Processing and Properties of Nanostructured Alloy Coatings in the Binary Ni-Cr System
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Processing and Properties of Nanostructured Alloy Coatings in the Binary Ni-Cr System Pablo A. Castro 1,2, Christopher A. Schuh2 and Hong Liang2,3 1
CINI Tenaris, Campana, ARGENTINA; Department of Materials Science and Engineering, MIT, Cambridge, MA. 3 Mechanical Engineering, Texas A&M, College Station, TX. 2
ABSTRACT Using a scalable electrodeposition process, nanocrystalline Ni-Cr alloy coatings have been produced on copper and steel substrates, and the roles of various processing variables on the resulting coating structure and properties have been investigated. The possibility of producing composite coatings of wear resistant nanostructured Ni-Cr alloys containing graphite or MoS2 particles has also been studied; the co-deposition of solid lubricant particles improved the tribological performance of the coatings in ballon-disk testing. The combination of a hard material such as nanocrystalline Ni-Cr alloy with a solid lubricant has a great potential for new surface modification developments. INTRODUCTION Ni-based nanostructured alloys such as Ni-W, Ni-Co, Ni-Cr, etc., are of great interest not only because of their enhanced mechanical properties when compared to conventional microcrystalline alloys, but also because they lend themselves to fundamental studies towards an improved understanding of material behavior in the nanoscale regime [1,2]. Many years ago, Lashmore et al. [3] presented a system for electroplating Ni-Cr alloys based on trivalent chromium, an alloy coating with high hardness, but without the environmental hazard associated with hexavalent chromium coating processes. In the original work of Lashmore et al., the grain sizes of the Ni-Cr deposits were not provided, but based on more recent work in other Ni-alloy electrodeposition systems, there is reason to expect that Ni-Cr would form a nanocrystalline structure. It is the purpose of this work to produce and more thoroughly characterize Ni-Cr deposits based on the solution and parameters proposed by Lashmore et al., and identify the presence of a nanocrystalline structure. Additionally, we present some modifications on the Lashmore process, including the co-deposition of lubricant particles incorporated to enhance wear and lubricity of the Ni-Cr deposits. Whereas solid lubricants are frequently used in industry as coatings, the incorporation of such lubricants into a hard metal coating is not yet well understood. The motivation of this research therefore stems both from fundamental aspects of the processing-structure properties of coatings, as well as the development of new composite coatings for tribological applications. EXPERIMENTAL The chemical composition of the plating bath used in this work is presented in Table I. For all of the experiments we have employed pulse plating, with the typical pulse cycle consisting of: • 21 ms forward: pulses 0.5 ms direct pulse and 0.2 ms off time. • An optional 3 ms reverse pulse (not used for every experiment). • The peak current density was in the range 10-20 A·dm-2.
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