Synthesis and mechanical properties of highly porous ultrafine-grain Si3N4 ceramics via carbothermal reduction-nitridation combined with liquid phase sintering

Highly porous Si 3 N 4 ceramics have been prepared via a novel carbothermal reduction-nitridation (CRN) reaction between carbon nanotubes (CNTs) and SiO vapor in a N 2 atmosphere. The green bodies consisted of 49.0 wt% CNT, 40.4 wt% α-Si 3 N 4 as seed and 10.6 wt% Y 2 O 3 as sintering additive, and SiO powders were located under the green bodies during sintering. By heating at temperatures ranging from 1550 to 1650 °C, fibrous β-Si 3 N 4 nanograins were obtained. Complete reaction was achieved after soaking at 1650 °C, and a mean grain diameter of approximately 290 nm and aspect ratio of as high as 20 were obtained, and a porosity of 73% was reached due to the low density of the green bodies. The bending and compressive strengths were up to 62 MPa and 82 MPa, respectively, which were higher than the values in the literature, as a result of the fine elongated fibrous β-Si 3 N 4 grains. It should be noted that the Si 3 N 4 ceramics exhibited a large strain to failure and ‘pseudoplastic’ deformation behavior during the compressive test, which was attributed to the high porosity.