Microhardness test results analysis is the systematic evaluation of Vickers (HV) and Knoop (HK) indentation data to assess material properties such as hardness gradients, case depth, coating quality, and microstructural uniformity. This article explains how microhardness values are calculated and interpreted in accordance with ISO 6507, ISO 4545, ASTM E384, and ASTM E92, covering sample requirements, measurement procedures, statistical evaluation, and common sources of error.
Technical Overview
Microhardness testing is an indentation-based measurement technique that applies low test forces—typically between 0.0098 N (HV0.001) and 9.807 N (HV1)—using a diamond indenter to characterize the hardness of very small volumes of material. Unlike macroscopic hardness tests, microhardness testing can resolve individual microstructural constituents, thin surface layers, coatings, and hardness gradients across heat-affected zones. The analysis of microhardness results involves converting measured indentation diagonal lengths into standardized hardness numbers, evaluating data scatter, and mapping hardness profiles to make engineering judgments about material condition and process quality.
Two indenter geometries dominate microhardness testing. The Vickers test, specified in ISO 6507 and ASTM E384/E92, uses a square-based pyramidal diamond with an apex angle of 136 degrees between opposite faces. The Knoop test, specified in ISO 4545 and ASTM E384, uses an elongated rhombic-based pyramid that produces a shallow, elongated indentation particularly suited to very thin layers and brittle materials. Correct analysis of results requires an understanding of the test force designation, indentation geometry, measurement resolution, and the metallurgical context of the specimen.
Testing Samples
Metallographic Cross-Sections of Hardened Layers
Case-hardened, nitrided, carburized, induction-hardened, and laser-hardened components are sectioned perpendicular to the treated surface, mounted, and polished to a mirror finish. A line of indentations is then placed from the surface inward to construct a hardness-depth profile. The analytical output is the effective case depth, such as the CHD value defined in ISO 2639 (the depth at which hardness reaches 550 HV1 for carburized steels) or the nitriding hardness depth defined in ISO 15787.
Coatings, Platings, and Surface Films
Electroplated layers, chemical conversion coatings, physical vapor deposition (PVD) and chemical vapor deposition (CVD) films, anodized layers, and thermal spray coatings are analyzed on polished cross-sections. The Knoop indenter is often preferred because its shallow elongated indentation stays within layers only a few micrometers thick. Result analysis verifies coating hardness uniformity, detects softening or interdiffusion at the interface, and confirms compliance with specified hardness windows.
Weldments and Jointsed Structures
Cross-weld sections are traversed with micro-indentations to profile the base metal, heat-affected zone (HAZ), and fusion zone. The resulting hardness maps reveal HAZ softening in precipitation-hardened alloys, hard martensitic zones in steels, and over-aging effects. Acceptance criteria are commonly defined in application standards such as ISO 15614 and AWS welding qualification codes, which set maximum hardness limits to reduce the risk of hydrogen-assisted cracking.
Small Components, Thin Materials, and Micro-Constituents
Fine wires, foils, thin strips, watch components, electrical contacts, solder joints, and individual microstructural phases (ferrite, pearlite, martensite, bainite, individual grains) are tested because bulk hardness methods cannot be applied to such small volumes. Individual phase hardness values support alloy development, heat-treatment studies, and failure investigations.
Testing Items
Typical analytical deliverables from a microhardness test program include the following:
- Vickers microhardness values (HV0.001 to HV1) on individual points, phases, or regions of interest
- Knoop microhardness values (HK0.01 to HK1) for thin layers and brittle materials
- Hardness-depth profiles and effective case depth determination (CHD per ISO 2639; NHD per ISO 15787)
- HAZ hardness traverses on welded joints with peak-hardness identification
- Decarburization depth assessment per ASTM E1077
- Coating and interface hardness characterization
- Statistical summaries: mean hardness, standard deviation, range, and coefficient of variation
- Hardness conversions to approximate tensile strength per ISO 18265 or ASTM A370 (where applicable)
- Measurement uncertainty evaluation for reported hardness values
Testing Methods
Calculation of Hardness Values
The Vickers hardness number is computed as HV = 0.1891 x F / d squared, where F is the test force in newtons and d is the mean of the two indentation diagonals in millimeters. Equivalently, HV = 1.8544 x F / d squared when F is expressed in kilogram-force. The Knoop hardness number is computed as HK = 1.451 x F / d squared (F in newtons), where d is the length of the long diagonal. The test force designation appended to the symbol (for example, HV0.3 denotes a force of 0.3 kgf, or 2.942 N) is an integral part of the reported result because hardness can vary with load, particularly at low forces where indentation size effects occur.
Sample Preparation Requirements
Accurate analysis depends on specimen quality. The test surface must be polished, free of deformation from grinding, and representative of the material volume of interest. Requirements specified in ISO 6507-1 and ASTM E384 include: surface roughness sufficiently low that diagonal endpoints are sharply resolvable; specimen thickness at least 1.5 times the indentation diagonal; indentation centers spaced at least 2.5 diagonals apart (3 diagonals for non-ferrous materials and 6 diagonals when testing layers, per applicable standards); and indentations positioned at least 2.5 diagonals from the specimen edge. Improper preparation—smearing, edge rounding, thermal damage, or etching that obscures the indentation—is a leading cause of erroneous results and must be addressed during analysis.
Measurement and Data Acquisition Procedure
A calibrated microhardness tester applies the selected force for a dwell time of typically 10 to 15 seconds. Both indentation diagonals are measured with a calibrated optical system, usually at magnifications of 400x or higher, and the mean diagonal is used in the hardness calculation. For depth profiling, a series of indentations is placed at defined depth increments (for example, 0.05 mm or 0.1 mm steps), and hardness is plotted against distance from the surface. Tester verification and indirect calibration using certified reference blocks, per ISO 6507-2/6507-3 or ASTM E384, must be current, and the analysis report should state the calibration status, test force, dwell time, and number of measurements.
Interpretation and Statistical Analysis
Reported values should never rely on a single indentation; standards and sound practice require at least three to five valid indentations per condition, with the mean and scatter reported. A coefficient of variation above roughly 5 to 10 percent on homogeneous material usually indicates preparation problems, poor focus, vibration, or indentations landing on grain boundaries, inclusions, or pores. When analyzing profiles, the case depth is obtained from the intersection of the hardness-depth curve with the specified limit hardness, using graphical or regression interpolation between adjacent measurement points. Load-dependence of results should be evaluated whenever comparisons are made between datasets obtained at different forces, because indentation size effects cause hardness to rise as test force decreases below approximately HV0.1 in many materials.
Sources of Error and Uncertainty
Principal contributors to measurement uncertainty include diagonal-reading error (dominant at small indentations, since a 1 percent error in d produces a 2 percent error in HV), optical resolution limits, indenter geometry deviation from calibrated angles, force calibration error, dwell-time effects, surface condition, vibration, and operator bias. A complete result analysis quantifies or qualifies these contributions and reports uncertainty consistent with the requirements of ISO 6507-1 or ASTM E384. Hardness conversions to other scales or to tensile strength must be treated as approximations and only applied within the material class and hardness range covered by ISO 18265 or ASTM A370.
Applications
Microhardness result analysis supports quality control, process development, failure analysis, and research across many industries:
- Automotive: verifying carburized and nitrided gear tooth hardness, CHD compliance, and grind-burn detection on camshafts and crankshafts
- Aerospace: qualifying coatings on landing-gear and turbine components, evaluating diffusion layers, and HAZ assessment of welded airframe alloys
- Tooling and die making: validating nitride layers, PVD coating hardness, and tempering response of tool steels
- Electronics and semiconductors: measuring solder joints, bond pads, plating hardness, and thin films
- Additive manufacturing: mapping hardness anisotropy, layer-to-layer uniformity, and post-treatment effects
- Failure analysis: detecting decarburization, soft zones, hydrogen-embrittlement-susceptible microstructures, and abnormal phases at fracture origins
- Metallurgical research: quantifying individual phase hardness to support alloy design and transformation studies
Accredited third-party laboratories routinely perform these analyses, providing traceable results that can be used for supplier qualification, customer acceptance, and certification against international standards.
FAQ
What does a designation such as HV0.2 mean in a test report?
HV0.2 indicates that the Vickers test was performed with a force of 0.2 kgf (1.961 N). Because microhardness values can vary with the applied force, the force level must always accompany the hardness number; comparing HV0.1 with HV1 results directly may be misleading due to indentation size effects.
How is effective case depth calculated from microhardness data?
A hardness-depth profile is generated from a series of indentations starting at the treated surface. The effective case depth is the depth at which the profile crosses the specified limit hardness—550 HV1 for carburized and hardened cases per ISO 2639—determined by interpolation between the last point above and the first point below the limit.
Why do my microhardness results show large scatter?
Common causes include inadequate polishing, indentations positioned over grain boundaries, inclusions or pores, edge rounding of mounted specimens, poor optical focus, vibration during indentation, or test forces so low that indentation size approaches the resolution of the measuring system. Reviewing indentation placement and preparation quality usually identifies and resolves the issue.
Can Vickers microhardness values be converted to Rockwell or tensile strength?
Approximate conversions exist for specific material families within defined hardness ranges, as published in ISO 18265 and ASTM A370. Conversions should be treated as estimates and clearly labeled as such, because no universal relationship links indentation hardness to tensile properties across all alloys and microstructures.
When should Knoop testing be chosen over Vickers testing?
Knoop indentation depth is only about one-third that of a Vickers indentation of equivalent long-diagonal length, and its elongated shape suits very thin coatings, brittle ceramics, and regions requiring closely spaced indentations. Vickers testing is preferred for general profiling and when direct comparability with conventional HV data is required.
How many indentations are required for a valid result?
Best practice and most specifications require a minimum of three to five valid indentations per measured condition, with the arithmetic mean, scatter, and test parameters reported. Depth profiles typically use indentation spacing and depth increments selected so that the limit-hardness crossing point is bracketed by at least two data points.
What sample preparation is needed before microhardness testing?
The specimen must be sectioned without altering its hardness, mounted, and ground and polished to a scratch-free finish, usually with final polishing at 1 micron or finer diamond suspension. Careful edge preservation is essential when testing coatings and case depths so that surface layers are not rounded or smeared.
