What Is Knoop Hardness?
The Knoop method uses a diamond pyramid whose long diagonal is about seven times its short diagonal. The resulting indentation is long and shallow; hardness is calculated by measuring only the long diagonal. This geometry leaves an indentation that penetrates less deeply than Vickers at the same load.
Purpose of Knoop Hardness
The purpose is to measure hardness in very thin coatings, brittle materials such as glass and ceramics, and narrow regions. The shallow indentation reduces substrate influence, and the elongated, narrow geometry allows measurement in narrow bands.
Which Products Is Knoop Hardness Applied To?
- Thin hard coatings (PVD/CVD)
- Glass and ceramic materials
- Narrow transition zones in a weld
- Electronic substrates and thin films
- Orientation-dependent measurements in anisotropic materials
In Which Sectors Is Knoop Hardness Used?
Standards Related to Knoop Hardness
- ASTM E384 (microindentation hardness — Knoop and Vickers)
- ISO 4545-1 (Knoop hardness test — test method)
- ISO 4545-2 (verification and calibration of testing machines)
- ISO 14577 (instrumented indentation — complementary method)
Knoop Hardness Conditions
- Indenter
- Diamond pyramid with a long-to-short diagonal ratio of about 7:1
- Test force
- In the micro range; selected per coating thickness
- Measurement
- Only the long diagonal is measured
- Orientation
- In anisotropic materials, indentation orientation affects the result and is recorded
Equipment for Knoop Hardness
- Microhardness tester with a Knoop indenter
- High-magnification measuring microscope
- Metallographic specimen preparation equipment
- Knoop reference blocks
Sample Requirements for Knoop Hardness
- The surface must be metallographically polished.
- For coating measurements, indentation depth must be limited relative to coating thickness.
- Spacing between indentations must meet the minimum defined in the standard.
- For brittle materials, cracking around the indentation must be recorded.
Duration of Knoop Hardness
A single measurement takes minutes; specimen preparation makes up most of the total duration.
Knoop Hardness Procedure
- 1The specimen is sectioned, mounted and polished.
- 2A suitable test force is selected.
- 3The machine is verified with a reference block.
- 4The indenter is pressed into the surface and held for the defined time.
- 5The long diagonal is measured.
- 6The Knoop hardness value is calculated.
- 7The measurement is repeated the defined number of times.
Knoop Hardness Acceptance Criteria
The method standard defines how the test is performed, not what the result should be. The acceptance limit comes from the material standard, the product specification or the customer's technical drawing; this page therefore gives no numerical limit.
Evaluation of Results
The result is reported with the HK designation together with the force. For brittle materials, cracks around the indentation are noted separately; these cracks provide additional information for fracture toughness assessment.
Knoop Hardness Report
- Specimen identification, material type and cast/batch number, if any
- Specimen geometry, dimensions and preparation method
- Standard and test conditions applied
- Test temperature and ambient conditions
- Measured values and calculated quantities
- Fracture surface or indentation photographs (where applicable)
- Measurement uncertainty (if requested)
- A statement that the result applies only to the specimen submitted for testing
Accreditation
EUROLAB holds internationally recognised accreditation obtained from accreditation bodies party to the IAF and ILAC multilateral recognition arrangements, under ISO/IEC 17025 for laboratory testing, ISO/IEC 17020 for inspection and surveillance, and ISO/IEC 17021-1 and ISO/IEC 17065 for system and product certification. This chain ensures the reports and certificates issued are accepted worldwide.
Whether this test can be offered within accredited scope depends on your product and the standard applied; since accreditation is method-based, the scope is confirmed in writing at the quotation stage.
Scope and authorisationsRelated Regulations
- EN 10204 (inspection documents for metallic products)
- EU 305/2011 Construction Products Regulation (for products used in construction)
- Harmonised standards applied according to the product family
Mekanik Testler — What Does This Testing Field Cover?
The section below describes the field that the Knoop Hardness test belongs to; the conditions and acceptance criteria of this particular test are given in the sections above. The scope specific to your product is confirmed in writing at the quotation stage.
Mechanical testing measures how a material or a part behaves under force. The family ranges from static loading (tension, compression, bending) to time-dependent behaviour (fatigue, creep), and from hardness and wear to fracture toughness.
What these tests have in common is that the result is reported as a MATERIAL PROPERTY. Yield strength or hardness is not a conformity decision; it is numerical data against which the design and the product standard will compare. Mechanical test reports are therefore usually an input to an engineering decision rather than to a product approval — material selection, safety factor, life prediction.
The distinction between static and dynamic behaviour is the key to understanding the field. A part may appear entirely unstressed under a static load yet break under repeated loading; fatigue fracture occurs at stresses well below the yield strength and gives no warning. For rotating, vibrating or cyclically loaded parts, a tensile test alone is therefore not sufficient evidence.
Test conditions determine the result, and this is critical for the readability of the report: specimen geometry, loading rate, temperature, conditioning and the gripping arrangement. The same material gives different values under different conditions. In plastics the rate effect is very pronounced, in metals the temperature effect; in composites the specimen orientation changes the result entirely.
Hardness is the most misused measurement in the field. Because it is fast, close to non-destructive and inexpensive, it is often used in place of strength. Conversion tables from hardness to strength exist, but they are APPROXIMATE and depend on the material group; using them as the sole evidence in material acceptance is wrong.
The fracture mechanics side requires a specific competence. Fracture toughness and fatigue crack growth rate measurements make it possible to calculate how long a cracked structure will last; they are the basis of damage-tolerant design in fields such as pressure equipment, aircraft structures and pipelines.
Method groups in this field
Static tests
Tension, compression, bending, shear and torsion. Yield and tensile strength, elongation and modulus of elasticity are obtained.
Impact tests
Charpy and Izod notch impact; the brittle-to-ductile transition behaviour of the material is determined as a function of temperature.
Hardness
Brinell, Vickers, Rockwell, Shore and micro hardness. Valuable for surface treatment depth and local differences in strength.
Fatigue
Force- and strain-controlled axial fatigue, fatigue in bending; S-N curves and the endurance limit.
Creep and stress relaxation
Behaviour under long-term load at high temperature; decisive for boiler, turbine and pressure equipment materials.
Fracture mechanics
Plane-strain fracture toughness, the J integral, CTOD and fatigue crack growth rate.
Wear and friction
Taber and rotating drum abrasion, coefficient of friction measurements.
Commonly confused in this field
- Calculating strength from hardness is approximate; it cannot be used as the sole evidence in material acceptance.
- Adequate static strength does not demonstrate fatigue strength; a cyclically loaded part requires a separate test.
- If test rate and temperature are not reported, the result cannot be compared — especially for polymers and composites.
- A test specimen and the actual part may not give the same value; moulding orientation, weld lines and differences in surface treatment all have an effect.
- A strength value given for a composite without stating the specimen orientation is meaningless.
- ASTM and ISO counterparts measure the same quantity but use different specimen geometries; the results are not treated as directly equivalent.
Which decisions these results support
- Design verification and material selection
- Incoming material control and supplier approval
- Meeting the mechanical requirements of the product standard
- Failure analysis, life prediction and damage tolerance calculation
- Verifying heat treatment and surface treatment processes
Related Tests
Frequently Asked Questions About Knoop Hardness
Should Knoop or Vickers be chosen?
Knoop is preferred when a shallower indentation is needed at the same load (thin coating) or when measuring in a narrow region. Vickers is more common for general-purpose measurement.
Can a Knoop value be converted to Vickers?
There is no exact conversion. The two methods create different stress states through different geometries; if a comparison is needed, values measured with the same method should be used.