Thermal Barrier Coatings (TBCs) have been used for several decades in the aeronautic industry to protect components exposed to high temperatures. The advancement of novel deposition techniques, such as axial-Solution Precursor Plasma Spraying (SPPS), has created new opportunities for achieving columnar microstructures. SPPS combines the advantages of traditional methods, such as Atmospheric Plasma Spraying (APS) and Electron Beam Physical Vapor Deposition (EB-PVD). In fact, SPPS allows for columnar coatings similar to those obtained through EB-PVD, while being as cost-effective as APS.

This study examines the microstructural evolution of Axial SPPS-deposited 8 wt.% YSZ coatings, focusing on the effects of bond coat roughness, substrate geometry, and in-flight particles on column formation. The results indicate that as-deposited bond coat roughness (Ra: 6.59μm) starts to yield columns at a thickness of 40 microns, while a finer bond coat roughness (Ra: 1.66μm) leads to vertical cracks. It was also observed that the column formation was accompanied by the development of a cauliflower-like structure at the sample’s surface.

Moreover, substrate dimensions significantly influenced the columnar microstructure, as different geometries affected the gas streamlines, thereby influencing the trajectories of the small particles. One of the consequences was a microstructural switch from columns to vertical cracks in the microstructure near the edge of the samples. This unprecedented phenomenon, which we named “Edge effect,” and an explanation theory is provided in this report.

By providing insights into the column formation of 8% YSZ top-coats deposited by axial-SPPS, this study aids in optimizing this emerging coating process for aeronautic applications, highlighting the essential role of bond coat roughness and substrate geometry in resulting coating microstructure