PHASE-STABILITY OF SCANDIA-YTTRIA-STABILIZED ZIRCONIA TBCS

The evolution in phase composition resulting from 1400 degrees C agein g 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 sync hrotron radiation X-ray diffraction (SR XRD) using whole powder patter n fitting (WPPF), a recently proposed Rietveld-based method for the si multaneous 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 plas ma-sprayed coating of SYSZ (6.57 mol.%Sc2O3-1.00 mol.%Y2O3) remained 9 6.7% in its original tetragonal phase (presumably t', c/a of 1.0058) a fter 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 tetrag onal 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 re garding lattice parameters, crystalline domain size and microstrain, a lso obtained by WPPF, are discussed in terms of the high-temperature p hase stability of the YSZ vs SYSZ systems. (C) 1998 Elsevier Science S .A. All rights reserved.