What Is Knoop?
The Knoop hardness test is a microhardness method that uses an elongated rhombic diamond indenter with a diagonal ratio of about 7:1. Because it leaves a shallow indentation, it is preferred for very thin layers and brittle materials.
Purpose of Knoop
The purpose is to determine hardness in materials such as thin coatings, glass and ceramics, too thin for a Vickers indentation to fit.
Which Products Is Knoop Applied To?
- Thin coatings
- Glass and ceramic
- Surface-hardened layers
- Cutting-tool coatings
In Which Sectors Is Knoop Used?
Standards Related to Knoop
- ASTM E384
- ISO 4545-1
Knoop Conditions
- Indenter
- Elongated rhombic diamond pyramid (diagonal ratio ~7:1)
- Load
- Generally micro loads; a range of 10 gf – 1 kgf
- Dwell time
- Defined in standard, typically 10–15 s
- Surface
- Polished metallographically
Equipment for Knoop
- Microhardness machine (with Knoop indenter)
- High-magnification optical measurement system
- Specimen-preparation rig
- Reference blocks
Sample Requirements for Knoop
- The surface must be polished and free of a deformation layer
- A cross-section may need to be taken for coating measurement
- The number of measurements is determined according to the application
Duration of Knoop
The measurement is short; specimen preparation determines the total duration.
Knoop Procedure
- 1The specimen is polished.
- 2The appropriate micro load is selected.
- 3The indenter is pressed for the defined duration.
- 4The long diagonal of the indentation is measured and hardness is calculated.
- 5The measurement is repeated the required number of times.
Knoop Acceptance Criteria
The acceptance range is given in the product or coating specification.
Evaluation of Results
Measuring the long diagonal is sensitive; optical clarity and polishing quality directly affect the result. Distorted or cracked indentations are excluded from evaluation.
Knoop Report
- Standard applied
- Load and dwell time
- Measurement positions
- Individual hardness values
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
- Coating and surface-treatment specifications
Mekanik Testler — What Does This Testing Field Cover?
The section below describes the field that the Knoop 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
How is a choice made between Knoop and Vickers?
At the same load, a Knoop indentation is shallower and longer, which is why it is preferred for very thin layers and brittle materials. Vickers is more widely used and has richer comparative data.