The Al₂O₃–Zr(Hf)O₂–La₂O₃ Phase Diagrams as a Scientific Basis for Developing New Thermal Barrier Coatings

Abstract

Comparison of the isothermal sections in the similar Al₂O₃–ZrO₂–La₂O₃ and Al₂O₃–HfO₂–La₂O₃ systems shows that equilibria between the α-Аl₂O₃, 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 the higher thermodynamic stability of the La₂Hf₂O₇ phase (melting point 2420 °С) compared to the similar La₂Zr₂O₇ phase (melting point 2280 °С). Other equilibria in both systems are of the same type. Lanthanum hafnate can be deposited onto the bonding coating without reacting with α-Al₂O₃ or destroying the thermal barrier coating. Stable performance of the lanthanum hafnate coating is expected.