Crack kills. Metals are able to deform plastically. A metallic bond is not so directional, and you typically thing of all the molecules in a metal as a big bl

Crack kills. Metals are able to deform plastically. A metallic bond is not so directional, and you typically thing of all the molecules in a metal as a big blob with a cloud of electrons that can freely move about (also the reason for good electrical conductivity). There are "elastic" and "plastic" deformations. Think about bending a paperclip. Use it gently, and you can open it up and get it to squeeze a few papers together. Pull it off and it returns to its original shape. That's elastic. Now open it wider or bend it into another shape, and it will no longer return to the original shape. That's a plastic deformation. Metals can deform plastically which means layers of atoms are sliding past one another in waves. Sometimes these waves are stopped by various irregularities in the crystal lattice. The waves start to pile up, preventing further plastic deformation. Keep bending that paperclip back and forth; it will eventually snap. But in the meantime, it absorbed a fair amount of energy. That's "toughness". In a ceramic, the electrons are not free to move about. If a crack develops, it doesn't take much for there to be terrific stress at the tip of the crack (much like a lever and fulcrum). The crack propagates through the material, and you have catastrophic failure. There are various ways to increase the toughness of ceramics. I'm not all that familiar with them, but a common one is Yttria-stabilized zirconia. That's zirconium oxide with added yttrium. Under stress from a crack, the material can undergo a phase transformation (from one crystal structure to another; not solid to liquid or anything like that). A phase transformation absorbs energy (just like boiling water into steam does) which may be enough to stop the crack from propagating. My ceramics professor had a YSZ hammer that was gifted to him by Coors ceramics (same family as the beer company). YSZ is not transparent however. http://en.wikipedia.org/wiki/Yttria-stabilized_zirconia http://en.wikipedia.org/wiki/Yttria-stabilized_zirconia