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.

Functionally graded thermal barrier coatings (FG-TBCs) with a gradual composition change from the MCrAlY bond coat (BC) layer to the yttria stabilized zirconia (YSZ) TBC layer is one way to decrease the residual stresses at the BC/TBC interface during a thermal cycling process. At high temperatures, the oxidation resistance of TBCs is also important which would affect the cyclic life of the coatings. In this paper, two YSZ materials were used to make FG-TBCs and conventional duplex TBCs by applying air-plasma spray (APS) technique. The coated samples were thermal cycled with 24-hours heating at 1100 °C followed by air cooling. The results showed that the FG-TBCs performed earlier failure than the duplex ones. Microstructure study indicated that the poor oxidation resistance of the graded mixed layer was responsible for the coating failure. In addition, the influence of spraying parameters on the coating life was discussed.

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.