Thermal Ageing Fatigue Behavior of Some Functionally Graded TBCs: Oxidation Failure

Functionally graded thermal barrier coatings (FG-TBCs) with a gradual composition change from MCrAlY layer to pure YSZ layer is one way to decrease the residual stresses at the layers’ interface during a thermal cycling process. At high temperature 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 coating samples were thermal cycled with 24 h heating at 1100 ºC followed by air cooling. The results showed that the FG-TBCs performed earlier failure than the duplex ones. Microstructure study indicated that the isolated oxidation of MCrAlY splats in the graded mixed layer to form spinels and Ni-Co-oxides was responsible for the poor oxidation resistance. In addition, the APS spraying parameters also influenced the ageing fatigue behavior of the FG-TBCs. The decreased plasma power can increase the volume fraction of MCrAlY in the mixed layer which gave a better oxidation resistance and longer coating life.