Vapor Deposited Coatings

Vapor-deposited coatings are sometimes used for the protection of steel, although the cost of such coatings can be prohibitive. In vapor deposition, whether it be physical vapor deposition (PVD) or chemical vapor deposition (CVD), the coating material is transported to the substrate in the form of individual atoms or molecules. A wide range of coating materials can be applied by vapor deposition. If applied to a sufficient thickness, the coating is essentially pore free and dense, thus providing excellent barrier protection. Processing details for various PVD/CVD coating methods can be found in the articles contained in the Section entitled "Vacuum and Controlled-Atmosphere Coating and Surface Modification Processes" in this Volume.

Ion plating of aluminum coatings on steel is used by the aircraft industry to protect high-strength steel components, such as landing gear, and as a fastener coating to reduce the galvanic effects of steel fasteners inserted in an aluminum structure. Ion plating has several advantages over other aluminum coating processes. The aluminum deposit is very adherent and passes the bend-to-break test. The deposit does not affect the mechanical properties of the substrate and can be applied with precise thickness control on a wide range of shapes. Both the coating and the coating process are nontoxic and do not contribute to pollution. Because of these advantages, ion plating of aluminum can be used in a wide range of applications and is particularly effective as a replacement for cadmium coatings, which can cause hydrogen embrittlement of high-strength steels. Vapor deposition of aluminum does not produce hydrogen; therefore, it does not cause embrittlement. Vapor-deposited aluminum protects against stress-corrosion cracking (SCC) and has a temperature limit of 495 °C (925 °F). It is less expensive than most other barrier coatings. Vapor deposition is also replacing hot dipping of aluminum on steel in some applications. During immersion in hot dipping, a brittle intermetallic compound layer of iron-aluminum is formed; this limits the formability of the steel. In addition, the coating contains iron impurities, which may be a severe shortcoming in highly corrosive environments (Ref 58).

Another important application area for vapor-deposited coatings on ferrous alloys is the use of PVD coatings to enhance the service life of high-speed tool steels. Examples of increased tool life obtained when using TiN and/or TiC PVD coated tools are described in the article "Surface Engineering of Specialty Steels" in this Volume.

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