Knoop hardness test: Procedure & application
The Knoop hardness test is one of the most important methods of microhardness testing. It is used when very small indentations, shallow penetration depths, and high spatial resolution are required. The Knoop hardness test offers particular advantages over other methods for thin films, coatings, ceramics, hard metals, and heat-treated surface areas. Hardness testing method.
the essentials in brief
- Knoop is a standardized Microhardness testing method using diamond indenters.
- The elongated, narrow shape is particularly suitable for narrow edge zones and the hardness testing of layers on cross-sections.
- With the same test force, the Knoop indentation has a significantly lower penetration depth than a comparable Vickers-Impression.
- The long diagonal is about three times longer than that of Vickers and allows for the precise measurement of the smallest impressions.
- Especially suitable for thin layers, coatings, ceramics, brittle materials and edge zones.
- The hardness will visually across the long impression diagonal certainly.
- The procedure is in DIN EN ISO 4545 .
- The result is indicated using the abbreviation HK.
What is the Knoop hardness test?
The Knoop hardness test is an optical indentation method for determining the hardness of metallic and non-metallic materials. The diamond indenter has an asymmetric pyramidal geometry and produces an elongated, diamond-shaped indentation. This results in a significantly shallower indentation depth than with the Vickers test for the same test force.
The method was specifically developed for applications where the test depth must be kept as small as possible (standard specification of minimum thickness to avoid penetrating layers or the test piece). Typical examples include electroplated layers, PVD and CVD coatings, nitrided layers, ceramics, or microstructural constituents in metallographic sections.
The Knoop hardness test is preferably used for these materials:
- thin coatings
- galvanic layers
- Hard coatings
- ceramics
- Hard metals
- brittle materials
- heat-treated edge zones
- metallographic specimens
- Microhardness profiles
In contrast, coarse or highly inhomogeneous materials may Brinell are often better suited because they capture larger material areas.
How does the Knoop hardness testing method work?
The procedure is similar to the Vickers hardness test:
- The sample is prepared and positioned securely.
- The Knoop indenter is subjected to a defined test force.
- The test force is applied for the prescribed loading duration.
- After the exoneration, a lingering impression remains.
- Only the long diagonal is measured optically.
- The Knoop hardness value is calculated from the test force and indentation length.
The quality of the optical measurement is crucial for the accuracy of the results. Here, Knoop is clearly superior to the Vickers method. The shallower penetration depth is also advantageous.
The geometry of the Knoop indenter
The Knoop indenter is made of synthetic diamond and has an asymmetrical pyramid shape. The opposing faces form angles of 172,5° and 130°, respectively.
This geometry leads to:
- elongated impressions
- low penetration depth
- reduced impact on the base material
- highly suitable for shift testing
What does a specification like 650 HK0,1 mean?
A typical result reads:
650 HK0,1
This means:
- 650 = determined hardness value
- HK = Knoop hardness
- 0,1 = Test force in kgf
If the standard loading duration is deviated from, the loading time will be specified additionally.
Testing force areas at Knoop
Knoop is primarily used in the microhardness range. Typical test loads range from a few grams to several kilograms.
Typical applications:
- HK0,01 to HK0,1: Very thin layers, structural components and microstructures
- HK0,2 to HK1: Small components, edge zones and hardness gradients
- HK1 to HK5: Larger test areas and classic microhardness tests
Why Knoop is particularly important for thin, extremely hard layers
The greatest advantage of the Knoop method lies in its shallow penetration depth and approximately three times longer diagonal. While the influence of the base material can distort measurements in many methods, the plastic deformation in the Knoop method remains comparatively shallow.
This makes the method particularly suitable for:
- PVD coatings
- CVD layers
- galvanic layers
- Nitriding layers
- Hard coatings
- oxide layers
- Thin films
Therefore, Knoop is explicitly recommended in many standards for layer testing.
Sample preparation and surface requirements
The Knoop hardness test is an optical method. The quality of the surface directly influences the result.
For reproducible results, the samples should:
- just be
- be clean
- be free of oxides
- be free from impurities
- be ground or polished – see Exception
Metallographic preparation is often necessary, especially for small test loads.
- Exception: Thin layers are often only about 3 to 20 µm thick. Grinding or polishing is generally unsuitable here, as the layer can be quickly removed or altered. Thin layers are therefore tested without additional preparation.
| Benefits | boundaries |
|---|---|
| Very shallow penetration depth – ideal for thin layers | Optical evaluation required |
| Approximately 3 times longer diagonal than Vickers – more precise measurement of the smallest impressions | Sensitive to surface defects |
| High spatial resolution and precise positioning | High demands on surface quality |
| Especially suitable for coatings, ceramics and hard metals | Thin layers often cannot be prepared. |
| Narrow indentation – ideal for edge zones and cross-cuts | Less suitable for coarse or inhomogeneous structures |
| Very small test impressions possible | Higher testing effort than depth measurement methods (e.g. Rockwell) |
Standards for the Knoop hardness test
The most important standard is DIN EN ISO 4545.
Important parts include:
- DIN EN ISO 4545-1: Test method for the Knoop hardness test
- DIN EN ISO 4545-2: Testing and calibration of testing machines
- DIN EN ISO 4545-3: Calibration of hardness comparison plates
Additionally, ASTM E384 is widely used internationally.
Common errors in the Knoop hardness test
Typical sources of error include:
- insufficient surface quality
- incorrect examiner
- Insufficient distances between test points
- unsuitable magnification
- inaccurate measurement of the diagonal
- Testing in porous areas
- unsuitable layer thickness ratio
Frequently asked questions
What other procedures are there?
Besides Knoop, Vickers, Brinell, and Rockwell are primarily used. Vickers is suitable for many materials and hardness profiles, Brinell for coarse or inhomogeneous structures, and Rockwell for rapid serial testing.
When is Knoop the right procedure?
Knoop is particularly suitable if:
- very thin layers are to be tested
- The penetration depth must be minimized.
- Ceramics are being examined
- Hard coatings are evaluated
- small structural components are analyzed
- Microhardness profiles are required
In many cases, Knoop is used as a complement to Vickers.
What is the difference between Vickers and Knoop?
Knoop inks produce an elongated indentation with shallower penetration. Vickers inks produce an almost square indentation and are more universally used.
Is Knoop suitable for coatings?
Yes. The shallow penetration depth makes Knoop one of the most important methods for coating testing.
Which standard applies to Knoop?
The international standard series DIN EN ISO 4545 describes the procedure.
When should Knoop be used instead of Vickers?
Especially for thin layers, brittle materials and applications with very small test indentations.
