Microstructure and Fracture Mechanism Investigation of Porous Silicon Nitride-Zirconia-Graphene Composite Using Multi-Scale and In-Situ Microscopy

Silicon nitride-zirconia-graphene composites with high graphene content (5 wt.% and 30 wt.%) were sintered by gas pressure sintering (GPS). The effect of the multilayer graphene (MLG) content on microstructure and fracture mechanism is investigated by multi-scale and in-situ microscopy. Multi-scale microscopy confirms that the phases disperse evenly in the microstructure without obvious agglomeration. The MLG flakes well dispersed between ceramic matrix grains slow down the phase transformation from alpha to beta-Si3N4, subsequent needle-like growth of beta-Si3N4 rods and the densification due to the reduction in sintering additives particularly in the case with 30 wt.% MLG. The size distribution of Si3N4 phase shifts towards a larger size range with the increase in graphene content from 5 to 30 wt.%, while a higher graphene content (30 wt.%) hinders the growth of the ZrO2 phase. The composite with 30 wt.% MLG has a porosity of 47%, the one with 5 wt.% exhibits a porosity of approximately 30%. Both Si3N4/MLG composites show potential resistance to contact or indentation damage. Crack initiation and propagation, densification of the porous microstructure, and shift of ceramic phases are observed using in-situ transmission electron microscopy. The crack propagates through the ceramic/MLG interface and through both the ceramic and the non-ceramic components in the composite with low graphene content. However, the crack prefers to bypass ceramic phases in the composite with 30 wt.% MLG.

Automated summary

The microstructure and fracture mechanism of composites with different levels of graphene content were investigated. Higher graphene content was found to hinder the growth of the ZrO2 phase. Cracks tended to bypass ceramic phases in composites with 30 wt.% MLG, while they propagated through the ceramic/MLG interface in composites with low graphene content.