Investigation of Plasma Spray Coatings as an Alternative to Hard Chrome Plating on Internal Surfaces

Hard chromium electroplating is extensively used by aircraft manufacturers and military maintenance depots to provide wear and/or corrosion resistance or to restore dimensional tolerance to components. However, chrome plating utlizes hexavalent chromium, which is a highly toxic carcinogen, with increasingly stringent government regulations making it more expensive for DoD. This document constitutes the final report on an investigation of deposition of coatings using miniature plasma spray guns that could replace hard chromium on internal surfaces where conventional thermal spray technologies could not be used. The coatings investigated included WC/Co, WC/Co with an Ni self-fluxing alloy, WC/CrC/Ni, and Tribaloy 400. Materials tests showed that all of the carbide coatings demonstrated sliding and abrasive wear performance equivalent or superior to hard chromium. Electrochemical corrosion measurements generally showed inferior corrosion performance. A cost analysis indicated that plasma spraying was comparable to hard chromium using larger guns on cylindrical components with internal diameters of the order of 4" but for smaller diameters the plasma spray costs would be somewhat higher. However, turnaround times could be greatly reduced because of fewer process steps, including the elimination of the 24-hour bakeout to remove hydrogen incorporated during chrome plating.