Brittle Material Removal Research Articles

The effects of scratching speed in ultrasonic vibration-assisted single diamond scratching process

Ultrasonic vibration-assisted machining has been widely used in manufacturing hard and brittle materials. Previous studies have shown that the vertical ultrasonic vibration in ultrasound-assisted single diamond scratching can affect the scratching forces, surface quality, and material removal mechanism by two mechanisms: changing the contact mode between the single diamond and the workpiece and increasing the speed of the single diamond relative to the workpiece. However, some studies point out that increasing the speed has little effect on the scratching force and material removal mechanism. Therefore, it is necessary to conduct an investigation that takes into account the effects of both scratching velocity and ultrasonic vibration on scratching force, surface quality, and material removal mechanisms. In this study, the single diamond conventional scratching (CS) and ultrasonic vibration-assisted scratching (UAS) experiments have been conducted at different scratching speeds to investigate the material removal mechanism of ultrasonic vibration-assisted machining. The effects of increasing the scratching speed on cutting forces, material removal mechanism, and machined surface quality are investigated. The results show that in both CS and UAS processes with low to medium scratching speeds (less than1000 mm/s), increasing the scratching speed leads to more brittle material removal and lower surface quality. At the same time, the scratching speed has almost no effect on the scratching speed force.

Super hard and brittle material removal mechanism in fixed abrasive lapping: Theory and modeling

Currently, the problems of inefficient fixed abrasive lapping (FAL) for super hard and brittle material wafers (such as sapphire) with fixed abrasive pads (FAPs), restrict the development of the semiconductor industry. Most importantly, the mechanisms of FAL processes are still unclear, often analyzed based on the theoretical models of chemical mechanical polishing (CMP). However, the material removal models of CMP are specific to loose abrasives, do not effectively apply to the lapping processes for super hard and brittle material wafers with a FAP. In this paper, the spherical crown model modified by elastic-plastic mechanics and brittle-plastic transformation mechanism was applied to super hard and brittle material removal mechanism in FAL processes. In consideration of the influences of the yielding abrasives embedded in the FAP matrix and the matrix surface topography on the wafer-pad contact, a material removal model suitable for super hard and brittle material wafers lapped by FAPs was obtained. The loads applied on the fixed abrasives and the matrix asperities during the elastic-plastic-brittle deformation processes of wafers were analyzed, respectively. The model indicates that the back-off of the abrasives embedded in the FAP matrix intensifies along with the decrease of the matrix hardness and the increase of the abrasive size, respectively. The penetrating depth of the fixed abrasive can be raised by increasing matrix hardness, applied pressure, abrasive size, and decreasing abrasive number, as same as the elastic recovery of the super hard and brittle material slid by the abrasive after unloading. The pad matrix surface dressed by loose grains greatly affects the MRR of wafers. The predicted MRR of sapphire wafers based on the FAL model is consistent with experimental data. Finally, the keys to improving the processing efficiency of super hard and brittle material wafers are clarified in detail.

A Novel Grain-Based DEM Model for Evaluating Surface Integrity in Scratching of RB-SiC Ceramics

A novel grain-based DEM (Discrete Element Method) model is developed and calibrated to simulate RB-SiC (Reaction-Bonded Silicon Carbide) ceramic and associated scratching process by considering the bonded SiC and Si grains and cementitious materials. It is shown that the grain-based DEM model can accurately identify transgranular and intergranular cracks, and ductile and brittle material removal modes. It also shows that by increasing the scratching speed or decreasing the depth of cut, the maximum depth of subsurface damage decreases, because the scratching force is relatively large under the low scratching speed or large depth of cut that facilitates the occurrence of transgranular cracks, large grain spalling from the target surface and the propagation of median cracks into the target subsurface. It has further been found that increasing the cutting-edge radius can enhance the target ductile machinability and reduce the target subsurface damage.

Surface Morphology Evaluation and Material Removal Mechanism Analysis by Single Abrasive Scratching of RB-SiC Ceramics

Reaction-Bonded Silicon Carbide (RB-SiC) ceramics possessing excellent mechanical and chemical properties, whose surface integrities have an essential effect on their performance and service life, have been widely used as substrates in the core parts of aerospace, optics and semiconductors industries. The single abrasive scratching test is considered as the effective way to provide the fundamental material removal mechanisms in the abrasive lapping and polishing of RB-SiC ceramics for the best surface finish. In this study, a novel single abrasive scratching test with an increasing scratching depth has been properly designed to represent the real abrasive lapping and polishing process and employed to experimentally investigate the surface integrity regarding different scratching speeds. Three typical and different material removal stages, including the ductile mode, ductile–brittle transition mode and brittle mode, can be clearly distinguished and it is found that in the ductile material removal stage by increasing the scratching speed would inhibit the plastic deformation and improve its surface integrity. It is also found that in the ductile–brittle transition and brittle material removal stages, to increase the scratching speed would inhibit the plastic deformation due to the fast scratching speed that limits the time of plastic deformation on the target, but it also results in the increased length of lateral cracks with the increased scratching speed which can reflect that the size of brittle chips, like brittle fractures and large grain fragmentations, increases as the scratching speed increases. It can provide the references for the optimization of the abrasive lapping and polishing of RB-SiC ceramics with high efficiency and surface quality.

Experimental and numerical investigation of heat generation and surface integrity of ZrO2 bioceramics in grinding process under MQL condition

Zirconia bioceramics has tremendous potential in medical applications owing to biocompatibility and high mechanical properties. Meanwhile, thermal damage and surface defects limit the grindability to achieve the desired properties. Therefore, this research provides an investigation of various grinding parameters on heat generation and surface morphology using a diamond wheel. In addition, a triangular and parabolic moving heat flux is used for heat distribution analysis based on FEM-model. The parabolic model more corresponds to experimental compared to the triangular heat flux, with an average deviation of <5% and 6.5% under dry and MQL, respectively. The response surface methodology is applied to extract a statistical representation of inputs and outputs. Dry grinding temperature obtained in range of 200–540 °C, which by applying MQL, it decreased by 16–35%. Increasing cutting depth would worsen the MQL efficiency in force and temperature. Results indicate the impact of cutting depth on temperature and force is greatest, followed by the effect of feed-rate, and that of wheel speed is the least. Thus, the increasing feed-rate should be utilized to preserve the high removal rate. SEM images indicate material removal mechanism is accomplished by plastic and brittle mode. Furthermore, MQL and a combination of low depth of cut could effectively decline the surface roughness and defects formation by decreasing the brittle material removal mechanism in one step. MQL reduced surface roughness by 46% compared with dry grinding, so that its performance increase in higher cutting depth. Because at higher cutting depths, the MQL changes the prevailing chip removal mechanism from brittle to ductile-regime grinding.

Material-related fundamentals of cutting techniques for GaAs wafer manufacturing

Abstract Driven by the requirement of high cutting efficiency and improvement of wafer flatness, wire sawing of GaAs single crystals under brittle material removal conditions has been studied. Crack nucleation and crack propagation were investigated by indentation and scratching tests on polished {100}-oriented semi-insulating GaAs wafers. Based on these results a concept has been developed that allows to control the force balance in the cutting slits so that the deflection of the wires perpendicular to the cutting planes is minimal resulting in cuts of high flatness. The concept has been successfully introduced in mass production of GaAs wafers.

Modelling and analysis of micro-grinding surface generation of hard brittle material machined by micro abrasive tools with helical chip pocket

The abrasive particles on the surface of micro abrasive tool are very small and tightly distributed, which is prone to appear debris blockage and accelerate tool wear, so the service life and performance of existing micro abrasive tools have become one of the key factors restricting the engineering application of micro-grinding technology. Compared to ordinary cylindrical micro abrasive tools, the micro abrasive tool with micro helical chip pocket (MAT-HCP) presents excellent performance in facilitating debris discharge and improving wear resistance, but the existence of helical chip pocket will significantly affect grains distribution, grinding contact area and surface generated mechanism in the micro-grinding process. In this work, the micro-grinding surface topography model of hard brittle materials obtained with MAT-HCP is built by considering grain size and distribution, dynamic impact, helical chip pocket structures, transient chip thickness and hard brittle material removal mechanism. The micro- grinding experiments are carried out to analyze surface morphology characteristics and surface generated mechanism of sapphire material machined by MAT-HCP. The contrast of simulated and experimental results showed that the average predicted error of contour supported ductility domain ratio and surface roughness respectively can be confined to be 10 % and 5%, which indicates that this surface topography predicted model can succeed in capturing the micro-grinding surface characteristics of sapphire material fabricated by MAT-HCP. Moreover, experimental results disclosed that the increase of pitch length will make MAT-HCP harder to realize the plastic region machining for sapphire, and this novel micro abrasive tool is potential to obtain better surface quality at high feed velocity compared with ordinary micro abrasive tools. The research results are of great significance for extending the service life of micro abrasive tools and promoting engineering application of micro-grinding technology.

Grain flash temperatures in diamond wire sawing of silicon

Diamond wire sawing has obtained 90% of the single-crystal silicon–based photovoltaic market, mainly for its high production efficiency, high wafer quality, and low tool wear. The diamond wire wear is strongly influenced by the temperatures in the grain-workpiece contact zone; and yet, research studies on experimental investigations and modeling are currently lacking. In this direction, a temperature model is developed for the evaluation of the flash temperatures at the grain tip with respect to the grain penetration depth. An experimental single-grain scratch test setup is designed to validate the model that can emulate the long contact lengths as in the wire sawing process, at high speeds. Furthermore, the influence of brittle and ductile material removal modes on cutting zone temperatures is evaluated.

Simulation of the ductile machining mode of silicon

Diamond wire sawing has been developed to reduce the cutting loss when cutting silicon wafers from ingots. The surface of silicon solar cells must be flawless in order to achieve the highest possible efficiency. However, the surface is damaged during sawing. The extent of the damage depends primarily on the material removal mode. Under certain conditions, the generally brittle material can be machined in ductile mode, whereby considerably fewer cracks occur in the surface than with brittle material removal. In the presented paper, a numerical model is developed in order to support the optimisation of the machining process regarding the transition between ductile and brittle material removal. The simulations are performed with an GPU-accelerated in-house developed code using mesh-free methods which easily handle large deformations while classic methods like FEM would require intensive remeshing. The Johnson-Cook flow stress model is implemented and used to evaluate the applicability of a model for ductile material behaviour in the transition zone between ductile and brittle removal mode. The simulation results are compared with results obtained from single grain scratch experiments using a real, non-idealised grain geometry as present in the diamond wire sawing process.

Finite element analysis on processing stress of polysilicon cut by diamond multi-wire saw

At present, diamond multi-wire sawing technology has been applied to the slicing of polysilicon wafers. The residual microcracks on the surface and subsurface of the silicon wafer due to material brittle removal are prone to expand and interfere under the sawing stress, which reduces the fracture strength of the silicon wafer and leads to breakage. However, the dynamic change and distribution of sawing stress is still difficult to measure by experiment. In this paper, the two ways of ductile and brittle material removal of abrasives at different azimuth angles on the saw wire surface were considered, the sawing force model of diamond wire sawing was established. Then based on the sawing force model, a finite element model of multi-wire sawing polysilicon was established. The distribution and coupling characteristics of sawing stress and the influence of process parameters and silicon wafer size characteristics on the stress value and distribution were studied. The research results show that the mechanical stress value during the sawing process is relatively small and accounts for a small proportion of the coupling stress, but it will affect the distribution of the coupling stress, and the thermal stress is dominant. The coupling stress increases with the increase of saw wire speed and feed speed. When the ratio of saw wire speed to feed speed is constant, the larger the value of saw wire speed and feed speed is, the larger the coupling stress is. The increase in the size and thickness of the silicon wafer will increase the coupling stress, while the change in the thickness of the silicon wafer has little effect on the value of the coupling stress. The research results of the paper provide a theoretical reference for understanding the changes and distribution of sawing stress, optimizing process parameters and improving the quality of sawing.

Analysis of crack-free surface generation of photovoltaic polysilicon wafer cut by diamond wire saw

At present, diamond wire sawing technology has been widely used in slicing photovoltaic polysilicon. Improving the surface quality of the slices to obtain a sawn surface without microcrack damage can greatly increase the fracture strength of polycsilicon wafers and reduce the cost of wet black silicon texturing, which is beneficial to improve the final photoelectric conversion efficiency of solar cells. In this paper, a mathematical model of diamond wire sawing was established based on the machining mechanism of brittle material removal and surface generation, and the numerical calculation of the sawing process was carried out. The validity of the model was verified by sawing experiments. The critical ratio of workpiece feed speed to saw wire movement speed was obtained when generating a crack-free slice surface under the combination of different process parameters and saw wire parameters. Based on the established theoretical model of sawing, the influence of saw wire movement speed, abrasives density and abrasives size on the critical speed ratio was analyzed, and the critical speed ratio (CSR) corresponding to the finer diameter wire and the higher wire speed which will be gradually applied in industry in the future was analyzed. The research results show that it is more conducive to obtaining a crack-free sawn surface by reducing the workpiece feed speed or increasing the saw wire movement speed, and the workpiece feed speed has a greater impact on the sawn surface quality. The CSR will slightly decrease within a small change range when increasing the saw wire movement speed. Increasing the density of abrasives on the saw wire surface within a certain range or decreasing the abrasives size is more inclined to obtain a crack-free sawn surface. The CSR will decrease slightly in a small range when decreasing the abrasives density or increasing the abrasives average size.

Prediction of crater depth, surface roughness and erosion rate during abrasive jet machining of glass

Understanding the mechanism for brittle material removal via solid particle impact is important for various areas such as energy and aerospace industries. The existing analytical formulation of erosion damage, consistent with indentation fracture theory, overestimates actual erosion by over 200%. This study complements an existing analysis by prescribing a realistic shape and mechanical properties to the impinging particle. We quantitatively show that a lateral crack in glass does not nucleate at the indenter's tip or the theoretical plastic depth, but at an intermediate depth. This depth can be found by implementing the actual particle penetration depth into Hill's ratio. Several new geometries of the particle's impacting tip are proposed as ways to achieve adequate particle penetration depth instead of an expanding hemispherical cavity. The crater depth, surface roughness, and erosion efficiency of borosilicate glass are predicted with 10%, 24%, and 23% error, respectively, using a simple analytical routine.

Experimental Study of the Impact of a Vibrating Wire on Free Abrasive Machining as Correlated to the Modeling of Vibration Subject to an Oscillating Boundary Condition

Abstract In this paper, we study experimentally the impact of a vibrating wire on the free abrasive machining (FAM) process in removing material from the surface of brittle materials, such as silicon. An experimental setup was designed to study the FAM process on silicon substrate surface by using a slurry-fed wire with a periodic excitation. An analytical solution of a wire moving axially, subject to an oscillating boundary condition with damping from abrasive slurry, was derived based on the partial differential equation of motion. Experiments were conducted on the apparatus using a wire with an oscillating boundary. It was found that the amplitudes of vibration were larger at the side of the oscillatory boundary, which caused more FAM interaction near the edge of the oscillatory boundary with larger material removal that was measured and validated. Furthermore, experiments were conducted to elucidate the effectiveness of brittle material removal using FAM with abrasive grits: (i) under dry condition, (ii) with water, and (iii) with abrasive slurry. Experimental results showed that the vibration of wire resulted in plastic deformation on the surface of silicon wafer. The abrasive grits in slurry driven by a vibrating wire generated material removal through observable grooves and fractures on the surface of silicon due to FAM in just a few minutes. The grooves from FAM process is an outcome of brittle machining through fracture formation and concatenation, generated by the indentation of abrasive grits on the silicon surface.

Slicing Ceramics on Material Removed by a Single Abrasive Particle.

Multi-wire saw machining (MWSM) used for slicing hard-brittle materials in the semiconductor and photovoltaic industries is an important and efficient material removal process that uses free abrasives. The cutting model of single-wire saw machining (SWSM) is the basis of MWSM. The material removal mechanism of SWSM is more easily understood than MWSM. A mathematical model (includes brittle fracture and plastic deformation) is presented in this paper for SWSM ceramic with abrasives. This paper determines the effect of various machining parameters on the removal of hard-brittle materials. For brittle fracture of SWSM ceramics, the minimum strain energy density is used as a fracture criterion. For plastic deformation of SWSM ceramics, the material removal is calculated using equations of motion. Actual wire-sawing experiments are conducted to verify the results of the developed mathematical model. The theoretical results agree with experimental data and practical experience. From the developed mathematical model, brittle fracture plays a major role in material removal of SWSM ceramics. Wire speed (S) and working load (P) are positively correlated with material removal of SWSM ceramics. The coefficient of friction is low, a lateral crack, which propagates almost parallel to the working surface, leads to more brittle fracture and material removal is increased.

Influence of grit geometry and fibre orientation on the abrasive material removal mechanisms of SiC/SiC Ceramic Matrix Composites (CMCs)

SiC/SiC Ceramic Matrix Composites (CMCs) have been identified as a key material system for improving aeroengine performance as they offer low density, high strength and stiffness, and superior environmental resistance at high temperatures. Nevertheless, due to their heterogeneous, hard and brittle nature, these materials are considered among the most difficult-to-machine, and grinding arises as one of the preferred choices for their processing. Therefore, understanding of the material removal mechanism and influence of the abrasive grit geometry when grinding CMCs is a critical enabler for achieving high component quality at highest efficiency and minimum cost.With the aim to reduce the uncertainties associated with the stochastic nature of the abrasive particles, grits of different shapes and sizes have been accurately created by Pulse Laser Ablation (PLA). In order to reproduce the grinding process kinematics, scratch tests in a circular trajectory have been carried out with single abrasive grain with different geometries and sizes, and arrays of overlapped grains to determine the influence of shape, size and spacing on the surface integrity of SiC/SiC CMCs after grinding. The morphology of the various constituents of the workpiece has been assessed regarding the direction of the scratch with respect to the orientation of the fibres. Results reflect a higher influence on the process forces by the grain shape rather than fibre orientation. Moreover, after the inspection of the abraded individual CMC constituents, a change in the mechanisms governing the process for the different abrasive grain geometries have been identified, despite the brittle material removal mode displayed by all of them. Explanation of the ground surface morphology in an analytical and comprehensive manner through a contact mechanics approach shows that the crack onset location is governed by the grains shape but its direction of propagation depends on the fibre orientation.

Evolution of chemo-mechanical effects during single grit diamond scratching of monocrystalline silicon in the presence of potassium hydroxide

Efficiency and workability of silicon wafers for various applications depend on theirs surface integrity and damage free structures. Productivity of mechanical processing on silicon wafers is always limited by occurrence of cracks and brittle mode material removal at high depth of cut. This study presents a method of using controlled concentration of KOH solution during diamond micro-scratching of silicon and investigates the chemo-mechanical effects of KOH solution with respect to cutting speed. To establish the chemo-mechanical theory of cutting, responses of physical and material properties of silicon in presence of KOH solution are captured and correlated with mechanical responses during micro-scratching. Zeta potential and nanohardness are greatly reduced in KOH environment. Ductile mode is found to be extended by using 10 wt% KOH. Crack/chipping length is observed to be first increasing with increase in cutting speed at low range (0.6–20 mm/min) and then decreasing on further increase in the cutting speed (20–200 mm/min). Stability in cutting force signature and behavior of material flow along the scratch length is also found to be improved by using KOH. Adhesive wear and abrasive wear are found to be the main forms of single grit wear, which is significantly reduced in the presence of KOH. The experimental results on different mechanical and physical phenomena in this study are of great importance for establishing the theoretical insight into material removal behavior and selection of machining parameters during micro-machining of silicon wafer.

A mechanistic cutting force model based on ductile and brittle fracture material removal modes for edge surface grinding of CFRP composites using rotary ultrasonic machining

Rotary ultrasonic machining (RUM) has been proven to be an effective method for surface grinding of carbon fiber reinforced plastic (CFRP) composites. Cutting force is considered as the main criterion to evaluate the performance of RUM surface grinding process. The cutting force modeling is essential to better understand such a process. All reported cutting force models for RUM of CFRP are developed based on brittle fracture material removal mode (brittle mode). However, it is recently found that both ductile material removal mode (ductile mode) and brittle mode exist in RUM of CFRP. Among surface grinding processes, edge surface grinding is the mandatory and primary process to remove an amount of composite material from the workpiece edge to achieve the desired workpiece with high precision. In edge surface grinding process, cutting forces in both feeding direction and in depth-of-cut direction (being perpendicular to feeding direction) play important roles in material removal. In addition, the understandings of material removal mechanisms will greatly benefit the modeling development of cutting forces and improve the RUM process. In this study, a mechanistic model based on both ductile mode and brittle mode is developed to predict cutting forces in both feeding and depth-of-cut directions for RUM edge surface grinding of CFRP composites. A series of experiments are conducted to verify this mechanistic model. The model prediction agrees well with the experimental results under different groups of input variables.

Experiment and theoretical prediction for surface roughness of PV polycrystalline silicon wafer in electroplated diamond wire sawing

The electroplated diamond wire sawing technology is the mainstream processing method of cutting PV polycrystalline silicon ingots. Surface roughness is one of the most significant evaluation indexes for wafers surface quality, and has an important influence on subsequent processes such as surface texturization, screen printing, subsurface damage layer prediction and so on. In this paper, a mathematical model, considering the random distribution characteristic of abrasive parameters and its location on saw wire surface, was developed. The influence of brittle and ductile material removal modes on the wafers surface morphology was taken into account, the model can obtain the cutting depths of abrasives by discretizing the cutting groove and wire profile during the sawing process, and then the material removal modes of abrasives are judged by cutting depth. According to the calculation nodes closest to the sawn surface, the wafer surface profile and roughness along the feed speed direction were calculated. The correctness and reasonability of the model were verified by experiments. Then, the model was used to predict the variation trend of surface roughness under different process parameters. The results showed that the relationships between surface roughness and feed speed Vf, wire speed Vs and their ratio Vf /Vs satisfied power functions. Finally, the calculation result of surface roughness was used to generate a 3D surface morphology of wafer.

Characterization of electroplated diamond wires and the resulting workpiece quality in silicon sawing

Abstract The aim of this research is to introduce a fast characterization method for diamond wires and the resultant silicon workpiece surface quality at high wire cutting speeds. The work is presented in two parts. Initially, four electroplated diamond wires with varying properties are characterized with the proprietary wire analysis algorithm and experimentally investigated at wire speeds up to 75 m/s. Following, the proprietary surface analysis algorithm which evaluates the brittle and ductile material removal characteristics of silicon workpiece surfaces is introduced and experimentally validated. Influence of the wire speed and feed rate on the material removal regime, surface roughness and process forces are studied on the Wire Wrapped Grinding Wheel setup, a novel-design experiment test rig designed to emulate the wire sawing process. The aspects and kinematics of the experiment setup are discussed in detail. Primary findings show that (1) better surface finish is achievable at higher wire speeds with ductile material removal as the dominant material removal mode (2) for wires of different grain sizes, at the cutting speed threshold of around 30 m/s, cutting force behavior changes and the difference between normal forces starts diminishing over this limit.

Stress interaction and crack propagation behavior of glass ceramics under multi-scratches

The interaction between scratches has a significant influence on the material removal mechanism during scratching glass ceramic. To clarify the interaction mechanism, an analytical model of stress interference considering scratch distance is established based on the interaction features between scratches to analyse the relationship between stress distribution and crack propagation. A double scratch experiment with different scratch distances was conducted on lithium aluminosilicate (LAS) glass ceramics. Experimental and analytical results show that the crack is further extended by the enhanced maximum principal stress in the case of the scratch interference. In the interaction range, the maximum principal stress decreases with increasing scratch distance. Furthermore, larger scratch distance in the interaction range is prone to achieve ductile material removal while smaller scratch distance makes the brittle material removal occurring.