Long-term behavior and application limits of plasma-sprayed zirconia thermal barrier coatings

Investigations of changes in phase composition, mechanical properties, and microstructure of ZrO2-based plasma-sprayed thermal barrier coatings (TBCs) with 8 mol% CeO2, 19.5 mol% CeO2/1.5 mol% Y2O3, 35 mol% CeO2, and 4.5 mol% Y2O3 after long-term heat treatments at typical operation temperatures (10 00 degrees -1400 degreesC) are presented. Experimental studies include X-ra y diffractometry, mechanical testing, and scanning electron microscopy. The rmal cycling experiments also have been performed. TBCs with g mol % CeO2 c ontain mainly the tetragonal equilibrium phase and, therefore, show rapid f ailure because of the high amount of tetragonal --> monoclinic phase transf ormation, even after relatively short heat treatments (1250 degreesC/1 h), In the case of the other systems that consist mainly of the tetragonal or c ubic nonequilibrium phases, TBCs with 19.5 mol% CeO2/1.5 mol% Y2O3 or 35 mo l% CeO2 reveal a smaller amount of monoclinic phase after long-term heat tr eatments (1250 degreesC/1000 h) compared with TBCs containing 4.5 mol% Y2O3 . TBCs containing 35 mol% CeO2 show a higher degree of sintering than the T BCs with 19.5 mol% CeO2/1.5 mol% Y2O3 and, therefore, a greater increase of the elastic modulus, Among the systems investigated, TBCs containing 4.5 m ol% Y2O3 exhibit the highest resistance to failure in thermal-cycling exper iments.