Knoop Hardness Testing (HK)

About 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.

The Knoop Hardness calculation

The Knoop Hardness (HK) is calculated by optically measuring the length of the longest diagonal of the impression left by the diamond indenter. The measured indentation is then converted into a hardness value using a standardized formula or reference table. Modern hardness testers automatically calculate and display the HK value through integrated software.

HK = 1,451 F/d^2

HK: Vickers hardness
F: Test load (gf)​
d: Length of the long diagonal (mm)

• Load range: From 1 gf up to 2 kgf​

Knoop 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.

STANDARD	LOAD RANGE
ISO 4545	1 gf - 2 kgf or more	(0.0098 - 19.613 N)
ASTM E384	1 gf - ≤1 kgf	(0.0098 - 9.807 N)

• 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.

Download our brochure covering our Knoop-Vickers hardness testers

Indent spacing

A Knoop indentation not only measures hardness but also changes the material structure around the test location. To avoid interactions between neighboring tests and ensure accurate hardness values, the standards prescribe specific spacing between indentations.

Edge spacing: ≥ 3.5× short diagonal for parallel orientation, or ≥ 1× long diagonal for perpendicular orientation.
Between indents: ≥ 3.5× short diagonal for side-by-side indentations, or ≥ 2× long diagonal for end-to-end indentations. Use the larger indentation if sizes differ.

Diagonal widths

Similar to Vickers, Knoop standards prescribe a minimum diagonal width of 20 µm.

Prepare for a Knoop hardness test

Specimen

The specimen needs to be properly prepared before placed on the micro-hardness machine. The requirements for the surface finish depend on the test load being applied.​

• The test surface should be mechanically polished or electropolished to ensure accurate measurement of the small indentation.​

Proper surface preparation is critical to achieving accurate, reliable, and repeatable Knoop hardness measurements.​

Equipment

To ensure reliable and traceable results, the Knoop hardness tester should undergo direct verification and calibration at least annually by an accredited calibration laboratory. In-between direct calibrations, operators are encouraged to perform regular indirect verifications using certified reference test blocks with known hardness values.

Environment

Among all hardness testing methods, the Vickers test is the most sensitive to shock and vibration, particularly when performing microhardness measurements. To ensure accurate and repeatable results, the instrument should be installed on a stable, rigid surface. Vibrations from nearby machinery, foot traffic, or other external sources should be minimized or eliminated whenever possible.​

ISO and ASTM standards specify an operating temperature between 10 °C and 35 °C.

Operator

The operator is responsible for ensuring that the specimen is correctly positioned and remains stable throughout the complete test cycle. To prevent measurement errors caused by material deformation, indentations should be spaced sufficiently apart and away from the specimen edges.

As a general guideline:​

• The distance between the centers of two adjacent indentations should be at least 3 times the diagonal length (3d) for steel and copper alloys.
• For light metals, the spacing should be at least 6 times the diagonal length (6d).
• The distance between an indentation and the edge of the test piece should be 2.5d to 3d, where d is the average indentation diagonal length.

For specific applications, always consult the relevant Vickers testing standard to confirm the required spacing and testing conditions.

Knoop versus Vickers method

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

	Knoop (HK)	Vickers (HV)
Standards	ISO 4545, ASTM E384	ISO 6507, ASTM E384
Indenter Geometry	Elongated diamond pyramid 172º	Symmetrical diamond pyramid 136º
Indent Shape	Rhombic, long and narrow	Square-shaped
Diagonal measurement	1 diagonal	2 diagonals
Indent depth	1/7 of diagonal length	1/30 of diagonal length
Main Application	Thin coatings and brittle materials (Ceramics)	Universal method with broad acceptance and wide load range, Industry standard for CHD, NHD, and SHD measurements, Welds
Limitations	Less commonly specified in industry standards and specifications	Smaller diagonals at low loads can increase measurement uncertainty.

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