Combining multiple strategies for fast densification and superior mechanical properties in 8YSZ with cold sintering pre-treatment

The cold sintering process (CSP), which has recently attracted significant research attention, is an emerging technique that enables the densification of high-melting-point ceramics at temperatures far lower than those required for conventional sintering by utilizing solvent-assisted mechanisms. In this study, CSP is employed as a pre-treatment strategy for 8 mol.% yttria-stabilized zirconia (8YSZ) to modify the initial microstructural and interfacial state prior to high-temperature sintering. In parallel, Fe₂O₃ was introduced as a sintering aid to enhance grain-boundary diffusion during the high-temperature stage. Rather than acting as an independent densification step, CSP establishes a path-dependent initial condition that governs the subsequent densification trajectory during conventional sintering. CSP at 180 °C for 1 h under a uniaxial pressure of 200 MPa, followed by post-annealing at 1200 °C for 30 min, resulted in a high relative density of 98.11% theoretical density (TD). Depending on the type of solvent, the treated samples exhibited either a high Vickers hardness (Hv = 14.99 GPa) or a high fracture toughness (K_IC = 5.56 MPa·m^1/2), reflecting differences in grain growth mode and crack-path geometry. Stage-resolved densification kinetics were directly monitored using in-situ laser dilatometry, enabling identification of pressure-driven rearrangement, solvent-mediated consolidation, and thermally activated diffusion contributions. The results demonstrate that CSP pre-treatment governs particle packing topology and interfacial chemistry, while Fe₂O₃ addition primarily amplifies high-temperature grain-boundary diffusion. This sequential coupling establishes a mechanistic framework for path-dependent densification in CSP-assisted ceramic processing.