Effect of Al powder content on the properties and microstructure of Y₂O₃-based ceramic core

In order to obtain ceramic core with high chemical reaction resistance for titanium alloy investment casting, the Al powders were introduced into Y₂O₃-based ceramic core as the sintering aids, and then the modified ceramic core was prepared by hot injection molding. The effects of Al powder content on the densification characteristic, mechanical properties, microstructure and phase composition of Y₂O₃-based ceramic cores were studied. The results indicate that the sintering shrinkage and high temperature deflection of Y₂O₃-based ceramic core decrease with the increasing of Al powder content. The oxidation of Al element led to the expansion of the Al powders, which is beneficial to resist the shrinkage of Y₂O₃-based ceramic core during the sintering process. The densification of ceramic core is enhanced by liquid-phase sintering of Al powder melting and the interface reaction sintering of Al-Y₂O₃, as well as the diffusion sintering between Al₂O₃ and Y₂O₃ matrix. The formation of Al₂Y₄O₉ crystals in matrix inhibits the secondary sintering of intergranular fine Y₂O₃ particles, which improves the high temperature creep resistance of the ceramic core. With a small amount of Al powder introducing, the Al₂Y₄O₉ and Y₂Al crystals are formed and then coated on the surface of the matrix Y₂O₃ particles, and the interfacial bonding strength is improved by the second phase strengthening. The Y₂O₃-based ceramic core modified by 2wt%Al powder, exhibits the optimal flexural strength about 34.38 MPa with a typical cleavage fracture, which is 49.15% higher than that of the ceramic core without Al powder. However, the expansion caused by excessive oxidation of Al powder can expands the distance between the ceramic particles. Therefore, the interfacial bonding strength of Y₂O₃-based ceramic core is weakened obviously, and then the ceramic core tends to fracture along the grain boundaries, resulting in the decrease of the bearing capacity under the load.