Plasma-spray application of coating materials that include tungsten carbide has been investigated as an alternative to electroplating of hard chromium onto internal metal surfaces that are required to be protected against wear or to be restored to within dimensional tolerances. Prime examples of such wear surfaces are the inner walls of cylinders in aircraft hydraulic actuators and dampers. The need for an alternative to chromium plating arises partly because chromium plating involves the use of hexavalent chromium, which is a highly toxic carcinogen subject to increasingly stringent government regulation and, therefore, increasingly expensive to use. Another reason for developing an alternative arises from a desire to reduce process time: To remove hydrogen that is unavoidably incorporated during chrome plating, it is necessary to perform a 24-hour bakeout. Process time could be reduced substantially if this bakeout could be eliminated. Plasma spraying involves fewer process steps than does electroplating, and for plasma-sprayed coatings, no hydrogen bakeout is necessary.

This Steel Cylinder, having a 3-in. (7.62-cm) inside diameter, was coated on its inside by plasma spraying of a WC/Co mixture incorporating a Ni-based self-fluxing alloy. At one end, the coating was ground with a diamond wheel in a test of finishing techniques.
What makes it feasible to consider plasma spraying of carbides as an alternative to chrome plating is the recent commercial development of miniature plasma spray guns. The limitations of the process are primarily the minimum inner diameter and the maximum axial length that can be coated. For a given plasma gun, the minimum coatable inner diameter is defined by the size of the gun plus the standoff (the required distance between the gun and the surface to be coated). The maximum coatable axial length is ordinarily determined by the length of an extension that carries the gun; however, if the extension is too long, the extensionand- gun assembly is not mechanically stable enough to be moved and operated with the necessary precision.

The coating materials investigated included a WC/Co mixture, two slightly different WC/Co mixtures incorporating a Ni-based self-fluxing alloy (see figure), a Co/Mo/Cr/Si alloy), and aWC/CrC/Ni mixture. In tests, the sliding and abrasive- wear performances of the plasmasprayed carbide-based materials were found to be, variously, equivalent or superior to those of electroplated hard chromium. Electrochemical corrosion measurements revealed corrosion performances inferior to those of electroplated hard chromium, indicating that these materials are not suitable for use in high-corrosion environments. However, they are well suited for such low-corrosion environments as the interiors of hydraulic dampers and actuators.

As part of the investigation, a cost analysis of plasma spraying of carbide coatings versus chrome plating was performed. It was found that for cylinders having diameters of 4 in., in which larger plasma spray guns can be used, the cost of plasma spraying is comparable to that of electroplating of hard chromium. For smaller diameters, the cost of plasma spraying was found to be somewhat greater than that of chrome plating. However, in some applications, the greater cost of plasma spraying might be justified by the reduction in process time.

This work was done by Keith O. Legg of the Rowan Technology Group; Bruce D. Sartwell and Jean-Gabriel Legoux of the National Research Council Canada; Montia Nestler and Christopher Dambra of Sulzer-Metco; Daming Wang and John Quets of Praxair Surface Technologies; Paul Natishan of the Center for Corrosion Science; Philip Bretz of Metcut Research Inc.; and Jon Devereaux of the Naval Air Depot of Jacksonville, FL, for the Naval Research Laboratory. For further information, download the free white paper at www.defensetechbriefs.com under the Materials category. NRL-0002


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Plasma-Spray Coating as an Alternative to Chromium Plating

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This article first appeared in the February, 2007 issue of Defense Tech Briefs Magazine.

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