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The failure mechanisms of air-plasma-sprayed ZrO 2 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 ϳ60% with thermal cycling because of sintering effects. However, after ϳ80 cycles (0.5 h at 980°C), the accumulated damage in the coatings led to a significant decrease of ϳ20% 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 ZrO 2 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.

The high-temperature creep behavior of two fine-grained (ϳ3 m) anorthite-rich glass-ceramics was characterized at ambient pressure and under a confining pressure of ϳ300 MPa. Experiments were done at differential stresses of 15-200 MPa and temperatures of 1200°-1320°C. Of the two materials, one had a tabular (lathlike) grain structure with finely dispersed second phase of mullite, mostly in the form of ϳ3-5 m grains comparable to that of the primary anorthite phase, whereas the other had an equiaxed grain morphology with fine (ϳ400 nm) mullite precipitates concentrated at the anorthite grain boundaries. The results of creep experiments at ambient pressure showed that the material with the tabular grain structure had strain rates at least an order of magnitude faster than the equiaxed material. Creep in the tabular-grained material at ambient pressure was accompanied by a significant extent of intergranular cavitation: pore-volume analysis before and after creep in this material suggested that >75% of the bulk strain was due to growth of these voids. The equiaxed material, in contrast, showed a smooth transition from Newtonian (n ‫؍‬ 1) creep at low stresses to non-Newtonian behavior at high stresses (n > 2). Under the high confining pressure, the microstructures of both materials underwent significant changes. Grain-boundary mullite precipitates in the undeformed, equiaxed-grain material were replaced by fine (ϳ100 nm), intragranular precipitates of silliminate and corundum because of a pressure-induced chemical reaction. This was accompanied by a significant reduction in grain size in both materials. The substantial microstructural changes at high confining pressure resulted in substantially lower viscosities for both materials. The absence of mullite precipitates at the grain boundaries changed the behavior of the equiaxed material to non-Newtonian (n ‫؍‬ 2) at a pressure of ϳ300 MPa, possibly because of a grain-boundary sliding mechanism; the tabular-grained material showed Newtonian diffusional creep under similar conditions.

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