Microstructure evolution upon annealing of a ZrB2–SiC composite containing lanthana and magnesia

The purpose of this work was to produce a dense ZrB2–SiC ceramic and to identify a suitable annealing cycle to crystallize the glassy phase and promote SiC growth along the c axis. The concept behind this work exploits the irreversible SiC β → α transformation occurring at temperature above 1900 °C, in such a way to have SiC platelets able to trigger effective toughening mechanisms. La2O3 and MgO were used as sintering additives. The as sintered and annealed materials were examined through X-ray diffraction (XRD), to identify the crystalline phases, scanning electron microscope (SEM), to study the distribution of the secondary phases, and transmission electron microscope (TEM), to analyze the microstructure at nanoscale level, with particular attention to new crystalline phases and to the interfaces in high resolution mode. A model for the microstructure evolution during densification and upon annealing is presented.