外文翻譯--在新材料精密加工的材料去除機制

在新材料精密加工的材料去除機制 摘要 現(xiàn)代產品的特點是高精密部件。廣泛的材料， 包括金屬及其合金，陶瓷，玻璃和半導體，完成給定的幾何形狀，光潔度，精度和表面完整性，以滿足服務需求。對于先進的技術體系， 較高的制造精度要求是通過使用脆性材料的是比較復雜的。對于高效和這些材料的經濟型加工 ，材料的去除機制的理解是必不可少的。這文章主要所涉及的脆性材料加工不同材料去除機制。 2001由 Elsevier 科學有限公司出版 關鍵字 ：脆 ；缺陷 ；延展性 ；材料去除 ；精密加工 1.介紹 超精密加工技術已經發(fā)展了近幾年為一些工業(yè) 應用，例如激光 ，光學，半導體，航空航天和汽車應用的許多功能成本效益和質量保證的精密零件。精密制造與實現(xiàn)產品的高形狀精度和表面質量。該準確性是在納米級。幾個加工技術可以在這里提到的像金剛石車削 ，磨削，研磨，拋光，珩磨，離子和電子束加工，激光加工等。該過程的效率的概述中給出的參考文獻。 1-3 。 金剛石由于超精密加工技術已經因為它的高精確度和高生產率的工業(yè)用光學， 力學電子元件制造業(yè)的 1980年代已經高度發(fā)達。對于許多先進的技術系統(tǒng)，較高的制造精度由使用的脆性材料的復雜化。在過去的十年里，中興在結 構應用中使用的陶瓷。由于近期發(fā)展的整體實力和先進的陶瓷均勻性優(yōu)良的熱，化學和這些材料的電阻可實現(xiàn) 4 。 陶瓷材料已被廣泛地適于作為功能材料，以及在各種工業(yè)領域中建筑材料及其應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質量不消失必然由陶瓷的陶瓷粉末。由于精密加工的常規(guī)成形和燒結方法得到的成形后，燒結是公認的關鍵技術來制造精密陶瓷部件 6 。 陶瓷材料的精加工過程中除去的量必須非常小，從而使裂紋不會殘留在成品的表面。研磨工藝如磨碎與金剛石磨料研磨已普遍采用陶瓷材料的精密 加工 7-9 。 然而，可以預期更好的表面完整性和更高的生產速率可以通過切割工藝實現(xiàn) 。與其他方法相比，切削也是有利于制造復雜形狀 .脆性材料可分為三組：非晶玻璃，硬晶體組成陶瓷。先進陶瓷是一家現(xiàn)代化的發(fā)展。它們是由形成，鞏固和精確受控制的情況熱處理的良好多孔顆粒制成。否則使用這些材料使高科技設備的開發(fā)和系統(tǒng)根本不能生產 10 。 同樣的情況可以作出有關 使用某些晶體材料（如半導體）和先進的高溫眼鏡。 2. 球墨鑄鐵加工 在加工公差的改善，使研究人員能夠揭露脆性材料的韌性材料去除 。在某些控制的條件下，可 以對機器脆性材料像陶瓷使用單點或多點金剛石工具，使得材料被移除，留下一個無裂紋的表面（圖 4） 。這個過程被稱為韌性政權加工。 韌性政權加工如下一個事實，即所有的材料將塑性變形如果變形非常小。在查看由宮下 17 中描述的，如圖韌性政權加工是另一種方式。 5 。材料的去除速率磨碎和拋光進行比較，并存在其中既不技術已被利用的間隙。這區(qū)域可以被稱為微研磨間隙，因為該區(qū)域位于磨削和切削 .這間隙之間是很重要的，因為它代表了韌性和脆性區(qū)域制度之間的閾值，適用范圍廣的象陶瓷，玻璃和半導體材料。 2.1.韌性材料加工 原理 脆性材料的加工過程中從脆性到韌性模式的轉換中的應變能和表面能 18之間的能量平衡方面進行說明。應用負載時本地化是脆性材料加工的興趣。制造壓痕過程中，這會產生壓痕裂紋，這些裂紋在塑性加工機制發(fā)揮一個很重要作用 19。 一個關鍵的穿透深度為直流裂描述如下 20 Kc 為斷裂韌度， H 是硬度， E 是彈性模量， b 是依賴于工具的幾何形狀。圖。圖 6示出沿垂直于切斷方向該工具的投影。根據(jù)能量平衡概念，斷裂損傷將啟動在有效的切斷深度和將傳播到平均深度 YC 。如果不繼續(xù)損害切面呈平面下方，球狀態(tài)的條件下得以 實現(xiàn)。橫進給量 f 決定直流沿刀尖的位置。 f 的舉動直流較大的值更接近韌轉變現(xiàn)象的工具中心 .另一個解釋是基于解理斷裂是由于時候。 21 。裂解和塑性變形的臨界值是由缺陷 /錯在加工材料的密度影響的。因為缺陷的密度沒有在脆性材料那么大 ，斷裂的臨界值取決于應力場的大小。 圖 7顯示了排屑與尺寸效應的模型。當未切割晶片厚度小，臨界應力場的小，以避免分裂。在芯片結果的過渡 2.2.在韌性加工材料去除機制 加工由兩個配合表面，即在工件和磨料工具的緊密接觸會產生有用的表面。然而，材料去除的微觀結構由材料而異 取決于兩個工件和刀具材料的微觀結構。 通常，在脆性材料的加工精度高，具有大的負前角的工具被使用（高達 -30 ） 。的負前角為使被加工材料的塑性變形的刀具半徑之下所需的靜水壓力。在用單刃刀具切削加工的前角為正或接近 0 因此，工具的變形提前將在濃縮剪切面或在一個狹窄的平面，如圖 8所示。在研磨過程中，人們普遍認為，該工具將有一個大的負前角，也使切削力是大約一半的推力 圖。圖 8（ b ）。在脆性材料在切削深度比刀沿半徑較小的超精密加工中，工具呈現(xiàn)一個大的負前角和刀具邊緣 行為的半徑為如圖所示的壓頭。圖 8（ c）所示。這代表縮進整個工件表面鈍壓頭滑動。這是類似的情況下被牢固地支承在工具和應力，從而產生不平均的通風口但工具下方的材料產生塑性由于大的靜壓力，如圖變形下切割工件。圖 8（ d）所示。 3.材料的去除在玻璃和陶瓷 光學玻璃的延性磨削被認為是一個加工方法最完美適配的材料 22。玻璃是從熔融狀態(tài)冷到固態(tài)無結晶無機材料。眼鏡的非結晶（或無定形）和響應的液體和固體之間的中間 ；即，在常溫下它們的行為在一個脆性的方法，但上述的粘稠方式的玻璃化轉變溫度。玻璃的脆性高是由于原子排 列不規(guī)則。在象金屬的結晶材料，該原子具有一個固定裝置和由密勒指數(shù)描述的規(guī)律性，而玻璃結構沒有顯示出任何明確的取向 23。 陶瓷，例如硬度和強度，化學惰性和高耐磨損性的獨特的物理和機械性能的機械和電子部件提供給其增加的應用程序。先進陶瓷的結構和磨損的應用包括氧化鋁（ Al2O3） ，氮化硅（ Si3N4 ） ，碳化硅（ SiC ） ，氧化鋯（氧化鋯）和塞隆。原子鍵合的性質決定了材料的硬度以及楊氏模量。對于韌性金屬粘合材料的比 E / H 為約 250，而對于脆性材料的比率為約 20 。的比例將位于這些值 硬質合金 材料之間。 低密度和位錯的流動性低的原因是高硬度的一些脆性物料。 4.研磨柔性 有所謂的 “ 溫和 ” 加工，其中據(jù)信，塑性變形是不參與只在材料去除 26另一種假說。根據(jù)這一理論，由于變形（塑性 /脆性）的模式依賴于應力，而不是在應力的大小的狀態(tài)下，也很難認為變形的模式將通過僅僅改變切削深度改變保持所有其他參數(shù)不變。調查表明，為了使脆性材料，以在一個塑性方式變形，相當大的靜液壓力和 /或溫度是必需的。減少切削深度只會降低應力不改變應力狀態(tài)。因此這個理論表明，在切下的深度所產生的表面的優(yōu)良品質，是由于上述的效果，而不一定塑性變形。在更 小的切削深度，裂紋可能形成，但他們可能無法傳播，以形成較大的裂縫。因此，在磨非常小切深可稱為溫和的打磨，而不是延性磨削。 5.材料去除與微 折斷 在對脆性材料的常規(guī)機械加工操作大部分材料是由脆性斷裂去除，從而實現(xiàn)了更高的去除率。圖。圖 10示出壓痕的不同階段。壓頭下方的材料最初經受彈性變形27,28 。作為壓痕的繼續(xù)，下面的材料經受高的靜水壓力，因此非彈性 /塑性變形區(qū)產生的圖。圖 10（ a ） 。在某些時候，變形引起的缺陷發(fā)展成一個中間排氣孔，并隨后可卸圖中發(fā)展成一個位數(shù)裂紋。圖 10（ b ） 。在負荷進一 步增加產生的排氣部的生長與圖第 10（ c ） 。在卸載發(fā)泄開始關閉 圖。第 10（ d ） 。在壓頭切除，側通風口開始啟動下面的聯(lián)系的塑性變形區(qū)的基地附近，展開橫向上飛機接近平行于試樣表面。這是由于殘余拉伸應力場的存在。一旦徹底清除壓頭，側通風口繼續(xù)向試樣表面延伸，并可能最終導致材料去除剝落。裂縫地層通常是由于殘余應力場，這會導致從一個不匹配的彈塑性變形過程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