Direct production of Si3N4 foams by carbothermal reduction and nitridation of SiO2

Abstract

In this study, Si3N4 foams are successfully produced by a combination of replica and carbothermal reduction and nitridation (CRN) methods. Slurries prepared from C, SiO2 (molar ratio of C:SiO2 was kept to 2.2), bentonite and three types of oxide additives (Y2O3, CeO2 and Yb2O3) are impregnated into polyether sponges. With the addition of bentonite, sufficient green strength is provided and durable foams are produced after the sponge burn-out process. Bentonite also enhances the CRN process by increasing the amount of eutectic melt and, hence, a higher amount of β-Si3N4 is obtained. In addition to bentonite, the yield of β-Si3N4 is highly dependent on the type of oxide additive. The highest yield is achieved when Y2O3is used as a result of the formation of a lower viscosity eutectic melt. Vapor-liquid-solid mechanism is favored by the lower viscosity. Foams have β-Si3N4 as the major phase with minor amounts of α-Si3N4 and secondary phases originate from the eutectic melt. The formation and growth mechanisms of β-Si3N4 are directly affected by the eutectic melt. SiO is deposited in the eutectic melt and super-saturation of β-Si3N4 grains occur. During the growth of these grains, they contact each other, hindering the formation of typical elongated β-grains. The final microstructures of the foams consist of hexagonal β-grains with a low aspect ratio.