Introduction
The hardness test is a mechanical test for material properties used in engineering design, structural analysis, and material development. The primary goal of the hardness test is to determine the suitability of a material for a specific application or treatment to which it has been subjected. Because of the ease with which the hardness test can be performed, it has become the most commonly used method of inspection for metals and alloys.
Hardness is defined as a material’s resistance to permanent deformation such as indentation, wear, abrasion, and scratch. The significance of hardness testing is primarily related to the relationship between hardness and other material properties. The hardness test and the tensile test, for example, both measure a metal’s resistance to plastic flow, and the results of these tests may closely parallel each other. Because it is simple, easy, and relatively nondestructive, the hardness test is preferred.
There are numerous hardness tests in use today. All of these different hardness tests are required to categorize the wide range of hardness from soft rubber to hard ceramics.
Hardness testing is currently divided into two categories:
Macro Hardness Test
Macro hardness testers are used to determine the hardness of a material using a large load and a relatively large indenter. This type of testing is useful for evaluating the bulk properties of a material and is commonly used in industrial applications. Some common types of macro hardness testers include:
Brinell hardness tester: The Brinell hardness tester uses a hardened steel ball as the indenter and applies a load of up to 3000 kgf (kilogram-force). The resulting indentation is measured and the hardness is calculated based on the load and the diameter of the indentation.
Rockwell hardness tester: The Rockwell hardness tester uses either a diamond cone or a hardened steel ball as the indenter and applies loads between 15 kgf and 150 kgf. The depth of the resulting indentation is measured and the hardness is calculated based on the depth and the applied load.
Vickers hardness tester: The Vickers hardness tester uses a diamond pyramid as the indenter and applies loads between 1 gf (gram-force) and 120 kgf. The resulting indentation is measured and the hardness is calculated based on the load and the surface area of the indentation.
Knoop hardness tester: The Knoop hardness tester uses a rhombic-based pyramid as the indenter and applies loads between 10 gf and 1000 gf. The resulting indentation is measured and the hardness is calculated based on the load and the projected area of the indentation.
Macro hardness testing is typically performed on larger specimens and can provide valuable information about the bulk properties of a material. However, it may not be as precise as micro hardness testing, which uses smaller loads and indenters to evaluate the properties of a material at a smaller scale.
Micro Hardness Test
Microhardness testers are used to determine the hardness of a material at a small scale, typically on thin sections or small samples. This type of testing is useful for evaluating the mechanical properties of a material at a localized level, such as near a surface or interface. Some common types of microhardness testers include:
Vickers micro hardness tester: The Vickers microhardness tester uses a diamond pyramid as the indenter and applies loads between 1 gf (gram-force) and 1 kgf. The resulting indentation is measured and the hardness is calculated based on the load and the surface area of the indentation.
Knoop microhardness tester: The Knoop microhardness tester uses a rhombic-based pyramid as the indenter and applies loads between 10 gf and 1000 gf. The resulting indentation is measured and the hardness is calculated based on the load and the projected area of the indentation.
Martens hardness tester: The Martens hardness tester uses a sharp diamond point as the indenter and applies loads between 0.1 gf and 1 kgf. The resulting indentation is measured and the hardness is calculated based on the load and the depth of the indentation.
Micro hardness testing can provide valuable information about the mechanical properties of a material at a small scale, such as near a surface or interface. It is commonly used in materials science and engineering research to study the effects of processing or microstructure on the mechanical properties of materials.
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