Tribological behavior of thin films on polymer substrates

Nowadays, more and more electronic consumer products are introduced that make use of flexible electronics including displays and touch panels. Shifting the design of the electronic components from rigid to flexible substrates like polymers, adds new requirements for the thin film materials. In flexible electronics the thin materials have to withstand frequent stretching and bending while still maintaining their functionality to ensure that the entire device remains operational. Such exposure has already been studied in detail and is well understood. However, in particular when wearable electronics are concerned, the flexible electronics devices are also exposed to frequent rubbing against a series of other objects and materials. Since such exposure has only been studied scarcely, the current project aims to characterize the friction and wear of thin films on flexible substrates, in this case polymers, in great detail. Of particular interest is the combined exposure to stretching and bending on one hand and to rubbing against other objects on the other hand as it would occur in real life situations. In order to simulate and study such realistic exposures, a test method will be developed and verified with simultaneous bending and rubbing of the thin films. All thin films will be synthesized by plasma-assisted deposition processes as it is frequently done in industry. The outcome of the project will enable the formulation of design rules for thin films used in flexible electronics that can guide the development of new components and devices for consumer products.

Thin films deposited on polymers are nowadays an integral part of flexible electronics, however the degradation of such films due to mechanical loading conditions is not yet fully understood. Here, the current project shined first light on how the exposure of thin films on polymers to straining and bending interacts with tribological loading conditions. In order to enable fundamental research in this field, suitable methods and procedures for investigation need to be developed, which was the main goal of this project using Mo and MoN thin films deposited on polyimide substrates as model systems. In a first step, a comprehensive investigation on the tribological properties of the films on polyimide needed to be conducted as such information was absent in literature. Here, different tribological loading conditions were achieved by using a variety of counterpart materials including rubber, polymers, glass, steel and alumina. The results revealed that fatigue of the thin film played a major role among the wear mechanisms present which was induced by frequent deformation of the underlying polyimide substrate. A second step focussed on combining straining and bending with tribological exposure by performing the tests subsequently. Even though though the cracks induced by straining and bending closed after relaxation of the sample, they influenced the wear behavior as enhanced wear was observed in places with oblique angles between the cracks and the sliding direction in the tribological test. In a final step, a recently developed bending apparatus was adapted to enable bending and tribological testing simultaneously. This experiment served as a proof of concept on how tests that mimic the exposure of thin films on polymers to various mechanical loading conditions can be performed. In addition to the obtained experimental results, modelling the test setup by the finite-element method provided beneficial insights in terms of load and strain distribution on the tested sample. In general, the results of the project showcase how to advance the investigation on thin films on polymers with respect to crack formation and wear, where a comprehensive understanding of these effects can enable the development of optimized thin film designs to hinder the degradation of these films.