Crack deflection and bridging: The presence of the soft shell can induce crack deflection and bridging, which effectively dissipates the energy of the propagating cracks. When a crack encounters a core-shell structural unit, it tends to deflect along the interface between the core and the shell, rather than propagating directly through the ceramic matrix. This crack deflection mechanism helps to increase the toughness of the material.
Energy dissipation: The soft shell can undergo plastic deformation or viscoelastic deformation, which consumes energy and dissipates the stress concentration at the crack tip. This energy dissipation mechanism helps to reduce the driving force for crack propagation and improve the toughness of the ceramic.
Phase transformation toughening: In some cases, the core-shell structural units can undergo phase transformation toughening. For example, when the core is made of a metastable phase, it can transform into a more stable phase under the stress field of the propagating crack. This phase transformation can induce volume expansion and generate compressive stresses around the crack tip, which can effectively arrest the crack propagation and enhance the toughness of the ceramic.
Crack bridging and pullout: The rigid core can act as a bridge to connect the crack surfaces and resist the crack opening. When a crack propagates through a ceramic matrix containing core-shell structural units, the rigid cores can bridge the crack surfaces and prevent the crack from further opening. Additionally, the soft shell can promote the pullout of the rigid cores from the matrix, which also contributes to the toughening of the ceramic.
By combining these toughening mechanisms, core-shell structural units can significantly improve the toughness of ceramics, making them more resistant to fracture and damage. This makes core-shell structural units a promising approach for the development of advanced ceramic materials with enhanced mechanical properties.
