Toughness: When a huge external force is applied on a metal, the metal will experience fracture. Toughness is the ability of a metal to resist fracture. The ability of a material to resist fracture due to high impact loads, is called toughness.

Hardness: Hardness is the ability of a metal to resist permanent change of shape caused by an external force.

Brittleness
Brittleness is a property of material which breaks with little or no deformation.

Ductility: Ductility is the property by which a metal can be drawn into thin wires. It is determined by percentage elongation and percentage reduction in area of a metal.

In Clearance fit, the largest permitted shaft diameter is smaller than the diameter of the smallest hole, so that the shaft can rotate or slide through with different degrees of freedom according to the purpose of mating members.