Factors inducing degradation of properties after long-term oxidation of Si_3N_4–MoSi_2 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 Si 3 N 4 –35 vol% MoSi 2 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 Y 2 Si 2 O 7 crystals. At the same time, MoSi 2 , assisted by oxygen, reacts with Si 3 N 4 forming Mo 5 Si 3 , Si 2 N 2 O, and SiO 2 . 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 10 7 –10 9 Ωcm.