Subsurface Damage Depth Research Articles

Development of an intelligent grinding system for fabricating aspheric glass lenses

Precision optical systems, such as lithographic projection lens or cameras for satellites, typically composed of glass or ceramics optical components. In the manufacturing processes of glass and ceramics components, grinding is a key technique for curvature generation (CG) of optical surface. However, dimension errors and sub-surface damage (SSD) of components occur during the CG process. High material removal rates during the CG leads to deeper SSD on optical surface. The SSD must be removed by subsequent lapping and polishing processes to ensure high optical surface quality. However, these processes are complex, and a precise process to remove SSD from the aspheric surfaces with tens or hundreds of microns of departure from best fit sphere (BFS) is required. Thus, an efficient precision lens fabrication method can reduce the depth of SSD and help to control the dimension error during the CG process. We developed a five-axis grinding system with intelligent machining techniques to improve the quality of CG process. The intelligent grinding system based on the Tongtai five-axis machine (GT-630) was combined with two measurement modules of the Renishaw measurement probe. A customized computer-aided manufacturing software was developed and applied to this grinding system. Spherical and aspherical components with a maximum diameter of 600 mm can be fabricated using the grinding system. To improve the grinding efficiency, an ultrasonic-assisted module was combined with the BBT 40 tool arbor. The frequency range, amplitude, and power of the ultrasonic device were 15–40 kHz, 0.2–2 μm, and 1000 W, respectively. The intelligent grinding system included several vibration, temperature, and acoustic emission sensors and a microphone to monitor the dynamic status of machine during the grinding process and help to quality control. The wear of the diamond grinding wheel could be predicted immediately by analyzing the collected data in this system. A five-axis machine was applied to an intelligent grinding system for large glass lenses. The fabrication time of 200 mm diameter aspheric lenses reduced about 3 weeks by using this intelligent grinding system to integrate aspherization processes into curvature generation to become an aspheric curvature generation step.

Smoothed-Particle Hydrodynamics Investigation on Brittle–Ductile Transition of Quartz Glass in Single-Grain Grinding Process

The smoothed-particle hydrodynamics (SPH) method was introduced to simulate the quartz glass grinding process with a single grain under micro-nano scale. To investigate the mechanism of brittle–ductile transition, such factors as the machining depth, grinding force, maximum equivalent stress, and residual stress were analyzed. The simulation results indicate that quartz glass can be machined in a ductile mode under a certain condition. In this paper, the occurrence and propagation of cracks in quartz glass at different grinding depths (0.1–1 μm) are observed, and the critical depth of brittle–ductile transformation is 0.36 μm. At different grinding depths, the grinding force ratio is greater than 1. When the cutting depth is 0.4 μm, the crack propagation depth is about 1.2 μm, which provides a basis for the prediction of subsurface damage depth. In addition, the correctness of the simulation result was verified by carrying out scratch experiments of varying cutting depth on optical quartz glass.

Study on cutting PV polysilicon with a new type of diamond abrasives-helix-distribution saw wire based on controlling the subsurface microcrack damage depth

There are large brittle fracture pits on the surface of photovoltaic polysilicon wafer cut by diamond wire saw, because of the problems such as poor flow of cutting fluid and difficulty in chip discharge. At the same time, the anti-corrosion amorphous layer on the wafer surface can obviously block the subsequent acid etching and texturing reaction. In order to solve these problems, a new type of diamond abrasives-helix-distribution saw wire is used as the sawing tool to obtain the surface characteristics of photovoltaic polysilicon wafer suitable for acid texturing, then research on wire sawing photovoltaic polysilicon based on controlling subsurface microcrack damage depth control is carried out. The concept of high-quality cutting abrasives (HQCA) is defined, and the cutting process of polycrystalline silicon is numerically calculated based on the established new type diamond wire model, the influence of the process parameters and the structural parameters of the saw wire on the cutting characteristics is analyzed, and the reasonable matching between the process parameters and the saw wire parameters is discussed. The study results show that the number and proportion of HQCA in all abrasive grains involved in sawing can be increased by increasing the workpiece feed speed or reducing the moving speed of saw wire, properly reducing the abrasive concentration on the saw wire surface or increasing the abrasives size, or setting the ratio of axial distance between the abrasive area on wire surface and the bare wire area as 1:1. In this case, the formation of the wafer surface tends to be removed in the way of material microbrittleness, and the subsurface crack depths are in the optimal etching depth range. The number and proportion of HQCA remain basically unchanged when the workpiece feed speed changes in constant proportion to the wire speed, and the screw pitch of the abrasive groups on the surface of the saw wire has little effect on it.

Ductile-brittle transition mechanisms of amorphous glass subjected to taper grinding experiment

Taper grinding experiments were conducted in this paper to investigate the continuous and complete ductile-brittle transition process of two kinds of amorphous glass: high purity fused silica (HPFS) which is silica rich glass and soda-lime silica glass (SLSG) which is low silica glass. The grinding force, ground surface morphology, surface roughness, and subsurface damage depth induced during different stages of taper grinding were all analyzed. A mathematical model describing the cutting force of a grit and micro-crack length was established to clarify the ductile-brittle transition mechanisms of isotropic material. The model revealed that material removal mechanisms and grinding force were mainly determined by the crack equivalent length in front of the grit and its equivalent cutting force. The ground surface roughness and subsurface damage depth were mainly affected by the cutting force of the grit and length of cracks behind it. The ductile machinability of SLSG was better than that of HPFS due to the bonding of metallic atoms in SLSG with nonbridging oxygens, as well as their packing into free volume in SiO2 network.

Research of Subsurface Damage Depth of Lithium Niobate Crystal by Fixed-Abrasive Lapping

The subsurface damage depth of optical material introduced by lapping determines the efficiency of lapping and polishing process. In order to explore the subsurface damage (SSD) of fixed-abrasive lapping process, angle polishing method was employed to evaluate the SSD depth and damage form of soft-brittle material lithium niobate (LN) crystal. The influences of the grit size, the applied load and the hardness of pad matrix on SSD were investigated experimentally. The results show that: with the grit size decreasing form W28 to W14, the damage depth decreases by 70%. With the applied load decreasing form 15 kPa to 7 kPa, the SSD decreases by 40%. The dense and deep subsurface micro-cracks are caused by the hard matrix pad during lapping processing, while the matrix with low hardness causes plastic scratches on the surface/subsurface of the workpiece without obvious micro-cracks around the scratches. This study affords effective method for SSD evaluation and control of soft brittle materials such as LN crystal.

Molecular dynamics study of the removal mechanism of SiC in a fixed abrasive polishing in water lubrication

The mechanical removal mechanism of SiC substrates in the water-lubricated environment is investigated by molecular dynamics (MD) simulation. The surface quality, subsurface damage, removal efficiency and structural phase transition of SiC substrates were analyzed in a fixed abrasive polishing (FAP) under various water film thickness and external load. In water-lubricated environment, a small number of water molecules are pressed on the interface between the SiC substrates and the diamond abrasive. The results show that the largest pressing depth decreases with the increase of the water film thickness, contrary to the effect of the external load. The phase transition and the frictional heat both decrease as the pressing depth decreases, which further reduces the stress and temperature of the substrate in the FAP. The water layer has positive impact on the surface roughness and subsurface damage depth. Furthermore, the removal efficiency in the water-lubricated nano-abrading is lower than that in the dry process. Our researches benefit to understand the mechanical removal mechanism of SiC substrates under a water-lubricated environment and give theoretical guide for improving the machining technology of FAP.

Effect of a Ti alloy layer on CFRP hole quality during helical milling of CFRP/Ti laminate

During the helical milling of carbon fibre reinforced polymer (CFRP)/Ti laminated structures, there are inevitable influences between the CFRP and Ti. This paper investigated the effect of Ti on hole quality in CFRP during the helical milling of a CFRP/Ti laminate. Cutting force and heat, as well as the surface and subsurface qualities of holes in CFRP, were observed with and without the presence of a Ti base layer. A subsurface damage assessment method is also proposed. The results show that Ti had slight influences on cutting force and surface quality, and significant influences on cutting temperature and subsurface quality. The cutting of Ti generates a great deal of heat, thus significantly increasing the temperature at the outlets of CFRP holes. Due to this high temperature, the rigidity and adhesive properties of the CFRP resin base decline, which causes serious subsurface damage to the fibre layers close to the hole outlets. Subsurface damage was evaluated by the proposed assessment method (maximum damage depth = 54.7 μm). Additionally, the fibre cutting angle greatly influences the depth of subsurface damage. Hence, CFRP hole quality is significantly influenced by the cutting of adjacent Ti layers during the helical milling of CFRP/Ti laminates. Subsurface damage is the best factor for assessing CFRP processing quality.

Analytical modeling of subsurface damage depth in machining of SiCp/Al composites

SiC particle reinforced aluminum matrix (SiCp/Al) composite has been one of the key light metal matrix composites, which is widely used in aerospace and electronics industries due to its excellent material properties. However, the abrasive particles within the materials easily lead to severe tool wear during machining, and the subsequent machining-induced subsurface damage (SSD) will significantly affect the fatigue properties of composite structures. In this paper, an analytical model considering the effects of reinforced particle, thermal-mechanical coupling, and tool wear is first proposed to predict the SSD depth during cutting of SiCp/Al composites. The two-body abrasion and three-body rolling mechanisms are introduced to describe the effects of the particles on the tool-chip interface friction. The mechanical stress and thermal stress induced by mechanical load and thermal load, respectively are simultaneously taken into account. Meanwhile, the tool wear is considered in modeling of friction characteristic of tool flank-workpiece. The SSD depth is evaluated as a measure of variation in microhardness along the depth normal to the cross-section of the machined surface. The correctness of the proposed model was validated by the orthogonal cutting experiments of SiCp/Al composites under different tool wear values and feed rates. A good agreement is found by comparing the predicted and experimental results, which proves that the proposed model can accurately predict the SSD depth with an error of 7.8% in average. Furthermore, the results indicate that even a new tool is used, the SSD depth can reach to hundred micros level during machining of SiCp/Al composites, which is significantly greater than homogeneous metal machining.

Optimization of chipping parameters to mitigate the damage in a concrete substrate using a discontinuum modelling approach

Concrete chipping is frequently employed to remove the surface of deteriorated concrete substrate to allow the placement of repair material. Previous studies have revealed that subsurface microfractures generated during chipping is one of the most influential factors that decreases the interfacial bond strength between the old concrete and the repair material. This study focuses on the damage mechanism in a concrete substrate caused by a breaker chisel and aims to propose a chipping method to control the damage severity. To achieve this, a numerical model simulating a concrete substrate composed of mortar and coarse aggregates was constructed in the framework of the discrete element method. Concrete chipping simulations with the numerical model illustrated the mechanism of microfractures extending to a deeper region from the chipped surface with the penetration of coarse aggregates impacted by the breaker chisel. The numerical simulation further demonstrated that the chisel tip shape, chipping depth, and chipping velocity play a pivotal role in controlling the near-surface damage, whilst chipping inclination does not. A stepwise chipping method that removes the concrete in layers with a sharp-pointed chisel tip was proposed, since based on the numerical result the subsurface damage depth was reduced. Consequently, using this approach may improve the bond strength for concrete repair.

Study on hole quality and surface quality of micro-drilling nickel-based single-crystal superalloy

Nickel-based single-crystal superalloy has no grain boundary. This makes the traditional cutting mechanism of shearing and slipping along grain boundary of polycrystalline material based on the theory of elastic–plastic deformation not suitable for drilling single-crystal parts. To achieve high-quality and low-damage micro-drilling of nickel-based single-crystal superalloy, the micro-drilling surface/subsurface quality of the (001) crystal plane of nickel-based single-crystal superalloy was studied in this paper. Firstly, the influence rules of the cutting speed (vs) and the feed rate (vw) on the micro-drilling hole size, surface morphology, surface roughness and subsurface damage depth were studied. Then, the micro-hardness on micro-drilling subsurface was analyzed. Finally, the subsurface recrystallization was explored. The results show that with the increase in vs, the hole diameter error and the surface roughness decrease, and the depth of subsurface plastic deformation layer decreases first and then increases; with the increase in vw, the error of hole diameter and the surface roughness increase, and the depth of the deformation layer decreases first and then increases. When vw > 0.33 μm/r, there are severe scratches on the surface of the hole wall, and the hole wall appears undulating stripes; white layer structure and severe plastic deformation layer appeared on the micro-drilling subsurface; the γ and γ’ phases' deformation is severe; the micro-hardness at the subsurface white layer is small, and the micro-hardness value at the plastic deformation zone is large; after the high-temperature treatment, γ phase dissolves on the subsurface of micro-drilling parts, and the cellular recrystallization and the complete recrystallization occur. The minimum thickness of recrystallization layer obtained is 1.43 μm at vs = 141 m/min and vw = 0.105 μm/r. This study provides a good guidance and reference for the machining of nickel-based single-crystal superalloy micro-hole parts.

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.

Effects of local strain rate and temperature on the workpiece subsurface damage in grinding of optical glass

Subsurface damage (SSD) generated during the grinding process directly affects the optical quality and the service life of optical components. It is of great significance to accurately predict and effectively control the depth of the SSD layer of the ground workpiece. However, most of the existing SSD depth prediction models are based on quasi-static and room temperature conditions. This paper presented a prediction model of SSD depth, which considered the combined effect of local strain rate and temperature on the SSD generation during grinding process. The variation of strain and strain rate in the local shear zone of an average abrasive grit on the grinding wheel was analyzed, considering the dynamic density of abrasive grits; Strain rate effect of the material was described by introducing a dynamic fracture toughness KID, which would increase with the strain rate; Grinding zone temperature and especially the temperature on the abrasive grits were analyzed. Combining the strain rate effect and temperature effect, a thermodynamic fracture toughness KIDT model was further developed. On the basis of classic brittle material crack system and the kinematics of grinding, the SSD prediction model considering the dynamic variation of mechanical properties in grinding process was derived. The effects of local strain rate and temperature in grinding of BK7 optical glass were systematically studied under different grinding wheel speeds. The predicted SSD values using a conventional static model and current models considering the strain rate or grinding temperature effect or both, were compared with the measured values. The predicted SSD values using the theoretical model including both the strain rate and grinding temperature effects, agree well with the measured values. The proposed model therefore is anticipated to be meaningful to understand of the SSD of hard and brittle materials involved in a precision grinding process.

Two-dimensional detection of subsurface damage in silicon wafers with polarized laser scattering

Abstract This study proposes a polarized laser scattering (PLS) method for detection of the two-dimensional distribution of the grinding-induced subsurface damage (SSD). This study also uses two destructive methods to observe the distribution of SSD in the ground silicon wafers by cross-sectional microscopy and by layer-by-layer microscopy. An experimental investigation is arranged for detecting the two-dimensional damage from diamond wheel grinding of silicon wafers through wafer sample preparation and SSD observations with the cross-sectional and layer-by-layer microcopy. It also includes PLS detection of the ground silicon wafers for SSD information. The results show that both the depth of SSD and the crack density contribute to the PLS detection signals. The study reveals the characteristics of the PLS detection, which illustrates the potential application of the PLS method to in-process detection.

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.

Prediction of subsurface damage depth in rotary ultrasonic machining of glass BK7 with probability statistics

Subsurface damage (SSD) generated in rotary ultrasonic machining (RUM) process significantly deteriorates the technological and structural performance of the optical components. However, the invisibility of subsurface cracks underneath the machined surface makes it difficult to accurately and online evaluate the SSD depth. In the present research, incorporated with the probability statistics of the abrasive heights and the indentation fracture mechanics of the brittle material, a theoretical prediction model was established by investigating the inherent correlation between the measured cutting force of the diamond tool and the maximum depth of the subsurface cracks. Utilizing this predictive method, the SSD depth could be rapidly and precisely calculated through the mechanical properties of the material, the cutting force of the diamond tool, and the geometrical characteristics of the abrasives. To validate the feasibility of prediction technique, the experimental measurements of the maximum SSD depths were compared with the predicted results, revealing the acceptable consistency in their values.

Study on subsurface damage and surface quality of silicon carbide ceramic induced by a novel non-resonant vibration-assisted roll-type polishing

Subsurface damage (SSD) and surface roughness (SR) induced by the finishing process significantly influence the industrial and technological application of optical components. In this paper, a novel non-resonant vibration-assisted roll-type polishing (NVRP) was presented to process the silicon carbide (SiC) ceramic workpiece. The feasibility of the two-dimensional vibration-assisted processing device (2D-VPD) was verified, and the predicted value of SSD was modelled. Besides, the surface deformation and SSD during the polishing process of the SiC workpiece were discussed by numerical simulation. To experimentally verify the proposed polishing method, a set of contrast experiments were conducted on SiC samples to evaluate the effects of different processing conditions on SR and SSD quantitatively. The experimental results further demonstrated a good consistency with the theoretical values in terms of SSD depth, considering the rotational angle of the grain and polishing depth. Also, relatively lower subsurface/surface damages and good surface uniformity were achieved through combining non-resonant vibration-assisted technology and roll-type polishing system. Finally, the comparative experiment of the material removal rate (MRR) between NVRP and non-vibration roll-type polishing (NRP) is conducted.

Effects of polishing parameters on surface quality in sapphire double-sided CMP

To reveal the influence of polishing process parameters on the surface quality of sapphire after double-sided chemical mechanical polishing (CMP), the orthogonal test of sapphire wafer double-sided CMP was carried out by YH2M77110 high-precision vertical double-sided grinding/polishing machine. The effects of polishing parameters, such as polishing pressure(p), polishing carrier rotation speed(vr), and polishing time(t), were investigated regarding their effects on material removal rate (MRR), surface morphology, surface roughness (SR) and subsurface damage (SSD) depth. The experimental results showed that p and vr have the similar and significant positive correlation effects on the MRR and the SSD depth. However, as the increase of p and vr, the SR tends to decrease first and then increase. Prolonging t can effectively reduce the SR and the SSD depth but has a minor positive correlation effect on the MRR. The orthogonal experiment result optimization method based on weight matrix is used to obtain the influence degree of each factor on the orthogonal test index value. The set of optimal process parameter combinations are p = 35.37kPa, vr = 30r/min, t = 50min, which presented a higher MRR, lower SR and SSD depth. The quality and efficiency of sapphire double-sided CMP can be improved through parameter optimization according to this study.

High-Efficiency and Low-Damage Lapping Process Optimization.

The silica opticsare widely applied in the modern laser system, and its fabrication is always the research focus. In the manufacturing process, the lapping process occurs between grinding and final polishing. However, lapping processes optimizations focus on decreasing the depth of sub-surface damage (SSD) or improving lapping efficiency individually. So, the optimum balance point between efficiency and damageshould be studied further. This manuscript establishes the effective removal rate of damage (ERRD)model, and the relationship between the ERRD and processing parameters is simulated. Then, high-efficiency, low-damage lapping processing routine is established based on the simulation. The correctness and feasibility are validated. In this work, the optimized method is confirmed that it can improve efficiency and decrease damage layer depth in the lapping process which promotes the development of optics in low-damage fabrication.

基于概率统计的BK7玻璃磨削亚表层损伤深度在线预测技术

基于概率统计的BK7玻璃磨削亚表层损伤深度在线预测技术

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.