Thermal cycling behavior and failure mechanism of Yb₂O₃-doped yttria-stabilized zirconia thermal barrier coatings

Yb2O3-doped yttria-stabilized zirconia (YbYSZ) thermal barrier coatings (TBCs) with different Yb2O3 and Y2O3 doping contents were fabricated via electron beam physical vapor deposition. Phase stability, thermal conductivity, and thermal cycling behavior as well as failure mechanism of three types of YbYSZ were comparatively studied. It was found that components of as-deposited ceramic coatings deviated from those of corresponding ingots (Yb and Y contents were 6.50 and 5.73 at%, respectively, for 4.5Yb4YSZ ingot). As-deposited 6.5Yb2YSZ and 4.5Yb4YSZ coatings consisted of tetragonal, tetragonal-prime, and cubic phases, while monoclinic phase was found in 8.5YbSZ. After 6000 h of thermal cycling oxidation at 1150 degrees C, 8.5YbSZ had the largest amount of monoclinic phase (69%), while 4.5Yb4YSZ had the minimum (24%). Delamination was found to occur inside YbYSZ ceramic layer, and transverse and vertical cracks induced by poor fracture toughness of doped ceramic are probably main cause of TBC failure. 4.5Yb4YSZ TBCs remained intact during the first 2000 h of cyclic oxidation, but cracks were then generated in ceramic layer. Moreover, thermal conductivities of 4.5Yb4YSZ, 6.5Yb2YSZ, and 8.5YbSZ ceramic coatings at 1200 degrees C were 1.32, 1.39, and 1.43 W center dot m(-1)center dot K-1, which are 20%, 15%, and 13% lower than that of 8YSZ prepared via the same process.