Utility Of Mohs Hardness Testing

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What is Mohs Hardness Testing?

Evolved by the German mineralogist Friedrich Mohs in 1812, Mohs Hardness Test is one among the earliest ways of measuring hardness and it revolves round the observation whether a materials surface is scratched by a substance of known or defined hardness.

This leads us to the concept of Mohs hardness, which is defined as 'how well a substance will resist scratching by another substance'. Mohs hardness gives a rough measure/gauging of the resistance of a smooth surface to scratching or abrasion with reference to the Mohs scale. The Mohs scale, composed of some 10 minerals that have been assigned arbitrary hardness values, quantifies the physical property of hardness. The 10 minerals in Mohs scale in decreasing order of the hardness values are,

Diamond: 10

Corundum: 9

Topaz: 8

Quartz: 7

Orthoclase(Feldspar): 6

Apatite: 5

Fluorite: 4

Calcite: 3

Gypsum: 2

Talc: 1

NOTE: Each mineral in the Mohs scale will scratch those with a lower number.

How does Mohs Hardness Testing quantify hardness?

Mohs hardness of a mineral is established through close observation of whether its surface is scratched by a standard substance of 'known or defined hardness'. The hardness of a material is ascertained by finding out which of the standard minerals, the sample material will scratch or not scratch and the hardness will lie between two standard values on the scale. Thus, a mineral surface, which gets scratched by Quartz but not by Topaz has Mohs hardness between 7 and 8. (The process involves careful scrutiny or meticulous observation for the 'scratch' has to be a true cutting or grazing and not simply a mark).

A vital point to note here is that Mohs Scale is not a linear scale - value progression and difference of hardness between two steps is not constant. Thus, while the step between 9 and 10 is indicative of a hardness increase of more than 300 percent, the value progression between minerals 3 and 4 indicate an increase in hardness of only 25 percent.

Is Mohs Hardness Testing ideal for application in all fields?

Mohs hardness testing undoubtedly facilitates the identification of minerals, however, it is not the ideal method for gauging the hardness of industrial materials such as steel, iron or ceramics. Metallurgists and Engineers undertake Indentation-hardness tests based on load-displacement methods to measure the hardness of these materials accurately even on the macro-, micro- or nano- scales.

Mohs hardness testing, however, is not totally obsolete; engineers and metallurgists have sub-divided the scale for some innovative uses.

Author: Robert Allen

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