Hardness is a measure of a material's resistence to an indention, scratch or abrasion.
Materials engineer Dr. Dmitri Kopeliovich defines hardness testing as a method to measure the resistance of a material to abrasion, scratching or indention. Hardness is dependent upon the material's elasticity, yield strength and tensile strength. Categories of hardness tests include those designed to measure macro-hardness, micro-hardness and nano-hardness. All three methods involve forcing an indenter into the surface and measuring the resulting indentation.
Categories of Hardness Tests
Macro-hardness tests are the most widely used. The test uses from 50 to 3,000 Newtons of pressure to indent the surface. Micro-hardness tests use a smaller force to press a diamond pyramid into the surface. Micro-hardness tests are used to test surfaces divided into different areas of hardness by processes such as heat treating or coatings. Nano-hardness tests use small amounts of pressure for precise measurement of hardness.
Rockwell, Brinell and Vickers Hardness Tests
The Center for Advanced Life Cycle Engineering at the University of Maryland divides hardness tests by measuring techniques. The Rockwell Hardness Test measures the material hardness by the depth of the indention. The Brinell Hardness Test measures the diameter of the indention. The Vickers Hardness Test uses a pyramid-shaped indentor and the diagonal of the indention is measured.
Knoop and Shore Hardness Tests
The Knoop Hardness Test uses an irregularly shaped pyramid indentor that leaves an indention with varied width, length and depth. Depth of the indention is measured to determine the hardness. The Shore Hardness Test measures the distance a diamond-tipped hammer will rebound after being dropped from a specified height above the material to be tested.
Tests to Measure Micro, Macro and Nano Hardness
The Rockwell, Brinel or Vickers hardness test can be used to measure macro-hardness. A modified Vickers or Knoop hardness test can be used to measure micro-hardness. The Shore Hardness Test can be used to measure nano-hardness. In some cases, different tests can be used to measure the same type of material. The cost of the testing equipment and potential damage to the material will help to determine the test chosen.
Applications of Rockwell and Brinell Hardness Tests
The Rockwell Hardness Test can be used to measure the hardness of metal, ceramics or plastic. The Brinell Hardness Test is the most widely used test of the hardness of metallic materials. The Brinell test creates the largest and deepest indention which yields the most accurate measure of hardness. However, this causes the greatest damage to the measured materials.
Applications of the Vickers, Knoop and Shore Hardness Tests
The Vickers Hardness Test can be used on a wide variety of materials since it uses a pointed indenter. However, the testing machine is more expensive than the Rockwell or Brinell machines. The Knoop Hardness Test can be used to measure coatings or insulation materials. The Shore Hardness Test is useful when measuring elastic materials.
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