Scratch hardness
Scratch hardness tests are used to determine the hardness of a material to scratches and abrasion. The earliest test was developed by mineralogist Friedrich Mohs in 1820 (see Mohs scale). It is based on relative scratch hardness, with talc assigned a value of 1 and diamond assigned a value of 10. Mohs' scale had two limitations; it was not linear, and most modern abrasives fall between 9 and 10.
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.[1] Charles E. Wooddell, working at the Carborundum Company, extended the scale further by using resistance to abrasion, and extrapolating the scale based on seven for quartz and nine for corundum, resulting in a value of 42.4 for South American brown diamond bort.[2] Resistance to abrasion is less affected by surface variations than other methods of indentations.
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).[3]
| Material | Mohs' scale | Ridgway's scale[1] | Wooddell's scale[2] | |
|---|---|---|---|---|
| talc | 1 | 1 | ||
| gypsum | 2 | 2 | ||
| calcite | 3 | 3 | ||
| fluorite | 4 | 4 | ||
| apatite | 5 | 5 | ||
| orthoclase | 6 | 6 | ||
| vitreous silica | 7 | |||
| quartz | 7 | 8 | 7 | |
| topaz | 8 | 9 | ||
| garnet | 8.91[2] | 10 | ||
| corundum | 9 | 9 | ||
| fused zirconia | 11 | |||
| fused alumina | 9.03–9.065[2] | 12 | 10 | |
| tungsten carbide | 9.09[2] | 12.0 | ||
| silicon carbide | 9.13–9.17[2] | 13 | 14.0 | |
| boron carbide | 9.32[2] | 14 | 19.7 | |
| diamond | carbonado | 9.82[2] | 15 | 36.4 |
| ballas | 9.99[2] | 42.0 | ||
| bort | 10[2] | 42.4 | ||
