DIFFUSION COATINGS are deposited either by heating the components to be treated in contact with the powder coating material in an inert atmosphere (solid-state diffusion) or by heating them in an atmosphere of a volatile compound of the coating material (out-of-contact gas-phase deposition, or chemical vapor deposition). Solid-state diffusion methods include pack cementation, which is the most widely employed diffusion coating method and the process described in most detail in this article, and various slurry or powder-paint methods of coating metal parts and assemblies. The most widely used coatings are those based on aluminum (aluminizing), chromium (chromizing), and silicon (siliconizing). Substrate materials include nickel- and cobalt-base superalloys, steels (including carbon, alloy, and stainless steels), and refractory metals and alloys.

This article is divided into two major Sections. The first Section describes the widespread use of diffusion coatings for elevated-temperature protection of turbine components for aircraft engines or ground- or marine-based gas turbines. The second Section reviews diffusion-coated ferrous alloys.

High-temperature oxidation-resistant diffusion coatings for molybdenum, niobium, tantalum, and tungsten substrates are described in the article "Surface Engineering of Refractory Metals and Alloys" in this Volume. In addition, information on the use of diffusion coatings for wear resistance, for example, coatings based on boron (boronizing) via pack cementation or complex carbides (vanadium carbide, chromium carbide, and niobium carbide) deposited from salt baths, can be found in Heat Treating, Volume 4 of the ASM Handbook; see the articles "Boriding (Boronizing)" and "Thermoreactive Deposition/Diffusion Process," respectively.

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