Hardness

Resistance to localized plastic deformation, typically measured by indentation. Measured by indentation test.

When 2 surfaces rub, the softer material wears off faster. A material can only cut another material with lower hardness. Diamond is the hardest known material.

Measurement Scales

Mohs Scale

Qualitative scale from 1 (talc) to 10 (diamond). A material scratches any material below it on the scale. Not suitable for metals.

Vickers

Diamond pyramid indenter pressed into the surface. Hardness value ($HV$) is:

Here:

• $F$: applied load (kgf)
• $d$: average diagonal length of indentation (mm)

Applicable across all material classes. Most widely used for metals.

Brinell

10 mm steel ball pressed at a fixed load. Gives $HB$ (Brinell Hardness Number). Not suitable for very hard or very thin materials.

Rockwell

Measures depth of penetration under a fixed load. Faster and simpler than Vickers or Brinell. Multiple scales (HRA, HRB, HRC) for different material ranges.