外文翻译--在新材料精密加工的材料去除机制

在新材料精密加工的材料去除机制 摘要 现代产品的特点是高精密部件。广泛的材料， 包括金属及其合金，陶瓷，玻璃和半导体，完成给定的几何形状，光洁度，精度和表面完整性，以满足服务需求。对于先进的技术体系， 较高的制造精度要求是通过使用脆性材料的是比较复杂的。对于高效和这些材料的经济型加工 ，材料的去除机制的理解是必不可少的。这文章主要所涉及的脆性材料加工不同材料去除机制。 2001由 Elsevier 科学有限公司出版 关键字 ：脆 ；缺陷 ；延展性 ；材料去除 ；精密加工 1.介绍 超精密加工技术已经发展了近几年为一些工业 应用，例如激光 ，光学，半导体，航空航天和汽车应用的许多功能成本效益和质量保证的精密零件。精密制造与实现产品的高形状精度和表面质量。该准确性是在纳米级。几个加工技术可以在这里提到的像金刚石车削 ，磨削，研磨，抛光，珩磨，离子和电子束加工，激光加工等。该过程的效率的概述中给出的参考文献。 1-3 。 金刚石由于超精密加工技术已经因为它的高精确度和高生产率的工业用光学， 力学电子元件制造业的 1980年代已经高度发达。对于许多先进的技术系统，较高的制造精度由使用的脆性材料的复杂化。在过去的十年里，中兴在结 构应用中使用的陶瓷。由于近期发展的整体实力和先进的陶瓷均匀性优良的热，化学和这些材料的电阻可实现 4 。 陶瓷材料已被广泛地适于作为功能材料，以及在各种工业领域中建筑材料及其应用的精密零件也在增加 5。 然而，所需的精密零件的尺寸精度高和良好的表面质量不消失必然由陶瓷的陶瓷粉末。由于精密加工的常规成形和烧结方法得到的成形后，烧结是公认的关键技术来制造精密陶瓷部件 6 。 陶瓷材料的精加工过程中除去的量必须非常小，从而使裂纹不会残留在成品的表面。研磨工艺如磨碎与金刚石磨料研磨已普遍采用陶瓷材料的精密 加工 7-9 。 然而，可以预期更好的表面完整性和更高的生产速率可以通过切割工艺实现 。与其他方法相比，切削也是有利于制造复杂形状 .脆性材料可分为三组：非晶玻璃，硬晶体组成陶瓷。先进陶瓷是一家现代化的发展。它们是由形成，巩固和精确受控制的情况热处理的良好多孔颗粒制成。否则使用这些材料使高科技设备的开发和系统根本不能生产 10 。 同样的情况可以作出有关 使用某些晶体材料（如半导体）和先进的高温眼镜。 2. 球墨铸铁加工 在加工公差的改善，使研究人员能够揭露脆性材料的韧性材料去除 。在某些控制的条件下，可 以对机器脆性材料像陶瓷使用单点或多点金刚石工具，使得材料被移除，留下一个无裂纹的表面（图 4） 。这个过程被称为韧性政权加工。 韧性政权加工如下一个事实，即所有的材料将塑性变形如果变形非常小。在查看由宫下 17 中描述的，如图韧性政权加工是另一种方式。 5 。材料的去除速率磨碎和抛光进行比较，并存在其中既不技术已被利用的间隙。这区域可以被称为微研磨间隙，因为该区域位于磨削和切削 .这间隙之间是很重要的，因为它代表了韧性和脆性区域制度之间的阈值，适用范围广的象陶瓷，玻璃和半导体材料。 2.1.韧性材料加工 原理 脆性材料的加工过程中从脆性到韧性模式的转换中的应变能和表面能 18之间的能量平衡方面进行说明。应用负载时本地化是脆性材料加工的兴趣。制造压痕过程中，这会产生压痕裂纹，这些裂纹在塑性加工机制发挥一个很重要作用 19。 一个关键的穿透深度为直流裂描述如下 20 Kc 为断裂韧度， H 是硬度， E 是弹性模量， b 是依赖于工具的几何形状。图。图 6示出沿垂直于切断方向该工具的投影。根据能量平衡概念，断裂损伤将启动在有效的切断深度和将传播到平均深度 YC 。如果不继续损害切面呈平面下方，球状态的条件下得以 实现。横进给量 f 决定直流沿刀尖的位置。 f 的举动直流较大的值更接近韧转变现象的工具中心 .另一个解释是基于解理断裂是由于时候。 21 。裂解和塑性变形的临界值是由缺陷 /错在加工材料的密度影响的。因为缺陷的密度没有在脆性材料那么大 ，断裂的临界值取决于应力场的大小。 图 7显示了排屑与尺寸效应的模型。当未切割晶片厚度小，临界应力场的小，以避免分裂。在芯片结果的过渡 2.2.在韧性加工材料去除机制 加工由两个配合表面，即在工件和磨料工具的紧密接触会产生有用的表面。然而，材料去除的微观结构由材料而异 取决于两个工件和刀具材料的微观结构。 通常，在脆性材料的加工精度高，具有大的负前角的工具被使用（高达 -30 ） 。的负前角为使被加工材料的塑性变形的刀具半径之下所需的静水压力。在用单刃刀具切削加工的前角为正或接近 0 因此，工具的变形提前将在浓缩剪切面或在一个狭窄的平面，如图 8所示。在研磨过程中，人们普遍认为，该工具将有一个大的负前角，也使切削力是大约一半的推力 图。图 8（ b ）。在脆性材料在切削深度比刀沿半径较小的超精密加工中，工具呈现一个大的负前角和刀具边缘 行为的半径为如图所示的压头。图 8（ c）所示。这代表缩进整个工件表面钝压头滑动。这是类似的情况下被牢固地支承在工具和应力，从而产生不平均的通风口但工具下方的材料产生塑性由于大的静压力，如图变形下切割工件。图 8（ d）所示。 3.材料的去除在玻璃和陶瓷 光学玻璃的延性磨削被认为是一个加工方法最完美适配的材料 22。玻璃是从熔融状态冷到固态无结晶无机材料。眼镜的非结晶（或无定形）和响应的液体和固体之间的中间 ；即，在常温下它们的行为在一个脆性的方法，但上述的粘稠方式的玻璃化转变温度。玻璃的脆性高是由于原子排 列不规则。在象金属的结晶材料，该原子具有一个固定装置和由密勒指数描述的规律性，而玻璃结构没有显示出任何明确的取向 23。 陶瓷，例如硬度和强度，化学惰性和高耐磨损性的独特的物理和机械性能的机械和电子部件提供给其增加的应用程序。先进陶瓷的结构和磨损的应用包括氧化铝（ Al2O3） ，氮化硅（ Si3N4 ） ，碳化硅（ SiC ） ，氧化锆（氧化锆）和塞隆。原子键合的性质决定了材料的硬度以及杨氏模量。对于韧性金属粘合材料的比 E / H 为约 250，而对于脆性材料的比率为约 20 。的比例将位于这些值 硬质合金 材料之间。 低密度和位错的流动性低的原因是高硬度的一些脆性物料。 4.研磨柔性 有所谓的 “ 温和 ” 加工，其中据信，塑性变形是不参与只在材料去除 26另一种假说。根据这一理论，由于变形（塑性 /脆性）的模式依赖于应力，而不是在应力的大小的状态下，也很难认为变形的模式将通过仅仅改变切削深度改变保持所有其他参数不变。调查表明，为了使脆性材料，以在一个塑性方式变形，相当大的静液压力和 /或温度是必需的。减少切削深度只会降低应力不改变应力状态。因此这个理论表明，在切下的深度所产生的表面的优良品质，是由于上述的效果，而不一定塑性变形。在更 小的切削深度，裂纹可能形成，但他们可能无法传播，以形成较大的裂缝。因此，在磨非常小切深可称为温和的打磨，而不是延性磨削。 5.材料去除与微 折断 在对脆性材料的常规机械加工操作大部分材料是由脆性断裂去除，从而实现了更高的去除率。图。图 10示出压痕的不同阶段。压头下方的材料最初经受弹性变形27,28 。作为压痕的继续，下面的材料经受高的静水压力，因此非弹性 /塑性变形区产生的图。图 10（ a ） 。在某些时候，变形引起的缺陷发展成一个中间排气孔，并随后可卸图中发展成一个位数裂纹。图 10（ b ） 。在负荷进一 步增加产生的排气部的生长与图第 10（ c ） 。在卸载发泄开始关闭 图。第 10（ d ） 。在压头切除，侧通风口开始启动下面的联系的塑性变形区的基地附近，展开横向上飞机接近平行于试样表面。这是由于残余拉伸应力场的存在。一旦彻底清除压头，侧通风口继续向试样表面延伸，并可能最终导致材料去除剥落。裂缝地层通常是由于残余应力场，这会导致从一个不匹配的弹塑性变形过程29。 Material removal mechanisms in precision machining of new materials Abstract Modern-day products are characterised by high-precision components. A wide range of materials, includingmetals and their alloys, ceramics, glasses and semiconductors, are finished to a given geometry, finish,accuracy and surface integrity to meet the service requirements. For advanced technology systems, demandsfor higher fabrication precision are complicated by the use of brittle materials. For efficient and economicalmachining of these materials, an understanding of the material removal mechanism is essential. This paperfocuses on the different material removal mechanisms involved in machining of brittle materials. 2001Published by Elsevier Science Ltd. Keywords: Brittle； Defects； Ductility； Material removal； Precision machining 1. Introduction Ultra-precision machining technology has been developed over recent years for the manufactureof cost-effective and quality-assured precision parts for several industrial applications such aslasers, optics, semiconductors, aerospace and automobile applications. Precision manufacturingdeals with the realisation of products with high shape accuracy and surface quality. The accuracymay be at the nanometric level. Several machining techniques can be mentioned here like diamondturning, grinding, lapping, polishing, honing, ion and electron-beam machining, laser machining,etc. Efficient overviews of the processes are given in Refs. 13. Ultra-precision machining technology has been highly developed since the 1980s mainlybecause of its high accuracy and high productivity in the manufacturing of optical, mechanicaland electronic components for industrial use. For many advanced technology systems, higherfabrication precision is complicated by the use of brittle materials. The past decade has seen atremendous resurgence in the use of ceramics in structural applications. The excellent thermal,chemical and wear resistance of these materials can be realised because of recent improvementsin the overall strength and uniformity of advanced ceramics 4. Ceramic materials have been widely adapted as functional materials as well as structuralmaterials in various industrial fields and their application to precision parts is also increasing 5. However, the high dimensional accuracy and good surface quality required for precision parts arenot necessarily obtained by the conventional forming and sintering process of ceramic powders.Thus precision finishing of the ceramics after forming and sintering is recognised as a key technologyto precision ceramic parts 6. The quantity of ceramic material to be removed by the finishing process must be very small,so that microcracks do not remain on the finished surface. Abrasive processes such as grindingor lapping with diamond abrasives have generally been adopted for precision finishing of ceramics79. However, it is expected that better surface integrity and higher production rates can berealised by cutting processes. Compared with other processes, cutting is also advantageous inmachining complex shapes.Brittle materials can be divided into three groups: amorphous glasses, hard crystals andadvanced ceramics. Advanced ceramics are a modern development. They are made from fineporous particles that are formed, consolidated and thermally treated under precisely controlledconditions. Use of these materials enables development of high-technology devices and systemsthat simply could not be produced otherwise 10. The same statement could be made about theuse of certain crystalline materials (e.g., semiconductors) and advanced high-temperature glasses. 2. Ductile regime machining Improvements in machining tolerances have enabled researchers to expose the ductile materialremoval of brittle materials. Under certain controlled conditions, it is possible to machine brittlematerials like ceramics using single- or multi-point diamond tools so that material is removed byplastic flow, leaving a crack-free surface (Fig. 4). This process is called ductile regime machining. Ductile regime machining follows from the fact that all materials will deform plastically if thescale of deformation is very small. Another way of viewing the ductile regime machining problemis that described by Miyashita 17, as shown in Fig. 5. The material removal rates for grindingand polishing are compared and there is a gap in which neither technique has been utilised. Thisregion can be termed the micro-grinding gap since the region lies in between grinding and polishing.This gap is important because it represents the threshold between ductile and brittle grindingregimes for a wide range of materials like ceramics, glasses and semiconductors. 2.1. Principle of ductile regime machining The transition from brittle to ductile mode during machining of brittle materials is described in terms of the energy balance between strain energy and surface energy 18. Localised fracturesproduced during application of load are of interest in machining of brittle materials. Machiningis an indentation process during which indentation cracks are generated, and these cracks play animportant role in ductile regime machining 19. A critical penetration depth dc for fracture initiation is described as follows 20 where Kc is the fracture toughness, H is the hardness, E is the elastic modulus and b is a constantwhich depends on tool geometry. Fig. 6 shows a projection of the tool perpendicular to the cuttingdirection. According to the energy balance concept, fracture damage will initiate at the effectivecutting depth and will propagate to an average depth yc. If the damage does not continue belowthe cut surface plane, ductile regime conditions are achieved. The cross-feed f determines theposition of dc along the tool nose. Larger values of f move dc closer to the tool centreline.Another interpretation of ductile transition phenomena is based on cleavage fracture due to thepresence of defects 21. The critical values of a cleavage and plastic deformation are affectedby the density of defects/dislocations in the work material. Since the density of defects is not solarge in brittle materials, the critical value of fracture depends on the size of the stress field. Fig 7 shows a model of chip removal with size effects. When the uncut chip thickness is small, thesize of the critical stress field is small to avoid cleavage. Consequently a transition in the chip 2.2. Material removal mechanisms in ductile regime machining Machining generates a useful surface by intimate contact of two mating surfaces, namely the workpiece and abrasive tool. However, the micromechanisms of material removal differ from material to material depending upon the microstructure of both workpiece and tool material. Generally, during high-precision machining of brittle materials, tools having large negative rake angles are used (as high as -30). The negative rake angle provides the required hydrostatic pressure for enabling plastic deformation of the work material beneath the tool radius. During conventional machining with a single-point tool, the rake angle will be positive or close to 0.With positive rake angle, the cutting force will generally be twice the thrust force. Hence the deformation ahead of the tool will be in a concentrated shear plane or in a narrow plane as shown in Fig. 8. During the grinding process, it is generally agreed that the tool will have a large negative rake angle and also that the cutting force is about half of the thrust force Fig. 8(b). In ultraprecision machining of brittle materials at depths of cut smaller than the tool edge radius, the tool presents a large negative rake angle and the radius of the tool edge acts as an indenter as shown in Fig. 8(c). This represents indentation sliding of a blunt indenter across the workpiece surface. This is similar to a situation where the tool is rigidly supported and cuts the workpiece under a stress such that no median vents are generated but the material below the tool is plastically deformed due to large hydrostatic pressure as in Fig. 8(d). 3. Material removal in glass and ceramics The ductile grinding of optical glass is considered as the most perfect adaptation of a machining method to the material 22. Glass is an inorganic material supercooled from the molten state to the solid state without crystallising. Glasses are non-crystalline (or amorphous) and respond intermediate between a liquid and a solid； i.e., at room temperature they behave in a brittle manner 1838 P.S. Sreejith, B.K.A. Ngoi / International Journal of Machine Tools & Manufacture 41 (2001) 18311843 but above the glass transition temperature in a viscous manner. The high brittleness of glass is due to the irregular arrangement of atoms. In crystalline materials like metals, the atoms have a fixed arrangement and regularity described by Miller indices, whereas glass structure does not show any definite orientation 23. The unique physical and mechanical properties of ceramics such as hardness and strength,chemical inertness and high wear resistance have contributed to their increased application in mechanical and electrical components. The advanced ceramics for structural and wear applications include alumina (Al2O3), silicon nitride (Si3N4), silicon carbide (SiC), zirconia (ZrO2) and SiAlON. The nature of atomic bonding determines the hardness of the material as well as the Youngs modulus. For ductile metallic-bonded materials the ratio E/H is about 250, while for covalentbonded brittle materials the ratio is about 20. The ratio will lie in between these values for ionicbonded materials. Low density and low mobility of dislocations are the reasons for the high hardness of some of brittle materials. 4. Gentle grinding There is an alternative hypothesis called “gentle” machining wherein it is believed that plastic deformation is not involved exclusively in the material removal 26. According to this theory, since the mode of deformation (plastic/brittle) depends on the state of the stress and not on the magnitude of the stress, it is difficult to assume that the mode of deformation will change by merely changing the depth of cut keeping all other parameters constant. Investigations have shown that, in order for brittle materials to deform in a ductile manner, considerable hydrostatic stress and/or temperature are required. Reducing the depth of cut will merely decrease the stress without changing the stress state. Therefore this theory suggests that the superior quality of the surface produced at lower depth of cut is due to the above effect and not necessarily to plastic deformation. At smaller depths of cut, microcracks may be formed but they may not propagate to form larger cracks. Hence grinding at extremely small depth of cut can be called gentle grinding rather than ductile grinding. 5. Material removal with microfracture During conventional machining operations on brittle materials most of the material is removed by brittle fracture, enabling higher removal rates. Fig. 10 shows the various stages of indentation. The material below the indenter is initially subjected to elastic deformation 27,28. As indentation continues, the material below is subjected to high hydrostatic pressure and hence an inelastic/plastic deformation zone is produced Fig. 10(a). At some point, a deformation-induced flaw develops into a median vent and subsequently can develop into a median crack during unloading Fig. 10(b). Further increase in load produces growth of the vent as in Fig. 10(c). On unloading the vent begins to close Fig. 10(d). During indenter removal, lateral vents begin to initiate near the base of the plastic deformation zone below the contact and spread out laterally on a plane closely parallel to the specimen surface. This is due to the presence of a residual tensile stress field. Upon complete removal of the indenter, the lateral vents continue to extend towards the specimen surface and may finally lead to material removal by chipping. Crack forma-tion is generally due to the residual stress field, which results from a mismatch in the elasticplastic deformation process 29. 6. Material removal without microfracture It is well known that the extent of plastic deformation is determined by the magnitude of the hydrostatic stress. Under high hydrostatic pressures brittle materials are capable of ductile behaviour 30 at room temperatures. Such a condition exists at light loads under the indenter in indentation testing. Immediately below the indenter, the material is assumed to behave as a radially expanding core exerting uniform hydrostatic pressure on its surroundings, encasing the core in an ideally plastic region. Beyond the plastic region lies the elastic matrix 31. Fig. 11 shows a model for elasticplastic inde