Hardness is a measure of resistance to localized plastic deformation induced by either mechanical indentation or abrasion. It is a quality that varies between different materials, with some being harder than others. Hardness can be measured in different ways, such as scratch hardness, indentation hardness, and rebound hardness. Scratch hardness is the measure of how resistant a sample is to fracture or scratching, while indentation hardness is the measure of resistance to indentation and rebound hardness is the measure of the height of the "bounce" of an object dropped onto a material.
Hardness is dependent on several factors, including 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.
In addition to materials, hardness can also refer to water quality. Hardness in water is due to high levels of minerals such as iron, calcium, and magnesium.
Overall, hardness is a measure of how difficult or easy it is for a substance to be penetrated or scratched.
