Hardness is a measure of how resistant solid matter is to various kinds of
permanent shape change when a force is applied. Macroscopic hardness is
generally characterized by strong intermolecular bonds, but the behavior of
solid materials under force is complex; therefore, there are different measurements
of hardness: scratch hardness, indentation hardness, and rebound hardness. Hardness is
dependent on ductility, elastic stiffness, plasticity, strain, strength, toughness,
viscoelasticity, and viscosity. Common examples of hard matter are ceramics, concrete,
certain metals, and superhard materials, which can be contrasted with soft matter.There
are three main types of hardness measurements: scratch, indentation, and rebound. Within
each of these classes of measurement there are individual measurement scales. For practical
reasons conversion tables are used to convert between one scale and another.