A TEM study of nanostructures and interfaces in the hot-press sintered ZrB۲–SiC–Si۳N۴ composites

A fully dense ZrB۲–۳۰ vol% SiC composite containing ۵ wt% Si۳N۴ and ۴ wt% phenolic resin (۱.۶ wt% carbon) was sintered using the hot-pressing route under the external pressure of ۱۰ MPa at ۱۹۰۰ ºC for ۲ h. The microstructural evolution and interfacial phenomena were scrutinized using advanced microscopy facilities such as high-resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FESEM). The FESEM images showed the ZrB۲ and SiC grains without any evidence of Si۳N۴. The formation of the hexagonal BN (hBN) phase was proven in the sintered composite. The hBN nanosheets had a graphite-like morphology with an average thickness of ۲۰ nm. This phase has a perpendicular orientation to the pressure direction and prevents abnormal ZrB۲ grain growth. Two types of ZrB۲/SiC interfaces were detected, which exhibited an amorphous phase along with the grain boundary and a clean/smooth interface, resulting from the Si۳N۴ addition. HRTEM and inverse fast Fourier transform (IFFT) observations disclosed that the d-spacing value in the ZrB۲ grain (۰.۳۳۵ nm) is higher than those reported in the literature. Furthermore, it was found that the exerted pressure during the sintering distorted atomic planes. The presence of numerous dislocations within the ZrB۲ grains confirmed dislocation creep as the main densification mechanism.A fully dense ZrB۲–۳۰ vol% SiC composite containing ۵ wt% Si۳N۴ and ۴ wt% phenolic resin (۱.۶ wt% carbon) was sintered using the hot-pressing route under the external pressure of ۱۰ MPa at ۱۹۰۰ ºC for ۲ h. The microstructural evolution and interfacial phenomena were scrutinized using advanced microscopy facilities such as high-resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FESEM). The FESEM images showed the ZrB۲ and SiC grains without any evidence of Si۳N۴. The formation of the hexagonal BN (hBN) phase was proven in the sintered composite. The hBN nanosheets had a graphite-like morphology with an average thickness of ۲۰ nm. This phase has a perpendicular orientation to the pressure direction and prevents abnormal ZrB۲ grain growth. Two types of ZrB۲/SiC interfaces were detected, which exhibited an amorphous phase along with the grain boundary and a clean/smooth interface, resulting from the Si۳N۴ addition. HRTEM and inverse fast Fourier transform (IFFT) observations disclosed that the d-spacing value in the ZrB۲ grain (۰.۳۳۵ nm) is higher than those reported in the literature. Furthermore, it was found that the exerted pressure during the sintering distorted atomic planes. The presence of numerous dislocations within the ZrB۲ grains confirmed dislocation creep as the main densification mechanism.

Keywords:

ZrB۲–SiC ceramics , Si۳N۴ additive , Hot-pressing , Interface , Microstructure , Dislocation