Simultaneous improvement in porosity and strength of porous β-Si3N4 ceramics by formation of ultrafine fibrous grains

Abstract Si3N4 ceramic with ultrafine fibrous grains are expected to exhibit remarkable mechanical properties. In this work, highly porous Si3N4 ceramic monoliths composed of ultrafine fibrous grains were developed via a novel vapor-solid carbothermal reduction nitridation (V-S CRN) reaction between SiO vapor and green bodies comprised of carbon nanotubes (CNTs), α-Si3N4 diluents and Y2O3 in a N2 atmosphere. The unique fibrous grains-interconnected structure was developed through in-situ formation of Si3N4 and following liquid phase sintering. The porous Si3N4 monoliths with porosity of 61–78% was developed by controlling the contents of α-Si3N4 diluents and densities of the CNT green bodies. With increasing of the α-Si3N4 contents, Si3N4 fibrous grains with an aspect ratio of approximate or higher than 20 could be achieved, and the grains were gradually refined. For the samples with 40 wt% α-Si3N4, the minimum mean grain diameter and pore size of 164 nm and 0.79 μm were achieved, respectively, and the resultant porous Si3N4 monolith exhibited a flexural strength of as high as 73–102 MPa with the porosity of 61–73%, which is much higher than that of the reported in literature. The improvement of mechanical strength could be attributed to the densely interconnected bird's nests structure formed by the ultrafine fibrous grains. The effects of the α-Si3N4 diluents on the resulting porous Si3N4 monolith via this method were analyzed.