Phase stability of scandia-yttria-stabilized zirconia TBCs

The evolution in phase composition resulting from 1400 degrees C ageing of ZrO2 samples stabilized by similar to 4-7 mol.% each of Y2O3 (YSZ), Sc2O3 (ScSZ), or a mixture of Sc2O3-Y2O3 (SYSZ) was studied by synchrotron radiation X-ray diffraction (SR XRD) using whole powder pattern fitting (WPPF), a recently proposed Rietveld-based method for the simultaneous refinement of phase composition and crystal microstructure. The results suggest that SYSZ has better high temperature tetragonal phase stability than the current state-of-art YSZ. For example, a plasma-sprayed coating of SYSZ (6.57 mol.%Sc2O3-1.00 mol.%Y2O3) remained 96.7% in its original tetragonal phase (presumably t', c/a of 1.0058) after being aged 100 h at 1400 degrees C and then 24 h at 1480 degrees C, whereas a 4.5 mol.% YSZ coating was transformed into two new tetragonal phases, one a low-stabilizer phase with cia of 1.0154 (44.8%) and the other a high-stabilizer zirconia phase with c/a of 1.003 (46.5%), with some monoclinic (8.7%) formed. The phase compositions found for the various powder and coating specimens, together with information regarding lattice parameters, crystalline domain size and microstrain, also obtained by WPPF, are discussed in terms of the high-temperature phase stability of the YSZ vs SYSZ systems. (C) 1998 Elsevier Science S.A. All rights reserved.