A Novel Single-Step Surface-Treatment Process for Forming Cr-Nitride Coatings on Steels
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nitride coatings have been applied to metal surfaces for decades, mostly to enhance the wear resistance of metal components.[1–3] More recently, such coatings have also been assessed as corrosion-resistant coatings for biological applications and for use on metal alloy bipolar plates in proton exchange membrane fuel cells where low interfacial electrical contact resistance is also of critical importance.[4–7] At present, such coatings are applied to metal surfaces almost exclusively by physical vapor deposition (PVD) methods such as magnetron sputtering. Such PVD methods offer advantages of low-temperature deposition but have line-of-sight restrictions, i.e., they are not easily applied to coat components of complex shapes or internal surfaces. In contrast, thermochemical surface-treatment process such as gas nitriding has no such limitations and is suited for high-volume production purposes[8,9] However, steels normally contain only up to 25 wt pct Cr, which is not high enough to allow an external layer of Cr-nitride to form by selective nitridation.[10–13] Thus, nitriding cannot be used alone to produce an external Cr-nitride layer on steel surfaces; it must be used in combination with other surface-treatment processes such as Cr-platting or chromizing.[14–16] Usually, one of the latter two processes
X.J. LU and Z.D. XIANG are with The State Key Laboratory of Refractories and Metallurgy, School of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, P.R. China. Contact e-mail: [email protected] Manuscript submitted August 25, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
is applied first to raise the surface Cr concentration prior to nitriding, although it has been shown more recently that nitriding followed by chromizing may also lead to the formation of a thin external Cr-nitride layer on steel surface under suitable conditions.[17] However, such a two-step process is inefficient, and hence, a single-step process would be preferred in practice. The powder–pack process or pack cementation process is a thermochemical surface-treatment process for forming diffusion coatings on metal alloy surfaces.[18–20] It differs from the nitriding process not only in the way the process is applied but also in terms of the numbers of coating elements that may be diffused into the substrate. In a nitriding process, only N is used to react with metal substrates to form a hard surface layer, whereas in a pack cementation process, multiple alloy elements can be diffused into the substrate to synthesize a coating with varying microstructures and functionalities.[20–24] However, this process has not been investigated as a viable method to produce an external Cr-nitride surface layer on metal alloys, probably because it is counterintuitive that the conditions necessary to form such a coating can be generated in such a process in which chromizing and nitriding must take place simultaneously. The results to be presented in the following will show that this is indeed possible if pack–powder mixtures are p
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