Glass, ceramic, adamas are hard but fragile, is there any relation inside between hardness and brittleness, for example the relation decided by the level of molecues and atoms? Actually, the relation between them is quite strong, but it is not all true. Generally speaking (according to small variation of exact atoms’ ), covalent bond is greater than metallic bond greater than ionic bond greater than molecular link.
Hard material (connecting to quick radiating and high-temperature resistance) is basically three-dimensional covalent bond. Soft material (plastic) is one or two-dimensional.Tje intermediate material is metallic bond and ionic bond.
There is no exact direction of low dimension material and metallic bond, therefore, the split would not spread easily after bursting. Ionic bond and covalent bond has rigid direction and distance, once split existing, the damage area would extend, this is what we call brittleness.
As a result, the most hard materials are most brittle. Meanwhile, some quite soft materials such as ionic bond, are soft and also very brittle.
Metal gives priority to metallic bond, most alloy rely on a large number of covalent compound to be hardened. In the case of the same metal( iron ), more harden it is, more brittle it will be.
However, some kinds of high-temperature alloy can have both nice toughness and high hardness for they have strong metallic bond. Covalent bond can be designed to become composite material or NM material, therefore they have certain tenacity.