Characterization of the degraded microstructures of a platinum aluminide coating

The degradation of a platinum modified aluminide (PtAl) coating and a CMSX-4 superalloy substrate were investigated for cyclic and quasi-isothermal heating to 1200 °C. To accelerate the oxidation of the specimens, the thermally grown oxide (TGO) was removed at 10 h intervals. For up to 80 h of exposure, comparisons of specimens with periodic oxide removal and those without oxide removal were made. Qualitatively, the major changes to the bond coat were associated with phase changes from β-(Ni,Pt)Al to γ′-(Ni3Al) and precipitate coarsening. This evolution was quantified through backscatter scanning electron microcopy and image analysis. With instrumented indentation, the room temperature coating modulus was also measured at 10 h intervals. Additional results include observations of differences in waviness of the bond coat surface for cyclic, quasi-isothermal, and true-isothermal heating, and observations of rafting near the substrate/coating interface. The differences between cyclic and quasi-isothermal heating indicate that stresses associated with cooling and heating significantly alter microstructural evolution.