Development of Tailored Metallographic Preparation Techniques for Thermally Sprayed Coatings

Abstract

Abstract In order to properly characterize the entire deposition process, evaluation of the coating, including a reliable metallographic preparation technique which reveals the true microstructure, must be performed. Often, recommended metallographic sample preparation methods for thermally sprayed coatings are generic and are not tailored to specific materials. They are time-consuming and, in some cases, may provide inaccurate details (pull-outs, smearing, etc). This could lead to a wrong interpretation of the coating quality. The aim of the investigation was to develop new metallographic sample preparation procedures tailored to different types of coatings (metallic, ceramic, multilayer and composites), in order to reveal a more representative microstructure. A comparative study of different preparation procedures for the examination of various as-sprayed coatings is presented using an optical microscope. The coatings were deposited by atmospheric and vacuum plasma spray (APS and VPS) and high velocity oxygen fuel (HVOF) processes. A separate approach is recommended for choosing the right metallographic preparation procedure for ceramic, metallic, or composite coatings. Applied load and positioning of the mounted sample during preparation are identified as key factors in developing proper procedures. The microhardness of the coating must be considered when determining the applied load. Interesting practical trends in preparation procedures that may lead to superior coating representation and, in some instances, cost and time savings are presented.