Ceramic materials are usually crystalline (having a repeating shape) or amorphous (no repeating shape). They are usually held together with covalent or ionic bonds. Thus, they tend to fracture (break) before any plastic deformation (permanent damage) happens. So they have poor toughness (resistance to breaking). Second, ceramics have pores (small holes) which focus stress on smaller areas.This decreases the toughness even more and reduces the tensile strength. For these two reasons, ceramics fail far more catastrophically (suddenly) than metals. 