Experimental and Computational Phase Studies of the ZrO2-based Systems for Thermal Barrier Coatings

TheZrO2-based materials are practically important as the thermal barrier coatings(TBC) for high temperature gas turbines, due to their low thermal conductivity, hightemperature thermal stability and excellent interfacial compatibility. Studies of the phaseequilibira, phase transformation, and thermodynamics of the ZrO2-based systems can providethe necessary basic knowledge to develop the next generation TBC materials. In the thesis, the systems ZrO2 −HfO2, ZrO2 −LaO1.5, ZrO2 −NdO1.5, ZrO2 −SmO1.5,ZrO2 −GdO1.5, ZrO2 −DyO1.5, ZrO2 −YbO1.5 and ZrO2 −GdO1.5 −YO1.5 wereexperimentally studied. The samples were prepared by the chemical co-precipitation method, with aqueous solutions Zr(CH3COO)4, HfO(NO3)2, and RE(NO3)3⋅xH2O (RE=La, Nd, Sm, Gd,Dy, Yb) as starting materials. Various experimental techniques, X-ray diffraction (XRD),scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmissionelectron microscopy (TEM), differential thermal analysis (DTA), and high temperaturecalorimetry were employed to study the phase transformation, phase equilibria between 1400and 1700°C, heat content and heat capacity of the materials. A lot of contradictions in theliterature were resolved and the phase diagrams were reconstructed. Firstly, the thermodynamic transformation temperatures(T0, where the Gibbs energiesof monoclinic and tetragonal phases are identical) were reviewed and extrapolated for pure ZrO2 (1367 ± 5 K) and HfO2 (2052 ± 5 K) from the DTA study of ZrO2 −HfO2 system. Thetemperatures(As, Af, Ms, Mf) on the martensitic transformation of the materials in ZrO2 −HfO2 system directly obtained by DTA measurements show well consistent behavior togetherwith the calculated T0 temperatures. Based on the present DTA results and literatureinformation on transformation temperatures of monoclinic ↔ tetragonal, tetragonal ↔ cubicand cubic ↔ liquid, as well as the thermodynamics of different structures, the thermodynamic parameters of the pure ZrO2 and HfO2 were assessed, and the ZrO2 −HfO2 phase diagram wascalculated without any fitting parameter. Within the scope of the thesis, the tetragonal + fluorite (or the tetragonal + pyrochlorefor RE=La) two-phase region, the phase equilibria between disordered fluorite and ordered pyrochlore (or the δ phase for RE=Yb), and the phase equilibria between fluorite and REO1.5terminal solutions were well established for ZrO2 −REO1.5 (RE=La, Nd, Sm, Gd, Dy, Yb)systems, and the enthalpy increments of the materials with 30 mol% REO1.5, and 50 mol%REO1.5 (57.14 mol% for RE=Yb) were determined in the temperature range 200-1400°C.