The Knoop hardness test is an alternative to the Vickers test in the micro hardness testing range. This page describes the Knoop hardness test and gives you practical information on how to apply it in practice.
The Knoop hardness test at a glance:
A micro hardness test used for brittle materials, such as ceramics
Suitable for small elongated areas, such as coatings
Applicable standards: ASTM E384, ISO 4545, JIS Z 2251
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.
APPLICATION OF THE KNOOP HARDNESS TEST
Before application of the Knoop hardness test, you must prepare the surface of the material to be tested.
The Knoop hardness test is used for micro hardness testing (loads below or equal to 1 kgf) and so the surface of the sample material should be highly polished or electropolished before it is introduced to the micro hardness tester.
The Knoop micro hardness test should not be used if it produces diagonal lengths less than 20 μm, as this will result in measurement inaccuracy.
Indentation time: 10-15 seconds
Sample thickness ASTM: At least 10 times the indentation depth
Sample thickness ISO: At least one-third of the length of the long diagonal
KNOOP VERSUS 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