iTSSe TSS A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 2 4 3 iTSSe TSS RECOGNIZING ARTIFACTS IN AIR PLASMA SPRAY THERMAL BARRIER COATING MICROSTRUCTURES Identifying metallographic TSC artifacts, specifically in air plasma spray zirconia-based thermal barrier coatings. The microstructure of a thermal spray coating (TSC) is typically a primary indicator of the coating quality, its functional properties, and proper deposition and processing. Due to the nature of the TSC deposition process, the microstructures are not homogeneous, comprising multiple phases and phase boundaries, as well as layers and directionality. As a result, TSC microstructures are prone to interactions with common metallographic procedures that may result in artifacts and misinterpretation of the TSC microstructure. Many technical papers have been published on the best practices for metallographic preparation of TSCs[1,2]. Any of several metallographic preparation methods have been shown to produce a true and reproducible TSC microstructure. For comparative purposes, the TSCs used in this case were deposited simultaneously and polished using variants of ASTM E 1920-03 Method-II[3]. Artifacts that result from specific sectioning and mounting practices, as well as from different polishing times, are presented. TYPICAL APS MICROSTRUCTURAL FEATURES The air plasma spray (APS) thermal barrier coating (TBC) is typically a bi-layer coating comprised of a softer, relatively ductile, metallic bond coating (BC) and a harder, porous, lower ductility ceramic top coating (TC). In the TC, porosity and cracks are key to its function as a thermal barrier layer in aero and power generation turbine engines. In the BC, features related to cohesion between splats and adhesion to the substrate (porosity, oxidized boundaries, unmelted particles, cracks) are indicators of TBC service life at high temperature. A simplified drawing of key microstructural TSC features highlights the degree of inhomogeneity present in these coatings (Fig. 1). In addition to three distinct layers that make up a TBC (the substrate alloy, the BC, and the TC), there are other features common to TSCs in general such as voids or pores, oxidized particles in the metallic BC, metallic inclusions in the TC, unmelted or spherical particles, grit particles at the substrate-BC interface, vertical micro-cracks, and horizontal debonding cracks. Some amount of each of these features are allowable in a TBC TSC and are defined in the specific quality requirements for the coating. TBC METALLOGRAPHIC PROCESS Metallographic processing of TBCs follows the same general steps as those of wrought material, that is, sectioning, mounting, planar grinding, polishing, microscopy, and analysis. An exception is that chemical etching of the TSC is not typically done prior to microscopy. As described in other sources, the magnitude and direction of forces during sectioning can damage the TBC significantly, such that subsequent metallographic steps cannot remove the damage. Similarly, mounting and polishing methods also affect loading on internal microstructure interfaces and phase boundaries; consequently, these features can be changed by the method of mounting and polishing that is employed. Accepted metallographic practices for thermal spray coatings are well documented[1,2] and specific deviations from accepted methods are used in this article to create artifacts in the TBC FEATURE 9 Fig. 1 — Key TSCmicrostructural features[1].
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