Enhanced Mechanical Properties of Al₂O₃ Nanoceramics via Low Temperature Spark Plasma Sintering of Amorphous Powders

In this work, Al₂O₃ nanoceramics were prepared by spark plasma sintering of amorphous powders and polycrystalline powders with similar particle sizes. Effective comparisons of sintering processes and ultimate products depending on starting powder conditions were explored. To ensure near-full density higher than 98% of the Al₂O₃ nanoceramics, the threshold temperature in SPS is 1450 °C for polycrystalline Al₂O₃ powders and 1300 °C for amorphous powders. The low SPS temperature for amorphous powders is attributed to the metastable state with high free energy of amorphous powders. The Al₂O₃ nanoceramics prepared by amorphous powders display a mean grain size of 170 nm, and superior mechanical properties, including high bending strength of 870 MPa, Vickers hardness of 20.5 GPa and fracture toughness of 4.3 MPa∙m^1/2. Furthermore, the Al₂O₃ nanoceramics prepared by amorphous powders showed a larger dynamic strength and dynamic strain. The toughening mechanism with predominant transgranular fracture is explained based on the separation of quasi-boundaries.