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Rockwell
Vickers
Brinell
Shore, Durometer
Scleroscope
Microhardness
Materials need to be evaluated for hardness for a variety of reasons. By and large, the
hardness of solids effects how they interact with other materials. Wear, machineability, absorbency and strength are examples of properties that are effected by the hardness.
The various instruments are generally calibrated on use by using test specimens, test blocks, of known hardness.
There are several typical scales and associated instruments that are used to determine
the material hardness. Here is a link to a site that may be useful if you are needing to convert hardness between a variety of scales. Hardness Conversions
For most metals, the Rockwell scales are used to assess the general hardness,
although other indenting scales such as Vickers and Brinell are sometimes used.  The three primary Rockwell scales are A, B, and C, with the appropriate scale being used depending on how hard the material is. The A scale is for softer materials where the C Scale is for the hardest
materials. The test unit consists of a precision tip or ball that enters the material surface under the force of a specific load. The distance
the point moves into the material is associated with the hardness scale based on tables (or integrated electronics). The higher the penetration, the softer the material. The A, B and C scales utilize
different combinations of tips and loads to cover the hardness ranges. The typical test machine is a bench type piece of equipment with costs typically being the the $1000 range for generic mechanical
units. Depending on features, electronics and automation, costs increase from there. The stationary tester requires that the sample to be evaluated be small and light enough to fit inside the test frame.
Portable units are available that produce Rockwell Scale results, but their accuracy and repeatability are less than the stationary unit.
The Rockwell scale provides good general hardness information, however, if the material hardness is not consistent through the thickness of the material, or the material
to be tested isn’t small or light enough to fit into a Rockwell test machine other methods need to be used. Often times, metals have very different hardnesses on their surface that on the interior. Scleroscope hardness is an option for these
situations. This method takes quite a bit of practice to yield accurate, repeatable results (although, there are some automated units that use
the same principle that work fairly well). The scleroscope test involves raising a precision weight with specific end shape to a specified height inside a tube. The tube is placed over the surface to be checked
, the weight is dropped, allowing the point to hit the surface of the material. The weight then bounces back up the tube and the height it bounces to is translated into a hardness value.
Vickers hardness is a popular method for hardness measurement in Europe and Asia.
The principle is somewhat related to the Rockwell methods - a known load applied via a closely controlled probe. In the case of Vickers, the probe shape is more of a diamond
than the Rockwell ball shape. Vickers has some advantages given its wide range of loads from micro through macro. As a note, also, the conversion scales that are
generally available to compare Rockwell and Vickers scale values are only estimates. They may be relatively correct but should not be used to validate requirements. If the
specification shows a Vickers hardness, you need to use the Vickers method of measurement - or get a written waiver from the customer for another method.
Sometimes material microstructure needs to be evaluated for hardness. The
microhardness tester is sometimes used for these tests. A small portion of the material is mounted and the surface polished. This sample is then put into the microhardness
tester. It consists of high power microscope with a diamond shaped probe tip. The specific location is selected for checking and similar to the Rockwell method, the tip
penetrates the material at this point by applying a specific load. The width of the diamond imprint is measured which is then interpreted using a table of values.
For soft materials such as plastics, rubber, wood and some composites, the Rockwell
and Scleroscope scales are too high for practical use. The Shore scales are appropriate. A durometer is used to evaluate shore hardness. The common scales are
A and D, with the A scale being for softer materials such as soft rubber, polyester, PVC, wax and felt. The principle of measurement is similar to the Rockwell test but different
point configurations and loads are used for the softer materials. The normal durometer units are hand held portable devices.
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