Coating of tools

The third part identifies and describes the present and potential applications of CVD in semiconductors and electronics, in optics and optoelectronics, in the coating of tools, bearings and other wear- and corrosion-resistant products, and in the automobile, aerospace, and other major industries. 

Chemical vapor deposition (C VD) is a versatile process suitable for the manufacturing of coatings, powders, fibers, and monolithic components. With CVD, it is possible to produce most metals, many nonmetallic elements such as carbon and silicon as well as a large number of compounds including carbides, nitrides, oxides, intermetallics, and many others. This technology is now an essential factor in the manufacture of semiconductors and other electronic components, in the coating of tools, bearings, and other wear-resistant parts and in many optical, optoelectronic and corrosion applications. The market for CVD products in the U.S. and abroad is expected to reach several billions dollars by the end of the century. 

Contrary to the new carbon materials presented so far, diamond films are a product that is already being employed with many variants in large scale applications. Especially the coating of tools and fast turning parts represents a considerable market, but there are also electronic uses and synthetic optical windows on offer. Some of the important fields of appHcation are presented in more detail below. 

Refractory compound coatings of carbides, nitrides, and oxides on cemented carbide cutting tools, mainly by the CVD process, are estimated at 300 X 10 annually worldwide. 

Gel coats are typicaHy used to provide a part with a finished surface directly from the mold. Various inserts, stiffeners, and mechanical attachments can be incorporated in the mol ding step, thereby further reducing secondary operations. Final edge trimming is accompHshed with a variety of tools such as... 

Fig. 12. (a) A cross section of multiple coatings of TiN on TiC on a silicon nitride-based tool material (b) multicoatings on a SiAlON-based tool material. 

Chromium carbide is important in powder preparations designed for thermal spray apphcations of corrosion and wear-resistant coatings on tool and machine parts. Lower carbon carbides of chromium are important in hardfacing tods and electrodes for weld-apphed ovedays on machine wear surfaces. However, these carbides are usually formed in situ from Cr and C in the rod and not added as preformed carbides. The properties of Ci2C2 are hsted in Table 2. 

In addition to electrical uses, epoxy casting resins are utilized in the manufacture of tools, ie, contact and match molds, stretch blocks, vacuum-forrning tools, and foundry patterns, as weU as bench tops and kitchen sinks. Systems consist of a gel-coat formulation designed to form a thin coating over the pattern which provides a perfect reproduction of the pattern detail. This is backed by a heavily filled epoxy system which also incorporates fiber reinforcements to give the tool its strength. For moderate temperature service, a Hquid bisphenol A epoxy resin with an aHphatic amine is used. For higher temperature service, a modified system based on an epoxy phenol novolak and an aromatic diamine hardener may be used. 

The Dilex Process utilises a molten lead bath as transfer medium and is applicable to diflfusion coatings of Cr, Al, Ti, Mo, Ni and Co. Finally, a Japanese fused borate bath process produces carbide coatings (Cr, V, Nb or Ta) on carbon and tool steels. The coatings are wear and corrosion resistant. The TD Process uses this technique. 

Hot-Wall Reactors. A hot-wall reactor is essentially an isothermal furnace, which is often heated by resistance elements. The parts to be coated are loaded in the reactor, the temperature is raised to the desired level, and the reaction gases are introduced. Figure 5.6 shows such a furnace which is used for the coating of cutting tools with TiC, TiN, and Ti(CN). These materials can be deposited alternatively under precisely controlled conditions. Such reactors are often large and the coating of hundreds of parts in one operation is possible (see Ch. 18). 

Figure 5.6. Production CVD reactor for the coating of cutting tools.

CVD suffers these limitations to a lesser degree and, as a result, is being used increasingly in many industrial applications, particularly those operating in extreme conditions. It is often the best solution to severe problems of erosion, friction, or hot corrosion. A special case must be made forthe coating ofcutting tools, which is amaj or industrial application of CVD and is reviewed separately in Ch. 18. 

Coatings play a vital part in the cutting-tool industry and this is where CVD technology has made some of its most important gains. As an example, CVD films of titanium carbide on cemented carbide tools were first commercialized in the early 1960s and their use has continuously increased ever since. Today, the percentage of tools that are coated by either PVD or CVD depends on the type of tool as shown in Table 18.1 (in 1996). 

Schintlmeister, W., Wallgram, W., and Kanz, J., Properties, Applications and Manufacture of Wear-resistant Hard Material Coatings for Tools, Thin SolidFilms, (107) 117-127 (1983)... 

Stjernberg, K., and Thelin, A., Wear Mechanisms of Coated Carbide Tools in Machining of Steel, Prac. ASM Int. Conf. on High Productivity Machining, Materials and Processing, Paper No. 8503-004, ASM, Metals Park, OH 44073 (May, 1985)... 

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