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:
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.