Nanoscratch and Nanoindentation of Oxide Coatings on Thin Film Polymer Substrates

The reliability of novel flexible opto-electronic devices relies greatly on the resilience of a thin ceramic oxide layer deposited on a polymer substrate. One of the most popular combinations that currently exists comprises of a thin layer of Indium Tin Oxide (ITO) on a polyester substrate, such as polyethylene terephthalate (PET). The ITO layer, generally about a few hundred nanometers in thickness, is extremely susceptible to cracking. The resistance of this layer sharply increases and it is rendered useless as this layer experiences cracking and delamination from the substrate.

Characterization of the mechanical properties of this oxide layer after deposition is extremely important. The properties of ITO deposited on glass have been earlier investigated, but these properties can be quite different than when deposited on glass as the ITO layer has an amorphous structure. A large mismatch in modulus between the polymer substrate and the ITO can also affect adhesion to the substrate and the measured hardness values. For this reason, scratch testing and indentation of the ITO-coated PET system is extremely valuable, but straightforward testing might not always be an option.

There are a number of challenges when carrying out both scratch testing and indentation on a system consisting of a thin hard coating on a soft polymeric substrate. It is essential to ensure that substrate effects do not influence the coating data.

Figure 1. Load depth curves for 3 coating thicknesses.

Figure 2. Optical micrographs of residual indents for 4 applied normal loads and 3 coating thicknesses (1000x magnification).

The techniques described in this article include nanoindentation with a spherical indenter to promote circumferential cracking of the brittle layer and nanoscratch testing to promote adhesive failure.

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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.

For more information on this source, please visit Anton Paar GmbH.