Extraordinary toughening enhancement and flexural strength in Si3N4 composites using graphene sheets

Two types of Si3N4 composites containing graphene nanostructures using two different graphene sources, pristine graphene nanoplatelets and graphene oxide layers were produced by Spark Plasma Sintering. The maximum toughness of 10.4 MPa m1/2, measured by flexure testing of pre-cracked bars, was achieved for a composite (∼60β/40α-Si3N4, ∼300 nm grain size) with 4 vol.% of reduced graphene oxide, indicating a toughening enhancement of 135% when compared to a similar Si3N4. This was also accompanied by a 10% increase in flexure strength (1040 MPa). For the composites with thicker graphene nanoplateletes only a 40% of toughness increase (6.6 MPa m1/2) without strength improvement was observed for the same filler content. The large difference in the maximum toughness values accomplished for both types of composites was attributed to variations in the graphene/Si3N4 interface characteristics and the extent of monolayer graphene exfoliation.