Rare-earth nitrate additives for the sintering of silicon carbide

Fourteen rare earth elements in their nitrate form were evaluated as sintering additives for β-SiC. All rare earth nitrates transformed to oxides by a reaction with the surface-adsorbed thin SiO2 during heat treatment, which enhanced the density of the SiC monolith without decomposing SiC. In particular, Sc, Yb, Tm, Er and Ho were quite effective sintering additives; a > 99% relative density was observed by the addition of 5 wt.% rare earth oxide, whereas the other rare earth additives (Lu, Dy, Tb, Gd, Eu, Sm, Nd, Ce and La) revealed 77-92% density. Moreover, a fine 156 nm-sized SiC grain could be acquired by Sc addition, whereas the other additives showed a SiC grain size of approximately 1 μm. The mean hardness and KIc of the dense SiC containing rare earth elements were 24-27 GPa and 3.3-5.0 MPa m1/2, respectively.