Understanding column-formation in axial-SPPS thermal barrier coatings: Evolution of microstructure and role of bond coat roughness

Columnar yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) are renowned for their exceptional resistance to thermal cyclic fatigue (TCF) and their consequent role in extending the service life of gas turbine components. Traditionally, such coatings have been produced by electron beam physical vapor deposition (EB-PVD) and, more recently, by suspension plasma spraying (SPS). Latest studies demonstrating the capability of the aqueous solution precursor route to fabricate columnar YSZ TBCs using axial plasma spraying indicate its considerable potential to overcome shortcomings associated with both EB-PVD and SPS methods. In this work, the microstructural evolution of axial solution precursor plasma-sprayed (SPPS) 8 wt.% YSZ coatings is investigated, with emphasis on the role of bond coat roughness. In-flight particle generation and splat formation have been carefully examined to obtain insights into column development. Results show that a coarse bond coat surface promotes column initiation, whereas a much smoother surface favours vertical cracking, not quite leading to column formation. Careful collection of particles generated in flight confirmed their size to be predominantly in the 100-500 nm range (d50 ≈ 280 nm), leading to largely sub-micron splats (d50 ≈ 465 nm). It is postulated that such fine sizes are inadequate to rapidly roughen the growing surface for spontaneous column initiation, thereby making the initial bond coat roughness crucial in tailoring TBC microstructures. In this context, the concept of a certain ‘threshold roughness’ being necessary to trigger column formation is also proposed and needs further investigation.