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TGO growth stress measurements

Using X-ray synchrotron radiation, growth strains as well as thermal mismatch strains on cooling to room temperature, have been measured in the TGO formed on FeCrAlY, NiCrAlY, NiAl single crystals of (111) and (110) orientation, and two industrial bond coats viz. General Electric's (GE) Pt-modified NiAl bond coat and Pratt and Whitney's (P & W) NiCoCrAlY bond coat.

Reliability studies were done using FeCrAlY specimen. Room temperature strain obtained at the synchrotron for FeCrAlY specimen oxidized at 1000?C is in excellent agreement with the data obtained for the same specimen by conventional laboratory XRD.

NiCrAlY substrates were oxidized at four different temperatures: 950, 1000, 1050, and 1100?C. Tensile growth strains develop in the oxide at all temperatures, corresponding to tensile stresses between ~100 and ~300MPa; these tensile growth strains are due almost certainly to initially formed metastable transition alumina transforming to the stable and denser A¨CAl2O3. While the strains remain persistently tensile at 950 and 1000?C, a slow creep relaxation at 1050oC and a rapid stress relaxation at 1100oC was observed. Another interesting observation in NiCrAlYs is the effect of phase transition in the NiCrAlY substrate on TGO stresses. The specimens oxidized at 1050 and 1100oC exhibited much larger compressive stresses, ~ 6 GPa, than did specimens oxidized at 950 and 1000oC, ~2.5GPa. Vacuum dilatometry shows a phase transformation in the substrate between 1000 and 1100?C with a large lattice expansion in the high temperature phase.

The initial stresses, though tensile in both the cases, they show a different stress relaxation behaviour. This is clearly related to the different crystallographic texture in the TGO for the two orientations and different kinetics of E u A transformation. Further studies are being done.

Prior to the in situ oxidation experiment at 1100oC, the samples had been cycled 24 times to 1150?C (heating and cooling times of ~ 2 minutes and 1 hr hold for each cycle). Growth strains are higher in P&W's NiCoCrAlY bond coat than GE's Pt-NiAl bond coat, approximately by a factor of two.

Reported by

Case Western Reserve University,
Material Science and Engineering, 500 White Building, 10900 Euclid Avenue
OH 44106-7 Cleveland
United States