ASTM E384 Microindentation Hardness Guide: Practical Testing and Reporting

Microindentation hardness testing is a small-scale mechanical test used to evaluate local resistance to indentation. This ASTM E384 microindentation hardness guide explains how ASTM E384 organizes the test method, what equipment and calibration checks are required, and how to report results that survive technical review. The standard is widely used for metals, ceramics, carbides, coatings, and small or thin specimens where a macroindentation test would be impractical.

Scope of this ASTM E384 microindentation hardness guide

ASTM E384 is the Standard Test Method for Microindentation Hardness of Materials. It covers Vickers and Knoop hardness testing at test forces up to 1 kgf (9.8 N). In practice, most laboratories use forces between 1 gf and 1000 gf, depending on the material and feature size. The method is maintained by ASTM Committee E28 on Mechanical Testing.

Microindentation versus macroindentation

Macroindentation methods such as Brinell, Rockwell, and Vickers at higher forces produce a larger impression and average over a bigger volume. Microindentation uses lower forces and a diamond indenter, so it can target a single phase, a heat-affected zone, a coating, or a thin sheet. The trade-off is greater sensitivity to surface preparation, vibration, and operator technique.

Apparatus, calibration, and verification

An ASTM E384 testing machine includes a rigid frame, a precision stage, a diamond indenter, and an optical measuring system. Vickers uses a square-based diamond pyramid with a 136° angle between opposite faces. Knoop uses a rhombic-based diamond pyramid with included longitudinal angles of 172.5° and 130°. The indenter must be free of chips, cracks, and lapping defects.

Calibration follows a two-part approach. Direct verification checks the indenter geometry and the force application system. Indirect verification uses certified reference blocks with known hardness. Laboratories typically perform daily or shift-based checks on a reference block, and full recalibration at defined intervals. Record the reference block serial number, certified value, measured value, and tolerance in the equipment log.

Environmental control matters. Keep the machine on a stable bench, isolate it from vibration, and maintain a consistent temperature. ASTM E384 does not require a specific laboratory temperature for all materials, but drift can change both the force train and the optical measurement. Follow the manufacturer's specification and your quality management system, such as ISO 9001 or ISO/IEC 17025.

Specimen preparation and test procedure

Surface preparation is the most common source of error. Cut, mount, grind, and polish the specimen so the test surface is flat and free of deformation, scratches, and pull-out. For coated or case-hardened parts, use a metallographic mount that preserves the edge. Etching can help locate a specific phase or layer, but it must not alter the surface hardness.

Select a test force that produces a diagonal large enough to measure accurately but small enough to stay within the feature of interest. The specimen thickness must be sufficient to prevent the anvil or mounting material from influencing the result. Space indentations according to ASTM E384 so that strain fields from adjacent indents do not overlap. A common rule is to keep the indent spacing at least three diagonal lengths for Vickers; confirm the exact requirement in the current edition.

Apply the force smoothly, hold it for the specified dwell time, and remove it without shock. Typical dwell times are 10 s to 15 s, but the standard allows other times if reported. Measure both diagonals for Vickers and the long diagonal for Knoop using a calibrated optical system. Focus carefully, use consistent lighting, and avoid measuring after the impression has relaxed or corroded.

Calculating and reporting hardness values

Vickers hardness is calculated as HV = 1.8544 × F / d², where F is the test force in kgf and d is the mean diagonal length in mm. 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. When F is expressed in newtons, use the appropriate conversion factor or convert to kgf first.

Report enough detail for another laboratory to reproduce the result. Include the ASTM E384 designation and edition year, material and specimen identification, surface preparation method, test force, indenter type, dwell time, number of indentations, individual hardness values, mean, standard deviation, and any excluded readings. Note the location and orientation of each indent, especially for anisotropic materials or weld cross-sections.

Hardness data rarely stand alone. For steel products, pair the hardness results with chemistry and tensile data. See ASTM A36 chemical composition limits and ASTM A36 tensile strength requirements for examples of how material specifications connect to test reports. When a project requires certified documentation, follow ASTM material test certificate requirements to ensure traceability.

Common pitfalls and compliance tips

First, do not treat microindentation as a substitute for macrohardness without checking the material specification. Some product standards require Brinell, Rockwell, or Vickers at a specific force. Second, avoid measuring too close to a free surface, coating interface, or prior indent. Third, verify the optical system with a stage micrometer and check the indenter regularly.

Fourth, control operator bias. Two technicians can measure the same indent differently if lighting, focus, or edge detection varies. Use a documented measurement procedure, train operators, and run repeatability checks. Fifth, keep calibration records and raw data. An ISO 9001 or ISO/IEC 17025 audit will expect traceability from the reference block to the reported value.

Finally, use the current edition of ASTM E384. The standard is revised periodically, and older editions may have different force ranges, spacing rules, or reporting requirements. If a customer specification cites a dated edition, confirm which edition applies before testing.

Conclusion

Use this ASTM E384 microindentation hardness guide as a practical checklist: verify the machine, prepare the surface, choose the right force, measure carefully, and report every variable that affects the result. Done well, ASTM E384 gives reliable local hardness data for materials that cannot be tested by macroindentation.