Scratch hardness refers to the

hardness In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard ...

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 The sclerometer, also known as the Turner-sclerometer (from grc, σκληρός meaning "hard"), is an instrument used by metallurgists, material scientists and mineralogists to measure the scratch hardness of materials. It was invented in 1896 ...

. 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 Carl Friedrich Christian Mohs (; 29 January 1773 – 29 September 1839) was a German chemist and mineralogist. He was the creator of the Mohs scale of mineral hardness. Mohs also introduced a classification of the crystal forms in crystal syst ...

in 1820 (see

Mohs scale The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by th ...

). The Mohs scale is based on relative scratch hardness of different materials; with

talc Talc, or talcum, is a clay mineral, composed of hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2. Talc in powdered form, often combined with corn starch, is used as baby powder. This mineral is used as a thickening agent ...

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 Norton Abrasives of Worcester, Massachusetts, USA is the world's largest manufacturer and supplier of abrasives for commercial applications, household, and automotive refinishing usage. Norton Company was founded in 1885 by a group of ceramist ...

, 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 Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal sin ...

, extended the scale further by using resistance to abrasion, and extrapolating the scale based on seven for

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical fo ...

and nine for

corundum Corundum is a crystalline form of aluminium oxide () typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the pre ...

, resulting in a value of 42.4 for South American brown diamond

bort Bort, boart, or boort is an umbrella term used in the diamond industry to refer to shards of non- gem-grade/quality diamonds. In the manufacturing and heavy industries, "bort" is used to describe dark, imperfectly formed or crystallized diamonds ...

. 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).

References

Hardness tests {{materials-sci-stub

See also

References