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Phase stability of co-doped TBCs

The t' zirconia phase is desirable in thermal barrier coatings because it combines the attributes of non-transformability, required for thermal cyclic applications, and adequate toughness. Unfortunately, the t' phase is metastable at the temperatures of interest in gas turbine operation and tends to separate into a cubic phase with low toughness, and a depleted tetragonal phase which is transformable to monoclinic upon cooling. This evolution is undesirable because it degrades the mechanical integrity of the coating.

Extensive studies on zirconia partially stabilized with various tri-valent cations at a constant concentration of 7.6mol% MO1.5 (M = Sc, Yb, Y, Gd, Sm, Nd, La)revealed a well behaved trend for the high temperature stability of the metastable t' phase, which was optimal for Yb. Both smaller (Sc) as well as larger cautions (Y to La) yielded reduced stability of the t' phase relative to Yb, but for different reasons. At the composition selected, which is the standard stabilizer content for thermal barrier coatings, Sc is stable against decomposition but its tetragonal/monoclinic partitionless transformation temperature (T0[t/m]) is substantially above ambient and renders the 7.6ScO1.5 transformable upon thermal cycling. This is at variance with the claim in the literature that Sc is a better stabilizer than Y, resulting from comparing dissimilar concentrations of stabilizer.

The T0[t/m] appears to be minimal for Y and increases with increasing caution size, as does the driving force for phase separation in response to shifts in the position of the tetragonal and cubic phase boundaries. Thermodynamic modelling of the ZrO2-YO1.5-GdO1.5 system has ascertained the increased in driving force with progressive substitution of Gd for Y. The combined effect rapidly decreases the stability with increasing caution size. For comparison, a 7.6%Yb doped zirconia is 4 times as durable as one doped with Y and 16 times as durable as one doped with Gd at the same concentration.

Co-doping studies also revealed that combinations of Y with the smaller cautions, including Sc, are generally more durable than those with the larger ones at the same level of addition. This was again correlated to the changes in the phase equilibrium with varying caution size. In general, these results provide guidance for the design of novel coating compositions based on combinations of rare earth stabilizers.

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Reported by

University of California, Santa Barbara
1361D Engineering II, Materials Department
93106-5050 Santa Barbara, California
United States
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