DEFINITION OF THE KNOOP HARDNESS TEST

The Knoop hardness test is an alternative to the Vickers hardness test in the micro hardness testing range, and it can be performed on the same universal or micro hardness testing machine. It is mainly used to overcome cracking in brittle materials, as well as to facilitate the hardness testing of thin layers.

As in the Vickers hardness test, the indenter used in the Knoop hardness test is a pyramidal diamond. However, instead of being symmetrical, the pyramid is elongated. Knoop Hardness (HK) is ascertained by measuring optically along the long diagonal of the indent.

 

Comparison of Knoop and Vickers Hardness Testing

When deciding whether to use the Knoop or Vickers micro hardness testing method, there are a few things to consider.

• The Knoop indenter penetrates about half as deep as the Vickers, making it suitable for brittle materials, such as ceramics.
• The Knoop test is more sensitive to surface conditions because the indenter penetrates less deeply.
• The Knoop major diagonal is ~3.0 times longer than Vickers diagonals. This minimizes errors when reading the Knoop indent, which is important when small indents are produced (with low loads and hard materials, for example).
• Due to the geometry of the indenter, the Vickers test is better suited to small rounded areas; the Knoop test is better suited to small elongated areas, such as coatings.
• Both tests can be performed on the same micro hardness testing machines.

Read more about Vickers hardness testing