Electron microscopy characterization of porous ZrB₂–SiC–AlN composites prepared by pressureless sintering

This study explores the impact of AlN on the propagation of residual porosity and microstructure of porous ZrB₂–SiC-based composites. Several ZrB₂–SiC composites were fabricated with various amount of AlN by pressureless sintering method at 1900 °C. The specimens were kept at the maximum temperature for 120 min. Although the AlN-free composite possessed ~13% porosity, the incorporation of AlN to the ZrB₂–SiC composites elevated this value for approximately twofold. The prepared porous samples were precisely characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and field emission scanning electron microscopy (FESEM). XRD analysis revealed the presence of ZrB₂, SiC, and C peaks in the pattern of the sample containing 5 wt% AlN. However, no Al compound could be detected therein. The thermodynamic study suggested the formation of a liquid phase comprising the SiO₂–B₂O₃ with Al-based ingredients for the ZrB₂–SiC–AlN composites. FESEM fractographs showed that the intergranular fracture was predominant in the specimens. Furthermore, the high resolution TEM images confirmed the progress of the liquid phase sintering, resulted in the formation of clean interfaces.