Transformation-Toughened Ceramic Multilayers with Compositional Gradients

The processing and the thermomechanical characteristics of zirconia‐toughened alumina (ZTA) ceramic multilayers, with through‐thickness gradients in the concentrations of the tetragonal and monoclinic zirconia phases, are studied both theoretically and experimentally. Analytical solutions for the evolution of thermal strains, internal stresses, and the overall curvature changes in response to various temperatures were derived using known plate theory formulations. Compositionally graded multilayer stacks and complex shapes of ZTA ceramics were fabricated by three‐dimensional printing (3DP). The relative fractions of the tetragonal to monoclinic ZrO2 phases were varied in a prescribed fashion. Phase transformation of ZrO2 in ZTA during cooling from the sintering temperature was controlled by doping different amounts of Y2O3 through the thickness of the ZTA multilayer plate. The local content of monoclinic ZrO2 through the thickness was characterized by X‐ray diffraction analysis. The microstructure, some basic mechanical properties, and thermal expansion behavior of the 3DP ZTA system were also characterized. Thermally induced curvature of the ZTA multilayer plate was measured, and the experimental results were compared with the predictions based on the analytical solutions. Symmetric ZTA multilayers with surface compression were also prepared by 3DP and tested for flexural strength. Results showed a significant increase in strength from the monolithic specimens to the compositionally controlled ones. Results from the compression tests of notched ZTA blocks demonstrated the role of m‐ZTA shield zones in enhancing the load‐bearing characteristics. This study also demonstrated the possibility that commonly available computational tools can be used to design and construct complex shapes with compositional variation to enhance and control mechanical properties.