Hardness scales

Hardness scales

Arbitrarily defined measures of the resistance of a material to indentation under static or dynamic load, to scratch, abrasion, or wear, or to cutting or drilling. Standardized tests compare similar materials according to the particular aspect of hardness measured by the test. Widely used tests for metals are Brinell, Rockwell, and Scleroscope tests, with modifications depending upon the size or condition of the material. Indentation tests compare species of wood or flooring materials, and abrasion tests serve as an index of performance of stones and paving materials.

Hardness tests are important in research and are widely used for grading, acceptance, and quality control of manufactured articles. The hardness designation or scale is associated with the test method or instrument used.

Resistance to scratching is defined by comparison with 10 selected minerals, which are numbered in the order of increasing hardness. This mineralogical scale, called Mohs scale, is 1, talc; 2, gypsum; 3, calcite; 4, fluorite; 5, apatite; 6, orthoclase; 7, quartz; 8, topaz; 9, corundum; and 10, diamond. Minerals lower in the scale are scratched by those with higher numbers.

Materials are differentiated qualitatively according to resistance to scratching or cutting by files especially selected for the purpose. Whether or not a visible scratch is produced on the material indicates its hardness in comparison with a sample of desired hardness.

Resistance to indentation by a hardened steel or tungsten carbide ball under specified load is the basis for Brinell hardness. Brinell hardness number (Bhn), expressed in kilograms per square millimeter, is obtained by dividing the load by the spherical surface area of the impression.

Indentation of a square-based diamond pyramid penetrator with an angle between opposite faces of 136° measures Vickers hardness. Vickers hardness number, also called diamond pyramid hardness, is equal to the load divided by the lateral area of the pyramidal impression.

Depth of indentation of either a steel ball or a 120° conical diamond with rounded point, called a brale, under prescribed load is the basis for Rockwell hardness. The depth of impression is indicated on a dial whose graduations represent the hardness number.

The pressure in kilograms per square millimeter required to embed a 0.75-mm (0.0295-in.) hemispherical diamond penetrator to a depth of 0.046 mm (0.0018 in.), producing an impression 0.36 mm (0.014 in.) in diameter, is the measure of Monotron hardness.

Height of rebound of a diamond-tipped weight or hammer falling within a glass tube from a height of 10 in. (25.4 cm) and striking the specimen surface measures Shore Scleroscope hardness.

Resistance to indentation over very small areas (as on small parts, the constituents of metal alloys, or for exploration of hardness variations) is called microhardness.