Long‐Term Behavior and Application Limits of Plasma‐Sprayed Zirconia Thermal Barrier Coatings

Investigations of changes in phase composition, mechanical properties, and microstructure of ZrO 2 -based plasma-sprayed thermal barrier coatings (TBCs) with 8 mol% CeO 2 , 19.5 mol% CeO 2 /1.5 mol% Y 2 O 3 , 35 mol% CeO 2 , and 4.5 mol% Y 2 O 3 after long-term heat treatments at typical operation temperatures (1000°-1400°C) are presented. Experimental studies include X-ray diffractometry, mechanical testing, and scanning electron microscopy. Thermal cycling experiments also have been performed. TBCs with 8 mol% CeO 2 contain mainly the tetragonal equilibrium phase and, therefore, show rapid failure because of the high amount of tetragonal → monoclinic phase transformation, even after relatively short heat treatments (1250°C/1h). In the case of the other systems that consist mainly of the tetragonal or cubic nonequilibrium phases, TBCs with 19.5 mol% CeO 2 /1.5 mol% Y 2 O 3 or 35 mol% CeO 2 reveal a smaller amount of monoclinic phase after long-term heat treatments (1250°C/1000h) compared with TBCs containing 4.5 mol% Y 2 O 3 . TBCs containing 35 mol% CeO 2 show a higher degree of sintering than the TBCs with 19.5 mol% CeO 2 /1.5 mol% Y 2 O 3 and, therefore, a greater increase of the elastic modulus. Among the systems investigated, TBCs containing 4.5 mol% Y 2 O 3 exhibit the highest resistance to failure in thermal-cycling experiments.