Strategy to Improve the High-Temperature Mechanical Properties of Cr-Alloy Coatings
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Electrodeposited chromium coatings are extensively employed across many industries and can be plated from either hexavalent or trivalent baths.[1] For many years, the hexavalent bath has been used to produce ‘‘hard chromium’’ coatings with good wear and corrosion resistance;[2,3,4] however, these are quite temperature sensitive, and their contact and wear properties decline rapidly at elevated service temperatures.[5,6,7] In contrast, chromium plated from the trivalent state frequently contains metalloid alloying elements, which fundamentally change the evolution of the coating microstructure upon heating;[6–9] unlike ‘‘hard chromium’’ coatings, such alloy deposits harden considerably upon heating. In particular, carbon incorporation from a trivalent bath increases hardness after annealing by suppressing chromium crystallization and by precipitation of carbides that contribute to grain boundary pinning. We have recently proposed that Cr-C coatings derived from a trivalent bath may have broad applications in elevated-temperature environments.[10,11] As an added benefit, the health and environmental concerns about trivalent baths are dramatically lower than for hexavalent ones.[12] M.J.L. GINES, Visiting Scientist, formerly with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, is Senior Researcher, Department of Surface Chemistry and Coatings, CINI-TENARIS, B2804MHA Campana, Buenos Aires, Argentina. F.J. WILLIAMS, Surface Chemistry and Coating Department Head, is with the Department of Surface Chemistry and Coatings, CINI-TENARIS. C.A. SCHUH, Salapatas Associate Professor of Metallurgy, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology. Contact e-mail: [email protected] Manuscript submitted January 8, 2007. Article published online June 13, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
In spite of the advantages of Cr-C alloy deposits, the range of temperatures for which these coatings are suitable remains limited (
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