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Lapping Lapping is a finishing operation used on flat and cylindrical surfaces. The lap, shown in Fig.9.1a, is usually made of cast iron, copper, leather, or cloth.The abrasive particles are embedded in the lap, or they may be carried through slurry. Depending on the hardness of the workpiece, lapping pressures range from 7kPa to 140kPa (1 to 20 psi). Lapping has two main functions. Firstly, it produces a superior surface finish with all machining marks being removed from the surface. Secondly, it is used as a method of obtaining very close fits between mating parts such as pistons and cylinders. The lapped workpiece surface may look smooth but it is actually filled with microscopic peaks, valleys, scratches and pits. Few surfaces are perfectly flat. Lapping minimizes the surface irregularities, thereby increasing the available contact area. The drawing in Fig.9.1a shows two surfaces. The upper one is how a surface might look before lapping and the lower one after lapping. Lapping removes the microscopic mountain tops and produces relatively flat plateaus. Entire microscopic mountain ranges may need to be ground down in order to increase the available contact area. Production lapping on flat or cylindrical pieces is done on machines such as those shown in Fig.9.1b and 9.1c. Lapping is also done on curved surfaces, such as spherical objects and lenses, using specially shaped laps. Polishing Polishing is a process that produces a smooth, lustrous surface finish. Two basic mechanisms are involved in the polishing process: (a) fine-scale abrasive removal, and (b) softening and smearing of surface layers by frictional heating during polishing.Electropolishing Electropolishing is an electrochemical process similar to, but the reverse of, electroplating. The electropolishing process smoothes and streamlines the microscopic surface of a metal object. Mirror-like finishes can be obtained on metal surfaces by electropolishing. In electropolishing, the metal is removed ion by ion from the surface of the metal object being polished. Electrochemistry and the fundamental principles of electrolysis (Faradays Law) replace traditional mechanical finishing techniques. In basic terms, the object to be electropolished is immersed in an electrolyte and subjected to a direct electrical current. The object is maintained anodic, with the cathodic connection being made to a nearby metal conductor. Smoothness of the metal surface is one of the primary and most advantageous effects of electropolishing. During the process, a film of varying thickness covers the surface of the metal. This film is thickest over micro depressions and thinnest over micro projections. Electrical resistance is at a minimum wherever the film is thinnest, resulting in the greatest rate of metallic dissolution. Electropolishing selectively removes microscopic high points or “peaks” faster than the rate of attack on the corresponding micro-depressions or “valleys”. Stock is removed as metallic salt. Metal removal under certain circumstances is controllable and can be held to 0.0001 to 0.0025 mm.Chemical Mechanical Polishing Chemical mechanical polishing is becoming an increasingly important step in the fabrication of multi-level integrated circuits. Chemical mechanical polishing refers to polishing by abundant slurry that interacts both chemically and mechanically with the surface being polished.During the chemical mechanical polishing process, a rotating wafer is pressed face down onto a rotating, resilient polishing pad while polishing slurry containing abrasive particles and chemical reagents flows in between the wafer and the pad.The combined action of polishing pad, abrasive particles and chemical reagents results in material removal and polishing of the wafer surface. Chemical mechanical polishing creates flat, damage-free on a variety of brittle materials and it is used extensively on silicon wafers in the manufacture of integrated circuits. Chemical mechanical polishing is a complicated multiphase process. It mainly includes the following two dynamics. First, the active component in polishing slurry reacts with the atoms of the wafer, and the process is chemical reaction step with oxidation-reductive reaction. The second step is the process of desorption, that is to say, the resultants gradually separate from the wafer surface and new surface is exposed to polishing slurry. If chemical reactive rate is smaller, the total removal rate of the wafer is also small; furthermore, the surface degree of finish is not good.On the contrary, even if chemical reaction is very rapid, but desorption is very slow, the total removal rate is not good. Because resultants connot separate from the wafer surface, the active component in the polishing slurry cannot expose and react with the atoms on the new surface, which holds up chemical reaction. The balance and compositive effects of two steps decide the total removal rate and its surface degree of finish. The processes of surface engineering, or surface treatments, tailor the surfaces of engineering materials to: (1) control friction and wear, (2) improve corrosion resistance, (3) change physical property, e.g., conductivity, resistivity, and reflection, (4) alter dimension, (5) vary appearance, e.g., color and roughness, (6) reduce cost. Common surface treatments can be divided into two major categories: treatments that cover the surfaces and treatments that alter the surfaces. Covering the Surface The treatments that cover the surfaces include organic coatings and inorganic coatings. The inorganic coatings perform electroplatings, conversion coatings, thermal sprayings, hot dippings, furnace fusings, or coat thin films, glass, ceramics on the surfaces of the materials. Electroplating is an electrochemical process by which metal is deposited on a substrate by passing a current through the bath. Usually there is an anode (positively charged electrode), which is the source of the material to be deposited; the electrochemistry which is the medium through which metal ions are exchanged and transferred to the substrate to be coated; and a cathode (negatively charged electrode) which is the substrate to be coated. Plating is done in a plating bath which is usually a non-metallic tank (usually plastic). The tank is filled with electrolyte which has the metal, to be plated, in ionic form. The anode is connected to the positive terminal of the power supply. The anode is usually the metal to be plated (assuming that the metal will corrode in the electrolyte). For ease of operation, the metal is in the form of nuggets and placed in an inert metal basket made out non-corroding metal (such as titanium or stainless steel). The cathode is the workpiece, the substrate to be plated. This is connected to the negative terminal of the power supply. The power supply is well regulated to minimize ripples as well to deliver a steady predictable current, under varying loads such as those found in plating tanks. As the current is applied, positive metal ions from the solution are attracted to the negatively charged cathode and deposit on the cathode. As a replenishment for these deposited ions, the metal from the anode is dissolved and goes into the solution and balances the ionic potential. Thermal spraying process. Thermal spraying metal coatings are depositions of metal which has been melted immediately prior to projection onto the substrate. The metals used and the application systems used vary but most applications result in thin coatings applied to surfaces requiring improvement to their corrosion or abrasion resistance properties. Thermal spray is a generic term for a broad class of related processes in which molten droplets of metals, ceramics, glasses, and/or polymers are sprayed onto a surface to produce a coating, to form a free-standing near-net-shape, or to create an engineered material with unique properties. In principle, any material with a stable molten phase can be thermally sprayed, and a wide range of pure and composite materials are routinely sprayed for both research and industrial applications. Deposition rates are very high in comparison to alternative coating technologies. Deposit thickness of 0.1 to 1mm is common, and thickness greater than 1cm can be achieved with some materials. The process for application of thermal spray metal is relatively simple and consists of the following stages.(1) Melting the metal at the gun.(2) Spraying the liquid metal onto the prepared substrate by means of compressed air. (3) Molten particles are projected onto the cleaned substrate. There are two main types of wire application available today namely arc spray and gas spray. ARCA pair of wires are electrically energized so that an arc is struck across the tips when brought together through a pistol. Compressed air is blown across the arc to atomise and propel the autofed metal wire particles onto the prepared workpiece. GASIn combustion flame spraying the continuously moving wire is passed through a pistol, melted by a conical jet of burning gas. The molten wire tip enters the cone, atomises and is propelled onto the substrate. Thin-Film Coatings. Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) are two most common types of thin-film coating methods. PVD coatings involve atom-by-atom, molecule-by-molecule, or ion deposition of various materials on solid substrates in vacuum systems. Thermal evaporation uses the atomic cloud formed by the evaporation of the coating metal in a vacuum environment to coat all the surfaces in the line of sight between the substrate and the target. It is often used in producing thin, 0.5m, decorative shiny coatings on plastic parts. The thin coating, however, is fragile and not good for wear applications. The thermal evaporation process can also coat a very thick, 1mm, layer of heat-resistant materials, such as MCrAIYa metal, chromium, aluminum, and yttrium alloys, on jet engine parts. Sputtering applies high-technology coatings such as ceramics, metal alloys, organic and inorganic compounds by connecting the workpiece and the substance to a high-voltage DC power supply in an argon vacuum system.The plasma is established between the substrate (workpiece) and the target (donor) and transposes the sputtered off target atoms to the surface of the substrate.When the substrate is non-conductive, e.g., polymer, a radio-frequency (RF) sputtering is used instead. Sputtering can produce thin, less than 3m (120in), hard thin-film coatings, e.g., titanium nitride (TIN) which is harder than the hardest metal. Sputtering is now widely applied on cutting tools, forming tools, injection molding tools, and common tools such as punches and dies, to increase wear resistance and service life. CVD is capable of producing thick, dense, ductile, and good adhesive coatings on metals and non-metals such as glass and plastic. Contrasting to the PVD coating in the “line of sight”, the CVD can coat all surfaces of the substrate. Conventional CVD coating process requires a metal compound that will volatilize at a fairly low temperature and decompose to a metal when it contacts with the substrate at higher temperature.The most well known example of CVD is the nickel carbonyl (NiCO4) coating as thick as 2.5mm (0.1in.) on glass windows and containers to make them explosion or shatter resistant. Diamond CVD coating process is introduced to increase the surface hardness of cutting tools. However, the process is done at the temperatures higher than 700 (1300) which will soften most tool steel. Thus, the application of diamond CVD is limited to materials which will not soften at this temperature such as cemented carbides. Plasma-Assisted CVD coating process can be performed at lower temperature than diamond CVD coatings. This CVD process is used to apply diamond coatings or silicon carbide barrier coatings on plastic films and semiconductors, including the state of the art 0.25m semiconductors. Altering the Surfaces The treatments that alter the surfaces include hardening treatments, high-energy processes and special treatments. High-energy processes are relatively new surface treatment methods. They can alter the properties of surfaces without changing the dimension of the surface. Common high-energy processes, including electron beam treatment, ion implantation, and laser beam treatment, are briefly discussed as follows: Electron beam treatment. Electron beam treatment alters the surface properties by rapid heatingusing electron beam and rapid coolingin the order of 106/see in a very shallow region, 100m, near the surface. This technique can also be used in hardfacing to produce “surface alloys”. Ion implantation. Ion implantation uses electron beam or plasma to impinge gas atoms to ions with sufficient energy, and embed these ions into atomic lattice of the substrate, accelerated by magnetic coils in a vacuum chamber. The mismatch between ion implant and the surface of a metal creates atomic defects that harden the surface. Laser beam treatment. Similar to electron beam treatment, laser beam treatment alters the surface properties by rapid heating and rapid cooling in a very shallow region near the surface. It can also be used in hardfacing to produce “surface alloys”. The results of high-energy processes are not well known or very well controlled. But the preliminary results look promising. Further development is needed in high-energy processes, especially in implant dosages and treatment methods.It has already been stated that the workpiece must be located relative to the cutting tool, and be secured in that position. After the workpiece has been marked out, it is still necessary to position it with respect to the machine movements, and to clamp it in that position before machining is started. When several identical workpieces are to be produced the need to mark out each part is eliminated by the use of jigs and fixtures, but if a casting or forging is involved, a trial workpiece is marked out, to ensure that the workpiece can be produced from it, and to ensure that ribs, cores, etc. have not become misplaced. Jigs and fixtures are alike in that they both incorporate devices to ensure that the workpiece is correctly located and clamped, but they differ in that jigs incorporate means of tool guiding during the actual cutting operation, and fixtures do not. In practice, the only cutting tools that can be guided while actually cutting are drills, reamers, and similar cutters; and so jigs are associated with drilling operations, and fixtures with all other operations. Fixtures may incorporate means of setting the cutting tools relative to the location system. The advantages of jigs and fixtures can be summarised as follows: 1)Marking out and other measuring and setting out methods are eliminated; 2)Unskilled workers may proceed confidently and quickly in knowledge that the workpiece can be positioned correctly, and the tools guided or set; 3)the assembly of parts is facilitated, since all components will be identical within small limits, and “trying” and filing of work is eliminated; 4)The parts will be interchangeable, and if the product sold over a wide area, the problem of spare parts will be simplified. Bolt holes often have 1.5mm or even 3.0mm clearance for the bolt, and the reader may doubt the necessity of making precision jigs for such work. It must be remembered that the jigs, once made, will be used on many components, and the extra cost of an accurately made jig is spread over a large output. Furthermore, it is surprising how small errors accumulate in a mechanism during its assembly. When a clearance is specified, it is better to ensure its observance, rather than to allow careless marking out and machining to encroach upon it 1) The location of workpiece. Fig.13.1 represents a body that is completely free in space. In this condition it has six degrees of freedom. Consider these freedoms with respect to the three mutually perpendicular axes XX, YY, and ZZ. The body can move along any of these axes; it therefore has three freedoms of translation. It can also rotate about any of the three axes; it therefore has three freedoms of rotation. The total number of freedoms is six. When work is located, as many of these freedoms as possible must be eliminated, to ensure that the operation is performed with the required accuracy. Accuracy is ensured by machining suitable location features as early as possible, and using them for all location, unless other considerations mean that other location features must be used. If it is necessary, the new location features must be machined as a result of location from the former location features. 2) The clamping of the workpiece. The clamping system must be such that the workpiece is held against the cutting forces, and the clamping forces must not be so great as to cause the workpiece to become distorted or damaged. The workpiece must be supported beneath the point of clamping, to ensure that the forces are taken by the main frame of the jig or fixture, and on to the machine table and bed. When jigs and fixtures are designed, the clamping system is designed to ensure that the correct clamping force is applied, and that the clamps can be operated quickly but with safety. Definition of a Drill Jig A drill jig is a device for ensuring that a hole to be drilled, tapped, or reamed in a workpiece will be machined in the proper place.Basically it consists of a clamping device to hold the part in position under hardened-steel bushings through which the drill passes during the drilling operation. The drill is guided by the bushings. If the workpiece is of simple construction, the jig may be clamped on the workpiece. In most cases, however, the workpiece is held by the jig, and the jig is arranged so that the workpiece can be quickly inserted and as quickly removed after the machining operation is performed. Jigs make it possible to drill, ream, and tap holes at much greater speeds and with greater accuracy than when the holes are produced by conventional hand methods. Another advantage is that skilled workers are not required when jigs are used. Responsibility for the accuracy of hole location is taken from the operator and given to the jig. The term jig should be used only for devices employed while drilling, reaming, or tapping holes. It is not fastened to the machine on which it is used and may be moved around on the table of the drilling machine to bring each bushing directly under the drill. Jigs physically limit and control the path of the cutting tool.。 If the operation includes machining operations like milling, planing, shaping, turning, etc., the term fixture should be used. A fixture holds the work during machining operations but does not contain special arrangements for guiding the cutting tool ,as drill jigs do. Typical Jigs and Fixtures Typical drill jig. Figure 13.2 illustrates a drilling jig for drilling four holes in the flange of a workpiece that has been completed except for the four holes.The workpiece has an accurately machined bore, and is located from the bore and the end face, from a cylindrical post. There is no need to control the rotational position about the axis of the bore, because up to the time when the holes are drilled, it is symmetrical about that axis. The four bushes used to control the drill are held in the drill plate, which, with the hand nut, is used to clamp the workpiece against the base of the fixture. Typical milling fixture. Figure 13.3 illustrates a simple milling fixture for milling the slot in the otherwise completed workpiece shown. The workpiece is located from two of the four holes in its base, and from the underside of the base. The workpiece is clamped in position, and cutter is located against the setting block, which provides setting or cutter position and depth of cut. The fixture must be positioned relative to the machine table, this is done by engaging the two tenons at the bottom of the fixture in the slot in the machine table. The fixture is secured to the machine table with T-bolts, also engaging in the slots in the table (Fig.13.3). 研磨 研磨是一种用于平面和圆柱面的精加工作业。研具，如图9.1a所示，通常用铸铁、铜、皮革或布制成。研磨微粒嵌入研具内，或者可以通过液体携带。根据工件硬度，研磨压力可在7kPa到140kPa(1到20psi)范围中取。 研磨有两个主要作用。首先，它通过去除所有机加工痕迹能产生较好的表面光洁度。其次，它能用作获得像活塞与气缸之类配件间过盈配合的方法。 研磨后的工件表面可能看似平滑，其实布满着微观峰、谷、划痕和凹陷。几乎没有表面是完全平整的。研磨使表面不规则最小化，因而增加了有效接触面积。图9.1a上显示了两个表面。上面是研磨前表面可能的外观模样而下面则是研磨后的模样。研磨去除了微观峰顶从而产生相对平坦的平台。整个微观山脉范围都需要磨去以增加有效接触面积。 研磨平面或圆柱面工件的生产过程是在如图9.1b和9.1c那样的机器上完成的。研磨也可采用特殊成型研具在诸如球形物体和透镜之类的曲面上进行。 抛光 抛光是生成平滑、有光泽表面光洁度的工艺。抛光工艺涉及两种基本机理： (a)精细等级磨粒去除，和(b)在抛光中通过摩擦生热软化并抹光表面层。电解抛光 电解抛光是一种与电镀相似的电化学工艺，但过程与电镀正好相反。电解抛光工艺使金属物体的微观表面平滑和简单化。通过电解抛光能在金属表面得到镜面光洁度。 在电解抛光中，金属是逐个离子地从被抛光金属物体表面去除的。电化学和电解基本原理(Faraday定理)取代了传统的机械精加工技术。用基本术语说，要电解抛光的物体被浸没在电解液中并且通上直流电。该物体为阳极，阴极连接到附近的金属导体上。 金属表面的平滑是电解抛光主要的和最有优势的效应之一。在此过程中，一变化着厚度的膜覆盖在金属表面上。该膜在微观凹陷处最厚而在微观凸出处最薄。电阻在膜最薄处最小，导致最大金属分解率。电解抛光选择性地去除微观高点或“峰” 快于对相应微观凹陷处或“谷”的侵蚀速率。原材料以金属盐的形式被去除。在特定环境下金属的去除是可控的并且保持在0.0001 到0.0025mm范围内。化学机械抛光 化学机械抛光正在多层集成电路制造领域成为日益重要的步骤。化学机械抛光是指大量抛光液与被抛光表面产生化学和机械作用的抛光。在化学机械抛光过程中，旋转晶片面向下压在旋转、有回弹力的抛光衬垫上，而同时含有研磨微粒和化学反应物的抛光液流过晶片与衬垫之间。抛光衬垫、研磨微粒和化学反应物的共同作用导致晶片表面的材料去除并抛光。化学机械抛光可使多种易碎材料平整且不受损害，因此在集成电路制造中被广泛地用在硅晶片上。 化学机械抛光是一种复杂的多相工艺。它主要包括下列两个动态过程：第一，抛光液中活性成分与晶片的原子发生反应，这是带有氧化-还原反应的化学反应步骤。第二步是解吸附过程，即反应产物逐渐从晶片表面分离并将新表面暴露给抛光液。如果化学反应速率较小，晶片的总去除率也较小，而且表面光洁程度不够好。与之相反，即使化学反应很快，但解吸附很慢，则总去除率也不够好。因为反应产物不能从晶片表面分离，抛光液中活性成分就不能暴露并与新表面上的原子起反应，这会抑制化学反应。这两个步骤的平衡与合成效应决定了总去除率和表面光洁程度。 进行表面工程或表面处理的目的是：(1)控制摩擦和磨损，(2)改善抗腐蚀性，(3)改变物理性能，例如，传导率、电阻系数和反射率，(4)修改尺寸，(5)变更外观，例如颜色和粗糙程度，(6)降低成本。 通常的表面处理可以分为两个主要类型：覆盖表面的处理和改变表面的处理。覆盖表面 覆盖表面的处理包括有机涂层和无机涂层。无机涂层有电镀、转化层、热喷涂、热浸渍、熔炉熔融、或在材料表面涂上薄膜、玻璃、陶瓷。 电镀是一种在电镀槽通上电流使金属沉淀在基体上的电化学过程。 通常有一个阳极(正电极)，是要沉淀材料的来源；电化学反应是使金属离子交换并迁移到要覆盖基体上的中间过程；以及一个阴极(负电极)，即要覆盖的基体。 电镀在通常为非金属容器(一般是塑料)的电镀槽中进行。该容器装满了含有离子态被镀金属的电解液。 阳极与电源正极相连。阳极通常为被镀金属(假定该金属能在电解液中腐蚀)。为了操作容易，该金属呈固体小块形式并置于由抗腐蚀金属(如钛或不锈钢)制成的惰性金属筐内。 阴极是工件，即要镀的基体，连接到电源的负极。很好地调节电源使波动最小化并在载荷变化情况(如同电镀容器中看到的那样)下提供稳定的可预知电流。 一旦通上电流，来自溶液的正的金属离子被吸引到带负电的阴极并沉淀在其上。作为这些沉淀离子的补充，来自阳极的金属被溶解并进入溶液平衡离子势能。 热喷涂工艺：热喷涂金属涂层是金属熔化后立即投射到基体上形成的金属沉积层。所用的金属和应用系统都可以变化，但大多数应用都是在要求改善抗腐蚀或耐磨性能的表面涂上薄层。 热喷涂是用于很大一类相关工艺的一个通用术语，喷涂到表面产生涂层的熔化小滴可以是金属、陶瓷、玻璃和/或聚合物，形成独立的近似纯形或产生具有独特性能的设计材料。 大体上，有稳定熔化状态的任何材料都可以热喷涂，范围宽阔的纯净和合成材料一般都能喷涂用于研究及工业目的。其沉积率与可供选择的涂层技术比较是很高的。沉淀厚度普遍为0.1到1mm，对某些材料则沉淀厚度可以达到1cm以上。 热喷涂金属的应用工艺相对简单并由下列阶段组成：(1)在喷枪内熔化金属。(2)通过压缩空气将液态金属喷涂在准备好的基体上。 (3)熔化微粒投射在清洁过的基体上。 现在有两种主要的金属丝应用类型可选用，也就是电弧喷涂和气体喷涂。 电弧喷涂当一对金属丝通过手持喷枪连到一起时，通上电横过其末端划燃电弧。压缩空气吹过电弧使其雾化并驱使自动送料金属丝微粒到准备好的工件上。 气体喷涂连续移动的金属丝在燃烧火焰喷射中通过手持喷枪，并被燃烧气体的锥形喷嘴所熔化。熔化后的金属丝顶端进入锥体雾化并驱使其到基体上。 薄膜涂层：物理蒸发沉淀(PVD)和化学蒸发沉淀(CVD)是两种最常见薄膜涂层方法的类型。 物理蒸发沉淀涂层涉及到在真空装置内各种各样的材料原子紧靠原子、分子紧靠分子或离子沉淀于固态基体上。 热蒸发利用涂层金属在真空环境中蒸发形成的微粒子雾将基体和靶材之间可见范围内所有表面覆盖。在塑料零件上生成较薄(0.5m)的、装饰性的、有光泽的涂层时常常用到它。 然而，这种薄涂层是易碎的并不适合用于磨损场合。热蒸发工艺也能在喷气发动机零件上覆盖很厚(1mm)的耐热材料涂层，例如MCrAIY一种金属、铬、铝和钇合金。 反应溅射法通过在氩真空设备中连接工件和具有特定成分的材料到高压直流电来应用诸如陶瓷、金属合金、有机和无机化合物之类的高技术涂层。等离子区形成于基体(工件)和靶材(原料物质)之间并将被溅射的靶材原子转移到基体的表面上。如果基体不导电，例如聚合物，则采用射频(RF)溅射代替。反应溅射法可以生成较薄(小于3m(120in)的、坚硬薄膜涂层，像比最硬金属还硬的氮化钛(TIN)。现在反应溅射法已被广泛应用于切削刀具、成型工具、注射模具和诸如冲头和冲模之类的通用器具，以增强其耐磨性和使用寿命。 化学蒸发沉淀能在
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