Microstructural Development and Secondary Phases in Silicon Nitride Sintered with Mixed Neodymia/Magnesia Additions

Abstract

The pressureless sintering of silicon nitride powder with mixed neodymia/magnesia additives is reported. Of the sixteen additive combinations studied the majority give rise to rapid densification after firing in 1 atm nitrogen for 30min at a temperature of 1650°C and densities of 3050kg m⁻³ are achieved. Such rapid densification is attributable to a large volume of liquid formed during densification. The α–β transformation is controlled by the additive level and combination ratio and, in certain compositions, the optimum requirement of high density and virtually complete transformation are obtained simultaneously. The resulting β-grain morphologies are acicular and interlocking suggesting that ceramic compositions within this easily densified silicon nitride / additive system should have good mechanical properties. Preliminary work shows that the intergranular glass can be devitrified to give more bencial crystalline phases.