The effect of different sintering additives on the electrical and oxidation properties of Si3N4–MoSi2 composites

The fabrication and properties of electrically conductive Si3N4–MoSi2 composites using two different sintering additive systems were investigated (i) Y2O3–Al2O3 and (ii) Lu2O3. It was found that the sintering atmosphere used (N2 or Ar) had a critical influence on the final phase composition because MoSi2 reacted with N2 atmosphere during sintering resulting in the formation of Mo5Si3. 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 MoSi2 content over the percolation concentrations due to the formation of a three-dimensional percolation network of the conductive MoSi2 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 Lu2O3-doped composites had superior oxidation resistance compared to the composites containing Y2O3–Al2O3. It is attributed to the higher eutectic temperature and crystallisation of the grain boundary phase and the oxidation layer in the Lu2O3-doped composites.