A novel model for coating design was proposed in this research, by considering both oxidation property and interdiffusion effect, corresponding to two factors of the Cr:Al activity ratio and the potential of Al, respectively. To verify this model, oxidation tests of coated superalloys were performed at 1000 ° C for up to 5000 h. The test results indicated a strong positive correlation between GPDZ and Al potential and a clear negative correlation between oxidation kinetics parameter k p and Cr:Al activity ratio. Our research opened up new ideas for the coating design.

The current investigation focuses on understanding the influence of a columnar microstructure and a sealing layer on the corrosion behavior of suspension plasma sprayed (SPS) thermal barrier coatings (TBCs). Two different TBC systems were studied in this work. First is a double layer made of a composite of gadolinium zirconate + yttria stabilized zirconia (YSZ) deposited on top of YSZ. Second is a triple layer made of dense gadolinium zirconate deposited on top of gadolinium zirconate + YSZ over YSZ. Cyclic corrosion tests were conducted between 25 °C and 900 °C with an exposure time of 8h at 900 °C. 75 wt. % Na 2 SO 4 + 25 wt.% NaCl were used as the corrosive salts at a concentration of 6 mg/cm 2 . Scanning electron microscopy analysis of the samples’ cross-sections showed that severe bond coat degradation had taken place for both TBC systems and the extent of bond coat degradation was relatively higher in the triple layer system. It is believed that the sealing layer in the triple layer system reduced the number of infiltration channels for the molten salts which resulted in overflowing of the salts to the coating edges and caused damage to develop relatively more from the edge.

Fabrication of Thermal Barrier Coatings (TBCs) with higher lifetime and relatively cheaper processes is of particular interest for gas turbine applications. Suspension Plasma Spray (SPS) is capable of producing coatings with porous columnar structure, and it is also a much cheaper process compared to the conventionally used Electron Beam Physical Vapor Deposition (EB-PVD). Although TBCs fabricated using SPS have lower thermal conductivity as compared to other commonly used processes, they are still not commercialized due to their poor lifetime expectancy. Lifetime of TBCs is highly influenced by the top coat microstructure. The objective of this work was to study the TBCs produced using axial SPS with different process parameters. The bond coat was deposited using High Velocity Air Fuel (HVAF) spray. Influence of the microstructure on lifetime of the coatings was of particular interest and it was determined by thermal cyclic fatigue testing. Thermal conductivity of the coatings was determined by laser flash analysis. The results show that axial SPS could be a promising method of producing TBCs for high temperature gas turbine applications.

A thermal cycling test was carried out in an EB-PVD MCrAlY – superalloy system, the result of which showed that a continuous β-NiAl layer formed in the MCrAlY coating near the coating–superalloy interface. Since β phase can be as a reservoir of Al, the formation of the β layer, in which much Al is reserved, is probably beneficial to the coating’s life. An oxidation-diffusion model was adapted to simulate the development of the microstructures in the coating-superalloy system. The simulation results indicate that the formation of the β layer was strongly related to the high Al activity in the substrate; if the Al activity of the substrate was high enough, a β layer could be built up in the coating near the coating-substrate interface.

To understand performance of thermal barrier coatings (TBCs) in various industrial applications of Siemens medium size gas turbines, effects of three types of thermal exposures i.e., high temperature isothermal exposure, thermal cycle fatigue (TCF) test, and burner rig test (BRT) on adhesion strength of an air plasma sprayed (APS) TBC have been studied and reported in this paper. It has been seen that the TBC adhesion strength is influenced by the type of thermal exposures differently. Together with a microscopic examination on TBC microstructures and fractography, a correlation between failure mechanisms and types of thermal exposures is discussed. In addition to the impact of various engine operation conditions on behavior of TBC, impacts of TBC surface roughness on turbine performance have also been evaluated. Surface profile and surface roughness on as-sprayed and polished TBC and cast metal (uncoated) have been measured and two different polishing methods have been compared. As a result, a requirement of TBC surface roughness and a preferable polishing method are suggested.