Factors inducing degradation of properties after long-term oxidation of Si3N4-MoSi2 electroconductive composites

The effects of heat treatments on strength and electrical conductivity after 100 h in air up to 1500 °C were evaluated on hot-pressed Si3N4-35 vol% MoSi2 composite. The long-term oxidation involves microstructural changes at the material surface and subsurface, such as the formation of oxide scales and of a multilayered microstructure. At T 1200 °C, a glassy silicate phase is formed, which embeds cristobalite grains and highly textured Y2Si2O7 crystals. At the same time, MoSi2, assisted by oxygen, reacts with Si3N4 forming Mo5Si3, Si2N2O, and SiO2. The decrease of the room temperature flexural strength reached about 25% in the samples exposed at 1000 °C for 100 h, compared to the as-produced materials. On the contrary, after treatments at higher temperatures, the strength decrease is lower at 1500 °C, the residual strength is 836 ± 62 MPa with a strength decrease of about 8%. The surface oxide scale is an insulator and, consequently, the electrical resistivity of the composite rises from 10-3 to 107-109 ohm cm.