Essay On Thermal Spraying

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Following its definition, thermal spraying is a process in which molten, semi-molten or solid particles are deposited on a substrate. Consequently, the spraying technique is a way of generating a ‘stream’ of such particles. Coatings can be generated if the particles can plastically deform at impact with the substrate, which may only happen if they are molten or solid and sufficiently rapid. Their heating and/or acceleration are practical if they occur in a stream of gas.1 Thus, an academic classification of spray techniques is based on the way of generation of such streams. Table 3.1 shows some of the methods of generation of streams of gases. Some other sources of energy input into the gas, other than those listed in the table, are also possible. Photonic heating by focusing of a halogen lamp or a laser beam may generate a small-volume plasma that can be ‘theoretically’ used for spraying. Spray techniques can also be classified following technical and economical demands related to the sprayed coatings. These demands generate the ‘axes’ around which new techniques are being developed. These axes are related to the techniques characterized by the following:

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The layered deposition can also entrap pores, oxides, and unmelted particles (Fig. 8.2), and overlapping splats lock onto one another to form a continuous coating layer. The microstructure of the resultant splats ultimately affects the properties of the coating. Furthermore, certain uncontrollable parameters, such as electrical fluctuations, wear and tear of the gun, choking of powders during feed, inhomogeneity in powder distribution, density of materials, and impurity of carrier gases, can also induce further complexity in controlling the final microstructure and properties of the deposited ceramics as shown earlier in Figure

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