Effect of Target TBC Thickness on the TBC Application Rate, Microstructure and Properties

Thermally sprayed TBCs are routinely applied on many hot gas path components in gas turbines. For a certain chemistry and microstructure, the thermal resistance of a TBC is proportional to its thickness. Therefore, depending upon the operating environment, different components (or different locations on a given component) may require different coating thicknesses. In production settings, the design required target TBC thicknesses are typically achieved by varying the number of spray passes (wherein higher thickness corresponds to higher number of passes). Interestingly, on real components requiring varying thicknesses, it was consistently observed that the TBC thickness buildup rate (application rate) depended on the ‘target’ thickness in such a way that the average application rate was lower for thicker coatings, especially when the coating thickness exceeds 1 mm. From a practical perspective, this behavior, leads to undesirable increase in processing time and cost. It is also important to understand the implications of this behavior for the models used in offline programming tools, coating microstructure and related properties. This study was undertaken to investigate the mechanisms leading to this nonlinear dependence of TBC thickness on the number of spray passes and its effects on the coating microstructure and properties.