Vickers hardness testing is the primary method for verifying the mechanical quality of cutting tools, from high-speed steel drills and solid carbide end mills to ceramic inserts and PVD-coated surfaces. Performed in accordance with ISO 6507, ASTM E92, ASTM E384, JIS Z 2244, and GB/T 4340, the method provides a single continuous hardness scale covering annealed tool shanks through superhard materials exceeding 5000 HV. This article explains the test principle, specimen preparation, force selection, testing items, and typical applications for cutting tool manufacturers, coating centers, and end users.
Technical Overview
The Vickers hardness test is a static indentation method in which a polished, diamond-based pyramidal indenter with a 136° angle between opposite faces is pressed into the surface of a specimen under a controlled test force. After a defined dwell time, the lengths of the two diagonals of the resulting square indentation are measured optically, and the hardness value is calculated from the applied force and the mean diagonal length. The Vickers hardness number is expressed as HV according to the relationship HV = 0.1891 × F / d², where F is the test force in newtons and d is the mean diagonal length in millimeters.
Vickers testing is particularly well suited to cutting tools because it uses one geometrically defined scale across the entire hardness range of tool materials, from annealed steel shanks at roughly 200 HV to polycrystalline diamond cutting edges above 5000 HV. Unlike Rockwell testing, the Vickers method produces a small, precisely measurable indentation and can therefore be applied to thin coatings, cross-sections, individual microstructural phases, and small finished components without damaging the tool. The optical diagonal measurement also makes the technique highly sensitive to differences in heat-treatment condition, coating quality, and material grade.
The test is governed by an internationally harmonized family of standards, including ISO 6507-1 (test method), ISO 6507-2 (verification of testing machines), ISO 6507-3 (calibration of reference blocks), ASTM E92, ASTM E384 for microindentation testing, JIS Z 2244, and GB/T 4340. Test forces are designated by the notation HV followed by the force in kilogram-force equivalents, for example HV30 for a 294.2 N (30 kgf) test. In practice, three force ranges are distinguished:
- Macro-Vickers, HV5 to HV120, used for bulk hardness of tool substrates and cemented carbides.
- Low-force Vickers, HV0.2 to below HV5, used for case depth profiling and thin sections.
- Micro-Vickers, HV0.01 to below HV0.2, used for coatings, nitrided layers, and individual microstructural phases.
Testing Samples
Specimens are typically prepared as metallographic cross-sections so that both the cutting edge region and the tool core can be evaluated. Sections are cut with abrasive or precision sawing under flood coolant, mounted in resin with good edge retention, ground with progressively finer abrasives, and polished to a mirror finish, usually with 3 µm and 1 µm diamond suspension, following the general practice of ASTM E3. Care is taken to avoid overheating or burnishing, which can locally alter the true hardness of the surface being measured.
High-Speed Steel (HSS) Tools
Twist drills, taps, reamers, end mills, hobs, and broaches made from high-speed steels such as M2, M35, or M42 are tested at HV10 to HV30 to verify that hardening and tempering have produced the required 62–68 HRC equivalent, approximately 745–940 HV. Cross-sectional hardness traverses also reveal quench uniformity, carbide distribution, and any decarburized or over-tempered zones near the cutting edge.
Cemented Carbide Tools and Inserts
Solid carbide end mills, drills, and indexable inserts made from WC-Co and WC-TiC-TaC-Co grades are commonly tested at HV10 or HV30. Depending on cobalt content and grain size, typical values range from about 1300 HV for tough coarse-grain grades to 1900 HV and above for ultrafine submicron grades. Because cemented carbide is a composite, indentations must be large enough relative to the microstructure to represent the bulk behavior, and free of excessive edge chipping around the indentation corners.
Ceramic, Cermet, and Superhard Materials
Alumina-based ceramics, silicon nitride ceramics, and cermets used for turning and milling inserts typically measure 1500–2200 HV and are tested at HV10 to HV30. Polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) compacts are evaluated with micro-Vickers forces on the superhard layer, generally 3000–5000 HV for PCBN and above 5000 HV for PCD; because both the indenter and the PCD layer are diamond, low forces and careful indentation placement are essential, and brittleness-related cracking must be assessed before accepting a measurement.
Coated Tools and Cutting Edges
PVD and CVD coatings such as TiN, TiAlN, AlCrN, CrN, diamond-like carbon, and CVD diamond are measured in cross-section at HV0.01 to HV0.05 because layer thicknesses are commonly 1–20 µm. Indentation depth, approximately one seventh of the diagonal length, is kept below about one tenth of the coating thickness so that the substrate does not influence the result. Typical coating hardness values range from about 1800 HV for CrN to more than 3000 HV for advanced AlCrN and nanocomposite nitrides.
Shanks, Holders, and Heat-Treated Components
Tool shanks, collet pockets, holder bodies, and other supporting steel components are tested at HV10 to HV30 to confirm heat-treatment condition, through-hardness, or surface-hardening quality, often alongside hardness-depth profiling of carburized or nitrided surfaces.
Testing Items
Accredited third-party laboratories and in-house metallurgical laboratories typically offer the following Vickers hardness test items for cutting tools:
- Bulk or core hardness of the tool substrate, reported as HV10, HV30, or another agreed designation.
- Microhardness of coatings, nitrided layers, and diffusion zones at HV0.01 to HV0.05.
- Hardness-depth profiles and effective case depth determination after carburizing or surface hardening, measured with HV1 or HV0.5 traverses in accordance with ISO 2639.
- Nitriding hardness depth and compound layer evaluation using low-force Vickers traverses on cross-sections.
- Decarburization depth assessment in accordance with ISO 3887 or ASTM E1077.
- Hardness uniformity mapping across transverse and longitudinal sections to detect quench gradients or over-tempered zones.
- Phase-specific hardness of carbides, binder regions, and transformed microstructural constituents in tool steels and cermets.
- Approximate hardness conversions to HRC or HB using the standardized tables of ASTM E140 and ISO 18265, reported as reference values only.
Testing Methods
The complete test sequence follows ISO 6507-1 or its ASTM, JIS, and GB equivalents. A force-applying machine verified in accordance with ISO 6507-2, and calibrated reference blocks traceable to ISO 6507-3, are prerequisites for valid results. The key procedural elements are summarized below.
- Specimen preparation: cross-sectioning under coolant, hot or cold mounting, progressive grinding, and final polish to a scratch-free reflective surface suitable for accurate diagonal measurement, especially at microhardness forces.
- Force selection: chosen so the indentation is large enough for reliable measurement yet small enough to fit within the feature of interest; for coatings the indentation depth must not exceed roughly one tenth of the layer thickness.
- Indentation placement: center-to-center spacing between adjacent indentations of at least three diagonal lengths, and at least two and a half diagonal lengths from the specimen edge, to avoid work-hardening interaction and edge effects.
- Dwell time: normally 10–15 seconds of force application, held constant within the standard tolerance, with longer dwell times only where specifically agreed and reported.
- Measurement and calculation: both diagonals of each indentation are measured and averaged; the HV value is computed from the standard formula and reported together with the test force and, where relevant, the measurement location.
- Curved surface correction: for measurements on round shanks, flutes, or margins, the correction factors of ISO 6507-1 are applied, or the surface is sectioned flat to eliminate geometric error.
- Repeatability: at least three valid indentations are taken per condition, and the spread of values is checked against the repeatability limits of the standard; obviously cracked or skewed indentations are discarded and repeated.
Test reports normally identify the standard applied, the test force and dwell time, the individual and mean hardness values, specimen orientation and measurement locations, and any conversions or corrections applied. Measurement uncertainty budgets account for force calibration, diagonal measurement resolution, indenter geometry, and surface finish effects.
Applications
Vickers hardness testing supports the full life cycle of cutting tool products:
- Incoming inspection of bar stock, carbide blanks, and compacts against specified hardness limits before machining.
- Qualification of heat-treatment processes, including vacuum hardening, tempering, cryogenic treatment, carburizing, and nitriding of tool steels.
- Development and production control of PVD and CVD coatings, including coating hardness verification, layer thickness correlation, and interfacial quality checks.
- Failure analysis of worn, chipped, or fractured tools, where a hardness traverse can reveal thermal softening from excessive cutting temperature, grinding burn, or insufficient case depth.
- Research and development of new carbide grades, cermet compositions, ceramic grades, and superhard composites.
- Batch release, customer certification, and independent verification by accredited third-party laboratories.
- Tool reconditioning and regrinding services, confirming that re-sharpening has not over-tempered the cutting edge.
FAQ
Which test force should be used for cutting tools?
Bulk hardness of high-speed steel and cemented carbide tools is normally measured at HV10 or HV30. Low-force and micro-Vickers ranges are reserved for case depth profiling, nitrided layers, coatings, and individual microstructural features. The chosen force must keep the indentation large enough for accurate measurement but confined to the region of interest.
How does HV compare with HRC for high-speed steel tools?
Approximate conversions are provided by ASTM E140 and ISO 18265; for example, 63 HRC corresponds to roughly 780 HV and 67 HRC to roughly 900 HV. Because the two methods measure hardness differently, conversions are indicative reference values and should not replace a direct Vickers measurement for certification purposes.
How is the hardness of a thin PVD coating measured?
The tool is sectioned, mounted, and polished with careful edge retention, and micro-Vickers indentations at HV0.01 to HV0.05 are placed within the coating layer in cross-section. Indentation depth is limited to about one tenth of the coating thickness to prevent substrate influence. For very thin layers below roughly 1 µm, nanoindentation techniques are recommended instead.
What specimen size and preparation are required?
A representative cross-section of the tool is required, mounted and polished to a mirror finish per ASTM E3 practice. The specimen must be rigidly supported, free of oil and debris, and thick enough that the indentation depth does not exceed one tenth of the material thickness beneath the indentation, as required by ISO 6507-1.
Can measurements be made directly on the curved surface of a finished tool?
Small curved surfaces such as round shanks can be tested using the curvature correction factors tabulated in ISO 6507-1, provided the specimen is rigidly supported. For flutes, margins, and complex geometries, flat cross-sectional preparation gives more accurate and reproducible results.
What typical hardness values are expected for common tool materials?
High-speed steels measure approximately 745–940 HV, cemented carbides 1300–1950 HV depending on grade, ceramics and cermets 1500–2200 HV, PCBN 3000–4500 HV, and PCD above 5000 HV. Common PVD nitride coatings fall between about 2000 and 3500 HV.
How many indentations are performed, and how are results reported?
At least three valid indentations are made for each condition, with five or more used where scatter must be tightly characterized. The report lists the applied standard, test force, dwell time, individual readings, mean value, measurement locations, and any conversions or curvature corrections applied.
How long does Vickers hardness testing of cutting tools take?
For a prepared specimen, routine testing is typically completed within one to two working days. If cross-sectioning, mounting, and metallographic polishing are required, or if depth profiling with many indentations is specified, several additional working days may be needed.