scratching processing

Ultra-precision grinding is a fundamental machining method as for silicon. In grinding, abrasive particles repetitively scratch on the workpiece surface, and the subsurface damage caused by the initial scratching exerts a remarkable effect on the subsequent processing. Nonetheless, the previous researches regarding the subsurface damage mechanism of silicon ignore the influence caused by the initial scratching. Herein, a damage model was constructed via the initial scratching method, and the influence mechanism of the secondary nano-scratching on the subsurface damage of silicon substrate under different scratching parameters was explored utilizing molecular dynamics simulation. The simulation results show that the dominating removal part under secondary scratching is the amorphous layer induced by the initial scratching, and the secondary scratching without feed can effectively remove prefabrication subsurface damage. The larger scratching depth and tool radius could lead to the increasement of scratching force and scratching temperature so as to increase the thickness of subsurface damage layer. Moreover, the high pressure generated by the collision between the abrasive particles and the substrate will induce the amorphous phase transformation. This study provides a new insight into the mechanism and evolution of subsurface damage during grinding process of silicon from an atomic perspective.

Enhancing concrete sulfate resistance by adding NaCl

This study aimed to investigate the effects of adding NaCl on sulfate resistance of ordinary concrete. Concrete specimens were fabricated with 0.1%, 0.5%, and 1.0% NaCl (by weight of cement) and subjected to 10% Na2SO4 solution. The visual appearance of concrete was observed regularly. The compressive strength, ultrasonic velocity, and SO42− ion concentrations in concrete were also determined. Results indicated that incorporating NaCl in concrete can effectively inhibit the entry of external SO42− ions, consume the corrodible cement hydration products, and reduce corrosion product formation in concrete, thereby substantially enhancing the sulfate resistance of concrete. Under either full or partial immersion corrosion test, adding NaCl can reduce the compressive strength loss of concrete, lessen the development of concrete damaged layer thickness, and prevent the decline of concrete’s relative dynamic modulus. The sulfate resistance of concrete is approximately linear with NaCl content, particularly for concrete under partial immersion sulfate attack condition.

Migration and Reaction of Sulfate Ions in Concrete under Stray Current

Abstract The article presents a study on the influence of stray current on the migration and reaction of sulfate ions in concrete. The amount of sulfate ions in concrete is determined using the barium sulfate weighing method. The thickness of the deterioration damage layer of concrete is studied using an ultrasonic flat measurement method, and the thickness prediction model of the concrete deterioration damage layer is established to consider the influence of the intensity of the stray current. The results indicate the following: (1) the greater the stray current is, the more obvious the effect; (2) the influence of the current intensity on the migration of sulfate ions in concrete is 1.55–2.57 times that of the control group without stray current; (3) under stray current, the ratio of the reaction amount of sulfate ions in the concrete to the total amount of sulfate ions is suppressed, whereas the ratio of the reacted sulfate ions to the total amount of sulfate ions increases from 58.8 to 74.5 % with an increase in the current intensity; finally, (4) under stray current and sulfate, the thickness of the concrete damage layer increases with an increase in the deterioration age and the current intensity. The influence coefficients of stray current on the sulfate ion migration and on the sulfate ion reaction are proposed, and the evolution equation of the amount of sulfate ions in the concrete under the action of stray current with the deterioration age is obtained.

Numerical simulation of laser-generated Rayleigh wave pulses propagation in the machined surface with residual stress using finite-difference method

Laser-excited broadband Rayleigh wave propagation behaviors can be used for characterizing material surface characteristics non-destructively. The present paper is concerned with the Rayleigh wave propagation in the machined surface with a damaged layer with residual stress. Biot’s theory of small deformations affected by initial stress forms the basis for this study. Taking ground silicon wafer as an example, we adopted a staggered finite-difference (FD) scheme for numerical simulation. By comparing the dispersion images with the theoretical dispersion curves of Rayleigh wave, the accuracy of the FD algorithm is verified. Then, some two-layer models without or with residual compressive stress are utilized to further analyze the dispersion energy characteristics of Rayleigh waves in the machined surface. The results indicate that damage layer thickness and damage degree determine the main Rayleigh wave high-order mode and the energy of the high-order mode, respectively. In addition, residual compressive stress can reduce the phase velocity of the Rayleigh wave to a small extent. Meanwhile it can significantly increase the total energy of the Rayleigh wave which excited by the same excitation source. This paper will provide a valuable reference for laser ultrasonic evaluation of residual stress on the machined surface.

Modeling of excimer laser ablation of silicon carbide

The physical model of nanosecond laser ablation of semi-insulating 4H-SiC irradiated by KrF excimer laser with a wavelength of 248 nm was studied. The etching depth was tested by a stylus surface profiler. The morphology of the ablation pit and the thickness of the damaged layer were observed through scanning electron microscope. The phases at the laser irradiated surface were analyzed by Raman spectroscopy. In situ x-ray photoelectron spectroscopy was used to obtain component distribution of the damaged layer and a reasonable thermophysical model was constructed. The temperature distribution of the substrate after the laser irradiation was calculated according to this model. It was found that the etching depth had a linear relationship with the number of laser pulses and the thickness of the uniform damaged layer was independent of pulse number $(&gt;10)$. The thickness of the ablated layer and the newly generated damaged layer are equivalent for each laser irradiation. The established laser ablation model deepens the understanding of physical process and mechanism of nanosecond laser etching of SiC and provides a theoretical guidance for laser processing of SiC.

Study on damage rules on concrete under corrosion of freeze-thaw and saline solution

Coastal concrete in cold area is subjected to chemical attacks and freeze-thaw cycles, which both considerably decrease the service life of concrete structures. In this research, we mainly analyzed and assessed the durability of concrete under the coupling action of chlorine ion, sulfate ion and freeze-thaw cycles by designing the orthogonal experiment. In the process of conducting the experiment, the relative dynamic elastic modulus, the mass loss rate, the thickness of damage layer, the velocity of ultrasonic wave and other damage indexes which reflect the damage law of concrete under the coupling action were measured or calculated and the range analysis and variance analysis were used to study the influence degree of different factors. Then the damage evolution equation and the damage constitutive model of concrete were established on the basis of the analysis for damage indexes and the results of mechanical tests. The comparison results showed that the theoretical values well agreed with the experimental ones and the damage model acquired can potentially be used to predict the damage degree of concrete under the coupling effect of chlorine ion, sulfate ion and freeze-thaw cycles.

Chloride ion transport performance of lining concrete under coupling the action of flowing groundwater and loading

In this study, the chloride ion transport behavior of lining concrete under the coupling action of flowing groundwater and loading is investigated. The results indicate that the damage layer thickness and chloride ion concentration of the lining concrete increases with the increase of erosion age. The flowing groundwater accelerates the dissolution of the lining concrete, alkaline substances such as CH and NaOH in the lining concrete dissolve out. Under the action of the loading, the micro-cracks in the tension zone of the lining concrete continue to expand and the pores are connected to each other. Therefore, for same erosion age and distance from the lining concrete surface, the chloride ion concentration in the tensile zone is the highest under the coupling action of flowing groundwater and loading; that is followed by the action of flowing groundwater alone, and then, by the action of static groundwater. The chloride ion concentration in the pressure zone is the lowest, and the addition of fly ash and silica fume is found to help in decreasing the total chloride ion concentration and increasing the bound chloride ion concentration. In this study, a chloride ion transport model is established considering the accelerated dissolution of flowing groundwater, damage caused by loading in terms of micro-cracks and pore structure, and the effect of bound chloride ion on the total chloride ion transport, further, the rationality of the model is verified.

Influence mechanism of defects on the subsurface damage and structural evolution of diamond in CMP process

Surface defects of materials will deteriorate their mechanical properties and limit the applications. Understanding the effect of surface defects on machining performance remains a challenge. Therefore, it is of paramount significance to insight into the structural evolution and subsurface damage induced by defects. Herein, we construct a polishing model with initial defects to make the simulation more realistic, and elucidate the effects of initial defects on the subsurface damage and structure evolution via ReaxFF molecular dynamics simulations. Simulation results show that the structural evolution starts from defect edge, and the substrate with initial defects will produce more amorphous damage layers compared with the ideal substrate. The thickness of the amorphous damage layer is roughly the same as the depth of the initial defects. Moreover, smaller pit defects have a little effect on the surface morphology. Nevertheless, larger pit defects will cause poor surface quality and more severe subsurface damage. Our simulation results are of significance for further understanding the influence of initial defects on subsequent machining quality and subsurface damage, and provide theoretical support for the processing of diamond from an atomic perspective.

Influence of Cutting Parameters on the Surface Quality of High Volume Fraction SiCp/Al2024 Composite

This paper studies the milling process of SiCp/Al2024 composite with high volume fraction and large particle size. The milling mechanism of SiCp/Al2024 composite is analyzed. The influence of cutting depth and feed per tooth on surface quality is analyzed. It reveals the influence of the feed per tooth on the subsurface damage of the workpiece. The results show that in the milling process of high volume fraction SiCp/Al2024 composite, because the cutting edge radius is much larger than the particle radius, the main form of particle removal is crushing. The surface defects of the workpiece mainly include pits, cracks, scratches and indentations. Appropriately increasing the depth of cut can improve the surface quality of the workpiece. The increase in feed per tooth will result in increased surface defects of the workpiece and deterioration of surface quality. When the feed per tooth increases from 4 μm to 8 μm, the thickness of the subsurface damage layer of the workpiece increases from 47.7 to 60.5 μm. The ratio between the minimum cutting thickness of the SiCp/Al2024 composite and the radius of the blunt circle should be less than 8%.

A novel slurry for chemical mechanical polishing of single crystal diamond

Single crystal diamond (SCD) has extremely high hardness and wear resistance. However, this hampers the development of surfaces with ultra-low surface roughness and ultra-thin damage layer. It is particularly challenging to achieve the chemical mechanical polishing (CMP) of SCD with a surface roughness less than 0.5 nm and a damage layer thickness <1 nm. In this study, the influence of the CMP slurries with different pH values and oxidants on the quality of polished SCD surface was investigated. The optimal slurry consisted of silica, ferrous sulfate, hydrogen peroxide, nitrilotriacetic acid, and deionized water achieved the lowest surface roughness of SCD. The average surface roughness Ra was 0.754 ± 0.062 nm over eight areas of 868 μm × 868 μm. The Ra value of 0.35 nm was obtained over a scanning area of 70 μm × 50 μm, while the thickness of the damage layer was 0.7 nm. The CMP mechanism was elucidated by X-ray photoelectron spectroscopy and infrared spectroscopy. Hydroxyl radicals and hydrogen ions were adsorbed on the surface, oxidation of the surface and generated C-H, C-O, and C = O groups, and they are eventually removed by silica abrasive, yielding an ultra-smooth SCD surface.

Deformation anisotropy of nano-scratching on C-plane of sapphire: A molecular dynamics study and experiment

Sapphire is a typical anisotropic material due to its unique crystal structure. The study on the influence of anisotropy on the deformation mechanism has significance in guiding the processing of sapphire. In this study, the scratching were carried out on C-plane sapphire along [1¯010], [2¯110], [1¯100] and [1¯21¯0] directions by molecular dynamics simulation. Moreover, scratching experiments were conducted on C-plane sapphire along [1¯010] and [1¯21¯0] directions to compare with the simulation results. The results show that the activation of the prismatic slip affects the morphology and stress distribution on scratching surface in simulation and present evidences that the prismatic slip system provides nucleation conditions for cracks in experiment. The lateral flow of removed materials of the scratches along [1¯010] and [1¯100] directions which are parallel to the A-plane is more significant than that along [2¯110] and [1¯21¯0] directions. This study also shows that the scratches along [1¯010] and [1¯100] directions which are parallel to the A-plane result in a smaller thickness of subsurface damage layer. Besides, the transmission electron microscopy images of the cross-section have been compared with the simulation images, which validate the subsurface defects and reveal the difference of the symmetry of subsurface damage along different scratching directions.