Plasma-Spray Coating as an Alternative to Chromium Plating

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.