EFFECT OF Y2O3 ADDITION ON ALUMINA-HEX BORON-NITRIDE COMPOSITES

Hot-pressing is the only process to densify alumina-hexagonal boron nitride composites. It must be applied at 1750 degrees C - 40 MPa, under nitrogen, but it does not reduce the porosity lower than 8% for composites with alumina contents lower than 70 vol%, and leads to anisotropic composites due to the orientation of the boron nitride platelets perpendicular to the pressing axis. Furthermore, the composites' behaviour during their use in the continuous steel casting is mainly due to the porosity and anisotropy. To increase their resistance to oxidation by oxygen and corrosion by molten steel, and to change the effect of their microstructure, some yttrium oxide additions have been used. Yttrium oxide, by helping the alumina sintering, increases the composite's densification and so its mechanical strength, and by increasing the grains cohesion, it allows us to change the hot-pressing programme to obtain a move isotropic composite. Also, by reaction with alumina, it forms yttrogarnet (YAG) which protects the composite from oxidation by forming a protective liquid phase at the surface, following the reaction with boron oxide formed by the boron nitride oxidation. However, this liquid phase is eroded by the molten steel and so, cannot increase the corrosion resistance, but it is possible to stop this corrosion by an addition of neodymium oxide.