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Surface Engineering - Manufacturing Processes

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    Surface Engineering - Manufacturing Processes Surface Engineering - Manufacturing Processes Document Transcript

    • Surface Engineering By Rasikh Tariq (ME113006) Khawar Shahzad (ME113009) Mohammad Adam (ME-113125) A project report submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the course of MANUFACTURING PROCESSES-I
    • Copyright  2013 MAJU Students All rights reserved. Reproduction in whole or in part in any form requires the prior written permission of Rasikh Tariq, Khawar Shahzad, and Mohammad Adam. Page 1 of 30
    • Declaration It is declared that this is an original piece of my own work, except where otherwise acknowledged in text and references. This work has not been submitted in any form for another internship program or diploma at any institution for tertiary education and shall not be submitted by me in future for obtaining any degree from this or any other Institution. Rasikh Tariq ME-113006 Khawar Shahzad ME-113009 Mohammad Adam ME-113125 November, 2013 Page 2 of 30
    • Table of Contents Chapter 1: Surface Engineering – An Introduction Definition .......................................................................................................... 7 Why Surface Engineering? ............................................................................... 7 Surface Integrity ............................................................................................... 8 Surface Finish Measurement ........................................................................... 11 Mechanical Cleaning and Finishing Blast Cleaning ........................................ 11 Blast Finishing ............................................................................................. 11 Shot Peening................................................................................................ 11 Tumbling or Barrel Finishing ....................................................................... 12 Vibratory Finishing...................................................................................... 12 Media .......................................................................................................... 12 Compounds ................................................................................................. 13 Summary of Mass-Finishing Methods .......................................................... 13 Chemical Cleaning ......................................................................................... 13 General Considerations in Cleaning ........................................................... 13 Chemical Cleaning Processes ..................................................................... 14 Alkaline cleaning ..................................................................................... 14 Solvent cleaning ....................................................................................... 14 Acid cleaning ........................................................................................... 15 Chapter 2: Coating Processes Painting, Wet or Liquid ................................................................................... 17 Paint Application Methods .............................................................................. 18 Dipping ....................................................................................................... 18 Spray Painting ............................................................................................. 18 Hand spraying ............................................................................................. 18 Drying ............................................................................................................ 19 Powder Coating .............................................................................................. 19 Hot-Dip Coating .............................................................................................. 21 Chemical Conversion Coatings ...................................................................... 21 Page 3 of 30
    • Blackening or Coloring Metals ....................................................................... 22 Electroplating ................................................................................................. 22 Chapter 3: Surface Enhancement Processes Vaporized Deposition Processes .................................................................... 25 Physical Vapor Deposition .......................................................................... 25 Chemical Vapor Deposition ........................................................................ 26 Surface Hardening .......................................................................................... 27 Carburizing ................................................................................................. 28 Nitriding ...................................................................................................... 28 Carbonitriding ............................................................................................ 29 Chromizing .................................................................................................. 29 Boronizing ................................................................................................... 29 Clad Materials ................................................................................................ 29 References ...................................................................................................... 30 Page 4 of 30
    • Abstract The pupils who were studying Manufacturing Processes-I course were supposed to do a project within the domain of manufacturing processes. We had elected “Surface Engineering” as our project. Surface engineering plays a dynamic part in all the manufacturing parts irrespective if it is as small as a nut, bolt or as large as a space-shuttle. All the mechanical modules, after their manufacturing, requires surface finishing. As a result, of this project we got acquainted with the fundamentals of surface engineering. We got acquainted with some of the industrialized machines used in surface engineering. We had started our project with the introduction of surface engineering leading towards surface texture, mechanical cleaning, coating processes, heat treatment then to clad materials. Page 5 of 30
    • SURFACE ENGINERRING – AN INTRODUCTION Chapter 1 Mohammad Adam ME-113125 Page 6 of 30
    • Surface engineering is the sub-branch of material sciences and manufacturing engineering which deals with the surface of solid material. It has applications to chemistry, mechanical engineering, and electrical engineering (particularly in relation to semiconductor manufacturing). Definition Surface engineering is a multidisciplinary activity intended to tailor the properties of the surfaces of manufactured components so that their function and serviceability can be improved. Why Surface Engineering? A manufacturing module usually fails when its surface cannot effectively resist the external forces or environment to which it is subjected. The selection of a surface material with the suitable thermal, optical, magnetic and electrical properties and sufficient resistance to wear, corrosion and degradation, is vital to its functionality. Sometimes technological progress and manufacturing competence may be embarrassed only by surface requirements. For example, the fuel efficiency and power output of gas turbines or diesel engines are limited by the ability of key components to withstand high temperatures. However, it is often not practical, inefficient or uneconomical to manufacture components from a bulk material merely for its surface properties - far better to use a cheaper, more easily formed underlying material and coat it with a suitable high performance film. The resulting product conserves limited material resources, performs better than the original and may well be cheaper to produce. Improving the functionality of an existing product is only one aim of surface engineering. New coatings and treatment processes may also create opportunities for new products which could not otherwise exist. For example, satellites could not function, nor could modern power plants operate safely, without the application of advanced surface engineering techniques. The economic benefits of surface engineering are vast. According to a report by Page 7 of 30
    • RCSE staff, in 2005 the value of the UK coating market is approximately £21.3 billion, and those coatings critically affect products with a value greater than £143 billion (Source: "2005 Revisited; The UK Surface Engineering Industry to 2010", A Matthews, R Artley and P Holiday). Many manufacturing processes influence surface properties, which in turn may significantly affect the way the component function in service. The demands for greater strength and longer life in components often depend on changes in the surface properties rather than the bulk properties. These changes may be mechanical, thermal, chemical, and/or physical and therefore are difficult to describe in general terms. For example, two different surface finishes on Inconel 718 can have a marked effect on the fatigue life, changing the fatigue limit from 69 ksi after gentle grinding to as low as 22 ksi using electrical discharge machining. In brief, surface engineering is relevant to all types of products. It can increase performance, reduce costs and control surface properties independently of the substrate, offering enormous potential for:      improved functionality the solution to previously insurmountable engineering problems the possibility to create entirely new products conservation of scarce material resources reduction of power consumption and effluent output Surface Integrity The term surface integrity was coined by Field and Kahles in 1964 in reference to the nature of the surface condition that is produced by the manufacturing process. If we view the process as having five main components (workspace, tool, machine tool, environment and process variables), we observer the following properties are altered by the following:     High temperature involved in the machining process Plastic deformation of the work material(residual stress) Surface geometry(roughness, cracks, distortion) Chemical reactions particularly between the tool and the work piece Surface integrity has two aspects:   Topography Surface layer characteristics Page 8 of 30
    • Topography is made up of surface roughness, waviness, errors of form and flaws. Machining processes produce surface flaws, waviness, and roughness that can influence the performance of the component. Surface layer characteristics includes the peaks and valleys that are considered from waviness. Changes in the surface layer, as a result of processing, include plastic deformation, residual stresses, cracks and other metallurgical changes like hardness, over aging, phase changes, recrystallization, inter-granular attack. The material removal processes generate a wide variety of surfaces textures, known as surface finish. The cutting process generate a wide variety of surface textures on the material of which three are the important terms; Page 9 of 30
    • surface roughness, waviness and lay. Roughness refers to the finely spaced surface irregularities (it results from machining operations in case of machined surfaces).Waviness is surface irregularity of greater spacing than in roughness (it results from warping, vibration or the work being deflected during machining).Lay is the term used to designate the direction of the predominant surface pattern produced by the machining process. The variety of surface instrument is available for measuring surface roughness and surface profiles. The majority of these devices use a diamond stylus that is moved at a constant rate across the surface, perpendicular to the lay surface, perpendicular to the lay pattern. The rise and fall of the stylus is detected electronically by LVDT (Linear Variable Differential Transformer) is amplified and recorded on a strip-chart or is processed electronically to produce average or root mean square readings for meter. In most cases, the arithmetical average (AA) is used. In terms of measurements, the AA or RA would be as follows: 𝑅𝐴 = 𝑛 ∑ 𝑖−1 𝑦 𝑖 𝑛 yi is the vertical distance from the center line and n is the total number of vertical measurements taken within a specified cut off distance. Cuttoff distance refers to the sampling length used for the calculation of the roughness height. When it is not specified, a value of 0.030in. (0.8mm) is assumed. Page 10 of 30
    • Surface Finish Measurement All of the processes used to manufacture components are important if their effects are present in the finished part. It is convenient to divide processes that are used to manufacture parts into three categories: traditional, nontraditional and finishing treatments. In traditional processes the tool contacts the work piece. Examples are grinding, milling and turning. In nontraditional processes have intrinsic characteristics even if well controlled, will change the surface; in these processes the work piece does not touch the tool. Electrochemical machining (ECM), Electrical Discharge machining (EDM), Laser machining are its examples. Finishing treatments can be used to negate or remove the impact of both the traditional and nontraditional processes as well as provide good surface finish. For example residual stresses can be removed by the shot peening. Chemical milling can remove the recast layer left by EDM. The objectives of the surface-modification processes can be quite varied. Some are designed to clean surfaces and remove the kinds of defects that occur during processing or handling (such as scratches, pores, fins). Others further improve or modify the products, bring smoothness, texture and color. Mechanical Cleaning and Finishing Blast Cleaning Mechanical cleaning involves the physical removal of soils, scales, or films from the work surface of the work part by means of abrasives or similar mechanical action. The processes used for mechanical cleaning often serve other functions in addition to cleaning, such as debarring and improving surface finish. Blast Finishing Blast finishing uses the high-velocity impact of particulate media to clean and finish a surface. The most well-known of these methods is sand blasting, which uses grits of sand (SiO2) as the blasting media. Various other media are also used in blast finishing, including hard abrasives such as aluminum oxide (Al2O3) and silicon carbide (SiC), and soft media such as nylon beads and crushed nut shells. The media is propelled at the target surface by pressurized air or centrifugal force. In some applications, the process is performed wet, in which fine particles in a water slurry are directed under hydraulic pressure at the surface. Shot Peening In shot peening, a high-velocity stream of small cast steel pellets (called shot) is directed at a metallic surface with the effect of cold working and inducing compressive stresses into the surface layers. Shot peening is used primarily to Page 11 of 30
    • improve fatigue strength of metal parts. Its purpose is therefore different from blast finishing, although surface cleaning is accomplished as a by-product of the operation. Tumbling or Barrel Finishing Tumbling (also called barrel finishing and tumbling barrel finishing) involves the use of a horizontally oriented barrel of hexagonal or octagonal crosssection in which parts are mixed by rotating the barrel at speeds of 10 to 50 rev/min. Finishing is performed by a ‘‘landslide’’ action of the media and parts as the barrel revolves. As pictured in Figure 28.1, the contents rise in the barrel due to rotation, followed by a tumbling down of the top layer due to gravity. This cycle of rising and tumbling occurs continuously and, over time, subjects all of the parts to the same desired finishing action. Diagram of tumbling (barrel finishing) operation showing ‘‘landslide’’ action of parts and abrasive media to finish the parts. Vibratory Finishing Vibratory finishing is a type of mass finishing manufacturing process used to deburr, radius, descale, burnish, clean, and brighten a large number of relatively small work pieces. In contrast to the barrel processing, vibratory finishing is performed in open containers as shown in the figure. Tubs or bowls are loaded with work pieces and media are vibrated at frequencies between 900 and 3600 cycles per minute. The process is less noisy and easily controlled and automated. Media Media serves the purpose of separating parts from each other and interacting with each individual part Page 12 of 30
    • to do the required finishing. Media is made from a variety of materials such as ceramic, plastic, carbon or stainless steel, wood, leather, corn cob, nut shells, river rock. The success of any mass finishing process is dependent on the media selection and the ratio of media to the parts, their ratios are represented in the following table. Natural abrasives include slag, sand, corundum; granite etc Synthetic media contain 50 to 70% of abrasives such as Alumina (Al2O3), flint, Silicon Carbide (SiC). Compounds A variety of functions are performed by the compounds that are added in addition to the media and work pieces. These compounds can be liquid or dry, abrasive or no abrasive and acid, neutral or alkaline. They are often designed to assist in debarring, burnishing and abrasive cutting as well as to provide cleaning, descaling. Summary of Mass-Finishing Methods The barrel and vibratory finishing processes are quite simple and economical and can process large number of parts. Soft, nonferrous parts can be finished in a little as 10 minutes, while the harder steels may require 2 hours or more. Sometimes the operations are sequenced, using progressively finer abrasives. The following figure shows the variety of parts before and after the mass finishing operation using the triangular abrasive shown with each component. Chemical Cleaning Chemical cleaning operations are effective mean of removing oil, dirt, scale other foreign material that may adhere to the surface of the product. Manufacturers must ask themselves if a part really has to be cleaned, what soils have to be removed, hoe clean the surface have to be. Selection of cleaning method will depend on the cost of equipment, power, cleaning material etc. General Considerations in Cleaning There is no single cleaning method that can be used for all cleaning tasks. Just as various soaps and detergents are required for different household jobs (laundry, dishwashing, pot scrubbing, bathtub cleaning, and so forth), various cleaning methods are also needed to solve different cleaning problems in industry. Important factors in selecting a cleaning method are (1) the contaminant to be removed, (2) degree of cleanliness required, (3) substrate material to be cleaned, (4) purpose of the cleaning, (5) environmental and safety factors, (6) size and geometry of the part, and (7) production and cost requirements. Page 13 of 30
    • A simple test is a wiping method, in which the surface is wiped with a clean white cloth, and the amount of soil absorbed by the cloth is observed. It is a non-quantitative but easy test to use. Chemical Cleaning Processes Chemical cleaning uses various types of chemicals to effect contaminant removal from the surface. The major chemical cleaning methods are (1) alkaline cleaning, (2) emulsion cleaning, (3) solvent cleaning, (4) acid cleaning, and (5) ultrasonic cleaning. In some cases, chemical action is augmented by other energy forms; for example, ultrasonic cleaning uses high-frequency mechanical vibrations combined with chemical cleaning. In the following paragraphs, we review these chemical methods. Alkaline cleaning Alkaline cleaning is the most widely used industrial cleaning method. As its name indicates, it employs an alkali to remove oils, grease, wax, and various types of particles (metal chips, silica, carbon, and light scale) from a metallic surface. Alkaline cleaning solutions consist of low-cost, water-soluble salts such as sodium and potassium hydroxide (NaOH, KOH), sodium carbonate (Na2CO3), borax (Na2B4O7), phosphates and silicates of sodium and potassium, combined with dispersants and surfactants in water. The cleaning method is commonly by immersion or spraying, usually at temperatures of 50_C to 95_C (120_F–200_F). Following application of the alkaline solution, a water rinse is used to remove the alkali residue. Metal surfaces cleaned by alkaline solutions are typically electroplated or conversion coated. Following are its types:   Electrolytic cleaning Emulsion cleaning Solvent cleaning Organic soils such as oil and grease are removed from a metallic surface by means of chemicals that dissolve the soils. Common application techniques include hand-wiping, immersion, spraying, and vapor degreasing. Vapor degreasing uses hot vapors of solvents to dissolve and remove oil and grease on part surfaces. The common solvents include trichlorethylene (C2HCl3), methylene chloride (CH2Cl2), and perchlorethylene (C2Cl4), all of which have relatively low boiling points.In vapor degreasing process consists of heating the liquid solvent to its boiling point in a container to produce hot vapors. Parts to be cleaned are then introduced into the vapor, which condenses on the relatively cold part surfaces, dissolving the contaminants and dripping to the bottom of the container. Condensing coils near the top of the container prevent any vapors from escaping the container into the surrounding atmosphere. Page 14 of 30
    • Acid cleaning It removes oils and light oxides from metal surfaces by soaking, spraying, or manual brushing or wiping. The process is carried out at ambient or elevated temperatures. Common cleaning fluids are acid solutions combined with water-miscible solvents, wetting and emulsifying agents. Cleaning acids include hydrochloric (HCl), nitric (HNO3), phosphoric (H3PO4), and sulfuric (H2SO4), the selection depending on the base metal and purpose of the cleaning. For example, phosphoric acid produces a light phosphate film on the metallic surface, which can be a useful preparation for painting. A closely related cleaning process is acid pickling, which involves a more severe treatment to remove thicker oxides, rusts, and scales; it generally results in some etching of the metallic surface, which serves to improve organic paint adhesion. Page 15 of 30
    • COATING PROCESSES Chapter 2 Rasikh Tariq ME-113006 Page 16 of 30
    • Each of the surface finishing methods is material removal process, designed to clean, smooth, and otherwise reduce the size of the part. Many other techniques have been developed to add material to the surface of a part. If the material is deposited as a liquid or organic gas (or from a liquid or a gas medium), the process is called coating. If the added material is a solid during deposition, the process is known as cladding. Painting, Wet or Liquid Most of today’s commercial paints are synthetic organic compounds that contain pigments and dry by polymerization or by a combination of polymerization and adsorption of oxygen. Heat can be used to accelerate the drying, but many of the synthetic paints and enamels will dry in less than an hour without the use of additional heat. The older oil-based materials have a long drying time and require excessive environmental protection measures. For these reasons they are seldom used in manufacturing applications. Paints are used for variety of reasons, usually to provide protection and decoration but also to fill or conceal surface irregularities, change the surface friction, or modify the light or heat absorption or radiation characteristics. Following table provides a list of the more commonly used organic finishes, along with their significant characteristics. COMMONLY USED ORGANIC FINISHES AND THEIR QUALITIES Material Durability (Scale of 110) Relative Cost (Scale of 110) Nitrocellulose lacquers 1 2 Epoxy esters 1 2 Akyd-amine 2 1 Acrylic lacquers 4 1.7 Acrylic enamels 4 1.3 Vinyl solutions 4-7 2 Silicones 4-7 10 Flouropolymers 10 10 Page 17 of 30 Characteristics Fast drying; low durability Good chemical resistance Versatile; low adhesion Good color retention; low adhesion Good color retention; though; high baking temperature Flexible; good chemical resistance; low solids Good gloss retention; low flexibility Excellent durability; difficult to apply
    • Paint Application Methods In most cases, at least two coats are required. The first (or prime) coat serves to:    Ensure adhesion Provide a leveling effect by filling in minor porosity and other surface blemishes, and Improve corrosion resistance and thus prevent from being dislodged in service. In manufacturing, almost all painting is done by one of four methods: Dipping is a simple and economical means of paint application when all surfaces of the part are to be coated. The products can be manually immersed into a paint bath or passed through the bath while on or attached to a conveyor. Dipping is attractive for applying prime coats and for painting small parts where spray painting would result a significant waste due to overspray. Conversely, the process is unattractive where only some of the surfaces require painting or where a very thin, uniform coating would be adequate, as on automobile bodies other difficulties are associated with the tendency of paint to run, production both a wavy surface and a final drop of paint attached to the lowest drip point. Good-quality dipping requires that the paint be stirred at all time and be of uniform viscosity. Spray Painting is probably the most widely used paint application process because of its versatility and the economy in the use of paint. In the conventional technique, the paint is atomized and transported by the flow of compressed air. In a variation known as airless spraying, mechanical pressure forces the paint through an orifice at pressures between 500 and 4500 psi. This provides sufficient velocity to produce atomization and also propel the particles to the work piece. Because no air pressure is used for atomization, there is less spray loss (Paint efficiency may be as high as 99%) and less generation of gaseous fumes. Hand spraying is probably the most adaptable means of an application but can be quite costly in terms of labor and production time. When air or mechanical means provide the atomization, workers must exercise considerable skill to obtain the proper Page 18 of 30 Basic Electroplating Process
    • coverage without allowing the paint to “run” or “drape.” Only a very thin fill can be deposited at one time, usually less than 0.001 in. As a result, several coats may be required intervening time for drying. Both manual and automatic spray painting can benefit from the use of electrostatic deposition. A DC electrostatic potential is applied between the atomizer and are therefore repelled. The oppositely charged work piece then attracts the particles, with the actual path of the particle being a combination of the kinetic trajectory and the electrostatic attraction. The higher DC voltage, the greater the electrostatic attraction. Over-spraying can be reduced by as much as 60 to 80%, as can the generation of airborne particles and other emissions. Unfortunately, part edges and holes receive a heavier coating than flat surfaces due to concentration of electrostatics lines of force on any sharp edge. Depressed areas will receive a reduced amount of paint, and a manual touch-up may be required using conventional spray techniques. Despite these limitations, electrostatic spraying is an extremely attractive means of painting complex-shaped products where the geometry would tend to create large amount of overspray. The process is particularly attractive for applying the prime coat to complex structures, such automobile bodies, where good corrosion resistance is requirement. Drying Most paints and enamels used in manufacturing require from 2 to 24 hours to dry at normal room temperature. This time can be reduced to between 10 minutes and 1 hour of the temperature can be raised to between 275o and 450o F. As a result, elevated temperature drying is often preferred. Elevated-temperature drying is rarely a problem with metal parts, but other materials can damage by exposure to the moderate temperatures. For example, when wood is heated, the gases, moisture, and residual liquid are expanded and driven to the surface beneath the hardening paint. Powder Coating It is a variation of electrostatic spraying, but here the particles are solid rather than liquid. Several coats, such as primer and finish, can be applied and then followed by a single baking, in contrast to the baking after each coat that is required in the conventional spray processes. In addition, the overspray powder can often be collected and reused. Page 19 of 30
    • Modern powder technology can produce a highquality finish with superior surface properties and usually at a lower cost than liquid painting. Powder painting is more efficient in the use of materials and lower energy requirements. The process is not good for large objects (massive tanks) or heatsensitive objects. It is not easy to produce film thickness less than 0.03mm. Schematic Machine Diagram of Powder Coating Following table shows some thermosetting powder and their useful properties: THERMOSETTING POWDER COATING (DRY PAINTING) HAVE A WIDE VARIETY OF PROPERTIES AND APPLICATIONS Epoxy/Po TGIC Polyester Acrylic Properties Epoxy lyester Polyester Urethane Urethane Hybrid 0.5-20 mils 0.5-10 Application Thickness a 0.5-10 mils 0.5-10 mils 0.5-10 mils mils 450oF – 3 450oF – 3 400oF – 7 400oF – 7 400oF – 7 Cure Cycle (Metal min min min min min temperatures)b 250oF – 325oF – 25 310oF – 20 325oF – 17 360oF – 25 30min min min min min Outdoor Very Poor Poor Very Good Excellent weatherability Good Pencil Hardness HB-5H HB-2H HB-2H HB-3H H-3H Direct Impact 80-160 80-160 80-160 80-160 20-60 resistance, in lbc Very Good Very Chemical Resistance Excellent Least Good Good Good Most expensive expensive Cost (Relative) 2 1 3 4 5 Car Water Architect Furniture, wheels/ri Washing heaters, ural cars, ms, machine, Applications radiators, aluminum ovens, playgroun refrigerato office , outdoor appliances d rs, ovens furniture furniture equipment Page 20 of 30
    • Thickness up to 150 mils can be applied via multiple coats in a fluidized bed. Time and temperature can be reduced, by utilizing accelerated curing mechanisms while maintaining the same general properties c Tested at a coating thickness of 2.0 mils a b Hot-Dip Coating Large quantities of metal products are given corrosion-resistant coatings by direct immersion into a bath of molten metal. The most common coating materials are zinc, tin, aluminum, and tene (an alloy of lead and tin). Hot-dip galvanizing is the most widely used method of imparting corrosion resistance to steel. After the products, or sheets, have been cleaned to remove oil, grease, scale, and rust, they are fluxed by dipping into a solution of zinc ammonium chloride and dried. Next, the article is completely immersed in a bath of molten zinc. The zinc and iron react metallurgical to produce a coating that consists of a series of zinc-iron compounds and a surface layer of nearly pure zinc. The primary limitations to hot-dip galvanizing are the size of the product and the “damage” that might occur when a metal is exposed to the temperatures of the molten material. Tin coatings can also be applied by immersing in a bath of molten tin with a covering of flux material. Because of the high cost of tin and the relatively thick coatings applied by hot dipping, most tin coatings are now applied by electroplating. Chemical Conversion Coatings In chemical conversion coating, the surface of the metal is chemically treated to produce a nonmetallic, nonconductive surface that can impart a range of desirable properties. The most popular types of conversion coatings are chromate and phosphate. Aluminum, magnesium, zinc and copper (as well as cadmium and silver) can all be treated by a chromate conversion process that usually involves immersion in a chemical bath. The surface of the metal is convened into a layer of complex chromium compounds that can impart colors ranging from bright color through blue, yellow, brown, olive drab, and black. Most of the films are soft and gelatinous when they are formed but harden upon drying. They can be used to:   Impart exceptionally good corrosion resistance. Act as an intermediate bonding layer for paint, lacquer, or other organic finishes, or Page 21 of 30
    •  Provide specific colors by adding dyes to the coating when it is in its soft condition. Blackening or Coloring Metals Many steel parts are treated to produce a black, iron oxide coating – a lustrous surface that is resistant to rusting when handled. Since this type of oxide forms at elevated temperatures, the parts are usually heated in some form of special environment, such as spent carburizing compound or special blackening salts. Chemical solutions can also be used to blacken, blue and even “brown” steels. Brown, black and blue colors can also be imparted to tin, zinc, cadmium, and aluminum through chemical bath immersion or wipes. The surfaces of copper and brass can be made to be black, blue, green, or brown, with a full range of shades in between. Electroplating Large quantities of metal and plastic parts are electroplated to produce metal coating that imparts corrosion or wear resistance, improves appearance, or increases the overall dimensions. Virtually all commercial metals can be plated, including aluminum, copper, brass, steel, and zinc-based die castings. Plastics can be electroplated, provided that they are first coated with an electrically conductive material. The most common platings are zinc, chromium, nickel, copper, tin, gold, platinum, and sliver. The electro-galvanized zinc platings are thinner than the hotdip coatings and can be produced without subjecting the base metal to the elevated temperatures of molten zinc. Nickel plating provides good corrosion resistance but is rather expensive Electroplating Process and does not retain its lustrous appearance. Consequently, when lustrous appearance is desired, a chromium plate is specified. An initial layer of copper provides a leveling effect and makes it possible to reduce the thickness of the nickel layer that typically follows to less than 0.0006 in. The final layer of chromium then provides the attractive appearance. Gold, silver, and platinum platings are used in both the jewelry and electronic industries, when Page 22 of 30
    • the thin layers impart the desired properties while conserving the precious metals. Hard chromium plate, with Rockwell hardnesses between 66 and 70, can be used to build up worn parts to larger dimensions and to coat tools and other products that need reduced surface friction and good resistance to both wear and corrosion. Hard chrome coatings are always applied directly to the base material and are usually much thicker layers than the decorative treatments, typically ranging from 0.003 to 0.010 in. thick. Even thicker layers are used in applications such as diesel cylinder liners. Such hard chrome plate does not have a leveling effect, defects or roughness in the base surface will be amplified. If smooth surfaces are desired, subsequent grinding and polishing may be necessary. The figure in the margin shows the base process of electroplating coating. A DC voltage is applied to the material that is to be coated and the metal that will be used as a coating. Coating metal is provided with anode voltage whereas material to be coated resides on cathode voltage. The container also contains a solution can conducts electricity such as brine solution. Electrons passes through the brine solution and making layers on the surface of cathode (material to be coated). The surface to be plated must also be prepared properly if satisfactory results are to be obtained. Pinholes, scratches, and other surface defects must be removed if a smooth, lustrous finish is desired. Combinations of degreasing, cleaning, and pickling are used to ensure a chemically clean surface, one to which the plating material can adhere. Page 23 of 30
    • SURFACE ENHANCEMENT PROCESSES Chapter 3 Khawar Shahzad ME-113009 Page 24 of 30
    • Vaporized Deposition Processes The vapor deposition processes form a thin coating on a substrate by either condensation or chemical reaction of a gas onto the surface of the substrate. The processes can be classified into two main categories: physical vapor deposition and chemical vapor deposition. Vaporized Deposition Processes Physical Vapor Deposition Chemical Vapor Deposition Vaccum Vapor Deposition Sputtering Ion Plating Physical Vapor Deposition Physical vapor deposition (PVD) is a group of thin film processes in which a material is converted into its vapor phase in a vacuum chamber and condensed onto a substrate surface as a very thin layer. In the Physical Vapor Deposition process, a neg-atively charged electrode is slowly disintegrated by molecu-lar bombardment. The PVD medium is typically argon because this gas generates sufficient momentum to free atoms from the target. In a vac-uum environment, these free target atoms deposit them-selves on the surface of the material and form the desired coating or plating. Maintaining a specified gas mass flow rate to the vacuum cham-ber is critical during the PVD process. Typically, vacuum pumping stations require a throttle valve or orifice-limiting device to control the pump's output when the PVD gas is introduced. This method is extremely pressure sensitive and can result in inefficient gas delivery and poor product quality. Page 25 of 30
    • Applications of PVD include thin decorative coatings on plastic and metal parts such as trophies, toys, pens and pencils, watchcases, and interior trim in automobiles. Chemical Vapor Deposition Chemical vapor deposition (CVD) involves the interaction between a mixture of gases and the surface of a heated substrate, causing chemical decomposition of some of the gas constituents and formation of a solid film on the substrate. The reactions take place in an enclosed reaction chamber. The reaction product (either a metal or a compound) nucleates and grows on the substrate surface to form the coating. Most CVD reactions require heat. However, depending on the chemicals involved, the reactions can be driven by other possible energy sources, such as ultraviolet light or plasma. CVD includes a wide range of pressures and temperatures; and it can be applied to a great variety of coating and substrate materials. Modern interest in CVD is focused on its coating applications such as coated cemented carbide tools, solar cells, depositing refractory metals on jet engine turbine blades, and other applications where resistance to wear, corrosion, erosion, and thermal shock are important. In addition, CVD is an important technology in integrated circuit fabrication. Advantages typically cited for CVD include  Capability to deposit refractory materials at temperatures below their melting or sintering temperatures;  Control of grain size is possible;  The process is carried out at atmospheric pressure it does not require vacuum equipment;  Good bonding of coating to substrate surface. Disadvantages include  Corrosive and/or toxic nature of chemicals generally necessitates a closed chamber as well as special pumping and disposal equipment;  Certain reaction ingredients are relatively expensive;  Material utilization is low. Summary of Physical Vapor Deposition (PVD) Processes PVD Process Features and Comparisons Coating Materials Vacuum Evaporation Equipment is relatively low-cost and simple: deposition of compounds is difficult: coating adhesion not as good as other PVD processes Ag, Al, Au, Cr, Cu, Mo, W Page 26 of 30
    • Sputtering Better throwing power and coating adhesion than vacuum evaporation, can coat compounds, slower deposition rates and more difficult process control than vacuum evaporation Al2O3, Au, Cr, Mo, SiO2, Si3N4, TiC, TiN Ion Plating Best coverage and coating adhesion of PVD processes, most complex process control higher deposition rates than sputtering. Ag, Au, Cr, Mo, Si3N4, TiC, TiN Surface Hardening Surface hardening refers to any of several thermochemical treatments applied to steels in which the composition of the part surface is altered by addition of carbon, nitrogen, or other elements. The most common treatments are carburizing, nitriding, and carbo-nitriding. These processes are commonly applied to low carbon steel parts to achieve a hard, wear-resistant outer shell while retaining a tough inner core. The term case hardening is often used for these treatments. Pack Carborizing Carburizing Carbonitriding Surface Hardening Treatments Gas Carborizing Liquid Carborizing Chromizing Bronizing Gas Nitriding Nitriding Liquid Nitriding Page 27 of 30
    • Carburizing Carburizing is the most common surface-hardening treatment. It involves heating a part of low carbon steel in the presence of a carbon-rich environment so that C is diffused into the surface. In effect the surface is converted to high carbon steel, capable of higher hardness than the low-C core. The carbon-rich environment can be created in several ways. One method involves the use of carbonaceous materials such as charcoal or coke packed in a closed container with the parts. This process, called pack carburizing, produces a relatively thick layer on the part surface, ranging from around 0.6 to 4 mm (0.025 to 0.150 in). Another method, called gas carburizing, uses hydrocarbon fuels such as propane (C3H8) inside a sealed furnace to diffuse carbon into the parts. The case thickness in this treatment is thin, 0.13 to 0.75 mm (0.005 to 0.030 in). Another process is liquid carburizing, which employs a molten salt bath containing sodium cyanide (NaCN), barium chloride (BaCl2), and other compounds to diffuse carbon into the steel. This process produces surface layer thicknesses generally between those of the other two treatments. Typical carburizing temperatures are 875 to 925 C (1600 to 1700 F), well into the austenite range. Carburizing followed by quenching produces a case hardness of around HRC=60. However, because the internal regions of the part consist of low carbon steel, and its hardenability is low, it is unaffected by the quench and remains relatively tough and ductile to withstand impact and fatigue stresses. Nitriding Nitriding is a treatment in which nitrogen is diffused into the surfaces of special alloy steels to produce a thin hard casing without quenching. To be most effective, the steel must contain certain alloying ingredients such as aluminum (0.85% to 1.5%) or chromium (5%or more). These elements form nitride compounds that precipitate as very fine particles in the casing to harden the steel. Nitriding methods include: gas nitriding, in which the steel parts are heated in an atmosphere of ammonia (NH3) or other nitrogen rich gas mixture; and liquid nitriding, in which the parts are dipped in molten cyanide salt baths. Both processes are carried out at around 500o C (950o F). Case thicknesses range as low as 0.025 mm (0.001 in) and up to around 0.5 mm (0.020 in), with hardnesses up to HRC 70. Page 28 of 30
    • Carbonitriding As its name suggests, carbonitriding is a treatment in which both carbon and nitrogen are absorbed into the steel surface, usually by heating in a furnace containing carbon and ammonia. Case thicknesses are usually 0.07 to 0.5 mm (0.003 to 0.020 in), with hardnesses comparable with those of the other two treatments. Two additional surface-hardening treatments diffuse chromium and boron, respectively, into the steel to produce casings that are typically only 0.025 to 0.05 mm (0.001 to 0.002 in) thick. Chromizing Chromizing requires higher temperatures and longer treatment times than the preceding surface-hardening treatments, but the resulting casing is not only hard and wear resistant, it is also heat and corrosion resistant. The process is usually applied to low carbon steels. Techniques for diffusing chromium into the surface include: packing the steel parts in chromium-rich powders or granules, dipping in a molten salt bath containing Cr and Cr salts, and chemical vapor deposition. Boronizing Boronizing is performed on tool steels, nickel- and cobalt-based alloys, and cast irons, in addition to plain carbon steels, using powders, salts, or gas atmospheres containing boron. The process results in a thin casing with high abrasion resistance and low coefficient of friction. Casing hardnesses reach 70 HRC. When boronizing is used on low carbon and low alloy steels, corrosion resistance is also improved. Clad Materials Clad materials re actually a form of composite in which the components are joined as solids, using techniques such as roll bonding, explosive welding, and extrusion. The most common form is a laminate, where the surface layer provides properties such as corrosion resistance, wear resistance, electrical conductivity, thermal conductivity, or improved appearance, while the substrate layer provides strength or reduces overall cost. Alclad aluminum is a typical example. Here surface layers of weaker but more corrosion-resistant single-phase aluminum alloys are applied to a base of high-strength but less Page 29 of 30
    • corrosion-resistant age-hardenable material. Aluminum-clad steel meets the same objective but with a heavier substrate, and stainless steel can be sused to clad steels, reducing the need for nickel- and chromium-alloy additions throughout. Wires and rods can also be made as claddings. Here the surface layer often imparts conductivity, while the core provides strength or rigidity. Copper-clad steel rods that can be driven into the ground to provide electrical grounding for lightning rod systems are one example. References               “Wood Modification - Chemical, Thermal and Other Processes” by C. Hill (Wiley, 2006) BBS. “Process Engineering for Manufacturing” by Eray Johnson. “Coatings Technology Handbook” by Arthur A. Tracton. BASF Handbook on Basics of Coating Technology (American Coatings Literature) Surface Engineering of Light Alloys - Al., Mg. and Ti. Alloys - H. Dong (Woodhead, 2010) BBS “Fundamentals of Modern Manufacturing – Materials, Processes, and Systems” by M.P. Groover. DeGarmo's Manufacturing Engineering and Technology, 10th Edition. Coating Tribology (Properties, Mechanisms, Techniques and applications in Surface Engineering) http://www.packaging-int.com/video/Slot-Curtain-Coating.html International Society of Coating Science and Technology (http://www.iscst.org/) http://www.plasmacoatings.com/coating-types.html http://www.tstcoatings.com/types-of-coatings.html http://www.tstcoatings.com/ http://www.gordonengland.co.uk/coldspray.htm Page 30 of 30