Knoop Hardness Test (HK): Method, Formula and Standards
The elongated-diamond microhardness method for thin coatings, brittle materials and measurements near edges.
The Knoop hardness test (HK) is a microhardness method that uses an elongated rhombic-based diamond indenter and measures the long diagonal of the indentation. Its shallow, slender impression makes it ideal for thin coatings and layers, brittle materials such as ceramics and glass, measurements near edges, and studying hardness anisotropy, cases where the deeper, more symmetric Vickers indentation is less suitable. For the wider method context, see the Hardness Testing Academy.
What is the Knoop hardness test?
Like Vickers, Knoop is an optical microhardness method, typically applied with low loads from a few grams up to about 1 kgf. A diamond indenter with a rhombic base and a long-to-short diagonal ratio of about 7 to 1 is pressed into the surface; only the long diagonal is measured, and the hardness number is calculated from the load and that diagonal.
How does Knoop differ from Vickers?
| Feature | Knoop (HK) | Vickers (HV) |
|---|---|---|
| Indenter | Elongated rhombic pyramid (about 7 to 1) | Square-based pyramid (136°) |
| Measured | Long diagonal only | Both diagonals (averaged) |
| Indentation | Long and shallow | Symmetric and deeper |
| Best for | Thin layers, coatings, brittle materials, near edges, anisotropy | General microhardness, broad range |
For the deeper theory of both methods, read the Vickers and Knoop hardness test theory.
What is the Knoop hardness formula?
Knoop hardness is calculated as HK = 14.229 × F / d², where F is the test force in kgf and d is the long diagonal in mm. The constant accounts for the projected area of the specific indenter geometry. Because only the long diagonal is measured, Knoop is less sensitive to small reading errors at low loads than Vickers.
When should you use Knoop instead of Vickers?
Choose Knoop for very thin coatings and case layers (the shallow indent stays within the layer), for hard and brittle materials prone to cracking under a Vickers indent, for measurements close to an edge, and to investigate directional differences in hardness. For general-purpose microhardness, the Vickers method remains the most common choice.
Step-by-step: how to perform a Knoop hardness test
This practical procedure describes a Knoop microhardness measurement, for example on a thin coating or a brittle material, in accordance with ASTM E384 and ISO 4545.
- Prepare the sample. For a coating or layer, cut and mount a cross-section with an edge-retention compound; for a bulk surface, prepare the face directly.
- Grind and polish to a mirror finish. A flawless polish is critical at low loads, because the indentations are very small and need sharp edges to be measured accurately.
- Verify the tester. Verify the microhardness tester at the chosen load with a certified test block and confirm calibration traceability.
- Set a low test force. Select the load (for example HK0.1) and the dwell time appropriate for the feature you are measuring.
- Orient the long diagonal. For a thin coating, align the long diagonal parallel to the layer so the indentation stays within it.
- Make the indentation. Apply the load and confirm the indentation stays inside the layer or feature, away from edges and defects.
- Measure the long diagonal. Measure the long diagonal of the indentation, automatically or manually.
- Compute and check. The hardness is calculated as HK = 14.229 × F / d²; confirm the value falls in a valid range for the material.
- Repeat and report. Take several readings, then record the method and standard, the load, the values, tester traceability, operator and date.
Best practices for Knoop hardness testing
- Follow ASTM E384 and ISO 4545 for the test cycle, the dwell time and the indenter geometry.
- Verify the tester with a certified test block at the working load before measuring; low loads are sensitive, so verification matters.
- Ensure a flat, well-polished surface, because at low loads the surface preparation dominates the accuracy.
- Respect the minimum spacing between indentations and from the edge required by the standard.
- Keep each indentation within the feature or layer being measured, away from edges, pores and second phases.
- Record the load (for example HK0.1), the result and the calibration traceability in the report.
- Choose Knoop over Vickers when the layer is too thin for a Vickers indent at the same load.
- Keep the long diagonal parallel to a thin coating to keep the whole indentation inside it.
- For brittle materials, the shallow Knoop indentation cracks less than a Vickers one.
- Invest in the polish: at low loads a poor surface is the main source of error.
- Respect minimum spacing and edge distance, which matter even more at low loads.
Which standards define the Knoop test?
The Knoop test is standardized by ASTM E384 (microindentation hardness of materials) and ISO 4545 (Knoop hardness test). Values can be approximately related to other scales using the hardness conversion table, but a conversion never replaces a direct measurement.
Frequently asked questions
What does HK mean?
HK is the symbol for Knoop hardness, usually written with the test force, for example HK0.5 for a 0.5 kgf load.
Is Knoop more accurate than Vickers?
Neither is universally more accurate. Knoop is advantageous for thin layers, brittle materials and near-edge measurements; Vickers is more general-purpose. The right choice depends on the sample.
Can Knoop values be converted to Vickers or HRC?
Approximate conversions exist for some materials, but they are estimates. Always measure on the required scale when a specification demands it.
Verify your micro Vickers and Knoop tester with traceable, ILAC-MRA certified reference blocks.