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 mu m voxel size during thermal cycles up to 1175 degrees 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 mu m), from approximately 5 to 11 vol.%. Besides, an in-situ characterization by HT-ESEM was performed during two thermal cycles, up to 1350 degrees C and 1200 degrees 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 reheating steps respectively. The evolution with temperature of the mechanical behavior of the material is discussed in light of microstructural changes.