Date of Completion
Thermal Barrier Coatings, CMAS Resistance, Thermal Conductivity, Plasma Spray, Thermal Cyclic Durability, Double Layer
Eric H Jordan
Field of Study
Materials Science and Engineering
Doctor of Philosophy
Advanced thermal barrier coatings (TBC) are crucial to improved energy efficiency in next generation gas turbine engines. The use of traditional topcoat materials, e.g. yttria-stabilized zirconia (YSZ), is limited at elevated temperatures due to (i) the accelerated undesirable phase transformations, and (ii) high temperature corrosive attacks from calcium-magnesium-aluminum-silicate (CMAS) deposits and moisture.
In this research, the Solution Precursor Plasma Spray (SPPS) process was employed first to further reduce the thermal conductivity of conventional YSZ TBCs by introducing a microstructural feature of layered porosity, called the inter-pass boundaries (IPBs), that disrupts heat conduction in the coatings. Process optimization involving extensive scanning electron microscopy (SEM) characterization and laser-flash measurements on hundreds of spray trials, yielded a thermal conductivity as low as 0.623Wm-1K-1 in SPPS YSZ TBCs, which equated an approximately 50% reduction than the standard air-plasma-sprayed (APS) TBCs. At the same time, other engine critical properties of the low thermal conductivity SPPS TBCs, such as cyclic durability, erosion resistance and sintering resistance, were characterized to be equivalent or better than the APS baselines.
To enhance the high temperature capability of SPPS TBCs, modifications were introduced to the SPPS YSZ TBCs so as to improve their resistance to CMAS and moisture under harsh IGCC environments. Several mitigation approaches were explored, including doping the coatings with Al2O3 and TiO2, applying a CMAS infiltration-inhibiting surface layer, and filling topcoat cracks with blocking substances. The efficacy of these modifications on CMAS resistance was assessed with a set of novel CMAS-TBC interaction tests, while the moisture resistance was evaluated in a custom-built high-temperature moisture rig.
In the end, the optimal coating system was evaluated to be consisted of a thick inner layer of SPPS YSZ having the low thermal conductivity, and a high-temperature stable CMAS resistant gadolinium zirconate (GZO) protective surface layer made by the SPPS process. Noteworthy was the fact that the YSZ to GZO interface made by the SPPS process was not the failure location as had been observed in similar APS TBCs.
Jiang, Chen, "Low Thermal Conductivity YSZ-based Thermal Barrier Coatings with Enhanced CMAS Resistance" (2015). Doctoral Dissertations. 871.