Abstract
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 α to β-Si3N4, subsequent needle-like growth of β-Si3N4 rods and the densification due to the reduction of 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 of graphene content from 5 wt% 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 appr. 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.