Adding depth to surfaces

© LFM Laser-Zentrum Münster

Microstructuring of materials using brilliant laser light is one of the key topics now being investigated by the Laser Center (LFM) at the Münster University of Applied Sciences. The center’s founder and director, Professor Klaus Dickmann, specializes in structuring surfaces and lending them new functions.

past / From a barebones tool to a diagnostic system

2009. After a decade of research, the ultra-short pulsed (USP) laser was about to experience an industrial breakthrough at the Laser World of Photonics — the world’s leading trade fair for the laser and photonics industry, held in Munich. Professor Klaus Dickmann had come to the exhibition seeking the best USP laser for his laboratory. At that time, USP still had an almost mythical status — a tool that could process material without heating it up. Dickmann had his eye on the brand new TruMicro 5000 series of lasers made by TRUMPF. Using extremely short laser pulses lasting just 0.00000000001 second, the TruMicro 5000’s laser beam could vaporize material in tiny, highly accurate portions, leaving no residue behind.

“Ultra-short pulsed lasers? From my very first encounter I saw them to be the perfect tool for micropro­cessing materials and structuring surfaces.”

Prof. Klaus Dickmann

TruMicro Serie 5000

The lasers of the TruMicro series 5000 are ultra-short pulsed lasers with power of up to 100 watts and pulse energy of up to 250 microjoules. The extremely short pulses, of less than 10 picoseconds vaporize nearly any material so quickly that no heat affected zone can be detected.

Münster. The TruMicro 5000 arrived at the LFM’s 600 square meter lab facility as a barebones system, a beam source in its purest state. But Prof. Dickmann had a whole lot more in mind for the empirical processes at his laboratory in Steinfurt. Conducting research with lasers means running through a virtually endless series of tests, especially if you are dealing with exotic materials. And that’s precisely where USP lasers really come into their own. The team led by Dickmann is familiar with just about every type of material, including copper, stainless steel, piezo ceramics, glass and many, many more.

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Modifying the surface of stainless steel at a structural level by varying laser parameters — in this case, laser fluency. This makes it possible to achieve different functional effects. (Photo: LFM Laser Center Münster)

Dickmann and his team got to work on transforming the beam source into a rather intelligent production system — boasting special optical features, technical optimization of the control system, and an in-line monitoring system. They even added high-resolution cameras to detect and correct processing deviations during processing.

And they never lost sight of their primary goal: to investigate the capabilities of ultra-short laser radiation in real-life applications and to channel their findings into industrial practice. This would ultimately enable clients to select the perfect production parameters so as to achieve optimum results in their manufacturing process — in terms of both quality and quantity.

“Automation? That’s just as essential in research as it is in other fields. By bringing industrial processes into the lab setting, we can achieve valid results faster.”

Prof. Klaus Dickmann

The empirical approach to the objective involved plenty of automation — for tasks such as gradually shifting the laser fluency or energy density. The sample surface was divided into individual fields, creating separate segments, which were then processed with different beam parameters. The system painstakingly documented all the parameters and results to ensure that the test setup could be replicated. And it generated results that would go on to prove their worth in industrial settings.

present / Searching for applications

So who benefits from this research? Applications for medical technology are inevitably right at the top of the list. These range from film perforation and microcomponents for medical instruments to absorbable coronary stents that can be produced at significantly higher quality levels using ultra-short laser pulses. But the application that reigns supreme in Dickmann’s mind is the adding functions to parts of surfaces in combination with microstructuring — and the process of generating microstructures using picosecond pulses (ps pulses) has already been implemented on an industrial scale.

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Generating a pyramid in copper through layer-by-layer rendering of a 3D model (Photo: LFM Laser Center Münster)

But there is so much more that is possible when researchers apply their imagination. For example, a picosecond laser can also be used to lend a functional structure to surfaces. This might involve a “lab-on-a-chip”, a tiny laboratory the size of a fingernail, which acts as a blood analysis platform. After taking a blood sample, it channels it to a tiny testing station and immediately displays the results.

Inside the mini-laboratory, the beam of a picosecond laser has carved hydrophilic channels in a hydrophobic environment. Interfacial forces drive the fluid sample along these channels to target destinations in the various reaction and analysis zones.

By now the researchers at the LFM can not only measure the flow resistance; they have also succeeded in setting all the parameters with absolute precision. This opens the door to multiple applications in medical diagnostics. A fascinating effect already inspiring researchers — and which will bene fit manufacturers in the future — is the ability of materials to form self-organized microstructures under the influence of ultra-short laser pulses. The size and shape of the nano-ripples and periodic microstructures produced in this manner have a direct impact on the properties of the surface and can be controlled by modifying the laser parameters. The effect of material-specific self-organization under the influence of ps laser irradiation can further simplify the task of structuring functional surfaces. Researchers at the LFM laboratory are now busy investigating what exactly happens when you vary the laser parameters.

“Why niche applications? Because that’s where interes­ting things are happening and where we can make our mark. And that’s why we’re not focusing on photovoltaics, because that market is very crowded right now.”

Prof. Klaus Dickmann

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Laser structured and functionalized microchannel with hydrophobic (left) and hydrophilic (right) surface properties (Photo: LFM Laser Center Münster)

One example of a niche application is the work currently being carried out by researchers in the field of tribology. For example, microscopically small lubrication pockets or freestanding microcones can reduce frictional resistance in mechanical drive systems and enhance performance. LFM researchers are seeking the most suitable parameters to optimize the speed and quality of the process. Microsieves and filters are another promising field. The LFM specializes in developing methods used to produce microfiltration membranes from stainless steel.

These microsieves are then used in biomedical and food technology, for instance. The primary goal in this case is to understand exactly how the process works and determine how USP laser radiation interacts with a wide range of different materials. Researchers hope that the results of these studies will lead to enhanced drilling speeds and quality at reduced hole diameters.

“Patents? They require tons of work and often clog up the process rather than yielding benefits. That’ s why we’re not focused on patents. We’re more excited about showing people the fascinating new technical possi­bilities for using the laser as a tool.”

Prof. Klaus Dickmann

LFM Laserzentrum FH Münster

The laser center is a facility within the department of Physical Science and Technology of the University of Applied Sciences Münster. The modern equipment is used for teaching and the realisation of research projects and industrial contracts. Various laser systems are located on 600 m² of laboratory space, complemented by an extensive analysis technology.

The LFM is also hard at work at the boundary between picosecond and femtosecond processing. They have already completed a project on microprocessing temperature-sensitive components. That was done in collaboration with the University of Applied Sciences at Mittweida. A novel ps laser system with an optical scanner was developed for the study, catering to several microstructuring processes using a variety of metallic and dielectric materials.

A chromatic sensor first recorded the topography. The team then used a scanning electron microscope to analyze aspects of specific applications. At the end of the study the researchers compared their findings with those of parallel studies on femtosecond laser processing, carried out at University in Mittweida.

The results were astonishing. With parameters optimized specific to the application, ps pulses achieve a level of quality that in many applications is barely distinguishable from the quality achieved by femtosecond processing. The project results made it possible to accelerate ablation processes for ps laser processing by up to 400 percent and significantly increase the level of precision.

forecast / Virtually unlimited opportunities

Material processing with ultra-short pulses has been gaining ground for a long time, making its way into more and more high-volume production applications and supplanting conventional methods such as mechanical drilling, EDM and chemical etching. The laser offers multiple benefits — it can be configured with absolute precision to ensure reliable processes, and it is often the only tool that provides the option of selective processing.

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Functionalized matrix on a stainless steel surface, intended to fix water droplets in place (Photo: LFM Laser Center Münster)

It also offers potential in fields such as security technology by incorporating counterfeit-proof structures inside materials, for example. Right now the LFM is working on the topic of beam forming for USP lasers. The team figures that it can optimize processes by using adaptive optics to vary the geometries of the focus spot, exploiting the fact that the material is photonically activated in a different way. The researchers hope this will enable them to achieve faster process rates.

The LFM is also involved in fascinating work for the LACONA (Lasers in the Conservation of Artworks) organization. Every two years LACONA organizes a scientific exchange between restoration experts and laser physicists. Klaus Dickmann has been a permanent member of the international conference committee for a number of years. He and his team use the appropriate lasers to restore valuable parchment and old frescoes.

“Limitations? What limitations? As Peter Leibinger said: ‘With the the ultra-short pulse laser we’ve opened a door into a new realm — and we won’t know its precise extent or all its details for a very long time’.”

Prof. Klaus Dickmann

The key challenge continues to be the task of discovering new applications. Thanks to the non-linear absorption processes, ps pulses can be used to process transparent materials, too. And the new class of coating systems and composites offers as much food for researchers’ imaginations as does the structuring of ceramic surfaces.

microstructuring-laser-surface(2)Prof. Klaus Dickmann, D.Eng., is the founder and director of the Laser Center at the University of Applied Sciences at Münster (LFM). He studied electrical engineering and physics and obtained his doctorate from the Hannover Laser Center (LZH), in the Institute of Production Engineering and Machine Tools (IFW) at the University of Hannover. His dissertation was on “Using Lasers to Cut Sheets of Electrical Steel.” He not only focuses on research into material processing with the laser; he is also passionate about safety. As a publicly appointed and certified expert in laser safety and laser technology, he spends much of his time working on unresolved safety issues, including those relating to ultra-short pulsed laser radiation.
www.fh-muenster.de/laserzentrum

Contact

Prof. Klaus Dickmann
Universität Münster
phone: + 49 (0) 2551 9 – 62322
dickmann@fh-muenster.de

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