Hardness test methods use an indenter probe that is displaced into a surface under a specific load. The indentation typically has a defined dwell time. In traditional mechanical testing, the size or depth of indentation is measured to determine hardness. Hardness testing is divided into two ranges: macrohardness and microhardness. Macrohardness covers testing with an applied load over 1 kg or about 10 Newton (N). Microhardness testing, with applied loads under 10 N, is typically used for smaller samples, thin specimens, plated surfaces or thin films. The two most common microhardness techniques are Vickers and Knoop hardness tests.
For more accurate and reproducible results, microhardness testing needs to account for effects of sample size, preparation and environment. Samples must fit in the sample stage and be perpendicular to the indenter tip. An extremely rough surface may reduce the accuracy of indentation data; a proven method for polishing samples is recommended. The microhardness tester needs to be isolated from vibrations. For samples with multiple phases or variation in grain sizes, statistical data is required.
Traditional microhardness test methods optically analyze the indented impression, convoluting data with operator bias. Unlike Vickers or Knoop hardness test methods, instrumented indentation (nanoindentation) uses a three-sided pyramidal (Berkovich) indenter. This shape allows the tip to be theoretically designed to an atomic point. Using a high load nanoindenter for microhardness testing with forces ranging up to 1 Newton (N), instrumented indentation with an array of indents using dynamic measurements yields unparalled accurate and reliable microhardness data with no operator bias.
The Vickers hardness test uses a Vickers indenter (below) pressed into a surface to a specified force. The force is usually held for 10 seconds. After the indentation is finished, the resulting indent is analyzed optically to measure the lengths of the diagonals to determine the size of the impression.
Schematic of Vickers indentation probe
There is a degree of operator bias in this method, especially in the lower range of the applied load. According to ASTM E384-11, indentation diagonals should be greater than 17 microns in length. For coated samples, this test is not valid for coating thicknesses under 60 microns.
Vickers indentation schematic
For many types of samples, the contact depth (hc) is not identical to the displacement depth (h) due to surrounding material getting elastically deflected during the indentation, as shown schematically (left). In addition to the above mentioned sample and environmental considerations, this effect also affects accuracy and precision for microhardness data.
The Knoop hardness test is also a microhardness technique that is similar to the Vickers hardness test method. A Knoop indenter is used to press into a surface to measure hardness. The Knoop indenter, however, is shaped differently than a Vickers indenter for microhardness or a Berkovich indenter used in nanoindentation. The shape for the Knoop indenter is more elongated or rectangular. The Knoop hardness test method is usually done with lighter loads for microhardness testing and careful sample preparation is required. Knoop hardness testing is applied to samples needing indentations close together or on the edge of a sample, both benefitting from the different probe shape.
Schematic of Knoop indentation probe
A designated load is applied for a specified dwell time. In contrast to the Vickers hardness method, the Knoop test method uses only the long axis. The resulting indentation measurements is then converted to a Knoop hardness number using a chart. For the Knoop indenter probe shown here, angles are d=172.50° and g=130.00°.
Due to the limitations with a lower applied load range, validity issues for thin films, and an increase in nanotechnology resulting in smaller dimensions, micro and nanoindentation methods have been developed.