The failure mechanisms of air-plasma-sprayed ZrO2 thermal barrier coatings with various microstructures were studied by microscopic techniques after thermal cycling. The elastic modulus (E) and hardness (H) of the coatings were measured as functions of the number of thermal cycles. Initially, both E and H increased by similar to 60% with thermal cycling because of sintering effects. However, after similar to 80 cycles (0.5 h at 980 degreesC), the accumulated damage in the coatings led to a significant decrease of similar to 20% of the maximum value in both E and H. These results were correlated with stresses measured by a spectroscopic technique to understand specific damage mechanisms. Stress measurement and analysis revealed that the stress distribution in the scale was a complex function of local interface geometry and damage in the top coat. Localized variations in geometry could lead to variations in measured hydrostatic stresses from -0.25 to -2.0 GPa in the oxide scale. Protrusions of the top ZrO2 coat into the bond coat were localized areas of high stress concentration and acted as damage-nucleation sites during thermal and mechanical cycling. The net compressive hydrostatic stress in the oxide scale increased significantly as the scale spalled during thermal cycling.