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In this paper, the concept of incorporating core–shell structured units as secondary phases to toughen Al2O3 ceramics is proposed. Al2O3 composite ceramics toughened by B4C@TiB2 core–shell units are successfully synthesized using a combination of molten salt methodology and spark plasma sintering. The synthesis of B4C@TiB2 core–shell toughening units stems from the prior production of core–shell structural B4C@TiB2 powders, and this core–shell structure is effectively preserved within the Al2O3 matrix after sintering. The B4C@TiB2 core–shell toughening unit consists of a micron-sized B4C core enclosed by a shell approximately 500 nm in thickness, composed of numerous nanosized TiB2 grains. The regions surrounding these core–shell units exhibit distinct geometric structures and encompass multidimensional variations in phase composition, grain dimensions, and thermal expansion coefficients. Consequently, intricate stress distributions emerge, fostering the propagation of cracks in multiple dimensions. This behavior consumes a considerable amount of crack propagation energy, thereby enhancing the fracture toughness of the Al2O3 matrix. The resulting Al2O3 composite ceramics display relative density of 99.7%±0.2%, Vickers hardness of 21.5±0.8 GPa, and fracture toughness 6.92±0.22 MPa·m1/2.

Previous studies have shown that an "intragranular" structure can greatly improve the properties of composite ceramics. In this paper, the influence of ZrO2 content changes and different preparation methods of the composite powder on the formation of the "intragranular" structure and mechanical properties of composite ceramics was investigated. The results showed that 3 vol% ZrO2–Al2O3 composite ceramics prepared by mechanical grinding and dispersing technology contained a large number of "intragranular" structures after microwave sintering at 1500 °C for 30 min, and its relative density, flexural strength, fracture toughness and Vickers hardness were 99.07%, 520.21 MPa, 7.64 MPa.m1/2 and 18.79 GPa, respectively. In addition, the flexural strength and fracture toughness were increased by 50% and 83%, respectively, compared with those of 3 vol% ZrO2–AlO3 ceramics prepared by the conventional direct doping ZrO2 powder method. The "intragranular" ZrO2 particles were dispersed in the Al2O3 ceramic matrix and formed a superlattice structure, and there was a coherent relationship between the two interfaces. The strengthening and toughening mechanism of the "intragranular" structure of ZrO2–Al2O3 composite ceramics is discussed. [ABSTRACT FROM AUTHOR]