Fabrication and testing of micro- and nanostructured composites produced by means of laser processing/writing

Project leader: Daniel Kiener

Laser processing applications are rapidly increasing in the field of micro-mechanics and material testing due to recent developments in laser technology. Ultra-short pulsed lasers reach extraordinarily high intensities for extremely short periods of time ranging between picoseconds (10^-12 s) and femtoseconds (10^-15 s). For reference, in such short time periods, light travels only micrometers. Consequently, unique interaction mechanisms with materials are triggered, which can be utilized for subtractive or additive manufacturing.

Very short interaction time and high energy deposited enable the nearly damage-free ablation of bulk materials. Within a first work different processing strategies were investigated for improving the surface quality after laser ablation with optimized parameters. A general strategy was developed to avoid the so-called laser induced periodic surface structures (LIPSS), which are mesoscale surface features on glancing incident trenches, leaving a residual surface roughness in just the nanometer regime. Electron backscatter diffraction analysis could be performed directly on such laser processed surfaces without further preparation. Thus, even ultra fine-grained copper could be mapped, which would be impossible using conventionally laser machined surfaces. [1]

Ultra-short pulsed laser ablation is in general less sensitive to the materials properties compared to other small scale subtractive techniques, such as electrode-discharge machining (EDM) or focused ion beam (FIB) milling, with the only drawback of precision residing in the range of a few micrometers, which is much better than EDM but inferior to FIB. Thus, it is a potentially beneficial method for destructive processing of composite materials consisting of different material types, such as polymers with metal coatings.

Otherwise, ultra-short pulsed lasers can uniquely cure photo-resins by the so called two-photon-absorption. Utilizing this principle for polymerization enables two-photon-lithography (TPL). TPL is therefore a technique to perform additive manufacturing on a small scale, even with sub-micrometer resolution. Within the current project the application field of this technique is explored for small scale material testing. Through great geometrical freedom of the manufacturing process, highly defined and reproducible specimen geometries can be manufactured on a large scale.

Within a recent work, a tuned manufacturing strategy for a common fracture mechanical specimen geometry, the notched cantilever, was introduced as well as demonstrated through SEM in-situ mechanical testing. Finite element simulation was employed to derive the geometry function and compare the tuned with the ideal specimen shape. Additionally, a completely new version of a push-to-pull device, incorporating a double edge notched tension specimen, was presented. The geometry is optimized for additive manufacturing and is presented along with demonstration of SEM in-situ experiments. Such a geometry cannot be produced in a reasonable manner with common manufacturing techniques and enables tension testing on an experimental less demanding compression setup [2].

If a high number of specimens is regularly arranged, automated testing via a common nanoindenter device is feasible. In an ongoing work, push-to-pull specimen arrays, such as shown on a recently published journal cover [3], are tested in a high throughput manner. Therefore, experimental approaches such as the determination of the essential work of fracture, which require many specimens to be tested, can easily be performed and backed with a statistical relevant number of specimens.

As the application of tuned cantilever and push-to-pull specimen for micromechanical testing were already demonstrated for in-situ SEM and common micromechanical testing, future work will focus on further development of such geometries for compound testing. Various approaches for applying metal and ceramic coatings are investigated to finally obtain compound structures. Such small sample dimensions enable straightforward electron microscopical in-situ investigations and material / compound characterization on accessible small dimensions. The results will enable new approaches for thin film characterization and assist upscaling towards technical relevant applications, such as for the design of improved flexible electronics.

Beside the main topics of the project, common issues such as adhesion between different interface types and photoresists (cured via two-photon-lithographical) are accounted, and possibilities for improvement investigated. Furthermore, in an ongoing cooperation with the Faculty of Chemical Engineering and Technology (University of Zagreb and founded by OeAD GmbH), different nanostructured surface structures are manufactured as basis substrates for organic solar cell prototypes or for a subsequent coating particle target.

Publications

[1] https://doi.org/10.1007/s11837-021-04963-w (open access)

[2] https://doi.org/10.1002/adem.202200288 (open access)

[3] https://doi.org/10.1002/adem.202370025 (open access)

Links

To News article: AEM Cover (follows soon)

To NanoScribe device