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Hardness Tests Explained

An Explanation of Various Hardness Tests
The Knoop, Rockwell, Vickers and Brinell tests are all similar in that all three tests employ a hardened probe which is forcibly pressed into the surface of the specimen being tested.  The resulting impression is measured for depth and that value is the resulting “hardness” of the specimen.  None of these tests provide an absolute value of hardness.  All are relative measures.  A value of Knoop 200 means nothing without context.  You need to know, from experience or experimentation that a particular specimen works in your application.  Once you have a process that repeatable produces a given hardness, you run the hardness test to see what you have.  That test may, for example, produce a result of Knoop 200.  Now you know that for the future, Knoop 200 is appropriate to your needs.  Once you know that, you can test further specimens to insure that they meet your Knoop 200 requirement.
The Knoop Hardness Test uses a micro indenter which is shaped as an elongated pyramid.  The Knoop test is used for thin plastic sheets, thin metal sheets, electrodeposits, brittle materials, or materials where the applied load must never exceed 3.6 kgf.
In the Knoop Hardness Test, a load (typically in the range of 25 ~ 3600 gf) is applied to the surface of the test specimen via a pyramid-shaped diamond indenting tool. The resulting indentation is shaped like the diamond tool with point angles of 130º and 172º respectively and with a long-to-short axis ratio of 7:1. Knoop indentations are about 2.8 times longer and shallower than Vickers indentations made at the same load. Limitations of optical microscope resolution may limit the application of the Knoop hardness test because of the resulting shallow indentions.
The Knoop hardness number is the ratio of the applied load to the area of the indentation, according to the formula:

The Rockwell Hardness (HR) Test presses a steel or diamond hemisphere or conical shaped penetrator.  The diamond conical indenter is known as a Brale indenter.  The indenter is pressed against a test specimen and the resulting indentation depth is a measured of the specimen hardness.  Shallower indentations occur with harder materials and result in a higher HR reading.
Initially a minor, 10 kgf, load is applied. Then the test dial, which measures the indention depth, is reset to zero. A major load of 60, 100, or 150 kgf is subsequently applied to create a maximum indention. The major load is then reduced back to the minor load, and the difference in indention depths is measured.
The penetrator is usually 1/16 inch in diameter, although larger diameters may be used for softer metals. Choosing the proper penetrator and load requires experience and experimentation. Some commonly used combinations are identified below:

The Vickers Hardness test can be applied to different materials across a broad range of harnesses. The Vickers test uses a square-bottomed diamond pyramid that has a 136º point angle. The load is usually 50 kgf, but can be 5, 10, 20, 30, or 120 kgf. The load is applied via the pyramid-shaped indenter against the well-supported, smooth, flat surface of the test specimen for 30 seconds. The resulting hardness reading is calculated based on the load and the area of the pyramid impression according to the formula:

The Brinell Hardness Test is commonly used for metallic materials and determines hardness by applying a known load of 500, 1500, or 3000 kgf to the test specimen via a hardened steel or diamond ball which is 10 mm in diameter.
The size of the impression on the specimen surface is calculated into a Brinell Hardness Number (HB) using the formula:

A measurement is not considered valid unless the diameter of the impression is in the range of 2.5 to 4.75 mm, although slightly exceeding this limit is tolerable. The 3000 kgf load yields Brinell hardness results from 160 and 600; the 1500 kgf load yields an HB value of 80 to 300; and the 500 kgf load will produce HB values of 26 to 100. Smaller loads of 100, 125 and 250 kgf can be utilized for softer metals.
According to American Society for Testing and Materials (ASTM) Standard E10-66, a steel ball may be used up to HB 450, and carbide should be used up to HB 630.  Use of the Brinell test on materials harder than HB 630 is not recommended as deformation of the ball indenter itself may occur, leading to errors in test result values.
The Mohs Hardness Test is applied to non-metallic elements and minerals. The Mohs test was devised by Austrian mineralogist Frederick Mohs in 1822. In this test, hardness is defined by how well a material resists scratching by another material. A rating scale of 1 to 10 with half-step increments is utilized. Materials with higher scale numbers will scratch the surface of materials in equal or lesser values.
The reference minerals for the ten scale values are:

 
The Mohs' testing apparatus is a kit consisting of low cost specimens of the 10 minerals in the Mohs' scale.  A 9 specimen kit which omits diamond is also frequently used.  The materials are labeled and stored in a wooden box. Specimens are sometimes attached to the end of metal rods; each rod containing a fragment of the reference mineral at its end.
The Mohs' Hardness for common materials is described below:

The many hardness tests listed here measure hardness under different experimental conditions, such as indenters made in different shapes and sizes and materials, and applied with varying loads.  Further, the experimental data is evaluated using different formulae. Accordingly, there is no direct conversion between hardness measures. One must correlate test results empirically across the multiple hardness tests.