Materials Performance

JUN 2019

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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32 JUNE 2019 W W W.MATERIALSPERFORMANCE.COM COATINGS & LININGS S This article describes using physical vapor deposition (PVD) coatings to provide corrosion protection for metallic substrates. Historically, the corrosion protection offered by PVD coatings has been inadequate due to the presence of defects in the coat- ings, including macroparticles, flakes, pores, and the porous columnar structure of the coatings. However, a modified PVD coating process was introduced that produces coatings containing a much lower number of macroparticles and other defects, thereby significantly improving corro- sion resistance. A case study is pre- sented where a CrN coating pro- duced by the novel PVD process was used by the U.S. Army to replace con- ventional plated coatings. St e el i s on e of th e m o st imp or t ant engineering materials due to its combina- tion of low cost and excellent performance in strength, ductility, machinability, and formability. 1 However, the corrosion resis- tance of most steels is relatively poor in corrosive environments and to provide adequate corrosion resistance, steels often require the use of a protective coating. Commonly used coatings include paints, p o ly m e r s , e l e c t r o p l a t i n g , c o nv e r s i o n coatings, and galvanizing. These types of coatings suffer from problems that place severe constraints on their application at high temperatures, lack of wear resistance, or environmental problems associated with the coatings themselves. Another class of coatings that can be applied to steels are thin-film, hard coatings produced by physical vapor deposition (PVD) processes, such as cathodic arc evap- oration (CAE). Examples of these types of PVD coatings include TiN, CrN, AlCrN, and TiAlN. These metal nitride coatings are chemically inert 2 and should, therefore, pro- vide excellent corrosion protection for steel substrates. In practice, however, numerous studies have shown that the corrosion pro- tection provided by most PVD coatings is relatively poor. The poor corrosion protec- tion of PVD coatings has been related to defects in the coatings, 1,3-4 such as macro - particles, f lakes, pores, and the porous columnar structure of the coatings. 1 One potential solution is the use of thicker PVD coatings (up to 18 µm), which can provide better corrosion protection. 3,5 However, in addition to the cost issue, high thickness PVD coatings can be problematic in many commercial applications, such as those requiring wear resistance due to the brittle nature of thick coatings. 3 So PVD coatings are normally not thicker than 5 to 6 µm. P VD c o ati n g s mu st b e e ss enti al ly defect-free to achieve their potential for providing excellent corrosion protection. The penetration of corrosive media through the coating to the substrate must be pre- vented 6 by coatings with a dense, equiaxed grain structure. 5 This article describes a High-Quality Physical Vapor Deposition Coatings for Corrosion Protection Applications d avid b ell and v iktor k hominich, Phygen Coatings, Inc., Minneapolis, Minnesota, USA s teve m idson, The Midson Group, Denver, Colorado, USA c hristo P her m ulli G an, U.S. Army Armament Research, Development, and Engineering Center—Benét Laboratories, Watervliet, New York, USA COATINGS & LININGS

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