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.
For nanoindentation testing, a 20 µm spherical indenter was loaded to normal loads up to 200 mN with a pause of 10 seconds. This style of indentation testing aimed to promote cracking of the ITO layer. In all cases, the first visible crack appeared at about 40 mN. At 100 mN a second circumferential crack was observed, while at 150 mN a third crack was present. Radial cracking was also observed at a load of 200 mN for the coating thicknesses of 50 nm and 100 nm. At 200 mN severe damage of the 50nm thick coating was observed. Figure 2 shows optical micrographs of each indent.
Penetration depths of several microns were observed for the films. The load-depth curves presented in Figure 1 show a small variation between samples due to the thickness of the coating. The diameter of each crack was measured optically. The primary circumferential crack diameter for all loads and samples was equal to the diameter of the indenter itself (20 µm). This means that cracking was promoted by the compliance of the polymeric substrate.
Figure 3. Panoramic comparison of scratches on each sample, (500x magnification). Applied load range was 0.08 - 5 mN.
A primary crack is formed when the indenter first makes contact and load is increased. Additional loading elastically deforms the substrate, resulting in cracks and delamination of the ceramic coating. Future work will help model this contact in order to understand this failure mechanism in a more detailed manner.
A 5 µm radius spherical diamond indenter was used for nanoscratch testing. Samples were adhered to glass slides for testing. The High Resolution cantilever of the Nano Scratch Tester (NST) was used for low-load scratching. Critical loads were established using optical methods and were compared for several coating thicknesses.
Two primary failure mechanisms were observed for all samples. Rupture of the ITO layer was the first mode of failure during testing. Further failure took place in the form of scarring of the PET substrate and spallation of the coating. Figure 3 shows a panoramic comparison of a scratch carried out on each sample.
Scanning Force Microscopy (SFM) was performed at the vital failure points of the sample with a coating thickness of 250 nm (Figure 4).
Figure 4. Optical and 2-D and 3-D AFM micrographs of the LC1 (a) and LC2 (b) for the sample with a coating thickness of 250 nm.
The load at failure was also plotted against coating thickness (Figure 4). This graph illustrates that the failure mechanism of spallation of the coating has a greater dependence on coating thickness than a failure characterized as rupturing. Scratch width at the critical loads was also measured using optical methods for each scratch and was plotted against film thickness (Figure 5). Scratch widths at the critical loads seem to depend less on film thickness than the critical load values themselves.
Figure 5. Plot of the load at failure for each failure mechanism as a function of film thickness.
It is essential to adapt indentation and scratch testing methods when attempting to determine the mechanical properties of a transparent oxide deposited on a thin polyester film. Indentation testing with the help of a spherical indenter in order to promote circumferential cracking and low-load scratch testing with a high-resolution friction table were used to characterize and compare the mechanical properties of the composite films. Results highlight that these methods can accurately characterize variations in film thickness. Additional developments in these testing methods will allow for a more flexible range of tests that can be performed on thin composite films. This will allow correlations to be made between laboratory sample testing and the actual in-service performance of devices, which employ ITO technology (e.g., touchscreens, flexible LED lighting, flexible solar cells, etc.)
Prof. Darran Cairns and Nick Morris of West Virginia University are acknowledged for providing these interesting results.
1) K. Zeng, et al. Thin Solid Films 443 (2003) 60-65
2) B. R. Lawn, Journal of Materials Research, Vol 17, No. 12, Dec 2002
3) H.Chai et al. Journal of Materials Research, Vol 19, No. 6, Jun 2004
This information has been sourced, reviewed and adapted from materials provided by Anton Paar TriTec SA.
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