Research Within TresClean
Over the first 30 months of the project, partners within TresClean have taken important steps towards developing and upscaling suitable ultrashort laser texturing technologies for producing fluid-repellent antibacterial surfaces. Both self-reorganisation via Laser Induced Periodic Surface Structures (LIPSS) and texturing via Direct Laser Interference Patterning (DLIP) have been exploited to produce hierarchical surface structures with micro- and nano-scale features that have been shown to be superhydrophobic and antibacterial. The project is currently focusing on upscaling of laser technology based on results obtained so far to achieve industrially relevant texturing rates and throughput. This will include testing a newly-acquired 350 W average power ultrashort pulsed laser source and development of a 1 kW average power ultrashort pulsed laser source, high-efficiency frequency conversion chains and high-speed scanning optics.
Work Package 1 - Definition of requirements
Activities performed within Work Package 1 (WP1) had been aimed to collect information and define requirements to be used to reach the final target of TresClean Project. Three main tasks were defined for this WP: Task 1.1 – Customer requirements; Task 1.2 – Requirements for the new parts, laser and scanning systems; Task 1.3 – Test rig and high throughput production requirements.
The main issues correlated to antibacterial and fluid repellent surfaces had been considered in Task 1.1: superhydrophobic surfaces, surface interactions of alimentary fluids, nanoscale roughness and bacteria interaction, superhydrophobic surface texturing. Collected data had been reported in a review deliverable aimed to treat and correlate all the main topics referred to antibacterial/fluid repellent surfaces. Wettability theories, properties of and methods to fabricate superhydrophobic surfaces, bacterial biofilms in food industry, surface properties inflluencing bacterial adhesion, case studies referred to texturing of stainless steels, nanotextured surface characterization had been discussed.
Two reference industrial applications were considered for customer requirements definition, one in the framework of filling packaging machines and one in the framework of white goods, respectively a fillling pipe (reference fluid: milk) in the former case and a dishwater tank (reference fluid: water with low rise agent content), in the latter case.
Mechanisms for bacteria adhesion, proliferation and biofilm formation referred to different bacteria and surface topography patterns (random, regular and hierarchical patterned surfaces) were listed.
Requirements for the new parts, laser and scanning systems had been considered in Task 1.2. Surface energy and topography, in terms of size, shape and spacing/density of the nanostructures, had been correlated to surface wettability and bactericidal activity. Surface structuring for air retention in water submerging components were discussed. Risks, occurence probability and mitigation measures were defined for the laser systems considered in the frame work of TresClean Project referring to the two reference industrial applications. In the case of the filling pipe prototype, application of laser texturing treatment on the body of the stainless steel pipe was considered. In the case of the dishwater tank, application of the laser texturing on the mould was managed.
A test rig, for evaluation of surface properties and high throughput production requirements, was discussed in Task 1.3. The test rig concept design had been completed. A preliminary testing procedure - consisting of samples and chamber cleaning, samples sterilization, spray contamination, samples drying, samples washing and/or sterilisation – and a laboratory procedure for evaluating test samples surface was designed. Three different test sample shapes - flat, cylinder and tube – and two different contamination conditions – static and sliding drop – can be investigated. The system allows the simulation of the in service conditions - fluid deposition, heating, sanitization, sterilisation - and measures the cleanability improvement.
Production requirements had been focused on production of hierarchical structures. To reach the objectives of productivity, acquisition of a high power femtosecond laser was considered (WP4-T4.3).
Reference industrial application: dishwasher tank
Reference industrial application: filling pipe
Test rig for simulation of the in service conditions - fluid deposition, heating, sanitization, sterilisation - and measure of the cleanability improvement
Work Package 2 - Surface Development & Characterization
Work Package 2, led by the University of Parma, has focused on characterisation of ultrashort laser-textured samples in terms of wettability, morphology and bacterial retention. Wettability was firstly quantified in terms of the static water contact angle and sliding angle with the sessile drop method. Morphology of laser-treated samples was then analysed via scanning electron microscopy (SEM), optical profilometry and scanning probe microscopy to quantify areal roughness parameters such as the average areal roughness, skewness, kurtosis and density of peaks. Bacterial retention tests were then performed on surfaces based on international standards for anti-bacterial performance.
Laser-textured samples were found to achieve reductions of up to 99.8 % in E. coli retention and 84.7% in S. aureus retention compared to control samples, which had been chosen to reflect current industry practises for food handling and packaging equipment. These results represent vast improvements over standard stainless steel surfaces and have huge implications for the future of food handling and other industries in which biofouling and bacterial contamination are problems. The ability of ultrashort pulsed laser treatment to achieve this outcome is based on two main phenomena. The first is water repellency, or superhydrophobicity, reducing contact between the fluid responsible for transporting micro organisms and the surface itself. The second is an extremely fine surface topography that is smaller than bacterial cell size, thus reducing available contact points. The intended effect is a “bed of nails” that does not permit cell adhesion. These principles have been exploited to design laser surface treatments within Work Package 2 that can passively inhibit bacterial adhesion and therefore biofilm formation.
Further information relating to the scientific aspects of Work Package 2 can be found in publications by members of the TresClean consortium.
Work Package 3 - Surface Engineering: Anti-microbial Texturation
Work package 3, led by ALPhANOV technological center for optics and lasers, has focused on the basic process development for the realization of different surface topographies to optimize the anti-microbial function of the surface. For this, different processes, so called LIPSS (texturing by Laser Induced Periodic Surface Structures) and DLIP (Direct Laser Interference Patterning) have been investigated.
Texturing by LIPSS generation is a well-known one-beam direct texturing process which allows to produce regular homogeneous structures on the surface of several materials (metals, dielectrics, ceramics and composites) with structure size from a few hundreds of nm up to tens of µm. The minimum obtainable feature size is strongly correlated to the laser wavelength of the irradiating laser light: for irradiation with linear polarized light at laser fluence around the threshold fluence for the material surface modification, periodic parallel lines are produced on the surface with a periodicity slightly smaller than the laser wavelength and orientation perpendicular to the laser polarization direction. The final geometry of the surface pattern can be further modified by using non-linear laser polarization, as for instance circular, azimuthal or radial, this way achieving the formation of so-called 2D LIPSS. Increasing the cumulative energy absorbed by the surface, thermal effects bring to the formation of larger grooves and finally to the formation of the so-called spikes, characterized by structure size larger than a few µm. Fig.1 shows (top) a scheme of a typical texturing setup and (bottom) SEM images of representative surface structures for LIPSS, 2D LIPSS and spikes upon IR laser light irradiation. These structures, as well as UV-LIPSS, were characterized in WP2 to define their wettability and antibacterial properties. The most performing structures, that is LIPSS and 2D LIPSS produced by IR laser light, were found to achieve reductions of up to 99.8 % in E. coli retention and 84.7% in S. aureus retention compared to control samples.
Fig 1: (top) Scheme of a typical texturing setup and (bottom) SEM images of representative surface structures for LIPSS, 2D LIPSS and spikes upon IR laser light irradiation
Direct Laser Interference Patterning (DLIP) is a method for direct ablation of periodic structures on material surfaces, with a periodicity near the wavelength. These structures are produced by periodic interference pattern, which occurs due to the overlap of coherent beams with an angle of incidence. For this project an ultra-short pulsed laser capable DLIP-setup was designed and build up (Figure 2 (a)). With this setup the period, polarization orientation and size of the interference pattern are variable. This setup was presented at Lasers in Manufacturing 2017 .
Within the investigation of the DLIP process, a dependence of structure geometry on the polarization orientation was observed. Figure 1 (b) and (c) show two different SEM images of two structure geometries, dependent on polarization orientation. All laser and process parameters between these two geometries stay the same, except of a polarization orientation difference of 90°.
 A. Peter, V. Onuseit, C. Freitag, T. Graf, „Flexible, compact and picosecond laser capable four-beam interference setup“ Proceedings of Laser in Manufacturing 2017 (2017)
Figure 2: Direct Laser Interference Patterning setup (a) and SEM images of the resulting 2D surface structures on steel (b), (c). Structures were produced with ultra-short pulsed laser with a wavelength of 1030nm and <10ps pulse duration. Dependent on the orientation of the polarization, different structure geometries are possible. Cone structure (b) and hole structure (c).
WP4 - Implementation of high-powered lasers
Work Package 4, led by the University of Stuttgart, is dedicated to the further development of an Yb: YAG thin-disk multipass amplifier delivering 1kW at 1030 nm, which is already available at the University, to bring it to a usable level for the demonstration of a high-throughput laser-based manufacturing process. The main tasks in this work package are to work on new mechanical and optical concepts to improve the overall stability of the system e.g. beam pointing and power stability to allow reliable material processing. This work package will also focus on the development of a concept to modulate the laser beam at high average and peak powers in order to allow pulses on demands at the work piece.
Photo: Array of 60-mirrors used in the TD-MPA.
Work Package 5 - Developemnt of High-Speed Scan Head and Controller for Surface Structuring
Work Package 5, led by Raylase GmbH, has focused on the development of a new scanner controller and a high-speed scan head.
Surface structuring based on LIPPS and DLIP methods with ultrashort pulsed lasers have been already demonstrated by the scientific community. But structuring rates have been slow so far (< 5 mm²/s). To upscale the LIPPS and DLIP surface processing methods from R&D level to a high throughput production of large scale surfaces, innovative high-power USP laser sources and ultra-high speed scanning solutions are needed.
The ultra-high speed scanning solution is based on a newly developed 2D-scanning unit containing 1 polygon wheel plus 2 galvanometer scanners and a newly developed scanning controller which is able to synchronize USP lasers with the scanning movement of the polygon wheel. Scanning speeds from 35 – 275 m/s (f-340) and 65 – 525 m/s (f-640) are possible. Laser power up to 1 kW (pulse energy 3,3 mJ, repetition rate 300 kHz, pulse width 5-10 ps) or 350 W (pulse energy 50-70 µJ, repetition rate up to 13 MHz, pulse width 500 -1000 fs) can be coupled into the 2D scanning unit with a full beam diameter of 15 mm.
Due to the support of a wide range of scanning speeds in combination with repetition rates from 300 kHz up to 13 MHz, surface structuring with variable pulse overlap can be achieved in order to upscale the throughput of the DLIP and LIPPS processing methods. First units are available now and ready for high-throughput tests.
Further developments and tests with beam shaping modulators to convert the Gaussian profile of the lasers into a flat top beam profile are scheduled till end of 2018.
WP6: High Throughput Application: Upscaling
Work Package 6 led by the University of Stuttgart has focused on developing methods for the upscaling of surface texturation. Side effects due to the high average power like particle sheidling, heat accumulation and massive dust created have also been evaluated.
Influence of the applied fluence:
Figure 1: Propagation of particxles formed immediatley after applying a laser pulse for a mean fluence of 0.95 J/cm2 (a-d), 4.74 J/cm² (e-h), and 45 J/cm² (i-l). The temporal distance is 60 ns and the illumination time of each recordinmg is 30 ns. The recroding strated 0 ns (a,e,i), 60 ns (b,f,j), 120 ns (c,g,k), and 180 ns (d,h,l) after applying a laser pulse. Laser parameter: λ = 800 nm, τ = 0.35 fs.
Figure 1 shows the propagation of the particles formed immediately after applying a laser pulse on the surface of the workpiece for a pulse duration of 0.35 ps and different mean fluences. The higher the applied mean fluence H the more particles are formed. The velocity of the particles can be higher than 1000 m/s.
Influence of the applied pulse duration:
Figure 2: Propagation of particles formed immediately after applying a laser pulse for a pulse duration of 10 ps (a-d), 1 ps (e-h), and 0.35 ps (i-l). The temporal distance is 60 ns and the illumination time of each recording is 30 ns. The recording started 0 ns (a, e, i), 60 ns (b, f, j), 120 ns (c, g, k), and 180 ns (d, h, l) after applying a laser pulse. Laser parameter: λ = 800 nm, H = 45 J/cm².
Figure 2 shows the propagation of the particles formed immediately after applying a laser pulse on the surface of the workpiece with a mean fluence of H = 45 J/cm² and different pulse durations. For a pulse duration of 10 ps and 1 ps the ablation mechanism is similar. This is attributed to the electron-phonon relaxation which was calculated for steel to be 0.5 ps . For a pulse duration below the electron-phonon relaxation more particles are formed which are also faster. When more particles are formed more ablation may be assumed. This was experimentally validated .
1. D. Breitling, A. Ruf, F. Dausinger, in , ed. by P.R. Herman, J. Fieret, A. Pique, T. Okada, F.G. Bachmann, W. Hoving, K. Washio, X. Xu, J.J. Dubowski, D.B. Geohegan, F. Traeger (SPIE, 2004), p. 49
2. T. Häfner, J. Heberle, M. Dobler, M. Schmidt, Journal of Laser Applications 28, 22605 (2016)
Validation of heat accumulation:
The derivation of the model can be found in [1, 2]. The model was further developed within the TresClean-project to predict the process-resulting surface structure , where the following content is described in detail.
According to  a surface covered with laser-induced periodic surface structures (LIPSS) is formed when the maximum temperature increase during processing is below 585 K (when the room temperature is 22 °C). When the temperature increase exceeds 585 K micro-grooves are formed. When the temperature increase during processing leads to a surface temperature above the melting temperature the resulting surface structure has to differ.
Figure 1 shows the calculated temperature increase for all applied laser parameters (see caption of Figure 1). By applying different feed rates, heat accumulation due to successive pulses (within one peak in Figure 1) as well as due to passes (between peaks in Figure 1) was varied resulting to a different maximum temperature increase during processing.
Figure 1: Calculated temperature increase caused by HAP and HAS-I as given by Eq. (3) for a point on the surface of polished (Sa = 0.2 µm) AISI 316L as a function of time processed with a mean fluence of 0.71 J/cm² per pulse and different feed rates. Further parameters: ηabs = 0.55, ηheat = 0.38, ρ = 8000 kg/m³, cp = 500 J/(kg*K), κ = 3.75*10-6 m²/s, f = 300 kHz, db = 500 µm, Npasses = 8.
Figure 2 shows SEM-images of the corresponding surfaces. The analytical results agree very well with the experimental results.
Figure 2: LIPSS on polished (Sa = 0.2 µm) AISI 316L formed by processing with a mean fluence of H = 0.7 J/cm² per pulse and the different feed rates of 20.0 m/s (a), 10.0 m/s (b), 5.0 m/s (c), and 1.0 m/s (d). Further parameters: λ = 1030 nm, f = 300 kHz, db = 500 µm, dℓ = 62.5 µm, Npasses = 8.
1. R. Weber, T. Graf, P. Berger, V. Onuseit, M. Wiedenmann, C. Freitag, A. Feuer, Optics express 22, 11312 (2014)
2. R. Weber, T. Graf, C. Freitag, A. Feuer, T. Kononenko, V.I. Konov, Optics express 25, 3966 (2017)
3. S. Faas, U. Bielke, R. Weber, T. Graf, Applied Physics A 124, 612 (2018)
4. F. Bauer, A. Michalowski, T. Kiedrowski, S. Nolte, Optics express 23, 1035 (2015)
Different process conditions were tested to achieve a set of cleaning parameters able to ensure a functional cleaning to be adopted for the measurements of dirty areas.
The results for different foaming times are shown in the following figures.
Corrosion tests were performed on laser textured surfaces of AISI 316L samples to evaluate if texturing could affect corrosion properties of stainless steel. The images show SIMUCORR plant used to perform the test in NaOH. SEM analysis of the textured samples supplied information regarding corrosive phenomena eventually occurring on the surfaces.
Automatic test rig
Research activities of work package 2 also focused on the set up of the test rig protocol. These activities were fundamental for the validation of the surface treatments developed in the project regarding the cleanability of surfaces. Steel samples were used for the procedure optimisation.
Standard clean in place (CIP) procedures, adopted on industrial plants, were the reference to estimate the process parameters, affecting the cleanability, to be optimize.
A specific Image analysis technique has been developed and adopted to evaluate the cleanability of surfaces.
WP7 - Productivity Demonstrator
WP7 is coordinated by BSH Home Appliances, and is focused on the final demonstration. The objective of this WP is the validation of the structures defined in the project and the new developed laser components according to the real market requirements in the Home Appliances and Liquid Food Packaging fields. The validation is focused on the Home Appliances parts injected using a mould structured, and the Liquid Food Packaging machine parts from ECOR in the pilot installation constructed in the other WPs. For this purpose, BSH is constructing the replication of the original injection mould that would be structured, prototyped and studied in the laboratory.
The demonstration will focus on three major steps:
Structuring of defined demonstration parts with the developed processes from WP6.
Validation of parts at laboratory scale: the antibacterial properties of parts produced by an industrial process will be fully tested and characterised in order to ensure the property transfer. This will include both injected and directly structured parts.
Life cycle and durability testing by the assembling of the parts on industrial machines and home appliances, following the pre-defined protocols that are currently used in the production sites.
At the end of the process, a final validation report will compile all this information.