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 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.
Highlights
The use of tetragonal zirconia as an energy-dissipation component can effectively result in an improvement of fracture toughness of Si3 N4 ceramics [8,9,10]
Energy dissipation and high fracture toughness of zirconia-containing ceramics can be related to the tetragonal-to-monoclinic phase transformation [9,10]
Multi-scale and in-situ microscopy as a combined methodology reported in this study provides a potential unique approach to understand the microstructure and mechanical behavior correlation for other complex ceramic systems
Summary
The use of tetragonal zirconia as an energy-dissipation component can effectively result in an improvement of fracture toughness of Si3 N4 ceramics [8,9,10]. Energy dissipation and high fracture toughness of zirconia-containing ceramics can be related to the tetragonal-to-monoclinic phase transformation [9,10]. In another aspect, composites with nanofillers (e.g., carbon nanotube, graphene and hexagonal boron nitride (h-BN) show the potential to improve the properties of Si3 N4 -based ceramic matrix composites [11,12,13]. The problem of these composites is that graphene or graphite reacts with the sintering additives This effect strongly reduces the densification of the material. Hvizdos et al [20] and
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