Zirconia has been difficult to obtain stable sintered products due to the significant volume change of approximately 3-5 vol% accompanying the transition from tetragonal to monoclinic during cooling. To solve the problem, stabilization of the cubic polymorph of zirconia over a broader range of temperatures is accomplished by substituting some of the Zr⁴⁺ ions (ionic radius of 0.82 Å) in the crystal lattice with slightly larger ions of Y³⁺ (0.96 Å). Further, the toughness of yttria-stabilized zirconia (YSZ) can increase due to a stress-induced transformation of a metastable phase near a propagating crack. In the present work, the sintering behavior of YSZ could be controlled by modification of the surface energy and structure via the adsorption of graphitic carbon nitride (g-C₃N₄) nanosheets in the previous sintering stage, which is a carbon nitride compound with a two-dimensional structure similar to graphene.
   We prepared a spherical YSZ powder of approximately 30 μm formed by an organic binder based on the sintered YSZ seed. Before sintering, a process of adding surface-modified g-C₃N₄ nanosheets with metal ions such as yttrium and then adsorbing spontaneously to the pores and surfaces of the beads was considered. Specifically, the bulk g-C₃N₄ sheets were prepared by calcinating melamine as raw materials and then surface-modified by protonation for zeta-potential change, exfoliated, and dispersed through an acid treatment process based on Hummers' method. Thus, yttrium-doped g-C₃N₄ nanosheets (Y³⁺/g-C₃N₄) adsorbed YSZ powder could be prepared by incorporating stable spontaneous adsorption by zeta-potential attraction onto the surface of YSZ particle in g-C₃N₄ nanosheets dispersed yttrium precursor aqueous solution.
   The surface-modified g-C₃N₄ nanosheets were confirmed for positively charged surface by FT-IR analysis and zeta potential measurement, and the thickness of the sheet was exfoliated to about 10 nm through AFM analysis. The YSZ beads synthesized according to process conditions were compared by sintering at 1200 °C and 1300 °C. The crystal structure and microstructure were analyzed using FE-SEM and XRD in the sintered ceramic beads. As a result of the Vickers microhardness for cross-sections of the specimen, the hardness was improved from a minimum of 10.1 % to a maximum of 32.3 % with g-C₃N₄ adsorption under each sintering condition.