Surface stresses of mixed-mode grinding materials on borosilicate glass

Mixed-mode grinding occurs when a bound abrasive works in both brittle and ductile regimes simultaneously. Substrates ground in a mixed-mode behavior show reduced curvature induced by compressive surface forces than loose abrasives as demonstrated by observing the Twyman effect. This reduction in bending corresponds to reduced subsurface damage. This is verified by controlled acid etching, which shows the exponential decay of the compressive force per unit length. Loose abrasive particles, added to maintain pad wear due to low pressures, have no effect on the measured stresses. If loose abrasive wear ceases, the pads glaze. Glazing creates near-specular surfaces while reducing measurable stress. These effects for borosilicate glass and Trizact grinding pads are explored and quantified.     

Grinding process for beveling and lapping operations in lens manufacturing

A grinding process that uses loose abrasives for the beveling of lenses is presented. Determination of the parameters of grinding tools with loose abrasives for beveling applications with various optical elements is discussed. The process of grinding with loose abrasives for a lapping operation is analyzed by examination of the influence of optical glass material parameters on material removal and surface roughness for lens manufacturing conditions. The model established for this analysis uses the concept of lateral fracture, which is based on removal of optical glass material by rolling abrasive particles. The particles remove material by lateral cracking. The abrasive mineral Barton Garnet was used in the lapping experiments. Under specific large-diameter lens manufacturing conditions, lapping time values at the conventional removal depth have been obtained for various optical glasses.     

Surface microroughness of optical glasses under deterministic microgrinding

Deterministic microgrinding of precision optical components with rigid, computer-controlled machining centers and high-speed tool spindles is now possible on a commercial scale. Platforms such as the Opticam systems at the Center for Optics Manufacturing produce convex and concave spherical surfaces with radii from 5 mm to ∞, i.e., planar, and work diameters from 10 to 150 mm. Aspherical surfaces are also being manufactured. The resulting specular surfaces have a typical rms microroughness of 20 nm, 1 μm of subsurface damage, and a figure error of less than 1 wave peak to valley. Surface roughness under deterministic microgrinding conditions (fixed infeed rate) with bound abrasive diamond ring tools with various degrees of bond hardness is correlated to a material length scale, identified as a ductility index, involving the hardness and fracture toughness of glasses. This result is in contrast to loose abrasive grinding (fixed nominal pressure), in which surface microroughness is determined by the elastic stiffness and the hardness of the glass. We summarize measurements of fracture toughness and microhardness by microindentation for crown and flint optical glasses, and fused silica. The microindentation fracture toughness in nondensifying optical glasses is in good agreement with bulk fracture toughness measurement methods.     

Loose abrasive lapping hardness of optical glasses and its interpretation

We present an interpretation of the lapping hardness of commercially available optical glasses in terms of a micromechanics model of material removal by subsurface lateral cracking. We analyze data on loose abrasive microgrinding, or lapping at fixed nominal pressure, for many commercially available optical glasses in terms of this model. The Schott and Hoya data on lapping hardness are correlated with the results of such a model. Lapping hardness is a function of the mechanical properties of the glass: The volume removal rate increases approximately linearly with Young's modulus, and it decreases with fracture toughness and (approximately) the square of the Knoop hardness. The microroughness induced by lapping depends on the plastic and elastic properties of the glass, depending on abrasive shape. This is in contrast to deterministic microgrinding (fixed infeed rate), where it is determined from the plastic and fracture properties of the glass. We also show that Preston's coefficient has a similar dependence as lapping hardness on glass mechanical properties, as well as a linear dependence on abrasive size for the case of brittle material removal. These observations lead to the definition of an augmented Preston coefficient during brittle material removal. The augmented Preston coefficient does not depend on glass material properties or abrasive size and thus describes the interaction of the glass surface with the coolant-immersed abrasive grain and the backing plate. Numerical simulations of indentation are used to locate the origin of subsurface cracks and the distribution of residual surface and subsurface stresses, known to cause surface (radial) and subsurface (median, lateral) cracks.     

旋转超声磨削Si3 N4陶瓷用金刚石磨具磨损行为研究

热压烧结Si₃N₄陶瓷材料常应用于航天飞行器中关键耐高温零部件,但由于高硬度和低断裂韧性,其加工效率和加工表面质量难以满足制造需求。为了提高热压烧结Si₃N₄陶瓷旋转超声磨削加工质量,减小由于金刚石磨具磨损带来的加工误差,开展了磨具磨损行为研究。基于热压烧结Si₃N₄陶瓷旋转超声磨削加工实验,分析了金刚石磨具磨损形式;基于回归分析建立了金刚石磨具磨损量数学模型,揭示了加工参数及磨具参数与金刚石磨具磨损量间映射关系;并研究了磨损形式与磨具磨损量及加工表面粗糙度影响规律。结果表明：磨粒磨耗是旋转超声磨削Si₃N₄陶瓷用金刚石磨具最主要磨损形式,比例超过50%;主轴转速和磨粒粒度对磨具磨损量影响最为显著;且磨损量较小时,加工表面粗糙度值反而增加。以上研究可为提高旋转超声磨削Si₃N₄陶瓷加工精度和加工质量提供指导。     

Wear of diamond grinding wheels and material removal rate of silicon nitrides under different machining conditions

Studies have been carried out to determine the influence of depths of cut of diamond grinding wheels on the wear of diamond abrasives and the material removal rate of silicon nitride during single pass grinding. The results suggest that there is an optimum depth of cut, which yields the greatest material removal rate. If the grinding depth is less than this optimum depth of cut, the grinding efficiency is low. If the grinding depth is larger than this optimum depth of cut, premature dulling of the diamond wheel occurs. Furthermore, some of the diamond abrasives break due to grain fracture, and the original sharp diamond abrasives become flat and smooth. As a result, the material removal rate of silicon nitride drastically decreases.     

Surface integrity and removal mechanism in grinding sapphire wafers with novel vitrified bond diamond plates

In order to improve machining efficiency of sapphire wafer machining using the conventional loose abrasive process, fixed-abrasive diamond plates are investigated in this study for sapphire wafer grinding. Four vitrified bond diamond plates of different grain sizes (40?µm, 20?µm, 7?µm, and 2.5?µm) are developed and evaluated for grinding performance including surface roughness, surface topography, surface and subsurface damage, and material removal rate (MRR) of sapphire wafers. The material removal mechanisms, wafer surface finish, and quality of the diamond plates are also compared and discussed. The experiment results demonstrate that the surface material is removed in brittle mode when sapphire wafers are ground by the diamond plates with a grain size of 40?µm and 20?µm, and in ductile mode when that are ground by the diamond plates of grain sizes of 7?µm and 2.5?µm. The highest MRR value of 145.7?µm/min is acquired with the diamond plate with an abrasive size of 40?µm and the lowest surface roughness values of 3.5?nm in Ra is achieved with the 2.5?µm size.     

Residual stresses in surface layers of carbide tool tips after diamond grinding

X-ray diffraction measurements were used to study residual stresses in the surface layers of WC-Co specimens after diamond wheel grinding. The presence of compressive macrostresses was established and the mechanism of their formation was considered. Data relating to grinding with diamond and other abrasive wheels were compared.     

Feasibility study of a flexible grinding method for precision machining of the TiAl-based alloy

This study presents detailed experimental investigations on precision machining of the TiAl-based alloy with an abrasive belt flexible grinding method. Subsequently, the feasibility of this precision machining method is evaluated with respect to the material removal rate, abrasive wear, machined surface roughness, and residual stress. The material removal rate and surface roughness were determined as experimental indicators and were measured via a three-coordinate measuring instrument and surface profiler, respectively. Micro-morphologies of the machined surface and worn abrasive belt were investigated via a scanning electron microscope. The residual stress distributions in the machined surface layer were detected by using an X-ray diffractometer. The experimental results revealed that the aforementioned evaluation indicators satisfied the desired requirements, thereby indicating that the abrasive belt flexible grinding technique was suitable for precision machining of the TiAl-based alloy. Additionally, the optimal combinations of grinding parameters were determined to obtain desirable material removal rate and machined surface roughness. The basic wear processes and characteristics of the abrasive belt were thoroughly examined. The formation of desirable residual compressive stresses in the machined surface layer was mainly attributed to low frequency and small amplitude vibration knocking at the grinding interface.     

纳米磨削过程中加工表面形成与材料去除机理的分子动力学仿真

目前，以线性断裂力学为基础的加工理论对解释微切削加工机理还存在不足．分子动力学分析的方法在研究纳米尺度或原子尺度下的固体变形方面具有独特的优势；辅以压痕挤压机理分析，解释纳米磨真削过程中加工表面形成和材料去除机理．研究表明：静水压力对非结晶变形程度影响很大；晶格重构原子与一部分非晶层原子堆积在磨粒的前上方。由于磨粒不断前移，最终形成磨屑而实现材料去除，处在磨粒前下方的非晶层原子在压应力的作用下与已加工表层断裂的原子键结合重构形成已加工表面变质层；变质层由内外两层组成，外层是非晶层，内层是晶格变形层。     

Imaging of surface strain and polishing artefacts in fused silica by environmental scanning electron microscopy

The recently developed application of charge contrast imaging (CCI), available in variable pressure or environmental scanning electron microscopes (ESEM), has been found to provide images of near surface strain around micro-indentations in fused silica glass. Results suggest this strain contrast information is derived from within a few nanometres of the material surface, making CCI an invaluable tool for the study of nanometre scale surface deformation. Images of indentation strain have also been imaged using backscattered electron (BSE) imaging. The CCI technique has also been applied to the study of fused silica surfaces polished by chemically active polishing abrasives. In the samples studied, CCI provides unique images of linear defects residing below layers of chemically adhered polishing compound. Visualisation of surface strain on sub-nm rms glasses provides supporting evidence for plastic (permanent) deformation of the glass surface at the nanometre level during polishing with certain abrasives and for chemical interactions between the polishing abrasive and glass surface.     

金刚石刀具单点切削单晶硅加工表面特性

本文利用超精密机床对单晶硅进行了斜切及车削实验,采用拉曼光谱仪测量单晶硅材料切削表面的损伤,利用高斯和洛伦兹分布拟合拉曼光谱得到单晶硅表面相变层厚度及残余应力信息．结果表明,单晶硅切削表面与磨削、纳米划擦表面不同,除了非晶相外,测不到其他高压相．随着切削厚度的增大,单晶硅表面非晶层的厚度和表层的残余应力也会相应增加．较大的切削厚度使得残余应力变得不均匀,最终导致单晶峰退简并分裂成2个或3个峰．一般单晶硅的塑性车削生成表面由于切削厚度相对较小,其表面非晶层相对较薄,表面存在轻微残余压应力．当车削生成表面的切削厚度较大时,表面有脆性凹坑,非晶层相对较厚,残余压应力较大．车削过程是对已加工表面的切削,由于已加工表面非晶硅的存在,采用较高的切削速度可以增加切削区域温度,提高单晶硅表面非晶层的塑性,可加工出更好的光学表面．     

The Nature of Mechanically Polished Surfaces of Copper

Surfaces of copper polished with diamond abrasives were examined by transmission electron microscopy; surfaces abraded on silicon carbide papes were also studied for comparison. There was no evidence of an amorphous layer, known as the Beilby layer, on any of the polished surfaces; the surface layers were crystalline and all showed evidence of plastic deformation. Slab-shaped cells were present at the surfaces that appear to correspond to the microbands or shear bands that have been observed in heavily cold-rolled copper. Some recrystallization occurred in the surfaces polished with 6-μm diamond abrasive, indicating some relaxation and modification of the microband-shear band structure; small subgrains were also observed, and it is concluded that they were also recrystallized grains. The degree of deformation at the surface decreased with increasing fineness of polish (i.e., from 6-μm diamond abrasive to 1 μm). The abraded surfaces consisted almost entirely of small subgrains, indicating that the shear band structure at these surfaces had nearly all recrystallized. It was concluded, however, that the highest surface temperature was attained when polishing with 6-μm diamond abrasive and then was only 100-150°C.     

界面反应对Cu35Ni25Co25Cr15多主元合金/金刚石复合材料磨损性能的影响

金刚石超硬磨具在高端芯片加工、3C陶瓷等领域发挥的作用日益重要,粘结相与金刚石的界面结合情况在很大程度上影响了金刚石超硬复合材料的力学和磨损性能。为了研究粘结相和金刚石的界面结合情况,采用放电等离子烧结方法制备了Cu₃₅Ni₂₅Co₂₅Cr₁₅多主元合金/金刚石复合材料,通过热力学计算和实验研究了粘结相和金刚石颗粒的界面反应。结果表明：烧结过程中,金属粘结相中的Cr元素与金刚石在界面处发生了化学反应,生成Cr-C化合物,且Cr-C化合物层的厚度随着烧结温度的升高而增加。当烧结温度达到950℃时,Cr-C化合物反应层均匀连续,厚度大约为1.1μm。复合材料粘结相与金刚石颗粒的粘结系数随着Cr-C化合物层厚度的增加而增大。摩擦磨损测试表明,在900℃和950℃烧结的样品表面,粘结相在摩擦过程中首先被磨除,金刚石随后露出,而Cr-C界面反应层有助于保持对金刚石颗粒的把持能力,提高复合材料的磨削性能。因此,适当的界面反应可提升金刚石复合材料的服役性能。     

Influence of thermal treatment conditions on two-dimensional crystal growth of nanocrystal corundum abrasives

The effect of thermal treatment on mechanical property and microstructure evolution in two-dimension of nanocrystal corundum abrasives has been investigated. The abrasives with and without presence of bond were calcined at different thermal treatment temperatures. The results indicated that thermal treatment conditions had obvious influence on crystal size and mechanical strength of corundum abrasives. Crystal size increased with increasing of the calcining temperature, and crystal growth of sample with presence of bond was more remarkable than that without presence of bond. And it was found that 950 °C is the critical temperature of abnormal crystal growth of corundum abrasives in the heated treatment conditions. Beyond this critical temperature, crystal size evidently increased while strength and wear resistance of abrasive decreased. In this case, the degradation of the abrasive in microstructure and strength should be important considerations for the preparation of ceramic corundum abrasive grinding tool.     

Subsurface damage and microstructure development in precision microground hard ceramics using magnetorheological finishing spots

We demonstrate the use of spots taken with magnetorheological finishing (MRF) for estimating subsurface damage (SSD) depth from deterministic microgrinding for three hard ceramics: aluminum oxynitride (Al(23)O(27)N(5)/ALON), polycrystalline alumina (Al(2)O(3)/PCA), and chemical vapor deposited (CVD) silicon carbide (Si(4)C/SiC). Using various microscopy techniques to characterize the surfaces, we find that the evolution of surface microroughness with the amount of material removed shows two stages. In the first, the damaged layer and SSD induced by microgrinding are removed, and the surface microroughness reaches a low value. Peak-to-valley (p-v) surface microroughness induced from grinding gives a measure of the SSD depth in the first stage. With the removal of additional material, a second stage develops, wherein the interaction of MRF and the material's microstructure is revealed. We study the development of this texture for these hard ceramics with the use of power spectral density to characterize surface features.     

On theoretical substantiation of the choice of effective grain profile shape in modeling of diamond layer in dressing tools. Part 1. Electroplating

The paper gives theoretical substantiation of the phenomenon of a circular shape of a scratch cross-section averaged over a set of cuts produced by far-from-round tips of diamond grains of electroplated dressing tools that work on the abrasive material of grinding wheels. This cross-sectional shape is identified as an effective shape of the grain that produces the scratch. Information on the scratch parameters and their relationship with the diamond grain size in the tool is needed for calculating individual and total cross-sections of cuts, dressing forces, and surface roughness of workpieces upon grinding with abrasive wheels dressed by a diamond roll.     

Experiments and observations regarding the mechanisms of glass removal in magnetorheological finishing

Recent advances in the study of the magnetorheological finishing (MRF) have allowed for the characterization of the dynamic yield stress of the magnetorheological (MR) fluid, as well as the nanohardness (H(nano)) of the carbonyl iron (CI) used in MRF. Knowledge of these properties has allowed for a more complete study of the mechanisms of material removal in MRF. Material removal experiments show that the nanohardness of CI is important in MRF with nonaqueous MR fluids with no nonmagnetic abrasives, but is relatively unimportant in aqueous MR fluids or when nonmagnetic abrasives are present. The hydrated layer created by the chemical effects of water is shown to change the way material is removed by hard CI as the MR fluid transitions from a nonaqueous MR fluid to an aqueous MR fluid. Drag force measurements and atomic force microscope scans demonstrate that, when added to a MR fluid, nonmagnetic abrasives (cerium oxide, aluminum oxide, and diamond) are driven toward the workpiece surface because of the gradient in the magnetic field and hence become responsible for material removal. Removal rates increase with the addition of these polishing abrasives. The relative increase depends on the amount and type of abrasive used.     

Empirical approach to develop a multilayer icebonded abrasive polishing tool for ultrafine finishing of Ti-6Al-4V alloy

This paper covers the development of a multilayer icebonded abrasive polishing (IBAP) tool for multistage polishing of Ti-6Al-4V alloy specimens based on a systematic study that determined the number of layers, thickness of each layer, and the type, size and concentration of abrasives in each layer. Based on this study, a three-layered IBAP tool with the bottom layer consisting of soft aluminum oxide abrasives of 3?µm size with 5% concentration, the middle layer with moderately hard silicon carbide abrasives of 8?µm size with 10% concentration and the top layer with hard boron carbide abrasives of 15?µm size with 30% concentration was formulated for ultrafine finishing of Ti-6Al-4V alloy specimen in a single setup. The performance of the three-layered IBAP tool assessed in terms of finish and morphology over the work surface showed 81% improvement in surface finish, showing its effectiveness over a single-layered IBAP tool. Polishing studies have clearly demonstrated the generation of ultrafine surfaces, yielding a finish of 37?nm while the morphological studies on the polished surface have shown a nearly scratch-free surface on the Ti-6Al-4V alloy.     

Effect of substrate pretreatment on diamond deposition in a microwave plasma

The evolution of substrate surface at an early stage of diamond formation in a microwave plasma was studied with a high-resolution scanning electron microscope. Changes in the shape, size and population of diamond particles at the same points were observed at prescribed time intervals. The substrate used was a mirror-polished Si (100) plate which was ultrasonically pretreated with diamond, c-BN or -Al₂O₃ powders prior to the deposition. The pretreatment introduced fragments of the abrasives as well as many scratches on the substrate surface. When the diamond and c-BN abrasive were used, diamond was formed on the surface of abrasive residues. With -Al₂O₃ abrasive powder, on the other hand, residues vanished in the plasma and no deposition was observed. These results suggest that the deposition site of diamond from the vapour phase is dependent on the type of abrasive powder used for substrate pretreatment.