Why are ceramics generally harder and more brittle than metals?

• A. Because ceramics have metallic bonding
• B. Because ceramics have ionic/covalent bonds and slip causes like-charged repulsion leading to fracture
• C. Because ceramics have strong ductile slip systems
• D. Because ceramics are always denser

Answer: (B)

The main idea is that the type of chemical bond in a material strongly governs how it deforms. Ceramics are held together mainly by ionic or covalent bonds, which are strong and highly directional. This makes the crystal lattice very stiff and difficult for atoms to rearrange, so there is little plastic deformation before fracture. Metals, by contrast, deform plastically because their bonds are non-directional metallic bonds and dislocations can glide on many slip systems, allowing significant strain before breaking. In ceramics, dislocation motion is strongly hindered; trying to slip would break bonds and create unfavorable charge interactions, so cracks initiate and propagate rather than accommodate deformation. That combination—very strong, directional bonds and limited dislocation mobility—yields hardness with brittleness. The other statements don’t fit: metallic bonding wouldn’t explain brittleness; having strong ductile slip systems would imply ductility instead of brittleness; density isn’t the fundamental reason for brittleness.