Hardness Testing of Materials
Rockwell, Vickers, Knoop, and Brinell Tests for Material Hardness
The hardness of a material is commonly defined as its resistance to scratching or to indentation under load.
A scratch resistance test, called the Mohs hardness test, compares the scratch resistance of the test material with ten different minerals arranged in sequence from the hardest to the softest. For most engineering purposes, the Mohs scale is too imprecise to be of value.
Indentation tests use an indenter that is pressed against the surface of a material in order to leave an indentation on that surface. The type and material of the indenter, the load applied to the indenter, and the length of time that the load is applied are all controlled so that the dimensions of the indentation that results give a direct and accurate value of the hardness of the material at that point
There are four types of indentation hardness tests: Brinell, Vickers, Knoop, and Rockwell.
Brinell Test
The Brinell test uses a 10 mm steel or tungsten carbide ball indenter, which leaves a round indentation on the surface of the test specimen. The hardness of the material is determined by a calculation that includes the measurement of the diameter of the indentation, the diameter of the indenter ball, and the load applied.
Vickers Test
The Vickers test is the most common hardness test used for metals. The indenter has a diamond tip shaped as a square pyramid, which leaves a square indentation, allowing for more precise measurement. The hardness of the material is determined using a calculation that includes the measurement of one diagonal axis of the indentation, the load applied, and the time interval of the load application.
Knoop Test
The Knoop test is a microhardness test used on thin sheet materials and brittle materials that can accept only very small indentations. The indenter has a diamond tip shaped as a rhomboid pyramid, with the long axis being roughly seven times as long as the short axis. Once more, hardness is calculated based on the indentation measurement and the load applied.
Rockwell Test
The previous tests involve the application and withdrawal of a single load and the measurement of the consequent indentation. However, this does not take into account any surface imperfections of the specimen or changes of the measurement over time due to possible elastic relaxation of the material after the load is removed.
The Rockwell test attempts to compensate for these possibilities by the initial application of a pre-load, followed by a main load, then followed once more by reduction to the preload. The measurement taken is of the depth of penetration of the indenter between the minor and major loads.
Ongoing Challenges in Hardness Testing
The Brinell, Vickers, and Knoop tests depend on the ability to see and measure tiny indentations. Hardness testers using these tests require optics and illumination, and the ability to take measurements in microns; a time-consuming process open to inconsistencies between different operators. The introduction of video capture software with automatic image measurement promises to reduce some of this inconsistency.
The Rockwell test does not require optics as it is based on measurement of indentation depth. It is therefore a much faster test, but the accuracy of depth measurement on hard materials is yet another technical challenge.
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