Coating Characteristics of Plasma-Sprayed Ceramic Thermal Barrier Coatings on Internal Diameter (ID) Surfaces
Maheshwar Rao Bagathi, Robert Vaßen, Olivier Guillon, Georg Mauer
Abstract
Abstract Thermal barrier coatings (TBCs) are essential for insulating and protecting components in high-temperature environments, with applications traditionally focused on external surfaces. However, the development of TBCs for internal diameters (ID), particularly for small bores below 200 mm, poses significant challenges due to the limited understanding of coating formation mechanisms in confined geometries and the constraints of process conditions. Therefore, this study aims to elucidate the characteristics of plasma-sprayed ceramic TBCs on ID surfaces, with the goal of establishing a relationship between process parameters, microstructure, and coating properties for 8 wt.% yttria-stabilized zirconia (8YSZ) topcoats applied via atmospheric plasma spraying (APS) using the SM-F100 CONNEX ID torch. The key APS parameters, namely the hydrogen flow rate, current, and spray distance, were investigated using a design of experiments (DOE) methodology to systematically study both the individual and interactive effects of these parameters on coating systems such as porosity and deposition efficiency, which are critical features of TBC performance. Since ID coatings are constrained by limited space for spray distance and torch placement inside the tubular component, the selection of the appropriate feedstock becomes challenging. Therefore, feedstocks with different morphologies and sizes were investigated and evaluated based on the resulting porosity and deposition efficiency. An agglomerated and sintered 8YSZ feedstock showed the most promising results. In addition, this study also examined the impact of substrate geometry (flat versus curved surfaces) on coating properties and process control. The results contribute to the broader scientific understanding of plasma spray technology for internal diameter (ID) surfaces.