Huacai Protective Coatings

Coatings for use at high temperatures on a superalloy substrate can be defined as a surface layer of material, either ceramic or metallic or combinations thereof, that is capable of precluding or inhibiting direct interaction between the substrate and a potentially damaging environment. This damage can either be metal recession due to oxidation or corrosion, or a reduction in substrate mechanical properties due to the diffusion of harmful species into the alloy at high temperature. Aerospace coatings used on superalloys do not function as inert barriers. Rather, they provide protection by interacting with oxygen in the environment to form dense, tightly adherent oxide scales that inhibit the diffusion of damaging species such as oxygen, nitrogen, and sulfur into the substrate. Coatings must therefore be rich in those elements (such as Al, Cr, or Si) that readily participate in the formation of these protective scales. Essentially, they are reservoirs of those elements; the supply is continually being used to reform new scale to replace that which spalls as a result of thermal cycling or mechanical damage. Thus, by the nature of its protective mechanism, the usable life of a huacai powder coating is governed by its ability to form the desired protective scale and to retain or replace that scale as needed.

By far the largest use of coatings on superalloys is on components in the hotgas section of turbine engines, hot section components, that is, combustors, blades, and vanes. The need for such coatings surfaced in the aircraft engine business in the 1950s when it became apparent that substrate compositional requirements for improved high-temperature strength and optimum high-temperature environmental protection were not compatible. Increasing operating temperatures caused excessive oxidation of the high-strength nickel- and cobalt-base superalloys being used for turbine blades and vanes. This led to the development of the simple aluminide diffusion coatings that solved the oxidation problem. Several of these aluminide coatings are still in use today.