Knoop Hardness Testing

The Knoop hardness test was developed as an alternative to the Vickers test. The Knoop indenter is an elongated diamond pyramid, and the Knoop Hardness (HK) is calculated by measuring the length of the long diagonal of the indent.


The Knoop test can replace a Vickers test, mainly to overcome the cracking in brittle materials, as well as to facilitate the possibility of testing thin layers.

Surface Preparation

Micro hardness test

  • highly polished
  • electropolished

Avoid tests producing diagonal lengths less than 20 μm; otherwise, the measurement inaccuracy will be too high.

Knoop Loads table

  • Indentation time: Ten to 15 seconds
  • Thickness of test material ASTM: At least ten times of the depth of indentation
  • Thickness of test material ISO: At least one-third times the long diagonal
Applicable Standards

Applicable Standards

  • ASTM E384
  • ISO 4545
  • JIS Z 2251

Knoop vs. Vickers

Comparing the indentations made with Knoop and Vickers Diamond indenters for a given load and test material:

  • The Knoop indenter penetrates about half as deep as the Vickers, making it suitable for hard and brittle materials, such as ceramics.
  • For the same reason, the Knoop test is more sensitive to surface conditions.
  • The Knoop major diagonal is ~3.0 times longer than Vickers diagonals. This minimizes the error in readings for the Knoop indent, something that is important when producing small indents (with low loads and hard materials, for example).
  • Due to the geometry of the indenter, the Vickers test is better for small rounded areas, whereas the Knoop test is better for small elongated areas, such as coatings.
Knoop versus Vickers

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