Long-term behaviour and application limits of plasma-sprayed ZrO2 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 (1000 degrees -1400 degreesC) are presented.
Experimental studies include X-ray diffractometry, mechanical testing,
and scanning electron microscopy. Thermal cycling experiments also have
been performed. TBCs with g mol % CeO2 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 degreesC/1 h), In the case
of the other systems that consist mainly of the tetragonal or cubic
nonequilibrium phases, TBCs with 19.5 mol% CeO2/1.5 mol% Y2O3 or 35
mol% CeO2 reveal a smaller amount of monoclinic phase after long-term
heat treatments (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 TBCs with 19.5 mol% CeO2/1.5 mol% Y2O3 and,
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therefore, a greater increase of the elastic modulus, Among the systems
investigated, TBCs containing 4.5 mol% Y2O3 exhibit the highest
resistance to failure in thermal-cycling experiments.