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Monday, May 14, 2007 - 2:50 PM

Standardization of Metallographic Practices for Thermally Sprayed Coatings on Common and State-of-the-Art Coating Applications

G. Lucas, Buehler Ltd., Lake Bluff, IL

Thermally-sprayed coatings are applied to various materials to provide protection for heat-loaded parts as well as corrosion and wear. They also serve as a thermal barrier to reduce the effects of high temperatures on the substrate material. Microstructural analysis is used to characterize the substrate, the coating, and the adherence of the coating to the substrate. Accurate evaluation of these coatings depends heavily upon the quality of the polished surface produced by metallographic preparation. A well-prepared surface must initially be free of plucked material and have minimal relief between the hard and soft constituents. The entire surface must be flat to allow a clear view of the interface and any reaction products that might be present in the coating or substrate. Furthermore, the final polished surface must be free of scratches and have no resolution destroying films or stains. Various coating and substrate materials were evaluated to demonstrate the excellent microstructural detail that is revealed when correct polishing procedures are used. These special techniques successfully address the problems normally encountered when preparing hard, brittle coatings or soft, ductile coatings and substrates. As a result, clear microstructures are revealed across the specimen surface and valuable information is obtained.


Summary: Proper metallographic techniques reduce the errors in evaluation of thermally-sprayed coatings and reveal the true characteristics of the coating and substrate.