The effect of different sintering additives on the electrical and oxidation properties of Si 3N 4–MoSi 2 composites

Abstract

The fabrication and properties of electrically conductive Si 3N 4–MoSi 2 composites using two different sintering additive systems were investigated (i) Y 2O 3–Al 2O 3 and (ii) Lu 2O 3. It was found that the sintering atmosphere used (N 2 or Ar) had a critical influence on the final phase composition because MoSi 2 reacted with N 2 atmosphere during sintering resulting in the formation of Mo 5Si 3. The electrical conductivity of the composites exhibited typical percolation type behaviour and the percolation concentrations depended on the type of sintering additive and atmosphere used. Metallic-like conduction was the dominant conduction mechanism in the composites with MoSi 2 content over the percolation concentrations due to the formation of a three-dimensional percolation network of the conductive MoSi 2 phase. The effect of the sintering additives on the electrical and oxidation properties of the composites at elevated temperatures was investigated. Parabolic oxidation kinetics was observed in the composites with both types of additives. However, the Lu 2O 3-doped composites had superior oxidation resistance compared to the composites containing Y 2O 3–Al 2O 3. It is attributed to the higher eutectic temperature and crystallisation of the grain boundary phase and the oxidation layer in the Lu 2O 3-doped composites.