What Are Knoop Testers and How Do You Choose One for Microhardness Testing?

A Knoop tester is a microhardness instrument that presses a rhombic-pyramid diamond indenter into a specimen and calculates hardness from the size of the resulting elongated impression. Choose one when you need to test thin materials, coatings, small parts, or case-depth profiles where a larger indenter would punch through the feature of interest. The main decisions are load range, whether you measure the indentation manually or with a computer-assisted system, and how you fixture the sample.

How Knoop testing works

The Knoop indenter is a diamond ground to a rhombic pyramid with one long diagonal roughly seven times the short diagonal. Because the impression is shallow relative to its length, the indenter penetrates only about one-third as deep as a Vickers indenter at the same load. That shallow depth is the reason Knoop testing suits thin and brittle specimens.

The tester applies a controlled load, holds it for a set dwell time, and withdraws the indenter. You then measure the long diagonal of the impression. Hardness is reported as HK (Knoop hardness), calculated from the applied load divided by the projected area of the impression, which is derived from the measured long diagonal.

Practical consequences of the elongated shape:

• Shallow depth: good for coatings, foils, and thin sections.
• Directional sensitivity: the long diagonal should be oriented relative to the feature you are measuring, so the impression stays inside the region of interest.
• Anisotropic materials: Knoop readings can vary with indenter orientation on materials with a preferred grain direction, so keep orientation consistent between readings.

Knoop vs. Vickers: when the choice matters

Both are microhardness methods, and both use a diamond indenter and optical measurement of the impression. The differences drive selection.

If your specimen is thick and homogeneous, either method works and the choice often comes down to which scale your specification or customer calls out. If the feature is thin or the material cracks easily under a deeper impression, Knoop is usually the better fit.

Typical applications

• Coatings and surface layers where the layer is too thin for a deeper indentation.
• Case depth measurement, using a series of Knoop or microhardness readings from the surface inward to build a hardness profile.
• Thin foils, sheet, and small components that cannot accommodate a larger impression.
• Brittle materials such as ceramics and carbides, where the shallower Knoop impression reduces cracking.
• Weld zones and heat-affected regions where hardness changes over short distances.

Selection criteria

Load range

Microhardness testers operate at low loads, and the load you can apply sets the smallest feature you can test. As a rule, the impression and the surrounding deformed zone must fit within the feature, and the specimen must be thick enough that the impression depth does not interact with the mounting or the opposite surface. NEWAGE publishes a Minimum Thickness Requirements Guide in its Learning Zone, which is the right reference for checking whether a given load and material combination is valid.

Match the load range to your thinnest expected feature, not to your average sample. A tester that covers your lightest required load with headroom is more useful than one that only reaches low loads at its limit.

Manual vs. computer-assisted measurement

• Manual optical measurement: you read the diagonal through a microscope eyepiece or on a screen and enter the value. Lower cost, but slower and more operator-dependent.
• Computer-assisted systems: the C.A.M.S. (Computer Assisted Microhardness System) automates indentation measurement and can drive a traverse for case-depth profiles. Better repeatability and throughput when you run many readings or profiles.

If your workload is occasional single readings, manual measurement is often sufficient. If you routinely produce case-depth curves or run batches, the automated path pays back in consistency.

Sample fixturing and stage

Knoop impressions are small, so positioning matters. Consider:

• X-Y stage for stepping between indentation positions, especially for case-depth traverses.
• Vise fixtures and clamping shields to hold irregular parts without movement during the load cycle.
• Roundness correction factors if you test curved surfaces — NEWAGE publishes a guide on this, because a curved surface changes the effective impression geometry.

Software and data handling

DataView hardness testing software is listed for recording and managing results. If you need traceable reports, conversion to other scales, or profile plots, confirm the software supports those outputs before selecting the instrument.

Interpreting and converting Knoop values

Knoop hardness is reported as HK with the test load, for example HK0.1 for a 100 gf load. Always state the load, because Knoop values are load-dependent at low loads (the indentation size effect).

Conversion to Rockwell C, Rockwell B, or Vickers is done through published conversion tables, not a single formula. NEWAGE provides conversion guides for the Rockwell C/high-hardness range and the Rockwell B/low-hardness range in its Learning Zone. Use the table that matches your material class and treat converted values as estimates, not equivalents — conversions are empirical and material-specific.

Common problems and troubleshooting

NEWAGE lists "Common Problems in Microhardness Testing" among its articles, and the recurring issues in Knoop work fall into a few categories:

• Impression too small or indistinct: load too low for the optical system, or the surface is not polished enough. Increase load if the feature allows, and improve specimen preparation.
• Impression larger than expected or irregular: sample moved during the load cycle, or the surface was not perpendicular to the indenter. Check fixturing and leveling.
• Cracking around the impression: load too high for a brittle material. Reduce load or switch to Knoop from a deeper-penetrating method.
• Inconsistent readings on the same sample: indenter orientation changing between readings on an anisotropic material, or inconsistent dwell time. Standardize both.
• Readings drift toward the mounting or backing: specimen too thin for the load. Consult the minimum thickness guide and reduce load or use a different method.
• Case-depth profile looks wrong: traverse spacing too coarse relative to the case depth, or the first reading is too close to the surface. Tighten spacing near the surface.

Verification and calibration

Hardness results are only meaningful if the instrument is verified. Use certified Knoop test blocks — NEWAGE lists Knoop Test Blocks under accessories — and check the tester at the loads and ranges you actually use. Calibration, repair, and factory or field service are offered through the site's Service & Support section, and the company lists accreditations and proficiency testing, which matter if your results feed into a quality system.

A practical selection sequence

1. Define the thinnest feature and the material; use the minimum thickness guide to set the maximum allowable load.
2. Decide whether you need single readings or case-depth profiles — this determines manual vs. computer-assisted.
3. Confirm the stage and fixturing can position and hold your parts.
4. Check that the software produces the records and conversions you need.
5. Verify against certified Knoop test blocks at your working loads before trusting production data.

If you are comparing Knoop against the other common methods before committing, the site's comparison material on Rockwell, Brinell, Vickers, and Knoop testing and its selection guides for microhardness testers for Knoop or Vickers and for case depth are the relevant references.