The influence of YSZ powder morphology and microstructure on the surface roughness of Thermal Barrier Coatings (TBCs) applied in Industrial Gas Turbines (IGTs)

The primary ceramic material used in Thermal Barrier Coatings (TBCs) is yttria-stabilized zirconia (YSZ), typically containing 7-8 wt.% yttria. Original Equipment Manufacturers (OEMs) generally require that the surface roughness of the ceramic top coat meets specific targets to ensure optimal turbine efficiency and aerodynamic performance. Achieving the desired roughness after spraying is uncommon, requiring so manual or automated finishing. Powder manufacturing and coating microstructure are known to affect the ceramic top coat's tribological properties and surface roughness.

This study aimed to examine the surface roughness of single-layer ceramic coatings in both as-sprayed and finished conditions. These coatings were deposited using Atmospheric Plasma Spraying (APS) with an advanced cascade plasma torch. Standard 7-8% YSZ powders from different manufacturing processes were employed, resulting in both porous and Dense-Vertically Cracked (DVC) coating microstructure. All coatings were applied on Hastelloy-X substrates with an HVOF + APS “Flash” NiCoCrAlY bond coat.

The surface roughness of the samples, both in their as-sprayed state and after the finishing process, was evaluated using a profilometer and a roughness measuring device. Furthermore, the FEG-SEM technique was utilized to analyze all coating systems, offering insights into the mechanisms that could affect their roughness characteristics.