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.
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 1820 (see 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 seven for quartz and nine 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).
- Akono, A-T.; P. Reis; F-J. Ulm. (2011). "Scratching as a Fracture Process: From Butter to Steel" (PDF). Physical Review Letters. American Physical Society. 106 (20): 2. Retrieved 2022-05-03.
We present results of a hybrid experimental and theoretical investigation of the fracture scaling in scratch tests and show that scratching is a fracture dominated process. Validated for paraffin wax, cement paste, Jurassic limestone and steel, we derive a model that provides a quantitative means to relate quantities measured in scratch tests to fracture properties of materials at multiple scales. The scalability of scratching for different probes and depths opens new venues towards miniaturization of our technique, to extract fracture properties of materials at even smaller length scales.
- David Tabor (1951). The Hardness of Metals. Retrieved 2022-05-03.
- Industrial Minerals and Rocks: (nonmetallics Other Than Fuels). American Institute of Mining, Metallurgical, and Petroleum Engineers. 1960.
The Mohs scale is inadequate both because the methods of testing are very crude and because the intervals between steps in the scale are not uniform.
- Kevin J. Anderson. "Hardness Testing" (PDF). MRS Bulletin. Historical Note (November 1994): 7. Retrieved 2022-04-21.
For all its usefulness, the Mohs scale is arbitrary and nonlinear. ... When synthetic abrasive materials become widely available at the beginning of this century, R.R. Ridgway and his co-workers, finding they needed more numbers at the high end of the scale, modified Mohs' scheme. C.E. Wooddell measured how much various minerals resisted wearing down with diamond abrasives, which allowed a finer categorization between the Mohs numbers of 9 and 10. Ridgway arbitrarily shifted the value of diamond to 15 on the scale instead of 10, which allowed them to assign hardness numbers of 12 to fused alumina, 13 to silicon carbide, and 14 to boron carbide.
- Ridgway, Raymond R; Ballard, Archibald H; Bailey, Bruce L. (1933). "Hardness Values for Electrochemical Products". Transactions of the Electrochemical Society. 63: 369. doi:10.1149/1.3493827. Retrieved 2022-04-22.
- Wooddell, Charles E. (1935). "Method of Comparing the Hardness of Electric Furnace Products and Natural Abrasives". Transactions of the Electrochemical Society. 68: 111–130. doi:10.1149/1.3493860. Retrieved 2022-04-22.
- Henry Chandler. "Industrial Diamond : A Materials Survey". information circular. United States Department of the Interior (8200): 6–7. Retrieved 2022-05-03.
- Plendl, Johannes N.; Gielisse, Peter J. (1 Feb 1962). "Hardness of Nonmetallic Solids on an Atomic Basis". Physical Review. 125 (3): 828–832. Bibcode:1962PhRv..125..828P. doi:10.1103/PhysRev.125.828. Retrieved 2022-04-22.