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Why are ceramics vulnerable to cracks under mechanical loading?

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advanced structural ceramics Figure 1: Resistance to cracking in transformation-toughened zirconia. In a ceramic composed of tetragonal zirconia dispersed in a zirconia matrix, the stress field advancing ahead of a propagating crack transforms the small tetragonal particles to larger monoclinic particles. The larger particles exert a crack-closing force in the process zone behind the crack tip, effectively resisting propagation of the crack.

Ceramics are inherently vulnerable to cracks under mechanical loading primarily due to their brittleness and a lack of plastic deformation mechanisms. Unlike metals, which are ductile because they contain common and mobile dislocations that allow the material to accommodate stress through slip, ceramics possess strong, directional chemical bonds that make dislocation motion extremely difficult.123

In ionically bonded ceramics, attempting to move a dislocation would force identically charged ions into proximity, causing them to repel each other and impede movement. Because ceramics lack the necessary number of independent slip systems to transfer deformations between grains, they cannot effectively redistribute stress. Consequently, when subjected to mechanical loading, ceramics cannot accommodate stress through structural rearrangement; instead, they reach a critical point where the internal structure fails, leading to sudden, catastrophic fracture.13

Furthermore, ceramics are characterized by inherent flaw sensitivity. Small structural defects or sharp notches act as sites for stress concentrations, where stress can become significantly higher than the nominal load, facilitating the nucleation and rapid propagation of cracks.45