MS-5-P-5861 Microstructure of Si₃N₄ with graphene multipatelets sintered with different sintering additives

Work shows the relation between the sintering additives, evolution of microstructure and some of the properties of Si₃N₄–GNPs composites. Si₃N₄ with graphene multiplatelets (GNPs) as the reinforcement were sintered with five different sintering additives. Initial ceramic powders containing 1 wt.% GNPs were milled in planetary ball mill in isopropanol for 2 hours and hot pressed at 1600°C or 1550°C for 60 min with an external load of 30 MPa and 40 mbar nitrogen overpressure. The relative densities of all Si₃N₄ composites with 1 wt. % of GNPs were above 98.5% of the theoretical density. The grain size distribution, the shape of silicon nitride grains and the location of graphene multiplatelets in the silicon nitride matrix has been studied using TEM and SEM. Observed changes in the microstructure depend on the used combination of sintering additives especially their viscosity at sintering temperature. Evolution of the grains and the phase transformation from α to β Si₃N₄ was suppressed by the presence of the graphene multiplatelets. The incorporation of such  carbon-nanostructures into a ceramic matrix inhibits the sintering driving force leading to a lower grain size in at higher temperatures and to a suppression of α to β transformation of silicon nitride at lower temperatures. Due to the relatively mild conditions of the sintering (1600°C/ 1 hour), no coarsening of the silicon nitride matrix or any significant change of this phase was observed. Young’s modulus measurement was performed in order to investigate the orientation of GNPs in the composites. Young’s moduli measured in both perpendicular as well as parallel directions to the sintering plane shows only slight difference which is caused by the arrangement of the silicon nitride grains due to the uniaxial stress applied during hot press sintering not due to the arrangement of graphene platelets. The difference of the grain evolution influences the brittleness, hardness and strength of composites. The presence of the elongated β Si₃N₄ grains increased the value of fracture toughness due to so called toughening mechanisms – crack bridging, deflection, mechanical friction.