Machining Of Brittle Materials Research Articles

Rotary ultrasonic surface machining of silicon: Effects of ultrasonic power and tool rotational speed

The surging demand for monocrystalline silicon materials in the production of microelectronic components highlights its crucial role in the semiconductor and optic industries. Hence it is inevitable to produce a silicon workpiece with high quality finish to meet the demand in semiconductor industries. Due to high brittleness, controlling the quality of silicon in surface machining is quite difficult. Traditional manufacturing processes induce issues like rough surfaces and edge chipping. It was reported that rotary ultrasonic surface machining (RUSM) can effectively reduce cutting force, roughness, and edge chipping in machining of brittle materials. There have been several studies on drilling and sliding silicon materials using rotary ultrasonic machining investigating the effects of machining parameters on the output variables such as cutting force, torque, edge chipping, surface roughness etc. However, to the best of the authors’ knowledge, there are no reported investigations on effects of machining variables (ultrasonic power and tool rotation speed) in surface machining of silicon materials using the rotary ultrasonic machining. This study aimed to investigate the impacts of ultrasonic power and tool rotation speed on the cutting force, edge chipping, and surface roughness. Experimental results show that the ultrasonic vibration and tool rotation speed had a notable impact on edge chipping and cutting forces. Lastly, the current research has paved the way for widening the research on investigating grinding of the silicon wafer in semiconductor manufacturing with ultrasonic vibration and high rotation speed. In semiconductor wafer manufacturing, grinding process is used to reduce the flatness but generate surface and subsurface damage. With further investigations, RUSM can contribute to reducing these damages.

Proposal of trapezoidal vibration-assisted diamond cutting for ductile-regime machining of brittle crystals: A case study on KDP crystal

Despite various vibration-assisted cutting techniques have been utilized to increase the machining performances of brittle materials, the highly dynamic variations of cutting process quantities lead to the complicated brittle-ductile transition (BDT) mechanism and the formation of undesired vibration marks on the machined surfaces. In this paper, a novel ultra-precision vibration-assisted cutting process, named trapezoidal modulation diamond cutting (TMDC), is firstly proposed for ductile-regime machining of brittle materials with significantly increased BDT cutting depth. By imposing a dedicate trapezoidal locus to the diamond tool, the unique invariable uncut chip thickness and cutting states were achieved for realizing stable vibration-assisted cutting without the formations of vibration marks. Systematic cutting experiments of KDP crystals were carried out to comprehensively investigate the influences of different process parameters on the machining performances of TMDC process. In addition, the underlying mechanisms of machining performance improvements have been discussed under the different combinations of process parameters, based on which the guidelines for optimal process parameter selection are given for increasing the BDT cutting depths. The outcomes of this study contribute to not only improving the ductile machining efficiency and machining quality of KDP crystals, but also help to deepen the understandings of BDT mechanism during vibration-assisted diamond cutting of common brittle materials.

Cracking behavior during scratching brittle materials with different-shaped indenters

The cracking behavior greatly influences the material removal and surface integrity during the abrasive machining of brittle materials. The abrasive machining can be simplified as a series of scratching by different-shaped indenters. In this paper, a scratch-induced stress field model is built to analyze the cracking behavior during scratching brittle materials. The model considers conical, pyramidal, and spherical (CPS) indenters and correlates the scratch-induced principal stress with workpiece material properties, indenter geometries, and scratch behavior parameters (e.g., scratch normal load, tangential load, friction coefficient, scratch depth). With the model, the effects of scratch depth and indenter geometries on the scratch behavior and cracking behavior are investigated. The results show that the scratch load decreases when a sharper indenter or a spherical indenter with a smaller radius is used. The larger the scratch depth, the easier it is for radial, median, and lateral (RML) cracks to initiate. There is an optimal indenter half-apex angle at which it is most difficult to initiate radial and median cracks. The lateral crack easily initiates when the indenter becomes sharp. The initiation position of radial crack changes from being in front of the indenter to being behind it as the scratch depth increases or the half-apex angle decreases. The propagation angle of radial crack decreases, while that of median crack increases with an increase in scratch depth. The sizes of RML cracks increase with an increasing scratch depth or half-apex angle. Finally, the model is validated through scratching experiments on fused silica using CPS indenters and by referencing previous research. It demonstrates that the model can accurately determine the sizes of RML cracks, with an average relative error of less than 11%. This research provides guidance on selecting suitable process parameters or indenter geometries to increase the material removal rate and mitigate the crack damage during the abrasive machining of brittle materials.

Nano-to-microscale ductile-to-brittle transitions for edge cracking suppression in single-diamond grinding of lithium metasilicate/disilicate glass-ceramics

To suppress grinding-induced edge cracks in dental lithium metasilicate/disilicate glass-ceramics (LMGC/LDGC), this paper established a contact stress model for single-diamond grinding (SDG) to relate their crack generation and ductile-to-brittle transition (DBT) thresholds with the mechanical properties, diamond tool profiles and process variables. Nanoindentation, friction test and SDG were conducted to unravel material responses and dynamic diamond grit-workpiece interactions to determine the DBT thresholds. The nanoindentation revealed significant indentation size effects (ISEs) on the hardness and elastic moduli of the ceramics. SDG clearly elucidated their DBT behaviors, wherein edge cracks initiated at diamond peripheries when the concurrent contact stresses reached the DBT thresholds. Accordingly, the indicated critical cutting depths for DBT may be changed from the nanoscale to the microscale by increasing the tool radius and reducing the machining speed. This research contributes to edge crack suppression for ductile machining of brittle materials at large removal rates.

Materials removal mechanism and multi modes feature for silicon carbide during scratching

Multi modes features during machining of brittle materials determine the process window. To reveal these fundamental features of SiC, a series of scratching experiments and atomistic simulations were designed. Real-time signals of scratching force and acoustic emission were analyzed in the time-domain and time-frequency domains to distinguish features. The birefringence of 4H-SiC in polarized light was non-destructively deployed to analyze the localized stress distribution and morphology after scratching. The experiments found four scratching modes – Hertz mode, quasi-ductility mode, high stress mode, and stylus failure mode. To reveal the atom-level behaviors inducing mode transition, atomistic simulations were conducted and post-analyzed from macro-level parameters, phase transition, dislocations, and so forth. The dynamic cycle mechanism – elastic deformation, plastic deformation, cracking – was revealed and the shear modulus, poisson's ratio, scratching speed, penetration depth, and deformation energy was recommended to optimize analytical model. Finally, we attempted to propose a unified qualitative multi layers model to explain different material removal mechanisms.

Processing and machining mechanism of ultrasonic vibration-assisted grinding on sapphire

Owing to outstanding mechanical, optical, thermal stability and chemical stability properties, sapphire has widespread use in semiconductors, aerospace and other fields. However, it is difficult to machine it efficiently and precisely because of its brittleness and hardness. In this work, the processing and mechanism of machining sapphire using ultrasonic vibration-assisted grinding technology have been investigated via experiment. The machining factors have been analyzed, including the condition of machined surface, specific grinding energy, force and force ratio. Referring to conventional grinding, the application of ultrasonic vibration reduces the force, force ratio, specific energy, and the reduction ratio is direction dependent. The effect on surface roughness and morphology is also anisotropic. Regarding the smoothness of the surface, the suitable directions were axial and tangential, while there was no noticeable improvement in the radial direction. Our results and insights could be beneficial for the precise machining of brittle materials and quality management.

Development of a methodology for evaluating the process window of ductile machining for brittle-hard materials

This paper presents a standardized methodology for determining the process window for ductile machining of brittle materials. Its application for CaF2 is reported, identifying an optimized process window for single-point diamond turning on UPM machines by determining optimized process parameters.

Machining of brittle materials: micro milling of glass

Machining of brittle materials: micro milling of glass

On the improvement of the ductile removal ability of brittle KDP crystal via temperature effect

Brittle KH2PO4 (KDP) crystal is difficult-to-machine because of its low fracture resistance whereby brittle cracks can be easily introduced in machining processes. To achieve ductile machining without any cracks, this type of materials is generally processed by some ultra-precision machining techniques at ambient temperature with nanoscale material removal, yielding low machining efficiency and high processing cost. Recently, thermal-assisted techniques have been used to successfully facilitate the machining of some difficult-to-machine materials, like superalloys, but little effort has been made to explore whether the temperature effect can contribute to the ductile machinability of brittle materials yet. Thus, the aim of this study is to figure out the specific role of temperature in the deformation behaviours of brittle KDP crystal by nano indentation/scratch methods. It is found that compared with those at ambient temperature (AT, i.e. 23 °C), the hardness and Elastic modulus of KDP crystal at elevated temperature (ET, i.e. 160 °C) decrease substantially by 21.4% and 32.5%, respectively, while the fracture toughness increases greatly by 15.5%, implying a higher ability of ductile deformation at ET. Meanwhile, the scratch length within ductile removal has been identified to be extended more than 4 times by increasing temperature from AT to ET. Both the quantity and size of brittle features (e.g., cracks and chunk removal) show a reducing trend with the increase of temperature. To uncover the underlying mechanism of this phenomenon, an updated stress field model is proposed to analyze the scratch-induced stress distribution by considering the evolution of material property at various temperature. These presented results are significant for the future design of specific thermal-assisted processing techniques for machining brittle materials efficiently.

Self-tuned ultrasonic elliptical vibration cutting for high-efficient machining of micro-optics arrays on brittle materials

Ultrasonic elliptical vibration cutting is a very promising technique for the machining of brittle materials. However, its machining performance is currently limited by the ductile machining model and the machining strategy with a constant feed rate, leading to low machining efficiency. To overcome this defect, this paper presents a novel self-tuned ultrasonic elliptical vibration cutting (SUEVC) technique to achieve high-efficient ductile-regime machining of the micro-optics array on brittle materials. The proposed SUEVC includes a ductile-regime machining model and a tool path generation method. In SUEVC, the feed rate adaptively changes with respect to the local shape variation of the desired surface along the feeding direction to ensure both crack-free surface and high machining efficiency. Finally, two 1 × 3 spherical micro-optics arrays were successfully fabricated on single-crystal MgF2 by SUEVC and the traditional machining strategy respectively. Results demonstrated that the SUEVC could enhance the machining efficiency by 30% relative to the traditional machining strategy, while maintaining similar surface roughness and a crack-free surface.

Design and experimental study of longitudinal-torsional ultrasonic transducer with helical slots considering the stiffness variation

The ultrasonic transducer employing helical slots to generate longitudinal-torsional (L&T) ultrasonic vibration is popular in the field of hard and brittle material machining. However, the design approach for this type of L&T ultrasonic transducer is not well established. An improved design model combining equivalent circuit theory and material parameter equivalent method based on stiffness variation was proposed in this study. The structure parameters of the transducer can be obtained according to given resonance frequency and nodal position. The model can further predict electrical characteristics and mechanical responses under specific excitation. For the preliminary verification of this theoretical model and the optimization of the transducer, finite element analyses were carried out to investigate the influences of slot structure parameters on dynamic performances in terms of resonance frequency, frequency separation, and amplitude of L&T vibration. After that, a prototype was manufactured according to the design results, and the measurements of impedance curve and vibration amplitude were conducted. The experimental results indicate that this prototype resonates at 24,920 Hz with an effective electromechanical coupling coefficient of 0.19. Meanwhile, the vibration amplitude in longitudinal direction is 8.1 μm and that in tangential direction is 5.6 μm under 70-V sinusoidal voltage excitation, which are consistent with the theoretical design and simulation results. Furthermore, machining tests demonstrate that surface roughness reduction and surface quality improvement can be gained by adopting this L&T ultrasonic transducer, compared to using the longitudinal vibration transducer. The proposed design approach was validated and this study can provide guidelines for the design of L&T ultrasonic transducer for rotary ultrasonic machining (RUM).

Machining alumina plates using abrasive jet of silicon carbide

The abrasive jet machining (AJM) is used in many engineering applications including brittle material machining and finishing, surface preparation of the metal for welding and thermal spray coatings, etc. In this process, fine grain abrasive particles are given high speed through carrier gas which is directed to impinge on a target surface to be machined. The high kinetic energy of the abrasive particles is responsible to erode materials from the surface. In this work, three input cutting parameters such as abrasive flow rate, abrasive grain size, and stand-off distance (SOD) are chosen to experiment on 99.7% pure alumina plate of 4 mm thickness, using silicon carbide (SiC) abrasive. Material removal rate (MRR) and nozzle wear rate are the output responses that are to be investigated using response surface methodology, keeping working pressure constant at 6 kgf/cm2 throughout.

Study on nano-cutting of brittle material by molecular dynamics using dynamic modeling

Nano-cutting of brittle material finds wide applications in scientific research and engineering. Numerical simulation is an important tool for understanding the process mechanism, which has many advantages over experimental approaches. Currently, molecular dynamics is the most used method for nano-cutting simulation owing to its explicit description of material at atomic level. However, the model scale in current studies is far smaller than the reality. As a result, cutting-induced fracture, the main surface damage in the machining of brittle material, is difficult to reveal and well understand, which has become a critical issue in the field. In this study, a novel simulation approach is proposed to improve the computing efficiency after analyzing the shortcomings of traditional model. The principle is to build a localized workpiece near the tool which is dynamically modified so that the length of workpiece in computation is reduced and the saved computer resource can be used to increase the depth of cut and the tool edge radius. A large scale simulation on germanium verifies the capability of this method and reveals the fracture damage formation during the cutting process.

Polycrystalline diamond tools fabrication by micro EDM and their application of brittle material machining

The polycrystalline diamond(PCD)tools are widely used for the brittle and non-ferrous materials such as glasses, ceramic and titanium alloy machining. Even many researches have been studied on PCD tools design for high efficient and high-quality brittle materials machining. However, the parameters of PCD tools cutting characteristics are not so clear in some previous researches. According to the former researches studies, one of the biggest challenges in the processing of brittle materials is design and selection of cutting tools, there are different tool design criteria in brittle material because of the different removal mode.In this paper, serval different types PCD end milling tools such as radial, side radial, dual-notch and pyramid-shaped types are designed for brittle materials machining. Electrical discharge machining (EDM) with RC pulse generation circuit was used to fabricate the PCD end milling tools. A dual spindle CNC machining center with both functions of EDM and mechanical cutting was setup to evaluate the brittle materials milling process. Experimental results show that the PCD end milling shape edge is changeable with different capacitance of the RC discharge circuit. It is possible to fabricate varieties of PCD tools by micro EDM and the brittle and non-ferrous materials milling or cutting process could be carried out on the same machine.

Fundamental study of ductile-regime diamond turning of single crystal gallium arsenide

Gallium arsenide (GaAs) components, ranging from the planar substrate to those possessing complicated shapes and microstructures, have attracted extensive interest regarding their applications in photovoltaic devices, photodetectors and emerging quantum devices. Single point diamond turning (SPDT) is regarded as an excellent candidate for an industrially viable mechanical machining process, as it can generate nano-smooth surfaces, even on some hard-to-machine brittle materials such as silicon and silicon carbide, with a single pass. However, the extremely low fracture toughness and strong anisotropic machinability of GaAs makes it difficult to obtain nano-smooth, crack-free machined surfaces. To bridge the current knowledge gaps in understanding the anisotropic machinability of GaAs, this paper studied the mechanical material properties of (001)-oriented GaAs through indentation tests, assuming the diagonals of the indenter acted in the similar way of the cutting edge of a diamond tool with a negative rake angle. The results showed that the (001) plane of the GaAs material displayed harder and more brittle when indented along direction I (one diagonal of indenter parallel to the <110> orientation) compared to direction II (one diagonal of indenter parallel to the <100> orientation), which coincides with anisotropic machined surface quality by SPDT. This finding reveals, for the first time, that the crystallographic orientation dependence of both hardness and fracture toughness represents the underlying mechanism for the anisotropic machinability of GaAs. The paper presents a novel approach to evaluate the critical depth of cut under a high cutting speed comparable to SPDT and to determine the maximum feed rate for ductile-regime diamond turning. The 26.57 nm critical depth of cut was obtained for the hardest cutting direction using a large negative rake angle diamond tool. Finally, a nano-smooth surface was successfully generated along all the orientations in ductile-regime diamond turning, in which the material removal mechanism is considered as plastic deformation caused by high-density dislocations. The subsurface layer remains to its single crystal structure and no cracks are found under a transmission electron microscope (TEM). The results proves the proposed evaluation approach for the critical depth of cut and the maximum allowed feed rate is highly effective for guiding the ductile-regime machining of brittle materials.

Enhancing Ductile-mode Cutting of Calcium Fluoride Single Crystals with Solidified Coating

Positive improvements have been observed during machining brittle materials under high hydrostatic pressure, but the techniques to achieve such desirable effects often utilize complex and expensive equipment or tools. This work presents a cost-efficient method to achieve ductile-mode machining of brittle materials at higher uncut chip thicknesses, by the application of a solidified coating on workpiece surface before a machining process. Orthogonal microcutting experiments were conducted on calcium fluoride single crystals oriented with the (111) plane and an increase in critical uncut chip thickness was observed with the solidified coating. The primary cause has been resolved to be mechanical-related and results in a stabilized microcutting process. Transmission electron microscopy provided evidence of slip deformation occurring in the machined subsurface regions and a layer thickness of subsurface damages reduced by ~ 45% under the influence of the solidified coating. In addition, erratic fluctuations in direction of the resultant machining force were subdued with the applied coating, which is proved to be caused by the compressive stresses induced from the sandwiching of the CaF2 material between the tool and the solidified coating. The proposed technique successfully reduces the cost and pollution in the fabrication process of optical components from the use of coolant in an ultraprecision machining process to the time consumed by eliminating the subsurface damage with abrasive slurries in post-machining polishing.

Investigation on residual scratch depth and material removal rate of scratching machining single crystal silicon with Berkovich indenter

Single crystal silicon is the basic material for the fabrication of micro-devices and micro-structures. It exhibits plastic behavior in micro/nano-machining. The key to obtain high-quality machined surfaces in ultra-precision machining of brittle materials is the material removal in ductile region. This paper focuses on single crystal silicon scratching machining in ductile region. The contact area and contact force of Berkovich indenter along two typical scratching machining directions (Φ=0° and Φ=60°) are analyzed. The residual scratch depth of typical scratching direction of Berkovich indenter is calculated theoretically. The experiments of scratching machining on (001) crystal plane of single crystal silicon using nanoindenter and Berkovich indenter are conducted. Based on experiments of nanoindenter scratching machining, the effects of normal load and scratching direction on the residual scratch depth and material removal rate in ductile region machining are investigated. Between the theoretical calculation results and the experimental results, the maximum deviation of residual scratch depth and material removal rate of Berkovich indenter scratching machining single crystal silicon is less than 3.5%. The residual scratch depth and material removal rate of Berkovich indenter scratching machining single crystal silicon along directions Φ=0° and Φ=60° are quite different. When the normal load is 10 mN and 20 mN, the material removal rate of single crystal silicon scratching machining along direction Φ=60° is more than 2 times that of along direction Φ=0°. The residual scratch depth of scratching machining single crystal silicon with Berkovich indenter varies nonlinearly with normal load, and the material removal rate is obviously affected by the direction of the scratching machining. The results in this paper are useful for the determination of process parameters for ultra-precision machining of single crystal silicon.

Finite Element Modeling of Orthogonal Machining of Brittle Materials Using an Embedded Cohesive Element Mesh

Machining of brittle materials is common in the manufacturing industry, but few modeling techniques are available to predict materials’ behavior in response to the cutting tool. The paper presents a fracture-based finite element model, named embedded cohesive zone–finite element method (ECZ–FEM). In ECZ–FEM, a network of cohesive zone (CZ) elements are embedded in the material body with regular elements to capture multiple randomized cracks during a cutting process. The CZ element is defined by the fracture energy and a scaling factor to control material ductility and chip behavior. The model is validated by an experimental study in terms of chip formation and cutting force with two different brittle materials and depths of cut. The results show that ECZ–FEM can capture various chip forms, such as dusty debris, irregular chips, and unstable crack propagation seen in the experimental cases. For the cutting force, the model can predict the relative difference among the experimental cases, but the force value is higher by 30–50%. The ECZ–FEM has demonstrated the feasibility of brittle cutting simulation with some limitations applied.

Scratching-induced surface characteristics and material removal mechanisms in rotary ultrasonic surface machining of CFRP

Rotary ultrasonic machining has been successfully explored in surface machining of carbon fiber reinforced plastic (CFRP) composites. It has been proven to be an effective and efficient CFRP machining method. Both theoretical and experimental investigations have been conducted with the assumption that the CFRP is removed by brittle fracture removal mode. However, in brittle material machining, ductile flow phenomenon still exists. Ductile scratching marks are also observed on the machined CFRP surfaces. It is still unknown that what actual material removal modes are under different machining variables. To investigate the material removal mechanisms in rotary ultrasonic surface machining (RUSM) of CFRP, single abrasive scratching tests were conducted. The scratching induced characteristics and scratching forces were analyzed. Both the ductile removal mode and the brittle fracture removal mode were observed and identified in both carbon fiber layers and epoxy resin layers on the machined marks by using scanning electron microscopy (SEM) imaging. With the increase of scratching depth, the material removal mode of CFRP was changed from the ductile removal mode to the brittle fracture mode. From the analysis of kinematic trajectory of diamond grain, the scratching cutting forces were decreased in the tests with the assistance of ultrasonic vibration under the same machining variables. The generation mechanisms of the delamination were analyzed and discussed.

Mechanical Behavior of Undoped n-Type GaAs under the Indentation of Berkovich and Flat-Tip Indenters.

In this research, the mechanical behavior of undoped n-type GaAs was investigated by nanoindentation experiments using two types of indenters—Berkovich and flat-tip—with force applied up to 1000 mN. From the measured force-depth curves, an obvious pop-in phenomenon occurred at force of 150 mN with the flat-tip indenter representing elastic–plastic transition. The Young’s modulus and hardness of GaAs were calculated to be 60–115 GPa and 6–10 GPa, respectively, under Berkovich indenter. Based on the observation of indent imprints, the fracture characteristics of GaAs were also discussed. A recovery of crack by the next indentation was observed at 1000 mN with Berkovich indenter. In the case of flat-tip indentation, however, surface material sank into a wing shape from 400 mN. In this sinking region, a density of fork-shaped sinking, slip lines, and crossed pits contributed to the slip bands, and converging crossed twinning deformations inside the GaAs material were generated. Since cracks and destructions on GaAs surface took place more easily under the flat-tip indentation than that of Berkovich, a machining tool with a sharp tip is recommended for the mechanical machining of brittle materials like GaAs.