Hardness

Hardness is a material property that characterizes its resistance to localized plastic deformation, such as indentation, scratching, or wear. It is commonly used in materials science, geology, and engineering to compare materials and guide selection for specific applications.

In mineralogy, hardness is often described on the Mohs scale, a qualitative 1 to 10 ranking based on a material’s ability to scratch another mineral. The scale provides a convenient relative measure but does not yield a direct, quantitative comparison of mechanical properties.

In engineering and quality control, hardness is typically assessed through indentation tests. Brinell hardness (HB) uses a steel or carbide ball under a specified load, with hardness calculated from the impression diameter. Rockwell hardness (HRA, HRB, HRC, etc.) uses different indenters and scales, measuring the depth of penetration under a heavy load. Vickers hardness (HV) employs a diamond pyramid indenter to produce an indentation whose diagonals determine the hardness value. Knoop hardness (HK) uses an elongated pyramid for very hard or thin specimens. Microhardness tests apply smaller loads to reveal hardness on a microscopic scale. In polymers, Shore hardness scales (Shore A, Shore D) are commonly used.

Hardness is related to, but not determined by, strength or toughness. A material can be very hard yet brittle, while another may be moderately hard but highly tough. Hardness depends on composition, microstructure, heat treatment, temperature, and orientation in anisotropic materials. Applications include wear resistance, machining and tool selection, coating durability, and quality assurance.