microhardheid

Microhardness (Dutch: microhardheid) is a measure of a material's resistance to localized plastic deformation at small scales. It is typically determined by pressing a diamond indenter into the surface under a controlled, small load and measuring the size of the resulting impression. This scale of hardness testing is distinct from conventional macrohardness tests, which use larger loads and larger indentations.

The most common microhardness methods are the Vickers and Knoop tests. The Vickers test uses a square-based pyramidal indenter and measurements of the diagonals of the resulting square impression to compute HV values. The Knoop test employs an elongated pyramidal indenter, producing an asymmetric indentation that can be advantageous for thin or brittle materials. Loads for microhardness testing are small compared with standard hardness tests, typically ranging from several milligrams to a fraction of a kilogram-force, allowing assessment of thin coatings, films, and microstructural features. Indentation measurements are usually performed with optical or electron microscopy, and results are reported as HV (Vickers hardness) or HK (Knoop hardness), sometimes with subscripted load designations such as HV0.1 or HK0.05.

Applications of microhardness testing include evaluation of surface coatings, hard coatings on tools, thin-film materials, microelectromechanical systems, and small or layered components where bulk testing is impractical. The technique is also used to study hardness variations across a microstructure, such as grain boundaries or phase distributions.

Interpretation of microhardness data must consider factors such as surface finish, residual stresses, anisotropy, and indentation size effects. Empirical correlations exist that relate hardness to material strength, though they vary by material and condition.