Glossary term
Hardness
A material property describing resistance to localized plastic deformation, indentation, scratching, or wear.
Definition
quantityHardness is a material's resistance to localized plastic deformation, usually measured by indentation, scratching, or rebound methods.
In engineering practice, hardness is most often measured by pressing an indenter of known geometry into a material under a specified load and measuring indentation depth or size. The resulting number is not a fundamental material constant like elastic modulus; it depends on test method, load, indenter, surface preparation, microstructure, and scale. Hardness is still valuable because it correlates with wear resistance, heat treatment condition, strength, case depth, and process consistency.
Hardness measures how strongly a material resists localized deformation. In most engineering contexts, this means resistance to indentation by a standardized indenter. A hard surface shows a small indentation under a given load; a softer surface shows a larger or deeper indentation. Hardness is important for wear surfaces, gears, bearings, cutting tools, shafts, rails, welds, coatings, castings, and heat-treated parts.
Hardness is not the same as strength, stiffness, or toughness. A hard material may be brittle. A tough material may not be especially hard. Elastic modulus controls elastic stiffness and is only weakly affected by heat treatment in most metals, while hardness can change strongly with microstructure, cold work, precipitation, or surface treatment.
Common test methods
Brinell hardness uses a relatively large spherical indenter and is useful for castings, forgings, and materials with coarse microstructure. Rockwell hardness measures indentation depth under a specified load and is fast for production testing; Rockwell C is common for hardened steels. Vickers hardness uses a diamond pyramid and can cover a wide range of materials and loads. Knoop hardness is used for thin coatings, brittle materials, and microhardness testing where shallow indentation is needed.
Each scale has its own procedure and number. Hardness values should not be compared across scales without a valid conversion, and conversions are approximate because different tests sample different deformation volumes and stress states.
Engineering interpretation
Hardness is often used as a proxy for strength, especially in steels where empirical correlations exist between hardness and tensile strength. It is also used to verify heat treatment, carburizing, nitriding, induction hardening, weld heat-affected zones, and surface hardening depth. In quality control, hardness testing can detect wrong material, insufficient tempering, decarburization, excessive cold work, or uneven heat treatment.
For wear, hardness is relevant but not sufficient. Wear resistance also depends on counterface material, lubrication, surface roughness, contact stress, debris, temperature, corrosion, and whether the wear mechanism is abrasive, adhesive, fretting, erosive, or fatigue-driven.
Common mistakes
A frequent mistake is reporting a hardness number without the scale and test load. “60 hardness” is meaningless; “60 HRC” and “60 HB” describe very different conditions. Surface preparation also matters. Roughness, scale, coating, curvature, thin sections, edge distance, and indentation spacing can invalidate results. For case-hardened parts, a surface hardness number alone does not prove adequate case depth or core toughness.