Multi-scale in-situ microstructure observation of sintering and thermal damage in a refractory ceramic

The evolution of the microstructure of an alumina-mullite-zirconia refractory ceramic was studied during successive thermal treatments and correlated with the thermomechanical behavior of the material. More specifically, sintering and microcracking were followed by in-situ X-Ray tomography and High Temperature Environmental Scanning Electron Microscope (HT-ESEM). A dedicated setup was developed at the European Synchrotron Radiation Facility (ESRF) to perform in-situ X-Ray tomography providing images with 0.35 µm voxel size during thermal cycles up to 1175 °C. The observations show that sintering is heterogeneous throughout the material, leading to an increase of the volume fraction of over-micronic pores (pores larger than 1 µm), from approximately 5 to 11 vol.%. Besides, an in-situ characterization by HT-ESEM was performed during two thermal cycles, up to 1350 °C and 1200 °C respectively. HT-ESEM offers a higher resolution and the possibility to study sintering at a lower scale and additionally microcracking during cooling. This complementary qualitative analysis confirmed the heterogeneous nature of the sintering, leading to an increase of over-micronic porosity, and allowed, in addition, to visualize in real-time the evolution of glassy phases, responsible for liquid phase sintering . This technique also enabled to follow the opening and closing of microcracks during cooling and re-heating steps respectively. The evolution with temperature of the mechanical behavior of the material is discussed in light of microstructural changes.