Ductile Removal Mode Research Articles

Material removal mechanisms affected by milling modes for defective KDP surfaces

Surface defects which are caused by diamond fly-cutting can degrade the optical characteristics of KDP crystals by inducing the laser damage and resultantly shortening the service life when working in the condition of extremely high-power laser irradiations. The ball-end milling repair can totally remove the defects and correspondingly enhance the ability of the KDP crystals to resist laser damage. However, because of the existence of pull-milling, push-milling, down-milling, and up-milling in the complex machining trajectory, it is of great challenge for soft-brittle KDP crystals to achieve full plastic domain milling of repair points for obtaining ultra-smooth repaired surface. In this study, theoretical models of the maximum uncut chip thickness (UCT) considering the defect are constructed in the various milling modes. The cutting force and surface morphology affected by milling modes on the defective surface are simulated using a three-dimensional (3D) finite element model. Combined with the simulations, micro-milling tests are employed to confirm the influence of milling modes on the material removal mechanisms for defective KDP crystals by cutting force, surface morphology, surface roughness and specific cutting energy (SCE). It is found that defects could reduce the maximum UCT to distinct degrees in the various milling modes. Defects have a greater influence on the maximum UCT in the push-milling mode than that in the pull-milling mode, while the influence is the same in the up-milling and down-milling modes. Interestingly, the ductile removal mode occurs on the both defect-free and defective surfaces in the pull-milling mode when the feed per tooth (fz) is 0.5 µm/z. In contrast, the brittle-ductile transition caused by surface defects occurs in the other three milling modes. Furthermore, defects would lead to severer size effect in the all milling modes. When the fz is 0.1 µm/z, for the defective surface, the SCE is larger and the ploughing effect is more significant in the push-milling mode compared to the pull-milling mode. More importantly, as a result of the difficulty of chip discharge, the SCE is greater in the up-milling mode than in the down-milling mode on the defective surface. This work reveals the influence mechanism of milling modes on the material removal process for defective KDP crystals and offers a theoretical foundation and technical value for improving the repaired surface quality of functional KDP crystals served as the optical devices.

Precision-improving manufacturing produces ordered ultra-fine grained surface layer of tungsten heavy alloy through ultrasonic elliptical vibration cutting

High-precision and ultra-fine grained surface of tungsten heavy alloy exhibits superior service performance that is useful for many applications and shows promises for use as key parts in nuclear protection and precision instruments. The present study concentrated on a kind of precision-improving ultrasonic elliptic vibration cutting approaches, which fabricated nanometer-level surface roughness, inhibited subsurface damages evolution, and formed continuous ultra-fine grained layer microstructure. The surface morphologies have been characterized by ultra-depth three dimensional microscope and white light interferometer. Excellent machined surface quality was achieved under ultrasonic elliptic vibration cutting condition, and an ideal surface roughness of Sa = 70.7 nm was obtained. Microstructural alteration studied using EBSD technique and TEM observation confirmed the generation of ultra-fine grained structure. Surface grain size has been reduced from 50 ∼ 100 μm to 50 ∼ 300 nm without cracks and other micro-damages. Research demonstrated that surface energy accumulation and dislocations clustering induced by high-strain rate diamond tool impact provided the primary driving force of ductile-mode removal and grain recrystallization. A dislocation density-based simulation model was carried out to complement the static experimental investigations. The present work on surface formation and microstructural evolution identified that ultrasonic elliptical vibration machining has potential to deliver improved tungsten-based alloys service performance.

Semiempirical heat flux model of hard-brittle bone material in ductile microgrinding

Biological compact bone tissue is a typical hard-brittle material, which is extremely sensitive to high temperature. For the calculation of bone microgrinding temperature field, most of the known methods are based on the constant heat flux of the average tangential force without considering the ductile grinding characteristics of bone, leading to large calculation error. A semiempirical heat flux theoretical model based on the dynamic grinding force of hard-brittle bone ductile microgrinding is proposed to solve this bottleneck problem. The mechanism of heat generation and consumption in microgrinding zone under ductile removal mode of bone material is studied first, and the probability statistics of effective cutting abrasive number is analyzed. The heat distribution coefficient in grinding zone is calculated, and the semiempirical heat flux model of the ductile removal of hard-brittle biobone material is developed. Microgrinding experiments are conducted to verify the effectiveness of ductile removal grinding heat flux model. The temperatures of different measuring points on the surface of bone samples and micro grinding force are measured. Results show that the theoretical calculation is consistent with the experimental results, with an average error of <7.7%. Mechanical processing theory is applied to medical rehabilitation for realizing the precise control of temperature field in hard-brittle bone microgrinding process.

Precision grinding of cemented carbides based on a multi-layer brazed diamond wheel using an electrolyte containing carbon nanotubes

Carbon nanotubes (CNTs) were added into the electrolyte to improve electrolytic dressing grinding performance of a brazed multi-layer diamond wheel containing titanium. The effect of CNTs on the electrolytic dressing performance of the grinding wheel was studied. The grinding force, residual stress, roughness and morphology of the machined surfaces were recorded in the grinding experiment of cemented carbides, and the grinding performance was evaluated based on these. The experiment results expressed that the CNTs improved the electrolytic dressing performance of the grinding wheel, which allows the worn grits to smoothly fall off and maintain the sharpness. The grinding force and residual stress were reduced when the CNTs were added into the electrolyte. Moreover, there was a grand advancement in the machined surface quality when the CNTs were used as an additive to the electrolyte. The machined surfaces produced by the brazed wheel using an electrolyteadded CNTs showed predominantly smooth grinding marks. Ductile-mode removal was predominantly observed in the grinding experiments. Adding CNTs to an electrolyte can help achieve high-precision grinding of difficult-to-cut materials with a brazed multi-layer diamond wheel.

Improved machinability of TiB2–TiC ceramic composites via laser-induced oxidation assisted micro-milling

Low manufacturing efficiency, poor processing controllability and deteriorated surface quality were main challenges in micro-milling high hardness TiB2-based ceramic composites. This research work investigated an innovative hybrid machining method named laser-induced oxidation assisted micro-milling (LOMM) of TiB2–TiC ceramic composites with hardness of 20 ± 0.7 GPa. Oxidation mechanism of the material under laser irradiation was studied. At single pulse energy of 0.275 mJ and scanning speed of 1 mm/s, the irradiated surface in an oxygen-rich atmosphere produced metamorphic layer including loose and porous oxide-layer as well as dense sub-layer with hardness of 9.6 ± 0.7 GPa. In addition, the main compositions of formed oxides were anatase TiO2 and rutile TiO2, which can be easily removed with an extremely low cutting force. For the investigated range of milling parameters, the milling force and the machined surface quality were studied in LOMM of TiB2–TiC ceramic composites. A significant reduction (more than 70%) of milling force during removing the oxide-layer was obtained, compared to that milling of the sub-layer. Besides, a well-machined surface quality was obtained (ap = 2 μm and fz = 0.5 μm/z) with a ductile removal mode, and surface roughness is 146 nm. As a case study, a microgroove having a width of 0.4 mm and an aspect ratio of 2.5 was fabricated successfully using LOMM. As a comparison, under identical cutting parameters conventional micro-milling (CONM) was performed. The results indicated that machined quality of the microgroove and perpendicularity of the sidewall obtained by LOMM were superior to those using CONM. Machining efficiency was also improved significantly by 106% using LOMM, compared to CONM. It confirmed that the LOMM process was an efficient and effective method to machine high hardness ceramic composites.

Mechanistic cutting force model for rotary ultrasonic machining of rocks

Cutting force is the predominant output variable in rotary ultrasonic machining (RUM). It dictates other output variables, such as tool wear, cutting temperature, edge chipping, etc. It is desirable to develop a mechanistic model to predict cutting force and reveal the underlying cutting tool-workpiece interaction in RUM. Numerous researchers have developed theoretical approaches to predict cutting force in RUM; nevertheless, the combined effects of material removal on cutting force model have not been investigated. RUM has been used for machining rocks in several recent experimental investigations. However, there are no reports on cutting force model for RUM of rocks. This work bridges the gap and reports an improved mechanistic cutting force model. The model is derived based on the ductile mode removal and brittle fracture mode removal of rock under the indentation of a single abrasive particle. The cutting force model for RUM of rocks is then developed by aggregating the effects of all active abrasive particles bonded to the tool end face. Based on this model, the relationships between input variables (tool rotation speed, feedrate, ultrasonic vibration amplitude, abrasive size, abrasive concentration, and tool size) and cutting force are predicted. Experiments have been conducted and the experimental results agree well with the model predicted trends.

Effects of wheel speed on surface/subsurface damage characteristics in grinding of glass-ceramics

Glass-ceramic is widely used in optics and other areas because of its good physical and mechanical properties. However, the accompanying hard and difficult to machine characteristic brings great challenges to the commonly used grinding in industry. Previous researches on the removal mechanism of glass-ceramic are basically traditional low-speed scratch and low-speed grinding, ignoring the effect of the wheel speed on surface/subsurface damage. In this study, the effects of the wheel speed on the surface morphology, surface roughness, and subsurface damage of Zerodur glass-ceramics were studied at different wheel speeds based on a multi-step grinding process. Experimental and analytical results showed that increasing wheel speed can restrain the occurrence and development of brittle fractures due to the effects of strain rate and grinding temperature, resulting in fewer and smaller micro-pits and micro-cracks, and thus more signs of plastic flow are observed. In addition, the effects of wheel speed on surface roughness under the semi-finishing grinding stage are not as clearly defined as on that under the rough and finishing grinding stages. The smeared layers on the ground surface, which are related to the compound effects of the grinding temperature and semi-finishing grinding conditions, are recognized as the major contributor to the contradictory surface roughness results. Furthermore, the effect of wheel speed can prevent crack from nucleating and propagating, and then leads to a smaller depth of subsurface damage. Accordingly, there is a general trend toward subsurface damage depth with the increase of undeformed chip thickness, regardless of the grinding conditions. In a word, higher wheel speeds are easier to achieve ductile removal mode and conversely, the glass-ceramic is more prone to a brittle removal mode. The research is expected to provide guidance for grinding-induced damage control and the engineering application in high precision optical parts of glass-ceramic.

Effect of scratch direction on densification and crack initiation of optical glass BK7

It is known from nano-indentation studies that optical glass material undergoes a certain densification deformation during ductile scratching. In this study, nano-scratch experiments were conducted under different scratch directions (face-forward and edge-forward of the same indenter) and normal loads to explore the differences in densification of material in ductile removal mode. Furthermore, inherent differences in densification deformation were utilized to explore the effect of scratch direction on the crack resistance (CR) or ductile-to-brittle transition (DBT) of material under brittle removal mode. Based on quantitative evaluation of densification deformation induced by indentation and scratching, a high-temperature annealing procedure was employed to determine the changes of the scratch groove geometry and volume before and after the release of the scratch-induced densification deformation under different scratch directions (edge-forward and face-forward). The geometric parameters of scratch grooves under edge-forward and face-forward directions showed similar changes after annealing. It is found that in ductile removal mode, the densification ratio to the total scratching volume in edge-forward scratching is greater than that in face-forward scratching. It is likely that the greater densification deformation of edge-forward scratching led to stronger crack resistance and smaller tensile residual stress. Therefore, lateral cracks are more likely to initiate and propagate under face-forward scratching under brittle removal mode, which is consistent with the experimental results.

Theoretical and experimental analysis of material removal and surface generation in novel fixed abrasive lapping of optical surface

Abstract Fixed abrasive machining, which overcomes the lack of determinacy and efficiency of current loose abrasive polishing or lapping processes, is an alternative technology for fabricating large-aperture aspheric surfaces with high finishing efficiency and high-quality surface finish. A novel fixed abrasive lapping (NFAL) tool, which combines computer-controlled optical surfacing and conventional fixed abrasive lapping process, is introduced in this study to produce off-axis aspheric surfaces efficiently while surface state is controlled and subsurface damage (SSD) is avoided. The removal mechanism analysis shows that fused quartz glass is primarily removed in ductile regime and the SSD depth is 14.7 μm when the tool load is 25 N. The material removal volume is linear with the machining time under the ductile removal mode. The material removal and surface generation models are developed on the basis of the calculation of the spatial distribution of abrasive particles, the pad–particle–workpiece interactions, the single-particle abrasion mechanism, and the linearly cumulative removal of surface generation in the NFAL process. The models are verified through a series of spot and surface lapping experiments. Results show that the theoretical model can be successfully used to predict and optimize the NFAL process. The spot lapping experiments indicate that the material removal volume is linear with the rotation speed, and the maximum depth is limited to the specific value that depends on the stiffness of lapping tool. The surface roughness Ra and Rz values of the measured and simulated surface data decrease with the increase in feed rate and increase with the increase in raster spacing. The power spectral density analysis indicates that high feed rate can distinctly improve the high-frequency errors, and the selection of the raster spacing can principally affect the low-frequency and middle-spatial frequency errors.

A cutting force prediction model for rotary ultrasonic side grinding of CFRP composites considering coexistence of brittleness and ductility

Cutting force has a great effect on the machining surface/subsurface damage. The prediction model of cutting force is necessary to optimize input variables. During rotary ultrasonic side grinding (RUSG), the indentation depth of a single abrasive grain increases (up milling) from 0 to maximum or decreases (down milling) from maximum to 0 with the motion of abrasive grains. Therefore, the ductile removal mode and brittle fracture removal mode coexist. In this research, a mechanistic cutting force model in RUSG of carbon fiber–reinforced polymer (CFRP) composites, considering both brittleness and ductility, has been developed for the first time. The ductile-brittle ratio K has been put forward and obtained through experiments. The cutting force perpendicular to feed direction has been calculated by integration method from the point of force analysis. The results showed that K increases with the rise of spindle speed or the decrease of feed rate and cutting width. The cutting force behaves oppositely with respect to K. Finally, orthogonal experiments have been conducted to verify the cutting force model and the results show good consistency between theoretical cutting force and measured cutting force. The developed model is capable to predict the cutting force in RUSG of CFRP composites and provides a support for optimizing the process.

Mechanism of material removal by fixed abrasive lapping of fused quartz glass

In this paper, a novel fixed-abrasive lapping process to finish off-axis aspheric surfaces efficiently is proposed. The lapping mechanism of fused quartz glass was evaluated using the novel fixed-abrasive pad tool. The applied load and not the rotation speed and feed rate is the key factor affecting the material removal mode of fused quartz glass in the fixed-abrasive lapping process. Analysis of surface characteristic and subsurface damage indicates that lower applied load produces more ductile mode removal and that the removal on fused quartz glass can be changed from brittle fracture to ductile regime if the applied load is less than 25 N. The material removal rate (MMR) increases as the applied load is increased, which further increases the surface roughness. In addition, the MMR increases and the surface roughness decreases as the rotation speed is increased or the feed rate is deceased. A theoretical model for predicting the maximum SSD depth of the specimens during lapping was developed. The model can predict well that the maximum SSD depth increases with increasing applied load.

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.

In situ experimental study on material removal behaviour of single-crystal silicon in nanocutting

Ductile-mode removal of single-crystal silicon can be achieved by strictly controlling the cutting parameters, which significantly affects the machining efficiency. To improve the surface quality without reducing the machining efficiency, nanocutting experiments were performed to study the material removal behaviour using a specially designed nanocutting platform with a scanning electron microscope (SEM). Diamond tools with different edge radii were fabricated by focused ion beam (FIB) technology. The initiation and propagation of microcracks were observed online by an SEM to analyse the material removal behaviour of single-crystal silicon in the brittle mode. The effects of the crystal orientation and tool edge radius on the critical thickness of the brittle-ductile transition were investigated. Additionally, in the ductile mode, the influence of the tool edge radius on the minimum cutting thickness (MCT) was analysed. It was determined that the ratio of the MCT to the tool edge radius was 0.32–0.50, regardless of the tool edge radius. This in situ experimental study can provide a direct verification for the material removal behaviour in nanocutting.

The effect of cut depth and distribution for abrasives on wafer surface morphology in diamond wire sawing of PV polycrystalline silicon

Due to the existence of an acid etch resistant thin amorphous silicon layer over the smooth grooves of the diamond wire sawing polycrystalline silicon wafer surface, the anti-reflection effect is usually not ideal using the mature acidic texturization. The amorphous silicon layer will be produced on the machined surface by material ductile removal. Therefore, during the process of cutting photovoltaic polycrystalline silicon wafers, the material removed in the brittle way is expected and the surface topography of the wafers formed with the brittle fracture is better for the texture fabricating. In this paper, a mathematical model considering the influences of process parameters and wire saw parameters was developed based on indentation fracture mechanics. The variations of cutting groove profile formed by different material removal modes were also included. The effect of abrasives distributed on the wire saw on material removal and surface formation of polysilicon was analyzed. The results showed that most of abrasives removed material with ductile removal mode, however, the volume of the material removed by abrasive in ductile mode is less than 10% of the total removal volume. Brittle fracture removal mode was still the major way of material removal in diamond wire sawing. With the same ratio of the feed rate and wire speed, the faster feed rate and wire speed will not only improve the cutting efficiency, but also is easier to obtain a brittle fracture surface. There is a critical angle θc for the distribution of abrasives on the wire saw surface. Only when the position angle of the abrasive removing material in brittle mode is less than θc, the brittle fracture can be formed on the wafers surface.

Finite element implementation of the tension-shear coupled fracture criterion for numerical simulations of brittle-ductile transition in silicon carbide ceramic grinding

Despite great efforts have been made to understand the machining mechanism of brittle materials, few numerical models are available to describe the brittle-ductile transition (BDT) in engineering ceramic grinding due to the fracture mechanism difference in brittle and ductile removal modes. This paper introduces the tension-shear coupled (TSC) fracture criterion for the finite element method (FEM) analysis of the brittle material machining that is capable of describing BDT. With TSC fracture criterion, simulation results of single diamond grain grinding of silicon carbide (SiC) ceramic, including the ground surface/subsurface morphology, grinding forces, and equivalent plastic strain, were compared with those under effective plastic strain (EPS) fracture criterion, the one widely adopted by commercial finite element software. The results show that TSC fracture criterion can better characterize BDT in SiC grinding than EPS criterion, via ground surface/subsurface morphology and tangential grinding force. To experimentally validate the proposed TSC fracture criterion, single diamond grain grinding tests were conducted on SiC with various maximum undeformed chip thickness to demonstrate BDT and compare with the simulation results. The measured ground surface morphology evolution and BDT with the increased undeformed chip thickness matched the results of the FEM simulation with TSC fracture criterion.

Force prediction model considering material removal mechanism for axial ultrasonic vibration-assisted peripheral grinding of Zerodur

Axial ultrasonic vibration-assisted peripheral grinding (AUPG) has the major advantages of simultaneously improving the grinding quality and material removal rate compared with conventional grinding methods. In this study, a grinding force prediction model for the AUPG of Zerodur was developed to investigate the generation mechanism of the grinding force for guidance in practical engineering applications. Taking into consideration the material removal mechanism, material properties and vibration impact, the three primary grinding force components were first separately modelled, namely, the ductile removal force in the ductile removal phase, the brittle removal force in the brittle removal phase and the frictional force of the friction process. The critical uncut chip thickness and maximum uncut chip thickness were subsequently researched to define the two material removal modes in the AUPG of Zerodur, namely, the ductile removal mode and the mixture of brittle and ductile removal mode. The grinding force models of these two material removal modes were developed using effective grinding force component models. The instantaneous variation of the grinding force with time and space was also analysed to derive models of the final time-averaged normal force, tangential force and axial force. Finally, grinding experiments were performed, the results of which showed that the prediction errors of the developed model were only 7.37 and 11.53% for the normal and tangential grinding forces, respectively. The axial ultrasonic vibration was also determined to reduce the surface roughness by 18.0% compared with conventional grinding, while the normal and tangential forces were reduced by 27.31 and 22.52%, respectively. This indicates that AUPG affords a significantly improved grinding surface quality. The developed model enables an understanding of the comprehensive mechanism of AUPG and provides a basis for the development of the grinding force models of the other brittle materials.

Study on micro-crack induced cutting of 4H-SiC, sapphire and Si along tetrahedral micro-crater tips

The precision cutting of brittle materials requires the accumulation of nanoscale cut depth for ductile-mode removal without any micro-cracks, leading to inefficiency. Hence, a rapid cutting of brittle material is proposed through the micro-crack propagation along micro-crater tips on workpiece surface. The objective is to understand the feasibility of micro-crack induced cutting for various difficult-cut materials such as 4H-SiC, sapphire and Si. First, a diamond cone micro-tip was used to fabricate the tetrahedral micro-crater array; then the cutting force was modelled in micro-crack induced cutting; finally, the cutting performance was evaluated. It is shown that the indentation force ranging from 0.1–10 N produces the integrated micro-cracks along micro-crater tips. An increase in the micro-crack length leads to a decrease in cutting force and cutting time and an increase in cut form error. Moreover, the micro-crack propagation is little related to the crystal orientation. Unexpectedly, the cutting force, the cutting time and the cut form error for the hardest 4H-SiC was less than the ones for sapphire and Si, respectively. Its precision and mirror cutting efficiency reaches 10 mm2 s−1 with the micro-indentation time of 12 s. The indentation force related to micro-crack propagation may be used to predict and control the micro-crack induced cutting performance.

Investigation on critical crack-free cutting depth for single crystal silicon slicing with fixed abrasive wire saw based on the scratching machining experiments

The experimental research of single crystal silicon scratching machining is crucial for micro component machining, precision grinding and fixed abrasive wire saw precision slicing of single crystal silicon. The fixed abrasive wire saw slicing is the main slicing technology of large diameter single crystal silicon slicing process. During the fixed abrasive wire saw slicing process, the brittle material removal mode results in micro crack damage in wafer surface layer, which reduces the strength of sliced silicon wafer, and restricts silicon wafer thinner. The crack-free slicing surface could be obtained while the material is in ductile removal mode. In this study, the scratching machining experiments with the Nano Indenter and Berkovich diamond indenter scratching on single crystal silicon (100) plane along [010] orientation is conducted. The constant normal force 20mN, 100mN, and varied normal force from 0mN to 50mN are applied on the Berkovich indenter. The material removal mode is mainly in ductile regime while 20mN constant normal force, and the material removal mode is mainly in brittle regime and cleavage fracture emerge while 100mN constant normal force. The tangential friction force of scratching machining process changes from smooth to fluctuate when the varied normal force from 0mN to 50mN, and material removal mode from ductile regime to brittle regime. The critical normal force of ductile transition to brittle is 26mN. However, the scratching machining experimental results near the ductile regime removal mode show that the scratch exists obvious elastic recovery. The elastic recovery of scratch affects the material removal amount and machining accuracy. Based on the scratching machining experimental results, the critical crack-free cutting depth per abrasive for single crystal silicon slicing with fixed abrasive wire saw is analyzed. The results in this paper are useful for the single crystal silicon micro component machining and slicing process parameters selection of fixed abrasive wire saw.

Precision and mirror micro-grinding of micro-lens array on macro-freeform glass substrate for micro-photovoltaic performances

A hybrid of micro-lens array and freeform surface produces a novel micro-optic and micro-photovoltaic behaviors, but it is difficult to control both micro-form and macro-form accuracies in micro-machining of brittle photovoltaic glass. Hence, a ductile micro-grinding is proposed to replicate the precision-trued diamond wheel V-tip on macro-freeform glass substrate of solar cell. The objective is to understand the formation mechanism of accurate micro-lens array on freeform glass surface for photovoltaic application. First, the micro-optic properties of freeform micro-lens array were analyzed with regard to micro- and macro-shapes. Second, the precision and mirror micro-grinding along with the form errors of freeform micro-lens array was observed on the base of the transfer from brittle-mode removal to ductile-mode removal. Finally, the photovoltaic properties were experimentally measured. It is shown that the freeform micro-lens array increases luminous flux by 53.8–54.5 % compared with the plane and micro-lens array. The micro-form accuracy and the micro-structure mirror are dominated by micro-meter-scale diamond wheel V-tip with accurate wheel V-tip profile and integrated micro-grains protrusion, respectively. The novel measuring model of freeform micro-lens array may be used to evaluate and control the curved micro-form accuracy. The freeform micro-lens array contributes to the improvement of photovoltaic performance in scattered light illumination.

Effect of lapping slurry on critical cutting depth of spinel

The critical cutting depth for lapping process is very important because it influences the mode of material removal. In this paper, a serial of microscopic indentation experiments were carried out for measuring spinel wafers’ hardness and crack length in different lapping slurries. Their critical cutting depth and fracture toughness were calculated. X-ray photoelectron spectroscopy (XPS) was also employed to study the surface chemical composition and softened layer thickness of wafers in different slurries. Experimental results indicate that the softened layers of spinel wafers are formed due to the corrosion of lapping slurries, which leads to a lower hardness and a larger fracture toughness of samples, and increases the critical cutting depth. Among them, the critical cutting depth in ethylene glycol solution is the largest and up to 21.8nm. The increase of critical cutting depth is helpful to modify the surface quality of the work-piece being lapped via ductile removal mode instead of brittle fracture mode.