Microhardness Testing

Microhardness testing is a destructive hardness measurement method based on pressing a pyramidal indenter into the material surface under low loads (typically 10 gf to 1000 gf, i.e. approximately 0.098 N to 9.8 N) and measuring the resulting indentation. Unlike conventional macro hardness measurements, the very small applied force makes it possible to obtain a local hardness value in sub-millimetre regions, in thin sections, in individual metallographic phases and in coating layers. The most common application is the Vickers microhardness method (HV), which uses a square-based diamond pyramid indenter with a 136° apex angle; the Knoop indenter (HK), which leaves a narrower and more elongated indentation, is preferred especially for brittle materials, thin coatings and cases where a hardness gradient must be measured directionally. The hardness value is defined as the ratio of the applied force to the surface area calculated from the indentation geometry and is physically a measure of the local resistance of the material to plastic deformation.

The physical principle of the method is to press the diamond indenter into the specimen surface with a controlled force and for a defined dwell time (typically 10-15 seconds), then to remove the load and measure the permanent indentation optically. In the Vickers method the two diagonals of the indentation (d1 and d2) are measured under a microscope and their average is used to calculate the hardness from the relation HV = 1.8544 × F / d², where F is the force (kgf) and d is the mean diagonal length (mm). In the Knoop method only the long diagonal is measured. Because the indentation is very small, the measurement requires a high-magnification (generally 400x-500x) metallographic microscope or an automatic image analysis system; since even an error of a few micrometres in reading the diagonal is directly reflected in the result, surface preparation and optical resolution are of critical importance.

The equipment consists of digital microhardness testers with a motorised load application mechanism, an angularly adjustable slide-mounted specimen stage and an integrated optical measuring system. Metallographic preparation of the specimen is the most decisive stage of the method: a section is cut, mounted (in cold or hot resin), and brought to a mirror finish by progressive grinding and polishing; traces of the deformation layer are removed so that the true hardness of the material is revealed. The standards require that the measurement surface be perpendicular to the indenter axis, that the spacing between indentations be a defined multiple of the diagonal length (to avoid edge effects) and that the specimen thickness be sufficient in relation to the indentation depth.

The method is at its strongest in structures where the hardness distribution varies with position. In welded joints, a hardness profile is produced by a series of indentations taken at defined intervals across the base metal, the heat-affected zone (HAZ) and the weld metal; this profile is decisive in detecting hard regions carrying a cracking risk (for example a martensitic HAZ), especially in steels prone to hardening. In surface hardening treatments (carburising, nitriding, induction hardening), the effective case depth (CHD/effective case depth) is measured by indentations taken from the surface inwards. In coatings, the layer hardness, local differences relative to the substrate and surface degradation such as decarburisation are assessed on the micro scale.

Reporting includes the hardness value at each measured point, the load used (indicated as an index in the HV notation, for example HV0.3 for a 300 gf load), the dwell time, the indentation positions and, where required, a hardness-versus-distance graph. Acceptance criteria are generally set by the product/manufacturing specification or the relevant construction code; for example, maximum HAZ hardness limits in welded joints matter in services carrying a risk of creep or sulphide stress cracking. The value of microhardness testing lies in its ability to map the hardness variation within the material's internal structure by position rather than to give a single average value, thereby enabling direct verification of heat treatment, welding procedure and surface treatment quality.