Phase Stability and Slag-Induced Destabilization in MnO₂ and CeO₂-Doped Calcia-Stabilized Zirconia

MnO₂ and CeO₂ were doped to improve the corrosion resistance of CSZ (calcia-stabilized zirconia), and we studied the phase formation, mechanical properties, and corrosion resistance by molten mold flux. The volume fraction of the monoclinic phase gradually decreased as the amount of MnO₂ doping increased. The splitting phenomenon of the t(101) peak was observed in 2Mn_CSZ, and in 4Mn_CSZ, it was completely split, forming a cubic phase. The relative density increased and the monoclinic phase decreased as the doping amount increased, leading to an increase in Vickers hardness and flexural strength. However, in 3Mn_CSZ and 4Mn_CSZ, where cubic phase formation occurred, the tetragonal phase decreased, leading to a reduction in these properties. MnO₂-doped CSZ exhibited a larger fraction of the monoclinic phase compared to the original CSZ after the corrosion test, indicating worsened corrosion resistance. These results are attributed to the predominant presence of Mn³⁺ and Mn²⁺ forms, rather than the Mn⁴⁺ form, which has a smaller basicity difference with SiO₂, and due to the low melting point. The monoclinic phase fraction decreased as the doping amount of CeO₂ increased in CeO₂-doped CSZ, but the rate of decrease was lower compared to MnO₂-doped CSZ. The monoclinic phase decreased as the doping amount increased, but the Vickers hardness and flexural strength showed a decreasing trend due to the low relative density. The destabilization behavior of Ca in SEM-EDS images before and after corrosion was difficult to identify due to the presence of Ca in the slag, and the destabilization behavior of Ce due to slag after corrosion was not observed. In the XRD data of the specimen surface after the corrosion test, the fraction of the monoclinic phase increased compared to before the test but showed a lower monoclinic phase fraction compared to CSZ. It is believed that CeO₂ has superior corrosion resistance compared to CaO because Ce predominantly exists in the form of Ce⁴⁺, which has a smaller difference in basicity within the zirconia lattice.