Load Selection, Indent Spacing, and Edge Effects in Microhardness Testing

Microhardness testing is widely used for the localized mechanical characterization of thin layers, small parts, coatings, and individual microstructural constituents. Load selection, indent spacing, and control of edge effects directly determine whether the data are valid. Testing shall follow GB/T 4340.1 (Vickers hardness) and ASTM E384 (microindentation hardness).

Principles of Load Selection

The Vickers hardness number is HV = 0.1891×F/d², where F is the test force in newtons and d is the mean of the two indentation diagonals in millimetres. On a given material, too small a load produces a short diagonal and a disproportionately large measurement error, while too large a load invites influence from adjacent phases or the substrate. A diagonal of at least 20 μm is generally required for reliable optical measurement. For thin layers and coatings, the empirical rule is that indentation depth shall not exceed one tenth of the layer thickness; since a Vickers indentation is about d/7 deep, the diagonal should be kept within roughly 70% of the layer thickness—otherwise reduce the load or switch to a Knoop indenter. The Knoop indentation is long and narrow, with a depth of only about 1/30 of its long diagonal, making it better suited to thin layers and brittle materials, though more sensitive to surface roughness and perpendicularity.

Indent Spacing and Edge Effects

A plastic zone surrounds every indentation, and indents placed too close together interfere with each other, biasing results high. GB/T 4340.1 requires the spacing between adjacent indent centers to be at least three times the mean diagonal, and the distance from an indent center to the specimen edge at least 2.5 times; ASTM E384 specifies 2.5 times for steel and 3 times for light metals such as aluminum and copper. Apply the requirements of the chosen standard and, where they differ, take the stricter value.

Specimen Preparation and Test Conditions

The test surface shall be polished until the indentation diagonals are clearly readable, and the perpendicularity between the test surface and the indenter axis shall be within 2°. For mounted specimens, the mounting material must be flush with the specimen to avoid edge bevel errors. Vibration, temperature fluctuation, and loading rate all affect the result; the dwell time is controlled per the standard (typically 10–15 s).

Practical Tips

• Derive the minimum load from the requirement of a diagonal no smaller than 20 μm; for thin layers keep indentation depth within 1/10 of the layer thickness.
• Apply 2.5–3 diagonal lengths for indent spacing and edge distance per the governing standard, taking the stricter value.
• Prefer the Knoop indenter for brittle thin layers.
• Polishing quality, perpendicularity, and dwell time follow GB/T 4340.1 and ASTM E384; the actual operating procedure shall prevail.