Isotropic Si3N4 ceramics fabricated by low-pressure spark plasma sintering in combination with direct coagulation casting

Isotropic Si3N4 ceramics were fabricated by low-pressure spark plasma sintering in combination with direct coagulation casting (DCC/LP-SPS). A novel phenomenon is observed that hardness and fracture toughness of Si3N4 lift simultaneously as temperature rises. It can be attributed to the enhanced densification and formation of β′-sialon. Rapid coagulation of the submicron ceramic suspension prepares green body with high relative density and offers the sufficient driving force for densification, while electric field facilitates particles rearrangement, thus Si3N4 ceramics can obtain the relative density above 97% under the axial pressure of 5 MPa at 1550 °C. As SPS temperature rises, there is the formation of β′-sialon phase in Si3N4 ceramics without AlN as sintering aid. This is most likely due to electric field promoting the motion and uniform distribution of charged species in liquid phase, which facilitates the replacement of Si4+ and N3− by Al3+ and O2− during the process of dissolution-precipitation. Additionally, compared with Si3N4 prepared by hot pressure sintering, there is no obvious directional alignment of grains in the samples that are spark plasma sintered under a low applied pressure. As a consequence, Si3N4 ceramics prepared by DCC/LP-SPS acquire complete transformation of α→β phase and full densification without abnormal grain growth, leading to ultimate formation of an isotropic self-reinforced bimodal structure containing β-Si3N4 and β′-sialon. Therefore, Si3N4 ceramics, coagulated by 45 vol% suspension and sintered at 1650 °C for 20 min with a uniaxial pressure of 5 MPa, simultaneously achieve the high Vickers hardness and fracture toughness as follows: 17.5 ± 1.0 GPa and 8.5 MPa m1/2.