Kinetics of Densification and Phase Transformation at Microwave Sintering of Silicon Nitride with Alumina and Yttria or Ytterbia as Additives

Abstract

Sintering of Si₃N₄-based ceramics using microwave power at frequencies of 28 and 30GHz has been studied. Sintering of materials with various oxide additives, 3 wt% Al₂O₃+5 wt% Y₂O₃ and 3 wt% Al₂O₃+5 wt% Yb₂O₃, and small amount of 1.5 wt% Al₂O₃+2.5 wt% Yb₂O₃ additives has been investigated. A comparison of the results obtained at different rates of heating, from 5°C/min to 90°C/min, shows that the apparent activation energy for sintering in the materials with large amount of additives decreases with an increase of the microwave power absorbed per unit volume of material. The rate of densification depends strongly on the rate of the α-to β-phase transformation at the initial stage of sintering. The results show that microwaves do not affect directly the rate of the α-to β-phase transformation. Therefore, pressureless sintering of the Si₃N₄ ceramics with low content of additives and low rate of phase transformation can not be enhanced by an increase in the specific microwave absorbed power.