The Vickers Hardness Scale
One scale for everything from lead to diamond.
Origin
Devised by Robert Smith and George Sandland at Vickers Ltd in 1921, and adopted internationally as one of the standard hardness measures.
The catch
Its rivals each need a different indenter or load for different materials, so their numbers do not compare across ranges. Vickers uses a single diamond pyramid for everything, which means one continuous scale from the softest metal to the hardest, and a number that means the same thing wherever it appears.
| Material | Vickers (HV) | Note |
|---|---|---|
| Pure lead | 5 | About as soft as a metal gets |
| Pure aluminium | 25 | Dents under a thumbnail |
| Mild steel | 140 | Structural and general engineering |
| Stainless steel, annealed | 200 | |
| Hardened tool steel | 700 | Files, chisels, cutting edges |
| Tungsten carbide | 2,400 | Industrial cutting tips |
| Diamond | 10,000 | The top of the scale, and the indenter itself |
The test presses a square-based diamond pyramid into the surface under a known load, then measures the diagonal of the impression left behind. Dividing load by the impression area gives the hardness number. Because the indenter is diamond it cannot itself be deformed by anything it tests, which is precisely what allows a single scale to span the whole range. Vickers Ltd was better known for warships and aircraft, so the company that built the Wellington bomber also gave metallurgy one of its most durable tools.