Gas-pressure assisted ultrafast high-temperature sintering of Si3N4 ceramics

Abstract

Fabrication of high density silicon nitride (Si₃N₄) ceramics by ultrafast high-temperature sintering remains challenges due to the decomposition of Si₃N₄ under high temperature at atmospheric pressure. Herein, a gas-pressure assisted ultrafast high-temperature sintering (G-UHS) was developed for the preparation of Si₃N₄ ceramics. The effects of sintering temperature (T), heating rate (HR), nitrogen pressure (P), holding time (t), and Y₂O₃-MgO composite sintering aids on the mass loss, oxygen content, phase evolution, and microstructure of Si₃N₄ ceramics were systematically investigated. The densification mechanism during ultrafast high-temperature sintering was further elucidated by thermodynamic analysis in terms of mass loss and oxygen content changes. It was found that under atmospheric pressure, the mass loss during the ultrafast high-temperature sintering process first originates from the decomposition of silicon oxynitride generated by the reaction between silicon oxide on the powder surface and internal Si₃N₄, followed by the direct decomposition of Si₃N₄. Increasing the gas pressure can effectively inhibit the decomposition of Si₃N₄ and the volatilization of liquid-phase sintering aids, thereby promoting the densification of Si₃N₄ ceramics. It was indicated that under ultrafast high-temperature sintering, the rapid formation of rod-like crystal structure by α/β phase transition through dissolution and precipitation inhibits particle rearrangement and restricts the densification of Si₃N₄ ceramics. Si₃N₄ ceramics with a relative density of 94.17% were successfully prepared under the optimal sintering conditions (sintering temperature 1750 °C, nitrogen pressure 0.2 MPa, heating rate 50 °C/s, holding time 180 s) with the addition of 4 wt% Y₂O₃ and 6 wt% MgO. This study can provide useful guidance for the optimization of the ultrafast high-temperature sintering process of high performance Si₃N₄ ceramics.