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As the direction of materials science continues towards studying the basis of properties on smaller and smaller length scales, different techniques are used to quantify material characteristics and tendencies. Measuring mechanical properties for materials on smaller length scales, like thin films, can not be done using conventional uniaxial tensile testing. As a result, techniques testing material 'hardness' by indenting a material with an impression have been developed to determine such properties. Image File history File links No higher resolution available. ... This Manual of Style has the simple purpose of making things easy to read by following a consistent format — it is a style guide. ...

Hardness measurements quantify the resistance of a material to plastic deformation. Indentation hardness tests comprise the majority of processes used to determine material hardness, and can be divided into two classes: microindentation and macroindentation tests. Microindentation tests are considered to be on the order of 200 gram-Force (g-F) or less. Hardness, however, can not be considered to be a fundamental material property. Instead, it represents an arbitrary quantity used to provide a relative idea of material properties. [1] As such, hardness can only offer a comparative idea of the materials resistance to plastic deformation since different hardness techniques have different scales.

Macroindentation Tests

There are various different used macroindentation tests. The most frequently used include the Brinell Hardness test, the Rockwell Hardness test, and the Vickers Hardness Test.

The Brinell hardness test involves pressing a steel sphere against the material surface for a given period of time, usually 10-15 seconds. The surface of indentation is measured, and the load divided by the area of the impression surface upon the material gives the Hardness, HB. Since different spheres produce different impressions, to maintain the same HB the ratio of the load to the square of sphere diameter needs to be constant.

The Rockwell hardness test is the simplest, and most popular, hardness test. In this test, there is no indentation depth or width measured. Instead, a preload is applied before the main load, and the machine dial gives a number related to the main load. The machine dial provides a number related to the indentation depth, using one of many Rockwell scales. The limitations of the Rockwell test depend on how much the test piece sags. Sagging in the sample causes changes in the hardness measurements.

The Vicker's test is one of the most widely applicable hardness tests; it uses a pyriamidal indenter mounted on a square diamond base. The Vickers test usually uses loads that vary from 1 to 120 kg-F. Requirements include having the surface ground and polished, and making sure work-hardening has not affected the material. Additionally, the mechanism of flow in the material under the indenter affects the shape of the indentation - the sides of the square impression sometimes deform into concave or convex curves, which can result in reading errors.

Microindentation Tests

Microindentation tests, as previously mentioned, are used with loads lighter than 200 g-F, which typically creates indentation on the order of 50 microns. Due to their specificity, microhardness testing can be used to observe changes in hardness on the microscopic scale. Unfortunately, it is difficult to standardize microhardness measurements - it has been found that the microhardness of almost any material is higher than its macrohardness. Additionally, microhardness values vary with load and work-hardening effects of materials.[1] Regardless, the two most commonly used microhardness tess are the Knoop and Vickers tests. The Vickers test is carried out in a similar manner to the Vickers macroindentation tests with the same pyramid. The Knoop test uses an elongated pyramid to indent material samples - the elongated pyramid creates a shallow impression, which is beneficial for measuring the hardness of brittle materials or thin components. Both the Knoop and Vickers indenters require prepolishing of the surface to achieve accurate results.

See also

• Rockwell scale

The Rockwell scale characterises the indentation hardness of materials through the depth of penetration of an indenter, loaded on a material sample and compared to the penetration in some reference material. ...

Sources

[1] Meyers and Chawla (1999): "Mechanical Behavior of Materials," 162-168.