KNOOP HARDNESS TESTING

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

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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.

APPLICATION OF THE KNOOP HARDNESS TEST

Surface preparation
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.

Loads
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

KNOOP MICRO HARDNESS TESTING MACHINES (HARDNESS TESTERS)

Hardness testing is key in most quality control procedures. That’s why we provide a complete range of hardness testing machines.

All our Knoop micro hardness machines are designed to deliver the long-term performance for which Struers is renowned.

Duramin-4: Manual Micro and Micro/Macro Hardness Testing Machine
Load ranges: 10 gf – 2 kgf, 1 kgf – 62.5 kgf

Duramin-40: Semi-Automatic and Fully Automatic Micro/Macro Hardness Testing Machine
Load ranges: 10 gf – 10 kgf, 10.0 gf – 31.25 kgf, and 1 gf – 62.5 kgf

Duramin-100: Fully Automatic Micro/Macro/Universal Hardness Testing Machine
Load ranges: 10 gf – 62.5 kgf, 10 gf – 150 kgf, 10 gf – 250 kgf

Duramin-600: Semi-Automatic Universal Hardness Tester
Load ranges: 1 – 250 kgf, 3 – 750 kgf, 5 – 3000 kgf

Duramin-650: Semi-Automatic and Fully Automatic Universal Hardness Tester
Load ranges: 1 – 250 kgf, 3 – 750 kgf, 5 – 3000 kgf

Check out our complete range of Duramin hardness testers.

DELVE DEEPER INTO HARDNESS TESTING

If you would like to know more about the hardness testing methods for metallic and other materials, including a full definition of hardness testing, the different applications of hardness tests and how to prepare for hardness tests, download our resources.

Find all you need to know about hardness testing

Download our Hardness Testing Application Note