Material Removal Mode Research Articles

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.

Fundamental study on material removal mechanism in single-crystal germanium

In this study, vary-loads and const-load nano-scratching tests were performed to investigate the material removal mechanism in single-crystal germanium. Based on the indentation size effect, the cutting force in the ductile regime was modelled analytically considering the elastic recovery and sliding friction. Subsequently, a novel specific cutting energy (SCE) model was proposed to quantitatively determine the ductile to brittle transition (DBT) depth. The surface topography of the workpiece was observed and measured using a scanning electron microscope (SEM) and an atomic force microscope (AFM). The surface morphology revealed the existence of a ductile regime, DBT regime, and brittle regime in the nano-scratching process. The DBT depth was calculated analytically using the proposed SCE model, which had a prediction error of less than 4.02%. Thus, the model results were in good agreement with the experimental results. The phase transitions of different material removal modes were characterised using Raman spectroscopy, which revealed that the phase transition from the crystalline phase to the amorphous phase dominated the material removal process. In addition, the feasibility of high-efficiency machining by increasing the cutting speed was verified: the surface integrity improved and the subsurface damage reduced at high cutting speeds. These findings provide a fundamental understanding of the ductile regime machining process for single-crystal germanium.

Experimental Investigation and Analysis on Borosilicate Glass Using Electrochemical Discharge Machining Process

Electrochemical discharge machining process (ECDM) is a non-traditional machining process which has widely used in medical, electronics and aerospace industry. This paper discuss about the various different methods using same process i.e. electrode immersion depths along with electrolytic stirring effects in ECDM are discussed. The various process parameters considered during the experiment (applied voltage, electrolyte concentration and electrode immersion depths); on the other side response parameters consider as material removal, tool wear and surface finish. Experimental results indicate the adequate material removal with equivalent electrode cross sectional areas as well as fine immersion depths. Electrode immersion depth ratios, electrolyte concentration and electrolyte-stirring effect on borosilicate glass using NaOH as the electrolyte mediums show some improvement in process results. The addition of stirring effect to the electrolyte along with proper immersion depths of the electrodes resulted in improvement of the process performance mainly the improvement in surface finish values up to around four times than the normal ECDM process. The FESEM micro graphs of the fabricated micro channels reveal some interesting modes of material removal. EDS were studied for analysis of debris through this process.

Process Parameters Optimization Using Taguchi-Based Grey Relational Analysis in Laser-Assisted Machining of Si3N4.

Despite extensive research over the past three decades proving that laser-assisted machining (LAM) is effective for machining ceramic materials, which are affected by many machining parameters, there has been no systematic study of the effects of process parameters on surface quality in LAM ceramic materials. In this paper, the effects and optimization of laser power, spindle speed, feed rate, and cutting depth on surface roughness and work hardening of LAM Si3N4 were systematically studied, using grey relational analysis coupled with the Taguchi method. The results show that the combination of machining parameters determines the material removal mode at the material removal location, and then affects the surface quality. In ductile material removal mode, both the value of surface roughness and work hardening degree are smaller. Decreased surface roughness and work hardening degree can be obtained with smaller cutting depth and higher laser power.

Investigation on nanoscale material removal process of BK7 and fused silica glass during chemical‐mechanical polishing

AbstractUnderstanding the nanoscale material removal process in chemical mechanical polishing (CMP) is of fundamental importance for the operation and further development of CMP. In this study, the nanoscale material removal processes of both fused silica glass and BK7 glass were investigated based on single‐pad‐asperity polishing experiments. The results indicate that the material removal characteristics are highly dependent on the composition and structure of glass materials. In the mechanically induced chemical bonding removal mode, only atoms locate near the outermost several layers can participate in the formation and breakage of interfacial bridge bonds. Moreover, the force on the abrasive particle must exceed a threshold value to induce significant removal of Si atoms from the glass substrate, because as the breakage of Siglass–O backbonds does not occur in low‐stress conditions. We reveal for the first time that the chemical and mechanical properties of the topmost layer, which is recognized as densification, hydrated, or redeposition layer, have not been significantly affected by the mechanical action of the abrasive particles when polishing in this kind of material removal mode. The results are expected to provide a deeper insight into the nanoscale material removal mechanism during glass CMP.

Slurry Erosion Resistance of Microwave Derived Ni-SiC Composite Claddings

Hydro machinery components such as turbines, pipes, guide vanes, and pumps suffer from severe slurry erosion caused by the abrasive particles entrained in working fluid. In the current work, an array of systematic slurry erosion tests were executed to study the influence of impact angle on slurry erosion rates of Ni-SiC based composite claddings developed on SS 316 L through microwave hybrid heating. Various microstructural and mechanical characterizations such as micro and nano-indentation, x-ray diffraction (XRD), and scanning electron microscopy (SEM) were performed on the developed claddings. The claddings contain 5 wt.% of SiC particles of different size fractions from nano to micro (unimodal) and combination of both in the matrix (bimodal). Slurry erosion studies were performed on the developed claddings at different impingement angles. The claddings exhibited good metallurgical bonding with the substrate without any pores throughout. The developed claddings exhibited high hardness and elastic modulus compared to the substrate material. A significant improvement in the erosion resistance of the claddings was observed with the addition of bimodal sized SiC secondary particles. The enhanced erosion resistance of the claddings could be attributed to the higher fracture toughness and hardness of the developed coating. Among the developed coatings, the bimodal sample with 5% SiC claddings showed at least two times better erosion resistance than hydro-turbine steel at almost all the impact angles. SEM studies performed on the eroded samples showed micro-cutting and plowing as the main modes of material removal at lower impact angles.

Experimental investigation on cutting mechanisms in fixed diamond wire sawing of bone

Bone sawing has been widely used in performing bone surgery. However, thermal necrosis, loss of cutting precision and surface damage may occur in cutting process. The primary objective of this research is to improve cutting performance of bone by advantages of diamond wire sawing. Mechanism of material removal, cutting force, temperature and surface quality are analyzed based on experimental results. It is indicated that wire sawing provides small depth of cut, which is effective to obtain ductile material removal mode. Due to small material removal rate per abrasive, thermal energy is low and most of the heat can be taken away by the cyclic wire and bone chips. Consequently, cutting force and temperature in cutting zone are lower than that of traditional sawing. Due to the high efficiency of chip ejection, burrs and fracture are reduced and a significant improvement in surface quality is achieved. Based on cutting experiments with various values of cutting parameters, it is observed that better performance is achievable at higher wire speeds. These results provide a valuable basis for application of wire sawing and understanding of bone cutting mechanisms.

Effect of a protective coating on the surface integrity of a microchannel produced by microultrasonic machining

In micro-ultrasonic machining (μUSM), slight lateral vibration at the end of the tool inevitably occurs due to tool manufacturing installation errors and mechanical system vibrations. These vibrations cause overcutting and edge breakage, creating undesirable effects on heat transfer and flow deflection. Hence, this paper presented a new fabrication method that ensures high surface integrity by applying a protective coating on the substrate. To study the material removal modes during fabrication and the causes of overcutting and edge damage, a mathematical model of the microchannel surface integrity was established. The experimental parameters—feed rate, ultrasonic power, slurry viscosity, and protective coating—had a large impact on the surface integrity. The optimal parameters were a PE-Wax coating of 400 μm thickness, feed rate of 4 μm/s, 60 % ultrasonic power and 25 % slurry concentration. Under these conditions, the amount of overcutting decreased from 74.3 μm to 28 μm (62.3 % improvement) and the weighted edge damage decreased from 52.3 μm to 16 μm (69.4 % improvement), which led to a 65.85 % improvement in the surface integrity index. The experimental investigation revealed that the surface integrity of silicon microchannels and the quality of μUSM can be significantly improved by applying the protective coating.

Micromechanics of Machining and Wear in Hard and Brittle Materials.

Hard and brittle solids with covalent/ionic bonding are used in a wide range of modern-day manufacturing technologies. Optimization of a shaping process can shorten manufacturing time and cost of component production, and at the same time extend component longevity. The same process may contribute to wear and fatigue degradation in service. Educated development of advanced finishing protocols for this class of solids requires a comprehensive understanding of damage mechanisms at small-scale contacts from a materials science perspective. In this article the fundamentals of brittle-ductile transitions in indentation stress fields are surveyed, with distinctions between axial and sliding loading and blunt and sharp contacts. Attendant deformation and removal mechanisms in microcontact processes are analyzed and discussed in the context of brittle and ductile machining and severe and mild wear. The central role of material microstructure in material removal modes is demonstrated.

Fabrication of micro-pillar with high aspect ratio on monocrystalline diamond by galvanometer-assisted femtosecond laser milling

• The GFM trajectory was modeled and found beneficial for promoting aspect ratio. • The aspect ratio of diamond micro-pillar reached 42.25 fabricated by optimized GFM. • Graphitization level of lateral surface by GFM was slight and non-linearly changed. Diamond micro/nano-pillar is widely applied in quantum transportation and nanomechanics, but it is very difficult to be machined by conventional mechanical methods, duo to its feature of high hardness, wear-proof and extremely small scale. Thus, a novel strategy of galvanometer-assisted femtosecond laser milling (GFM) was proposed to overcome the difficulty of fabricating the micro-pillar with high aspect ratio on monocrystalline diamond. Through numeric calculation, it is found that GFM trajectory possesses the special distribution consisting of separate pulse-concentrated regions and expanded flat-distributed region, which was then experimentally proved beneficial for improving the energy consumption and highly promoting the material removal rate. Moreover, the GFM was found to be much easier to generate micro-pillar, whereas in the case of conventional non-galvanometer milling (NGM) micro-pillar underwent severe degeneration or height loss under various average laser powers. Through optimizing the machining parameters, the micro-pillar was successfully prepared with an aspect ratio of as high as 42.25 by GFM. A 5 × 5 micro-pillar array of monocrystalline diamond was also fabricated with consistent diameter, height and aspect ratio. With Raman spectroscopy, the graphitization induced by GFM was slight, though it was unavoidably introduced with relatively high average laser power. Besides, the graphitizing level on the lateral face of the machined micro-pillar was found to non-linearly change with different machining parameters, because the material removal mode underwent a transformation from graphitization dominated to sublimation dominated with the increasing of applied energy density.

Experimental investigation into material removal mechanisms in High Speed Wire EDM

High Speed Wire EDM (HSWEDM) is characterized by high relative velocities between its electrodes which appear in almost no other field of electrical discharge machining (EDM). Also, it previously has been described as a hybrid process. Consequently, material removal mechanisms show significant differences compared with other EDM processes. In this research, single discharge craters of HSWEDM processes were examined, and their geometrical features were associated with the underlying parameters. Discharge crater geometries on the workpiece electrode (anode) could be investigated by measurements whilst those on the wire electrode (cathode) were calculated. Results show that the high wire velocity leads to moving foot points on both electrodes. Pulse duration and the type of working medium influence the generation of discharge craters and thus their geometry and the modes of material removal. Static discharges and anodic dissolution could also be identified as material removal mechanisms and characterized in their geometrical properties. However, they are of secondary importance.

High temperature solid particle erosion behaviour of SS 316L and thermal sprayed WC-Cr3C2–Ni coatings

The boiler tubes, steam and gas turbines are commonly affected by severe high-temperature erosion wear due to impingement of solid particles entrained in the stream of fluid. The investigation focuses on the applicability of thermal sprayed WC based coatings for turbines and boiler tubes by improving the erosion resistance of the base material. The present work includes high-temperature solid particle erosion behaviour of WC-Cr3C2–Ni coatings deposited by atmospheric plasma spray (APS) and high-velocity oxy-fuel (HVOF) processes. The effects of temperature and impact angle on erosion performance of uncoated and coated specimens were comparatively studied using air-jet erosion tester (ASTM G76). The eroded surface morphology was analysed using a scanning electron microscope (SEM). Uncoated steel exhibit ductile mechanisms; however, both coated specimens follow the mixed mode of material removal under the same operating conditions. Erosion resistance of APS and HVOF coatings is approximately two and three times higher than uncoated specimens at higher temperatures under 30° and 90° impact angles. The HVOF coating offers higher erosion resistance than APS coating due to lower porosity, greater splat adhesion, a lower degree of decarburisation and higher inter-splat sintering at elevated temperatures.

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.

Effects of Depth of Cutting on Damage Interferences during Double Scratching on Single Crystal SiC

In this work, the damage interference during scratching of single crystal silicon carbide (SiC) by two cone-shaped diamond grits was experimentally investigated and numerically analyzed by coupling the finite element method (FEM) and smoothed particle hydrodynamics (SPH), to reveal the interference mechanisms during the micron-scale removal of SiC at variable Z-axis spacing along the depth of cutting (DOC) direction. The simulation results were well verified by the scratching experiments. The damage interference mechanism of SiC during double scratching at micron-scale was found to be closely related to the material removal modes, and can be basically divided into three stages at different DOCs: combined interference of plastic and brittle removal in the case of less than 5 µm, interference of cracks propagation when DOC was increased to 5 µm, and weakened interference stage during the fracture of SiC in the case of greater than 5 µm. Hence, DOC was found to play a determinant role in the damage interference of scratched SiC by influencing the material removal mode. When SiC was removed in a combined brittle-plastic mode, the damage interference occurred mainly along the DOC direction; when SiC was removed in a brittle manner, the interference was mainly along the width of cutting; and more importantly, once the fragment of SiC was initiated, the interference was weakened and the effect on the actual material removal depth also reduces. Results obtained in this work are believed to have essential implications for the optimization of SiC wafer planarization process that is becoming increasingly important for the fabrication of modern electronic devices.

Adjustment of Hardness of (CrFeMn)x(NiyAl)1−x, (y = 1,3 and x = 0.6,0.72,0.80) High Entropy Alloys by Deliberate Control of Intermetallic Phase Formation: Microstructural Evolution, Hardness and Dry-Sliding Wear Response

In this work, the softening of the hard, NiAl containing, Cr–Fe–Mn–Al–Ni high entropy alloy system by the introduction of Ni3Al, an intrinsically ductile intermetallic compound, is reported. The alloys were prepared by vacuum arc melting and their phase formation prediction models were verified in terms of the actual presented microstructural features. X-ray analysis revealed the presence of NiAl and Ni3Al intermetallic compounds in each HEA system group. The microstructure of all six examined alloys in their as-cast condition consisted of two phases (NiAl-rich B2 and FeCr-rich A2) featuring various eutectic morphologies, while spinodal decomposition was evident in every case. The NiAl–Cr pseudo-binary phase diagram was used for the explanation of the solidification sequence in each alloy. In terms of mechanical properties, HRC hardness measurements showed that higher amounts of Ni3Al can lower the alloys’ hardness up to half its initial value. Finally, dry-sliding wear testing revealed an oxidation-delamination mode of material removal in both HEAs group, while the Ni3Al containing alloys also exhibited parts of abrasive wear mode. Hardness vs composition diagram. It can be seen that by adjusting the type and the extent of the intermetallic phase being formed (a—NiAl, b—Ni3Al), the hardness can be controllably modified within a range of 21 to 58 HRC. The related wear rates are also included

Strain Rate Effect on the Ductile Brittle Transition in Grinding Hot Pressed SiC Ceramics

Surface and subsurface damage are still persistent technical challenges for the abrasive machining hot pressed-silicon carbide (HP-SiC) ceramics. Therefore, an investigation of the material behavior and critical depth of ductile to brittle transition (DBT) is essential for improving high precision and quality grinding HP-SiC ceramics. In this paper, single-grit grinding experiments with different scratch speed were conducted to study strain rate effect on the critical depth of DBT. The nanoindentations were performed to test the hardness and Young’s modulus changes of DBT position under different scratch speeds. The material removal mechanism and phase changes underneath the scratch groove were investigated using Raman tests. Based on the specific energies consumed in ductile and brittle modes of machining, a theoretical model of the critical depth of DBT was developed. The experimental results suggest that high scratch speeds generate high nanohardness, high Young‘s modulus and high critical depth of DBT of HP-SiC ceramics. The measured critical depth of DBT shows a good agreement with the predicted value calculated by the developed model. The subsurface damage depth reduced with high strain rate. Furthermore, the Raman results revealed that dislocations and amorphous transformation dominated the ductile removal mechanism of HP-SiC grinding. The fracture chips and subsurface damage depth was determined by the lateral crack and median crack, respectively. This paper’s results provide a fundamental understanding of the effect of grinding speed on the material removal mode of HP-SiC ceramics.

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 Study on Work Hardening in the Laser-Assisted Machining of Si3N4 Ceramics under Different Material Removal Modes

In order to understand the work hardening phenomenon and mechanism of laser-assisted machining (LAM) of Si3N4 ceramics, the work hardening degree of LAM Si3N4 under different material removal modes was studied. Two sets of single-factor experiments were performed in which the laser power and the cutting depth were changed respectively. The results show that work hardening is the result of the combination of heat and cutting deformation during cutting. The work hardening degree decreases with the increase of material softening degree. When the material is removed plastically, the work hardening degree is 110–115%.

Preparation of nanotwinned cBN cutting edge by combining mechanical lapping and ion beam polishing

Nanotwinned cubic boron nitride (nt-cBN) is a promising tool material for ultra-precision cutting of ferrous metals. However, high hardness and excellent thermal stability make it difficult to obtain sharp cutting edge by using conventional methods. In this study, the factors influencing the material removal modes and edge sharpness in mechanical lapping process have been investigated, and a novel method combining mechanical lapping and ion beam polishing is proposed for achieving the sharp nt-cBN cutting edge. The results show that the material removal in mechanical lapping is mainly contributed by the transgranular ductile failure at the nanometer scale. Besides, high material hardness with a low Poisson's ratio and small impact force on the cutting edge are beneficial for obtaining sharp cutting edge. Therefore, the ion beam polishing process is adopted to sharpen the mechanically lapped cutting edge. By using a steel baffle plate with straight profile and eliminating the clearance between the baffle plate and rake/flank face with resin filler, the edge radius is observed to decrease from micrometre scale to about 60 nm. The proposed method represents a promising strategy for the preparation of ultra-sharp cutting edge with low cost and high accuracy.