Vickers Hardness Number (HV): Microindentation Testing
The Vickers hardness number (HV) quantifies a material’s resistance to plastic deformation under a diamond pyramid indenter. Defined as the applied test force divided by the surface area of the resulting indentation, HV offers a continuous scale that covers soft solders, hardened tool steels, and advanced ceramics alike. Because the Vickers method employs a geometrically self-similar indenter across a broad load range, it provides a unified framework for correlating hardness with tensile strength, wear performance, and heat-treatment response.
This article formalises the HV definition, retraces the historical transition from Brinell balls to diamond pyramids, and highlights measurement protocols that secure reproducibility. You will find cross-links to the Brinell explainer and to calculators such as the spring force tool that simplify test setup, alongside broader discussions of mechanical property integration in manufacturing quality systems.
Definition and Governing Relationships
Vickers hardness number equation
HV is defined as HV = 1.8544 F / d², where F is the applied force in newtons and d is the mean length of the two impression diagonals in millimetres. The coefficient 1.8544 stems from the geometry of the square-based diamond pyramid whose face angle is 136 degrees. Reporting HV requires specifying the test force, typically expressed in kilograms-force (e.g., HV10 for 98.07 N) or directly in SI units. Modern standards such as ISO 6507 and ASTM E384 recommend expressing F in newtons and documenting dwell time to maintain SI coherence.
Microhardness versus macrohardness regimes
The Vickers method spans test loads from a few grams-force (microhardness) to over 100 kilograms-force (macrohardness). Selecting load influences penetration depth and sensitivity to microstructural features. Microhardness testing resolves case depths, weld heat-affected zones, and thin films, while macrohardness assesses bulk properties. Maintain consistent load levels when comparing datasets and note the indentation depth relative to coating or grain sizes to avoid substrate or edge effects. Linking HV data to elastic properties via the Young’s modulus article supports deeper mechanical analysis.
Historical Evolution of the Scale
Origins in early twentieth-century metallurgy
The Vickers test debuted in 1921 at Vickers Ltd., responding to the need for a single hardness method applicable across metals of varying strength. Engineers Smith and Sandland devised the diamond pyramid to avoid the indenter size changes inherent in Brinell testing. Their approach delivered consistent geometry and enabled accurate measurements on thin sections. Within a decade, national standards bodies formalised the method, and the HV scale became a staple of metallurgical laboratories evaluating armor plate, turbine blades, and bearing steels.
Modern standardisation and automation
Today, ISO 6507 and ASTM E384 define the Vickers procedure, covering load application, dwell time, optical measurement, and uncertainty estimation. Automated hardness testers employ motorised stages, autofocus systems, and digital imaging to measure indentation diagonals with sub-micrometre precision. Instrumented indentation extends the method further by recording load-displacement curves, allowing simultaneous extraction of hardness and modulus. Calibration traceability relies on reference blocks certified by national metrology institutes, tying HV values to SI force and length realisations documented in the SI defining constants guide.
Measurement Techniques and Best Practices
Sample preparation and surface quality
Reliable HV measurements demand mirror-polished surfaces free from scratches, embedded abrasive, and residual stresses. Follow progressive grinding and polishing sequences, then perform final polishing with colloidal silica to remove deformation. Clean specimens with deionised water or solvents specified by your process engineers to prevent contamination. The water hardness converter helps maintain rinse water quality that avoids mineral residue compromising optical contrast.
Indentation measurement and error sources
Optical microscopes capture indentation images; measurement uncertainty arises from focus, illumination, and operator interpretation. Employ image analysis software calibrated with traceable stage micrometers, and verify measurement repeatability through gauge R&R studies. Document dwell times, typically 10 to 15 seconds, and note environmental conditions such as temperature, which can influence polymer and composite hardness. For thin films, ensure indentation depth is less than 10 percent of coating thickness or use cross-sectional mounts to isolate the layer.
Applications Across Industries
Heat treatment validation and metallurgical analysis
Hardness profiles reveal the effectiveness of carburising, nitriding, induction hardening, and quenching operations. Plotting HV versus depth identifies case-hardened layer thickness, compares furnace loads, and diagnoses thermal gradients. Combining HV data with the heat treatment calculator allows predictive modelling of diffusion-controlled processes, ensuring uniform hardness across complex geometries.
Quality assurance for coatings, welds, and additive manufacturing
Microhardness mapping verifies coating adhesion, assesses weld heat-affected zones, and characterises additive manufacturing builds layer by layer. Engineers correlate HV values with tensile tests, fatigue life, and wear rates to qualify materials for aerospace, biomedical, and energy applications. Coupling hardness maps with microstructural imaging and compositional analysis yields a comprehensive picture of performance drivers, fulfilling traceability requirements from industry regulators.
Importance and Reporting Considerations
Traceability and uncertainty budgets
Report hardness results with full test identifiers: load, dwell time, temperature, number of indents, surface preparation, and calibration references. Maintain control charts of certified reference block measurements to detect drift. When converting between HV and approximate tensile strength, state empirical correlations and applicable material classes. Align data storage with the mass metrology and length realisation guides to preserve SI consistency.
Integrating hardness into design decisions
Designers use HV to predict component lifespan, select surface treatments, and balance strength with toughness. Communicate hardness trends alongside complementary properties such as modulus, fracture toughness, and corrosion resistance. Linking to resources on propulsion materials or thermal management helps multidisciplinary teams interpret HV in context. Provide digital data packages with structured metadata so simulation analysts can incorporate hardness gradients into finite element models.