| For application to thermal barrier coatings (TBCs), ytterbia (Yb 2O3), ceria (CeO2) and niobia (Nb2O 5) are investigated for their effect as co-dopants on the stability of 7 wt% yttria (Y2O3) stabilized zirconia (ZrO 2), known as 7YSZ. Zirconia TBCs must be stabilized to prevent transformations between cubic fluorite (f), tetragonal (t) and monoclinic (m) phases during operation, since the latter causes cracking and failure. However, with an increasing use of co-dopants, very little data exists on the cooling rate sensitivities of the stabilizing mechanisms involved. Together with microstructural effects, this is the focus of the current study. It is found that slower cooling rates consistently result in more m phase formation, and are therefore not desirable in general. Further, lattice distortions are the most sensitive mechanism to cooling rate, and oxygen vacancies are the least sensitive. The addition of Yb2O3 stabilizes f phase by dilative lattice distortions and the production of oxygen vacancies. CeO2 stabilizes t phase with similar distortions and also produces metastable oxygen vacancies which are retained depending on cooling rate. Nb2O5 results in more m phase by annihilating oxygen vacancies produced by Y 2O3. Microstructural analyses additionally reveal that m, t and f phases can be identified qualitatively according to whether a crack produces intergranular, textured transgranular, or smooth transgranular fracture through grains, respectively. A mostly connected network of porosity of up to 16 vol% is also revealed in the manufactured ceramics, and improvements are suggested based on sintering theory. |
