Development of Optimized Thermal Barrier Coatings by Suspension Plasma Spraying

Yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) deposited by suspension plasma spraying (SPS) show benefits compared to conventional atmospheric plasma sprayed (APS) coatings. They provide a higher density of vertical segmentation cracks, enabling higher strain tolerance and lower Young’s modulus, with still relatively high porosity for good thermal insulation. Advantages in terms of coating properties were demonstrated, such as reduced thermal conductivity, improved optical properties and enhanced thermal cycling lifetime at moderate surface conditions. A key approach to optimize the coating microstructure and especially to reduce the amount of embedded unmolten material (overspray) is to improve the injection of the suspension into the plasma jet. Particularly, the characteristics of the suspension atomization obtained by a two-phase atomizer were investigated and the effects of nozzle geometry and atomization parameters were studied.

Since the requirements in gas turbines are constantly increasing and demand higher inlet temperatures, investigations focus on the evolution of YSZ coating microstructures during exposure to very high temperature (up to 1400°C). The advantages of the improved microstructure provided by SPS tend to be sensitive to these temperatures when YSZ is chosen as coating material. Our investigations pointed out shortcomings of the coatings in terms of sintering behaviour. Nevertheless, these aspects are mostly an issue of materials and do not represent a general limitation of the SPS process. It is known that YSZ has a high sintering tendency at such high temperatures. Therefore, new promising TBC materials such as e.g. lanthanum zirconate (La₂Zr₂O₇) are being developed for SPS coatings in order to circumvent this issue. The strategy is to combine process-related advantages of SPS with advanced properties of new TBC materials.