Dense t′ ZrO 2-8 mol.% YbO 1.5 candidate thermal barrier material was aged for 192 h at 1150 and 1250 °C. The high initial indentation toughness (∼87 J m -2) reduced by ∼56% in early-stage aging at both temperatures. The toughness increased gradually with aging at 1150 °C and more rapidly at 1250 °C. Precipitation of the cubic phase and its transformation to t″ during cooling was observed. Transformation toughening was activated by 192 h at 1250 °C. Ferroelastic toughening was dominant in the absence of transformation toughening. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.