Functionally gradient Al ₂ O ₃ ceramics with tailored high‐entropy carbide hardening phase and ZrO ₂ toughening phase

Abstract Al 2 O 3 , as a representative structural ceramic, suffers from intrinsic brittleness that severely limits its engineering applications. ZrO 2 , as a common toughening phase, has long been used to toughen Al 2 O 3 ceramic matrices. Although the phase transformation toughening mechanism of ZrO 2 can effectively improve the fracture toughness of Al 2 O 3 ceramics, it typically leads to a significant reduction in hardness. In this study, we innovatively introduced high‐entropy carbide (HEC) as a hardening phase combined with ZrO 2 toughening phase to construct a novel Al 2 O 3 –ZrO 2 –(HfNbTaTiZr)C functionally graded ceramic system, achieving an excellent combination of surface hardness (19.49 GPa) and fracture toughness (7.40 MPa·m 1/2 ). Mechanistic investigations reveal that the thermal expansion coefficient mismatch between HEC and ZrO 2 induces beneficial compressive residual stress fields in the surface layer. Besides, the phase transformation toughening mechanism of ZrO 2 further enhances the fracture toughness, and the introduction of the HEC phase effectively compensates for the hardness loss caused by ZrO 2 addition. This study provides a new strategy to overcome the hardness‐toughness trade‐off in conventional toughening approaches and establishes a promising pathway for developing advanced structural ceramics, demonstrating significant potential for engineering applications requiring both high hardness and toughness.