The initial hardness test is used to determine the hardness of the material against scratches and abrasions. The earliest test was developed by Friedrich Mohs mineralogy in 1820 (see Mohs scale). This is based on relative scratch hardness, with talc being given a value of 1 and diamonds given a value of 10. The Mohs scale has two limitations; it is not linear, and most modern abrasives fall between 9 and 10.
Raymond R. Ridgway, a research engineer at Norton Company, modified the Mohs scale by giving garnet hardness 10 and violent diamonds. 15. Charles E. Wooddell, working at Carborundum Company, expanded the scale further by using resistance to abrasion, and extrapolating scales based on seven for quartz and nine for corundum, yielding a value of 42.4 for South American brown bort diamonds. Resistance to abrasion is less affected by surface variation than other indentation methods.
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).
Video Scratch hardness
References
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