hard

Hard is an adjective that describes a material or surface that resists deformation, scratching, or penetration. It is also used more generally to describe something difficult or demanding. In materials science, hardness is a property that characterizes resistance to localized plastic deformation, typically under an indenter or a scratching force.

Hardness is measured by several types of tests. The Mohs scale ranks minerals by their scratch resistance, from talc (1) up to diamond (10). Other widely used methods are indentation tests that produce numerical values, such as Brinell, Vickers, and Rockwell hardness tests. These tests apply defined loads with specific indenter geometries to produce an impression, which is then related to a hardness value.

Hardness provides information about wear resistance and durability, but it does not directly measure a material’s strength, ductility, or toughness. A material can be very hard yet brittle, and some tough materials may not rank extremely high on hardness scales. Therefore hardness is often considered alongside other mechanical properties when selecting materials for a given application.

In other contexts, hardness describes water quality (water hardness) due to dissolved calcium and magnesium ions, which affects scaling and soap performance. The term is also used in science and engineering to describe problems that are computationally difficult, such as NP-hard problems.

Etymology traces hardness to Old English heard, meaning firm or severe. In common usage, the term spans physical properties, water chemistry, and abstract notions of difficulty.