Knoop

Knoop Hardness Testing Method

The Knoop method is a static hardness testing technique designed as an alternative to the Vickers method, particularly for evaluating thin films, coatings, and brittle materials. It is characterized by the following detailed technical features:

  1. Standardisation:
    • Governed by ISO 4545, ASTM E92, and ASTM E384, the Knoop method provides a reliable and standardised approach to microhardness testing.
    • The method accommodates both micro-hardness and low-load hardness testing scenarios, catering to a wide range of delicate materials.
  2. Test Load Range:
    • The Knoop method supports low applied loads, from 1 gram-force (gf) to 2 kilogram-force (kgf), with common loads such as 25 gf, 50 gf, and 100 gf for specific applications.
    • Low loads minimise the risk of damaging the specimen, making the technique ideal for thin layers and fragile materials.
  3. Measurement Principle:
    • This is an optical method, where the indenter creates a long, shallow indentation to minimise material distortion.
    • Only the long diagonal of the rhomboidal indentation is measured, as the short diagonal is too small for accurate optical evaluation.
    • The hardness is calculated using the formula: HK=FL2HK = \frac{{F}}{{L^2}}HK=L2F​ where FFF is the applied load in kgf, and LLL is the length of the indentation’s long diagonal in millimeters.
  4. Indenter Design:
    • The indenter is a diamond-shaped pyramid with a rhomboid base tailored for high precision and minimal specimen deformation.
    • The longitudinal edge angle is 172.5°, while the transverse edge angle is 130°.
    • This geometry allows for greater measurement sensitivity, especially when working with very small or thin samples.
  5. Specimen Requirements:
    • The test area must be smooth, polished, and free of debris or coatings that could affect the indentation visibility.
    • Minimum thickness recommendations ensure that the indentation does not interact with the substrate, which could skew results. For thin films, substrate hardness and elasticity must be accounted for.
  6. Optical Evaluation:
    • A high-resolution optical microscope with a calibrated stage is used to measure the long diagonal of the indentation, typically with a magnification range of 400x to 1000x.
    • Advanced systems may incorporate digital image processing for improved measurement accuracy and repeatability.
  7. Applications:
    • Commonly applied in testing the hardness of thin films, coatings, and surface layers.
    • Ideal for brittle materials such as ceramics, glass, and minerals, as well as composites and polymers.
  8. Advantages:
    • The elongated indentation shape offers higher resolution for hardness differences in small areas compared to the Vickers method.
    • The shallow penetration depth minimises interference with underlying materials.
  9. Limitations:
    • The method is less effective for bulk materials due to the need for highly polished surfaces and the difficulty of optical measurement on rough or textured specimens.
    • Errors can arise from uneven surfaces, improper lighting, or operator-dependent optical readings.

By offering precise measurement capabilities in the micro and low-load hardness ranges, the Knoop method is invaluable for applications involving thin, fragile, or brittle materials, especially in research and quality control environments.