Achieving high fracture toughness in Al2O3 ceramics via engineered B4C@ZrB2 core-shell toughening units

In this study, B 4 C@ZrB 2 core-shell powders were synthesized via a molten-salt method and subsequently consolidated with an Al 2 O 3 matrix by spark plasma sintering (SPS) to fabricate Al 2 O 3 -B 4 C@ZrB 2 composite ceramics. The core-shell architecture, consisting of a micron-sized B 4 C core coated with a submicron-thick ZrB 2 shell, was preserved in the sintered composites, forming well-defined composite toughening units. This unique geometric configuration, combined with multi-dimensional heterogeneities in phase composition (Al 2 O 3 /B 4 C/ZrB 2 ), grain size (micron/submicron), and thermal expansion coefficients (7.2/4.5/6.2 ×10 −6 K −1 ), actively induces multi-dimensional crack propagation: first along the Al 2 O 3 -ZrB 2 interface, then through the submicron‑thick ZrB 2 shell, along the micron‑sized B 4 C-ZrB 2 interface, out through the ZrB 2 shell again, and finally along the ZrB 2 -Al 2 O 3 interface before re-entering the Al 2 O 3 matrix. Such a shuttling behavior across multiple interfaces collectively dissipates substantial crack-propagation energy. The composite sintered at 1500 °C reached near-full densification (relative density 99.78 ± 0.12%) and exhibited excellent mechanical properties: Vickers hardness of 19.23 ± 0.19 GPa, flexural strength of 548 ± 31 MPa and fracture toughness of 6.79 ± 0.11 MPa·m 1/2 (indentation method) and 6.27 ± 0.42 MPa·m 1/2 (SENB method). This work provides a new strategy for developing high‑performance structural ceramics.