1. Why are ceramics brittle and most metals not?
Ceramics materials have very small and omnipresent flaws as minute surface or interior cracks (microcracks), internal pores, and grain corners, which are virtually impossible to eliminate or control. These flaws serve as stress raisers such that depending on crack orientation and geometry, applied stress may be amplified or concentrated at the tip of the crack. When the magnitude of a tensile stress at the tip of one of these flaws exceeds the value of this critical stress which is large enough to break apart the interatomic bonds, a crack forms and then propagates, which results in fracture. Crack then propagates through the cross section of material in a direction perpendicular to the applied load. The energy required simply to break the interatomic bonds is much less than that absorbed by ductile tearing in a tough material, and this is why materials like ceramics and glasses are so brittle.
In more detail:
Bond type: The atoms in ceramic materials are most commonly held together by the two covalent and ionic primary bonds (or) mix of them. Covalent and ionic bonds are much stronger than metallic bonds.
Slip systems: Both these bond types have a very few/limited slip systems (crystallographic planes and directions within those planes) as a consequence dislocations are limited and results in a negligible or no plastic deformation at room temperature.
Stress raisers: The effect of a stress raiser is more significant in brittle than in ductile materials. For a ductile material, plastic deformation ensues when the maximum stress exceeds the yield strength. This leads to a more uniform distribution of stress in the vicinity of the stress raiser and to the development of a maximum stress concentration factor less than the theoretical value. Such yielding and stress redistribution do not occur to any appreciable extent around flaws and discontinuities in brittle materials; therefore, essentially the theoretical stress concentration will result.
Fracture strength: The measured fracture strengths for most brittle materials are significantly lower than those predicted by theoretical calculations based on atomic bonding energies.