Hardness tests are also frequently called indentation tests. During these tests a tool is used to force an impression on the surface of the material; generally, one material is used to scratch another material or a series of materials. These tests are mostly done on metal or stone. The results are compared and examined. Sometimes bending, scratching, cutting, abrasions or penetration is used to evaluate hardness as well. Hardness tests can be done manually by a worker using a sharp tool to gouge at the material’s surface or it can be performed by a machine.
Hardness cannot be automatically determined by calculating the fundamental units of mass, length and time. Instead, a hardness value is the result of a certain procedure that provides accurate responses instead of estimations. The Mohs Scale ranks materials on their ability to resist scratching by another material, one of the most common, basic and longest known techniques of taking a hardness test.
The results of hardness tests are used as basis for the comparison of materials, heat treatment, quality control and more. This is necessary knowledge for industrial and manufacturing companies to determine materials and specifications for parts and products.
Hardness tests usually measure the depth or area of an indentation left by a tool of a specific shape with a certain force applied to it for a period of time. There are three main testing methods that use this basic procedure. One of the most common is the Rockwell hardness test which uses a small steel ball for soft material or a diamond cone for harder surfaces. The depth of penetration is measured automatically by the machine and displayed as a Rockwell hardness number.
Another widely used method is the Brinell test. It also uses a steel ball which averages 10 millimeters in diameter. The Brinell hardness number (BHN) is closely related to the tensile strength of the material; this test, like the Rockwell, is simple, fast and does not destroy the product being tested. The Vickers test can be a microhardness test; that is, the indentations made during the testing process are so small that a microscope is required to take a measurement. On the other hand, a macroindentation can be seen with the unaided eye.
The Vickers hardness test uses a triangular shaped tool to impress a pyramidal shape into the material if the metal or stone surface will allow it. Like other calibration services, hardness testing provides data and numerical discrepancies between samples; however, unlike machine or speedometer calibration the strength of the material cannot be quickly adjusted but rather must be reformulated or recreated.