Effect of cooling rate on phase transformation in 6–8 wt% YSZ APS TBCs

In properly produced as-sprayed thermal barrier coatings of yttria partially stabilized zirconia (7YSZ) the only phase that can be found is the metastable tetragonal prime structure, t′. Even though t′ is sometimes called “not transformable”, because it behaves as practically stable up to rather high temperature, long term exposure above 1200 °C produces its transformation in tetragonal, t, and cubic, c, phases. During cooling down to room temperature the t phase will transform in monoclinic, m, one. Although it is considered a martensitic transformation, high cooling rate, after prolonged high temperature exposure (over 1300 °C), can avoid or limit the evolution from t to m structure. The effect of the cooling rate on this transformation has been investigated in free standing TBCs both with porous microstructure and dense vertically cracked one, exposed at 1400 °C for 100 h. The samples have been analyzed by XRD and subsequent Rietveld refinement analysis to quantify the phase content: the results highlight that different cooling rates give different monoclinic contents, confirming the cooling rate effect on this transformation. Nevertheless, if the equilibrium, prevented by fast cooling, is restored, the transformation to m occurs; in fact it is sufficient to put in a furnace at low temperature for short duration the samples fast cooled down from 1400 °C in order to delete or weaken the cooling rate effect.