29 Scratch and Spark Testing
Scratch hardness refers to the hardness of a material in terms of resistance to scratches and abrasion by a harder material forcefully drawn over its surface.
Scratch hardness test or scratch test refers to any of a number of methods of measuring scratch hardness. Resistance to abrasion is less affected by surface variations than indentation methods. Scratch hardness is measured with a sclerometer.
Attempting to scratch a surface to test a material is a very old technique. The first scientific attempt to quantify materials by scratch tests was by mineralogist Friedrich Mohs in 1812, who established the Mohs scale. The Mohs scale is based on relative scratch hardness of different materials; with talc assigned a value of 1 and diamond assigned a value of 10. Mohs’s scale had two limitations: it was not linear, and most modern abrasives fall between 9 and 10; so, later scientists attempted to increase resolution at the harder end of the scale.
Raymond R. Ridgway, a research engineer at the Norton Company, modified the Mohs scale by giving garnet a hardness of 10 and diamond a hardness of 15. Charles E. Wooddell, working at the Carborundum Company, extended the scale further by using resistance to abrasion, and extrapolating the scale based on 7 for quartz and 9 for corundum, resulting in a value of 42.4 for South American brown diamond bort.
There is a linear relationship between cohesive energy density (lattice energy per volume) and Wooddell wear resistance, occurring between corundum (H=9) and diamond (H=42.5).
Mohs
The Mohs scale of mineral hardness is based on the ability of harder material to scratch softer material. This technique is very old. Theophrastus writes about it in his treatise On Stones, c. 300 BC, followed by Pliny the Elder in his Naturalis Historia, c. AD 77. In modern times, it is called a qualitative method because it relies on scratch marks and not measurement.
There are 10 different minerals or stones in a Mohs kit. Each stone is given a number from softest to hardest. The stone that scratches the metal is the number that metal is given, from 1 to 10. This is called an ordinal scale. The scale was introduced in 1812 by the German geologist and mineralogist Friedrich Mohs, in his book “Versuch einer Elementar-Methode zur naturhistorischen Bestimmung und Erkennung der Fossilien”.
Each of the ten hardness values in the Mohs scale is represented by a reference mineral, most of which are widespread in rocks.
The Mohs scale is an ordinal scale. For example, corundum (9) is twice as hard as topaz (8), but diamond (10) is four times as hard as corundum. The table below shows the comparison with the absolute hardness measured by a sclerometer, with pictorial examples.
| Mohs hardness | Reference mineral | Chemical formula | Absolute hardness | Image |
|---|---|---|---|---|
| 1 | Talc | Mg3Si4O10(OH)2 | 1 | |
| 2 | Gypsum | CaSO4·2H2O | 2 | |
| 3 | Calcite | CaCO3 | 14 | |
| 4 | Fluorite | CaF2 | 21 | |
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