Strength and failure modes of ceramic multilayers

A model was developed for the prediction of the tensile strength of thin, symmetric 3-layer sandwich specimens. The model predictions rationalize the effect of heat-treatment temperature on the strength of sandwich specimens consisting of an YSZ (Yttria-Stabilized Zirconia) substrate coated with porous NiO-YSZ layers. The model accounts for two different failure modes. Coated YSZ specimens heat-treated below 1200 °C were predicted to fail from flaws in the substrate while specimens heat-treated at higher temperatures were predicted to fail from channeling cracks forming in the coating and propagating into the substrate. These predictions are consistent with microstructural observations of the fracture surfaces. A good agreement was found between the measured strength values and model predictions.