Web Based Learning Object
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Assessing Hardness of Materials Level 2

A hard material is difficult to scratch, wear away by abrasion or to indent.

Hardness is not a fundamental property of a material: for each method of measuring it, it is some combination of elastic, plastic, and (in some cases) fracture properties.

Hardness can be measured only by comparison with a material used as a scratcher or indenter and has objective meaning only in terms of a specific type of test.

For example, glass will scratch steel but fractures more readily under indentation; nylon has a high resistance to wear but not to indentation.

Mohs proposed the first systematic hardness scale in 1822.

Ten standard minerals, ranging minerals, ranging in hardness from talc to diamond, were used as the reference scale.

The hardness of a material under test was determined by which of the reference minerals could scratch the material. The scale was not very sensitive to different degrees of hardness.

Nowadays hardness is usually measured in terms of resistance to indentation, and the most common test is the Vickers hardness test. A diamond indenter, in the form of a square pyramid with an included face angle of 1360, is pushed with constant force into the surface of a sample.

The resulting "square" impression is viewed in a measuring microscope and the two diagonals measured.

diagram of a Vickers hardness tester

The hardness is given a number Hv that is calculated as the load (in kg) on the indenter divided by the area of the faces of the indentation. Most hardness testing machines have a set of tables from which, having determined the average of the two diagonals of the "indent", the user can read off the Vickers hardness number.

The Vickers test is more common than the earlier Brinell test (BH), which uses a steel ball as the indenter.

Another test, the Rockwell, uses different loads with a diamond indenter for hard materials and a range of steel balls for softer materials


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Assessment:
The hardness of a material may be determined by using either a scratch test, or by making a surface indentation. This latter types of test may only be used for materials, which are capable of , namely metals and materials. The resistance of a material to indentation is not necessarily the same as its resistance to abrasion, but hardness measurement obtained from an indentation test can be used as an empirical check for abrasion resistance.

In general, metals possessing high hardness will have a high to abrasive wear. Indentation type hardness tests are widely used for checking of metal samples, as they are easy to make and information on heat treatment condition. There are also empirical relationships between the hardness of a metal and its strength.
 

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