Abstract
Comparison of the isothermal sections in similar Al2O3–ZrO2–La2O3 and Al2O3–HfO2–La2O3 systems shows that equilibria between the α-Al2O3, Hf(Zr)O2, La2Zr2O7, La2Hf2O7, and LaAlO3 phases in these systems differ fundamentally. While the ZrO2−LaAlO3 equilibrium is observed in the system with ZrO2, the alternative La2Hf2O7–Al2O3 equilibrium takes place in the system with HfO2. The triangulation in the system with HfO2 changes. This phenomenon is probably due to higher thermodynamic stability of the La2Hf2O7 phase (melting point 2420°C) compared to the similar La2Zr2O7 phase (melting point 2280°C). Other equilibria in both systems are of the same type. Lanthanum hafnate can be directly deposited onto the binding coating without reacting with α-Al2O3 or destroying the thermal barrier coating. Stable performance of the lanthanum hafnate coating is expected.
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