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Kalpakjian 20 Surface Treatments Coatings, and Cleaning 34.I Introduction 973 1 The preceding chapters have described methods of producing desired shapes 34.2 Mechanical Surface from a Wide variety of materials; although material and process selection is Treatments 974 very important, often the surface properties of a component determine its 34.3 Mechanical Plating performance or commercial success. and Cladding 976 34.4 Case Hardeningand ' This chapter describes various surfacefinishing operations thatcan be performed Hard Facing 976 for technical and aesthetic reasons subsequent to manufacturing a part. 34.5 Thermal Spraying 977 34.6 Vapor Deposition 979 ° The chapter presents the surface treatment, cleaning, and coating processes 34.7 Ion lmplantation and that are commonly performed and includes a discussion of mechanical surface Diffusion Coating 982 treatments such as shot peening, laser peening, and roller burnishing, with the 34.8 LaserTreatments 982 benefit of imparting compressive residual stresses onto metal surfaces. 34.9 Electroplating, Electroless Plating, ° Coating operations are then examined, including cladding, thermal spray and Electroforming 983 operations, physical and chemical vapor deposition, ion implantation, and 34.l0 Conversion Coatings 986 electroplating. 34.I! Hot Dipping 987 34.l2 Porcelain Enameling; ° The benefits of diamond and diamond-like carbon coatings are also investigated. Ceramic and Organic ° Finally, surface-texturing, painting, and cleaning operations are described. Coatings 988 34.I3 Diamond Coating and Diamond-like Carbon 989 34.I4 Surface Texturing 990 34.I Introduction 34.I5 Painting 990 34.I6 Cleaning of Surfaces 99| After a part is manufactured, some of its surfaces may have to be processed further EXAMPLES: in order to ensure that they receive certain properties and characteristics, It may be 34.I Repair ofaWorn necessary to perform surface treatments in order to Turbine-engine Shaft by Thermal Spraying 979 ° Improve resistance to wear erosion, and indentation (e.g., for machine-tool 34.2 Applications of Laser slidevvays, as shown in Figs. 23.2 and 35.1, Wear surfaces of machinery, and Surface Engineering 983 shafts, rolls, cams, and gears) 34.3 Ceramic Coatings for High-temperature ° Control friction (on sliding surfaces of tools, dies, bearings, and machine Applications 989 Ways) ° Reduce adhesion (of electrical contacts) ° Improve lubrication (surface modification to retain lubricants) ° Improve resistance to corrosion and oxidation (on sheet metals for automobile bodies, gas-turbine components, food packaging, and medical devices) 973 CHAPTER Chapter 34 Surface Treatments, Coatings, and Cleaning ° Improve fatigue resistance (of bearings and shafts with fillets) ° Rebuild surfaces (on worn tools, dies, molds, and machine components) ° Modify surface texture (appearance, dimensional accuracy, and frictional characteristics) ° Irnpart decorative features (color and texture). Numerous techniques are used to impart these characteristics to various types of metallic, nonmetallic, and ceramic materials on the basis of mechanisms that involve (a) plastic deformation of the workpiece surface, (b) chemical reactions, (c) thermal means, (d) deposition, (e) implantation, and (f) organic coatings and paints. We begin with surface-hardening techniques and then continue with descrip- tions of different types of coatings that are applied to surfaces by various means. Some of these techniques also are used in the manufacture of semiconductor devices, as described in Chapter 28. Techniques that are used to impart texture on workpiece surfaces and types of organic coatings used for various purposes are then described. The chapter ends with a discussion of methods used for cleaning manufactured surfaces before the components are assembled into the completed product and made ready for service. Environmental considerations regarding the fluids used and the waste material from various surface-treatment processes are also included, as they are an important fac- tor to be considered. 34.2 Mechanical Surface Treatments Several techniques are used to mechanically improve the surface properties of man- ufactured components. The more common methods are the following: Shot Peening. In shot peening, the workpiece surface is impacted repeatedly with a large number of cast steel, glass, or ceramic shot (small balls), which make over- lapping indentations on the surface. This action, using shot sizes that range from 0.125 to 5 mm in diameter, causes plastic surface deformation at depths up to 1.25 mm. Because the plastic deformation is not uniform throughout the part’s thickness, shot peening causes compressive residual stresses on the surface, thus improving the fatigue life of the component by delaying the initiation of fatigue cracks. Unless the process parameters are controlled properly, the plastic deformation of the surface can be so severe that it can damage the surface. The extent of deformation can be re- duced by gravity peening, which involves larger shot sizes, but fewer impacts on the workpiece surface. Shot peening is used extensively on shafts, gears, springs, oil-well drilling equip- ment, and jet-engine parts, such as turbine and compressor blades. However, note that if these parts are subjected to high temperatures, the residual stress will begin to relax (thermal relaxation) and their beneficial effects will be diminished greatly. An example is gas-turbine blades performing at their operating temperatures. Laser Shot Peening. In this process, also called laser shock peening and first devel- oped in the mid-19605 (but not commercialized until much later), the workpiece surface is subjected to planar laser shocks (pulses) from high-power lasers. This surface-treatment process produces compressive residual-stress layers that are typi- cally 1 mm deep with less than 1% of cold working of the surface. Laser peening has been applied successfully and reliably to jet-engine fan blades and to materials such as titanium, nickel alloys, and steels for improved fatigue resistance and some corro- Section 34.2 Mechanical Surface Treatments sion resistance. Laser intensities necessary for the process are on the order of 100 to 300 ]/cmz and have a pulse duration of about 30 nanoseconds. Currently, the basic limitation of laser shot peening for industrial, cost-effective applications is the high cost of the high-power lasers (up to 1 kW) that must operate at energy levels of 100 J/pulse. Water-jet Peening. In this more recently developed process, a Water jet at pres- sures as high as 400 MPa impinges on the surface of the workpiece, inducing com- pressive residual stresses and surface and subsurface hardening at the same level as in shot peening. The water-jet peening process has been used successfully on steels and aluminum alloys. The control of process variables (jet pressure, jet velocity, the design of the nozzle, and its distance from the surface) is important in order to avoid excessive surface roughness and surface damage. Ultrasonic Peening. This process uses a hand tool based on a piezoelectric trans- ducer. Operating at a frequency of 22 kHz, it can have a variety of heads for differ- ent applications. Roller Burnishing. In this process, also called surface rolling, the surface of the component is cold worked by a hard and highly polished roller or set of rollers. The process is used on various flat, cylindrical, or conical surfaces (Fig. 34.1). Roller burnishing improves surface finish by removing scratches, tool marks, and pits and induces beneficial compressive surface residual stresses. Consequently, corrosion resistance is improved, since corrosive products and residues cannot be entrapped. In a variation of this process called low-plasticity burnishing, the roller travels only once over the surface, inducing residual stresses and minimal plastic deformation. Internal cylindrical surfaces also are burnished by a similar process, called ballizing or ball burnishing. In this process, a smooth ball (slightly larger than the bore diameter) is pushed through the length of the hole. Roller burnishing is used to improve the mechanical properties of surfaces as Well as their surface finish. It can be used either by itself or in combination with other finishing processes, such as grinding, honing, and lapping. The equipment can be mounted on various CNC machine tools for improved productivity and consis- tency of performance. All types of metals (soft or hard) can be roller burnished. Roller burnishing is typically used on hydraulic-system components, seals, valves, spindles, and fillets on shafts. "2 Holler Burnished el Roller <a> <b> <c> FIGURE 34.I Burnishing tools and roller burnishing of (a) the fillet of a stepped shaft to induce compressive surface residual stresses for improved fatigue life; (b) a conical surface; and (c) a flat surface. Chapter 34 Surface Treatments, Coatings, and Cleaning Explosive l-lardening. In explosive hardening, the surfaces are subjected to high transient pressures through the placement and detonation of a layer of an explosive sheet directly on the workpiece surface. The contact pressures that develop as a re- sult can be as high as 35 GPa and can last about 2 to 3 /as. Major increases in sur- face hardness can be achieved with this method, with very little change (less than 5%) in the shape of the component. Railroad rail surfaces, for example, are explo- sively hardened. 34.3 Mechanical Plating and Cladding Mechanical Plating. In mechanical plating (also called mechanical
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