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Knoop Hardness Number

Knoop hardness test is a microhardness testing method specifically designed for measuring the hardness of brittle materials, thin films, surface coatings, and small, delicate components.  Unlike other hardness tests that might cause cracking or significant deformation, the Knoop test uses a unique elongated, rhomboid-based diamond indenter.  This indenter creates a shallow, asymmetrical, diamond-shaped impression with a long and a short diagonal when pressed into the material's polished surface under a precisely controlled, light load (typically ranging from 1 gram-force to 2 kilogram-force) for a specified dwell time.    

Knoop Hardness Test Formula

\( HK \;=\;  \dfrac{ 14.229 \cdot P }{ L^2 }   \)     (Knoop Hardness Test)

\( P \;=\; \dfrac{ HK \cdot L^2 }{ 14.229  }\) 

\( L \;=\;  \sqrt{ \dfrac{ 14.229 \cdot P }{ HK } }\) 
Symbol English Metric
\( HK \) = Knoop Hardness Test - \( kgf\;/\;mm^2 \)
\( P \) = Applied Force - \( kgf \)
\( L \) = Length of the Long Diagonal Identification - \( mm \)
After the load is removed, the length of the long diagonal of the indentation is measured using a microscope. This measured length, along with the applied load, is then used in a specific formula to calculate the Knoop hardness number (HK or KHN).  The shallower penetration depth and elongated shape of the Knoop indentation make it particularly suitable for applications where minimal damage to the sample or accurate measurement of very thin layers is crucial, such as in ceramics, glass, microelectronics, and various coatings.
 
Compare with Standards  -  Compare the calculated HK value to known hardness values for similar materials to assess whether the material meets expected hardness specifications.
Consistency Across Measurements  -  Take multiple measurements (typically 5–10 indentations) and calculate an average HK value to account for material variability or measurement errors.
 

Knoop Hardness Test Interpretation

Low Knoop Hardness Number  -  Might be acceptable for ductile materials or applications where toughness is prioritized over hardness.
High Knoop Hardness Number (HK > 1000)  -  Might indicate suitability for wear-resistant coatings.

Factors Affecting Interpretation

Load Sensitivity  -  The Knoop test typically uses low loads (e.g., 10 g to 1 kg).  The hardness value may vary with the applied load due to the indentation size effect, where hardness appears higher at lower loads.
Material Properties  -  The test is ideal for brittle or anisotropic materials because the Knoop indenter produces shallow indentations, minimizing cracking.
Surface Condition  -  The material’s surface must be polished and flat to ensure accurate measurements.  Surface roughness or defects can skew results.
Anisotropy  -  For crystalline materials, the orientation of the crystal lattice relative to the indenter can affect hardness values, so multiple measurements at different orientations may be needed.

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