Instrumented indentation test

The procedure and the most important parameters.

Instrumented indentation testing, also called nanoindentation, is one of the hardness measurement methods. As an important component of material testing, it is used to determine plastic and elastic material properties such as Martens hardnessHM, Indentation modulus EIT, Indentation hardness HIT and Indentation creep CIT.

In contrast to the classical hardness measuring methods – for example according to Vickers or Martens, in which only a single characteristic value is determined – nanoindentation enables very precise depth-dependent measuring of various material-specific parameters. The main field of application of nanoindentation is the testing of paint, electroplating layers, hard materials and polymers.

This is how nanoindentation works.

How nanoindentation works

In the instrumented indentation test, an indenter is pressed into the test specimen with a defined force curve. When the specified maximum force is reached, the indenter is unloaded again in a controlled manner. The depth of the indentation is recorded both during loading and unloading. Various material-specific parameters can be calculated from the force applied, the shape of the indenter and the depth of the indentation.

For most materials, the indentation test shows an elastic and a plastic component. The test specimen does not return to the initial value of the depth of the indentation after unloading. The figure shows this by the non-congruence of the loading curve (blue) and the unloading curve (orange).

The most important parameters.

Hardness and elastic properties are parameter-dependent properties of the materials. This means that the measured values depend on the experiment that was carried out. To ensure that the results are comparable, ISO 14577-1 requires that the test conditions are also specified. This is done in the following universal form:

Nanoindentation the most important parameters
  • Indentation hardness

      The indentation hardness HIT is a measure of the material's resistance to permanent (= plastic) deformation. It is determined by tangent formation from the unloading curve and applies to the maximum test load Fmax. The indentation hardness HIT can be converted into a Vickers hardnessHV, but this conversion must be clearly marked.

  • Martens hardness

      In contrast to the indentation hardness HIT, the Martens hardnessHM provides information not only about the plastic but also on the elastic material properties. The Martens hardness is calculated from the course of the depth of the indentation during loading.

  • Indentation modulus

      The indentation modulus EIT is an elasticity value and the most important parameter for all applications with elastic materials. The modulus EIT is calculated from the unloading curve of the indentation. In many cases, EIT values are comparable to the classic modulus of elasticity, but should not be equated with it.

  • Indentation creep

      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 the same force over a longer period of time (minutes to hours). Polymers and other materials with a tendency to creep continuously yield and the depth of the indentation increases.

  • Storage and loss modulus

      The storage modulus and the loss module (E' and E'') describe the material behavior under oscillating force action (dynamic mode). The storage modulus represents the elastic component. It is proportional to the portion of the deformation energy that is stored in the material and can be recovered from the material after unloading. The loss module, on the other hand, represents the viscous component. It corresponds to the loss fraction of the energy which is converted into heat during compression.

Measurement modes.

In order to be able to determine a wide range of parameters, our nanoindentation instruments offer different measurement modes.

  • Enhanced Stiffness Procedure

      In the Enhanced Stiffness Procedure method, or ESP method for short, the indentor is gradually loaded and (partially) unloaded again. This happens with increasing force until the defined maximum force is reached. This enables rapid force- and depth-dependent determination of parameters such as elastic indentation modulus (EIT), indentation hardness (HIT) or Vickers hardness (HV) at one and the same specimen location.

      This ESP method is particularly interesting for testing thin films. Measuring depth-dependent allows parameters of the coating to be determined at very low forces without the influence of the substrate. With increasing force, the transition from coating to base material can be analyzed.

  • Dynamic mode

      The dynamic measurement mode is based on dynamic mechanical analysis (DMA). While DMA focuses on testing solid materials, our dynamic mode also allows characterization of materials in much smaller dimensions, such as coatings like automotive paints. Here, an indentor is pressed into the surface with sinusoidally increasing and decreasing force - with an amplitude of only a few nanometers. In this way, properties such as elastic modulus, storage modulus and loss module can be determined.

    Where is this process used?

    • Testing of paint, electroplating, hard materials and polymers

    What factors can influence the measurement?

    With all methods, there are factors that can influence the measuring. In nanoindentation, in addition to indentor wear and temperature, vibration and roughness are particularly critical.

    • Indentor wear

        We only use natural diamond indentors because they are particularly resistant. Nevertheless, they 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 measuring on reference material, for example borosilicate glass. In the case of more severe wear, the indenter must be replaced.

    • Temperature

        Temperature plays an important role in measuring hardness and elasticity. Many materials, especially soft polymers, change their properties even with relatively small temperature fluctuations. That is why the ambient temperature must be defined during measuring.

        In addition, the measurement technology itself reacts to temperature. Especially when measuring over several hours, heat can develop in the device. If various components expand, this falsifies the results.

        Thanks to the construction with a plate made of natural hard stone, our devices are very stable in terms of shape and temperature. This means that temperature-independent measurements are possible even over several hours.

    • Vibrations

        The most common cause of measurement errors is vibration. With low test loads, even small air movements from air conditioning systems or floor vibrations caused by footsteps can falsify the results. For sensitive measurements, we recommend choosing a low-vibration location (such as a basement) and using enclosed measuring boxes with damping tables. We offer customized solutions for this purpose.

    • Roughness

        With rough surfaces, the indentor does not always have the same contact area with the test part. Therefore, the results are often poorly reproducible. If possible, it is important to polish rough surfaces before measuring, or to perform several comparative measurements.

    Which standard is applied here?

    Measuring and calculation of material properties according to DIN EN ISO 14577-1 Annex A and ASTM E 2546