The reliability of novel flexible opto-electronic devices depends
heavily on the resilience of a thin ceramic oxide layer deposited
on a polymer substrate. Currently, one of the most popular
combinations consists of a thin layer of Indium Tin Oxide (ITO)
on a polyester substrate, such as polyethylene terephthalate
(PET). The ITO layer, usually around a few hundred nanometers
in thickness, is very susceptible to cracking. As this layer experiences
cracking and delamination from the substrate, the resistance
of this layer sharply increases and it is rendered useless.
Characterization of the mechanical properties of this oxide
layer after deposition is very important. The properties of ITO
deposited on glass have been previously investigated, but because
the ITO layer has an amorphous structure, the properties
of the ITO can be quite different than when deposited on
glass. A large mismatch in modulus between the ITO and the
polymer substrate can also affect adhesion to the substrate and
the measured hardness values. For this reason, indentation
testing of the ITO-coated PET system is very valuable,
but straightforward testing might not always be an option.
There are a number of challenges when performing both indentation
testing on a system consisting of a thin hard
coating on a soft polymeric substrate. Care must be taken to
ensure substrate effects do not influence the coating data.
The techniques described here include nanoindentation
spherical indenter to promote circumferential cracking of the
brittle layer and nanoscratch
testing to promote adhesive failure.
testing, a 20 µm spherical indenter was
loaded to normal loads up to 200 mN with a pause of 10 seconds.
The goal of this style of indentation
testing was to promote
cracking of the ITO layer. In all cases, the first visible crack
appeared at approximately 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. Severe damage
of the 50nm thick coating was observed at 200mN. Optical
micrographs of each indent can be seen in Figure 1.
Figure 1. Optical micrographs of residual indents
for 4 applied
normal loads and 3 coating thicknesses (1000x magnification).
Penetration depths of several microns were observed for the
films. The load-depth curves presented in Fig. 2 show a small
variation between samples due to coating thickness. The diameter
of each crack was measured optically. The primary circumferential
crack diameter for all samples and loads was equal to
the diameter of the indenter itself (20µm). This shows cracking
was promoted by the compliance of the polymeric substrate.
Figure 2. Load depth curves for 3 coating thicknesses.
As the indenter first makes contact and load is increased, a primary
crack is formed. Further loading elastically deforms the
substrate while causing cracking and delamination of the ceramic
coating. Future work will model this contact with the goal
of understanding this failure mechanism in more detail.
testing was performed using a 5 µm radius spherical
diamond indenter. Samples were adhered to glass slides for
testing. Low-load scratching was performed using the High Resolution
cantilever of the Nano
Scratch Tester (NST). Critical loads
were determined using optical methods and were compared for
several coating thicknesses.
Two primary failure mechanisms were observed for all samples.
The first mode of failure during testing was rupture of the ITO
layer. Further failure occurred in the form of spallation of the
coating and scarring of the PET substrate. A panoramic comparison
of a scratch performed on each sample is presented in
Figure 3. Panoramic comparison of scratches
on each sample,
(500x magnification). Applied load range was 0.08 - 5 mN.
Scanning Force Microscopy (SFM) was performed at the critical
failure points of the sample with a coating thickness of 250 nm
and is presented in Figure 4.
Figure 4. Optical and 2-D and 3-D AFM micrographs
LC1 (a) and LC2 (b) for the sample with a coating thickness of
The load at failure was also plotted against coating thickness and
is shown in Figure 4. This graph shows 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.
This plot can be seen in Figure 5. Scratch widths at the
critical loads appear to be less dependent on film thickness than
the critical load values themselves.
Figure 5. Plot of the load at failure for each
as a function of film thickness.
When attempting to determine the mechanical properties of a
transparent oxide deposited on a thin polyester film, it is necessary
to adapt indentation
testing methods. Indentation
testing utilizing a spherical indenter 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
show that these methods can accurately characterize differences
in film thickness. Further developments in these testing
methods will allow for a more flexible range of tests that can be
conducted on thin composite films. This will allow correlations
to be made between laboratory sample testing and the actual
in-service performance of devices which utilize ITO technology
(e.g., touchscreens, flexible solar cells, flexible LED lighting, etc.)
Prof. Darran Cairns and Nick Morris of West Virginia University
are acknowledged for providing these interesting results.
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