Nanoindentation

NANOINDENTATION

Morphology and mechanical properties determine how and where materials, such as polymer blends, filled polymers and metals, or ceramics are used. For instance, nanoparticles are increasingly being used in the development of coatings for the automotive industry to optimize mechanical properties.

The combination of nanomechanical measurements in the form of nanoindentation and atomic force microscopy (AFM) opens up new avenues for material development and testing, and defect analysis. It makes it possible to visualize deformation processes locally and measure mechanical properties close to the surface. In addition to these quasi-static indent measurements, it’s also possible to carry out frequency-dependent dynamic measurements. Other testing methods, such as the scratch and wear test, are used to determine the scratch resistance or material wear of a coating.

Scratch test on the surface of a coating with crack formation following material failure.
See figure: Scratch test on the surface of a coating with crack formation following material failure.

Morphology and deformation of coatings

When applied to coatings, atomic force microscopy (AFM) provides morphological information, including information about the distribution of nanoparticles. It can also be used to visualize the deformation processes that occur on the sample surface. Traditional methods, such as the Vickers hardness test, are not suitable for measuring thin layers close to the surface due to the heavy load that must be applied. The method with the best load and displacement resolution is nanoindentation.

Impressions made with a diamond indenter on a coating surface following a nine-point indentation
See figure: Impressions made with a diamond indenter on a coating surface following a nine-point indentation

Depth-sensing indentation techniques

Depth-sensing testing procedures determine the mechanical properties of a material by shifting the indenter tip as a function of the force so that even impressions with penetration depths in the nanometer range can be taken into account. Nanoindentation measurements register mechanical properties within the uppermost layer of the sample without affecting the substrate layer. During indentation, a load-displacement curve is recorded, which is evaluated to determine the contact area between the indenter and the sample. The maximum penetration depth can be derived from this curve. The contact area is proportional to the material’s hardness. Once the load has been applied, a controlled unloading is carried out, whereby the sample springs back fully elastically. This makes it possible to determine the elasticity of the surface, using the principle of contact mechanics, in addition to the hardness. Over time, the method developed by Oliver and Pharr for evaluating load-displacement curves has established itself as the standard process.


See figure: Load-displacement curve mapping the loading and unloading of a coating layer

Load-displacement curve mapping the loading and unloading of a coating layer
See figure: Load-displacement curve mapping the loading and unloading of a coating layer
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