Thermochemical compatibility between alumina and ZrO2-GdO3/2 thermal barrier coatings

Gd2Zr2O7 and Gd co-doped zirconias are notable examples of materials under consideration for the next generation of thermal barrier coatings, wherein thermochemical and thermomechanical compatibility are critical to the durability of the system. The thermochemical compatibility of GdO1.5-ZrO2 compositions with Al2O3 has been investigated using powder compacts and diffusion couples. A tentative isothermal section for the AlO1.5-GdO1.5-ZrO2 system at 1200 DGC is reported, with additional discussion of the ordering transformation between the fluorite and pyrochlore phases. New light is shed on the long-standing uncertainty on the latter issue. It is shown that addition of GdO1.5 to ZrO2 above ~ 32 % results in the formation of a GdAlO3 interphase, with convoluted morphology and a Gd-depleted fluorite layer above it. The results provide guidelines for the compositional design of emerging thermal barrier materials. (Example substrates: sapphire, ferrous alloy.)