The instrumented indentation test, also called nanoindentation, is a method for measuring hardness. An important part of material testing, it serves to determine plastic and elastic material properties such as the elastic indentation modulus EIT, the indentation hardness HIT and the indentation creep CIT.
Unlike the classic hardness measurement methods – for example according to Vickers or Martens – that can determine only a single characteristic value, nanoindentation enables a very exact, depth-dependent measurement of several material-specific parameters. The main areas of application for nanoindentation are in testing paint, galvanic coatings, hard materials and polymers.
In the instrumented indentation test, an indenter is pressed into the test object with a defined force curve. When the specified maximum force is reached, the indenter is released again in a controlled manner. The penetration depth is recorded during loading and unloading. Various parameters can be calculated from the force applied, the shape of the indenter and the depth of the indentation.
Hardness and elasticity are material characteristics. This means that the measured value depends on what experiment was conducted. To make the results comparable, ISO 14577-1 requires specification of the test conditions.
The indentation hardness HIT is a measure of the material’s resistance to permanent (= plastic) deformation. It’s usually determined at maximum force. Indentation hardness can be converted into Vickers hardness, but this conversion must be clearly marked.
In contrast to the indentation hardness, the Martens hardness HM provides information about both the plastic and the elastic material properties. The Martens hardness is calculated from the course of the indentation depth under load.
The modulus of indentation EIT is an elasticity value and the most important parameter for all applications with elastic materials; it’s calculated from the course of the indentation while unloading.
The creep behavior CIT describes the further deformation of the material under constant force. To determine this value, the indenter is pressed into the sample with constant force over a longer period of time (minutes to hours). Polymers and other materials prone to creep continuously yield to the pressure, so the indentation depth increases.
The storage modulus and the loss modulus (E’ and E”) describe how the material behaves under an oscillating force. The storage modulus stands for the elastic component; it’s proportional to the fraction of deformation energy that is stored in the material and can be recovered from the material after unloading. The loss modulus, on the other hand, represents the viscous portion; it corresponds to the part of the energy that is lost as it’s converted into heat during compression.
To be able to determine a wide range of parameters, Fischer's nanoindentation devices offer different measuring modes.
In the ESP method, the indenter is repeatedly loaded and unloaded. This happens with increasing force until the pre-defined maximum force is reached. This allows for quick force- and depth-dependent determination of parameters such as the elastic modulus of indentation (EIT), the indentation hardness (HIT) and the Vickers hardness (HV), all at the same place on the sample.
This method is particularly useful when testing thin layers. The depth-dependent measurement allows the parameters of the coating to be determined at very low forces without being influenced by the substrate. As the force increases, the transition from coating to base material can also be analyzed.
The dynamic mode is based on dynamic-mechanical analysis (DMA). While DMA is meant for solid materials testing, Fischer's dynamic mode allows for the characterization of materials on much smaller scales, e.g. coatings like car paints. To this end, an indenter is pressed into the surface with sinusoidally increasing and decreasing force – all with an amplitude of just a few nanometers. This allows determination of properties such as the elastic modulus and the storage and loss moduli.
As with all methods, there are factors that can influence the measurement. For nanoindentation, besides indenter wear and temperature, the most important factors are vibrations and roughness.
Fischer only uses indenters made of natural diamonds, because they are exceptionally resistant. Nevertheless, they do wear out after many measurements. The tips become rounder and lose their clearly defined shape. To a certain extent, this effect can be compensated by taking measurements on a reference material, e.g. borosilicate glass. Once it’s worn down, though, the indenter must be replaced.
Temperature plays an important role in all measurements of hardness and elasticity. Many materials, especially soft polymers, experience changes in their properties even with relatively small temperature fluctuations. This is why the ambient temperature must be defined during the measurement.
In addition, the measurement technology itself reacts to the temperature. Particularly when measuring over the course of several hours, heat can develop in the device. When different parts expand, it distorts the results.
The natural hard stone plate in the HM2000 and PICODENTOR HM500 instruments make them very stable with respect to both form and temperature. This allows measurements to be conducted over several hours without being influenced by temperature.
The most common cause of incorrect measurements is vibration. At low test loads, results can be distorted by even the gentle airflow from ventilation systems or vibrations in the floor due to footsteps. For delicate measurements, Fischer recommends choosing a low-vibration location (e.g. a basement) and using closed measuring boxes with damping tables.
With rough surfaces, the indenter doesn’t always make uniform contact with the surface of the sample. That’s why the results are often difficult to reproduce. If possible, rough surfaces should be polished before measuring, otherwise several comparative measurements should be carried out.