Laser cleaning of insect residue with a TEA-CO2 -laser in shockwave regime

doi:10.21203/rs.3.rs-5267684/v1

Nowadays, much effort is put into the development of laminar flow technologies for next generation aircrafts creating laminar flow in areas around the leading edge of wings of an airplane. Laminar flow reduces drag at aero plane wings thus reducing environmental pollution and costs due to saved kerosene. Nevertheless, three dimensional disturbances like insect residues, if reaching a threshold height, cause a breakdown in laminar air flow turning it into a turbulent one. Consequently, these insect residues need to be removed repeatedly. Therefore, insect residues were artificially applied on 1.4544.9 steel and Titanium Grade 5 and removed using a TEA-CO₂ laser in multiple cleaning cycles. The cleaned area was investigated in terms of process influence on the base material using micro hardness measurements, CLSM and SEM imaging of the topography and grain structure. This paper presents a way of significantly reducing (by 84%) the inhomogeneously distributed insect residue without any identified alteration of the base material regarding hardness, topography, or grain structure.

Laser beam machining (LBM)

Material removal

Laser shock wave cleaning

Studies about fuel consumption found that aviation is one of the main contributors to global greenhouse gases, accounting for 5% [Lai22]. Civil aviation contributed to this with a consumption of 360 billion liters in 2019 [Iat23]. At a time when climate change and the resulting social and environmental challenges are of great global concern and one of the greatest issues in the 21st century [Die20], immense efforts are being made to reduce this consumption through new technologies. One of these technologies is laminar airflow technology. Research demonstrates that if this technology is applied to the surface of an airplane wing not only the drag is reduced but simultaneously airframe aerodynamic efficiency increases and thus the overall fuel consumption decreases [Kok15]. In order to develop its potential, the technology requires a defined surface condition, which must be maintained at all times or at least restored repeatedly. A repetitive cleaning is needed since insects adhere to the airplane’s surface forming three dimensional disturbances during the flight and especially during the take-off and landing. The latter two phases are characterized by the highest insect population density [Kok15] and are therefore predestined for a failure of the technology due to large accumulations (above 100 µm in height [Pet78]) of insect residues on the surface.

Such contaminants can be cleaned conventionally with cleaning agents and a brush. This is time-consuming and the cleaning agents used are also hazardous to the environment and the user's health due to the chemicals they contain. An alternative to this is laser-based cleaning. It can be accomplished via pulsed laser in three typical ways. Firstly, dry laser cleaning that bases on the thermal expansion of the substrate in a short time. Consequently, a lifting force arises spraying residues from the substrate surface. The disadvantages of this process are the rapid thermal expansion of the substrate and the local matrix ablation. [Che20]. Secondly, liquid assisted laser cleaning that bases on the creation of a pressure wave generated by overheating and explosive evaporation of auxiliary liquid film. The high-pressure wave then separates the dirt from the substrate surface [Mos20]. However, the use of an auxiliary liquid film makes it difficult to clean surfaces that require working overhead. Thirdly, laser-based shockwave cleaning. This method overcomes the shortcoming of the former mentioned processes [Zhu22] and bases on the creation of a rapidly expanding plasma, generated at the focus, causing a plasma shockwave generated above the particles through the air breakdown phenomenon. It is therefore a dry, non-contact and large-area cleaning technology [Jan²21].

Hence, this study deals with the feasibility of shockwave-based laser cleaning of insect residue with a pulsed CO₂-laser on 1.4544.9 stainless steel and Titanium Grade 5. It is hypothesized that this cleaning method removes adhered residue and without the alteration of the surface. Therefore, the cleaning results and the surfaces are investigated using microscopy (reflected light microscopy, confocal laser scanning microscopy (CLSM) and, scanning electron microscopy (SEM)) and micro indentation measurements.

2.1 Shockwave based laser cleaning

Shockwave-based laser cleaning of insect residue was performed using a pulsed nanosecond TEA-CO₂-laser. Its properties are listed in Table 1.

Table 1

Properties of the laser performing shockwave based laser cleaning
type	ML105E by SCLR Lasertechnik GmbH	pulse frequency	10 Hz
wavelength	10600 nm	spot geometry	rectangular
pulse duration	100 ns

The beam was focused by concave mirror onto the targets’ surface. A xy-table of type KT310-200 produced by Steinmeyer-Mechatronik, where the target was placed on, enabled the movement of the target underneath the laser beam. Table 2 summarizes information on the scanning strategy to clean the polluted surfaces.

Table 2

Properties of the scanning strategy in order to clean the polluted surfaces.
pulse overlap	33%	geometry of hatch	45 mm x 45 mm
path overlap	30%	number of passes	1; 2; 3
hatch	bi-directional, parallel lines

2.2 Generation of insect residue

The insect residue on the targets’ surfaces was artificially created and consists of a single flesh fly that was anesthetized and subsequently shot onto the surfaces. Table 3 summarizes information on the process.

Table 3

Information on artifically creating insect residue on targets’ surfaces
insect velocity	70 mm/s to 90 mm/s
angle of incidence	45°
distance between nozzle and target’s surface	300 mm

2.3 Microscopy

The documentation of the cleaning process was done by an optical microscope using 25-fold magnification and stitching of single images to provide an overview of the initial and afterwards radiated area. More detailed images of characteristic locations on the surface were taken using 50-fold magnification. This kind of images were taken using an Olympus BX53M microscope.

The evaluation of the roughness regarding areal, volumetric, and functional parameters was performed using a VK-X 300 confocal laser scanning microscope made by Keyence at 10-fold magnification. Single images of the former identified characteristic locations were stitched to one. Subsequently, the MultiFileAnalyzer software from Keyence was used to evaluate the images regarding the arithmetic mean height (Sa), the maximum peak height (Sp), and material peak volume (Vmp).

2.4 Materials

On the one hand the stainless steel 1.4544.9 (X10CrNiTi18-9) and on the other hand Titanium Grade 5 (Ti6Al4V) were used as targets’ material. Both are commonly used in the aerospace industry. Their chemical composition is depicted in Table 4 according to DIN EN 10088-3:2014-12 [Din14] (1.4544.9) and DIN 17860:2023-10 [Din23] (Titanium Grade 5).

Table 4

Chemical composition of 1.4544.9 [Din14] and Titanium Grade 5 [Din23]
1.4544.9	Cr in mass-%	C in mass-%	Ni in mass-%	Ti in mass-%	others
1.4544.9	15.0–17.0	max. 0.07	3.0–5.0	max. 0.7	bal.
Titanium Grade 5	Al in mass-%	V in mass-%	Fe in mass-%	C in mass-%	others
Titanium Grade 5	5.50–6.75	3.50–4.50	max. 0.40	max. 0.08	bal.

Figure 1 on the left (1.4544.9) as well as Fig. 2 on the left (titanium grade 5) depict an inhomogeneous distribution of artificially created insect residue on the surface prior to shockwave-based laser cleaning with a pulsed CO₂-laser. In both cases, the residue can be distinguished into three different contaminants by color: the exoskeleton (insect shell), the organs and the hemolymph. They are concentrated in several locations of which three are marked on 1.4544.9 (Fig. 1 on the right) and four on titanium grade 5 (Fig. 2 on the right).

The results of the first cleaning cycle on 1.4544.9 and titanium grade 5 are depicted in Fig. 3 and Fig. 4 respectively. It seems that the cleaning was qualitatively more pronounced on stainless steel than titanium as the comparison of both results suggests. Even a slight spreading of the insect residue seems to have happened on titanium if compared to the initial state in Fig. 2. On stainless steel, on the other hand, the cleaning removed the insect residue nearly completely. Nevertheless, in both cases the exoskeleton is removed while hemolymph and organs remain on the surface.

A quantitatively view on the cleaning process provide Fig. 5 for the cleaning of stainless steel and Fig. 6 for titanium grade 5. On the right side of both figures specific surface roughness parameters are depicted in dependency of the surface state like the initial state before cleaning, after the first cleaning cycle or after the second. The right side of each figure shows height images in false colors.

The impression that the cleaning on stainless steel is more pronounced is quantified based on the relative reduction of the insect residue in terms of roughness parameters Sa, Sp and Vmp (Fig. 5 and Fig. 6). While Sa decreases between 73% and 97% after the second cleaning cycle on stainless steel, it decreases between 63% and 97% on titanium. The maximum peak height is even more reduced on stainless steel: between 87% and 94%. On titanium, on the other hand, the cleaning ranges between 61% and 92%. In absolute values, the residue is reduced by 604 µm (94%, 1.4544.9) respectively 672 µm (92%, titanium grade 5) in its maximum peaks. The cleaning effect could be measured to 96–98% (stainless steel) and to 71–83% (titanium grade 5) of the peak material volume Vmp.

According to the demonstrated results, the material of the probes seems to have an influence on the cleaning results. An additional influence could result from the moisture of the adhered residue. Therefore, two probes were dried at normal atmosphere for different periods. The results of shockwave-based laser cleaning of these two probes are shown in Fig. 7. The left image depicts the result of a polluted surface that was exposed to normal atmosphere for less than five hours while the right image shows the result of a cleaned surface that was exposed to normal atmosphere for more than three weeks. As it is demonstrated, the differences in the cleaning results are marginal and therefore can be neglected. Apart from the cleaning effect, it is also important that the surface is not altered in any way by the laser irradiation, the plasma, or the shockwave. Therefore, SEM images of the grain structure and CLSM images of the surface topography (Fig. 8: surface topography, Fig. 10: grain structure) were taken and analyzed and micro hardness measurements (Fig. 9) were performed.

In case of the micro hardness measurements, it can be said that for both materials there is no difference in the indentation hardness HIT and indentation modulus EIT under consideration of the standard deviation between a non-irradiated location and the irradiated surface. This applies also for the surface topography in Fig. 8. The plasma and the shockwave do not lead to a noticeable alteration of the peaks and valleys which form the relief of the surface. The arithmetic mean height Sa as well as the maximum height Sz are equal within the standard deviation as Table 5 shows. The depictions of the grain structure of each material in Fig. 10 before and after it is irradiated via laser show also no sign of alteration or influence of the plasma or the shockwave on the grains and their structure.

Table 5

Arithmetic mean height Sa and maximum heigt Sz before and after shockwave-based laser cleaning of insect residue from different metallic surfaces after the second cleaning cycle
	1.4544.9		Titanium Grade 5
	before cleaning	after cleaning	before cleaning	after cleaning
Sa	(0.33 ± 0.01) µm;	(0.35 ± 0.02) µm	(0.46 ± 0.05) µm	(0.50 ± 0.02) µm
Sz	(7.48 ± 0.53) µm	(8.85 ± 2.69) µm	(5.58 ± 0.43) µm	(5.73 ± 0.17) µm

The results of this study provide clear evidence that neither the laser pulse nor the following plasma and shockwave alter the mechanical properties in terms of indentation hardness and modulus, the surface topography or the grain structure of 1.4544.9 stainless steel and titanium grade 5. The primary objective was to demonstrate the feasibility of shockwave-based laser cleaning of insect residue on metallic surface with a pulsed TEA CO₂-laser.

While other laser-based cleaning processes like dry laser cleaning and liquid-assisted laser cleaning come with disadvantages like surface ablation and handling problems while cleaning downward facing surfaces, shockwave-based laser cleaning is reported in the literature to be directly applicable to a polluted surface and without any liquids or noticeable ablation. Though ablation is avoided in dry laser cleaning, it is mentioned that a change of the surface in terms of topography and near-surface areas regarding grain structure occurs [Li23]. This is contrary to the findings in this study, which clearly shows that neither the surface topography (Fig. 8) and the micro hardness (Fig. 9) nor the grain structure (Fig. 10) are altered. The possibility of shockwave-based cleaning of surfaces and simultaneously avoiding any surface damage is also shown by [Kum14] but with a different experimental setup. There, a laser beam propagates parallel to the surface to initiate the plasma. Furthermore, it is presented that the cleaning efficiency depends on the gap between the laser focal spot and the target’s surface, the pulse energy and the number of exposures. The latter proves the need of multiple cleaning cycles to fully restore a clean surface though a remarkable cleaning effect can also be achieved within the first cleaning attempt (Fig. 3). Alternatively, the use of a geometrical confinement during laser-induced shockwave creation increases significantly the intensity of the shockwave created and proves to remove particles from a surface better than a process without the confinement [Jan09]. The idea of a geometric confinement resulted in a so-called pressure reflection cell which was used in [Czo18] for the same purpose: significant increase in laser-induced shockwave intensity. According to the literature, it can be assumed that the process has a dual cleaning effect. On the one hand, the mechanical effect caused by the shock wave and on the other, a thermal effect due to the plasma and pulse energy. Studies on the thermal effect of laser radiation and plasma on targets have shown that there is a significant increase in target temperature of around 200°C [Czo18]. Depending on the plasma temperature and the duration of the plasma the evolving surface temperature could be even higher [Oh08]. This would likely lead to damage of the surface but is not the case here. A third influence from the laser beam cannot be ruled out at this point either. In comparison to other test arrangements (for example in [Li23]), the laser beam is arranged parallel to the surface. In this study, however, the laser beam is perpendicular to the surface. It is therefore assumed that the laser beam reaches the target’s surface before the plasma shields it and absorbs a large amount of the incident beam. It therefore remains to be clarified in future work what the dominant effect is when cleaning surfaces contaminated by insects to increase process efficiency based on process understanding and to be able to clean thermally and mechanically sensitive surfaces such as films and paints.

Nevertheless, the shockwave-based laser cleaning of insect residues on metal surfaces was presented as a process that has a significant cleaning effect in the first cleaning cycle without altering or damaging the surfaces.

The present work deals with the shockwave-based laser cleaning of artificially generated insect residues on stainless steel and titanium in multiple cycles using a pulsed CO₂-laser. The irradiated area was investigated regarding an influence of the base material using micro hardness measurements as well as CLSM and SEM imaging of the topography and grain structures.

The results demonstrate that the inhomogeneous distributed insect residue could be in terms of the material peak volume Vmp significantly reduced by 85% in terms of stainless steel and 73% regarding titanium without any identified alteration of the micro hardness and topography of the surface or even the grain structure of the material below. The present work thus provides the basis for a laser-based cleaning procedure that allows regularly repeated cleaning of polluted metallic surfaces without surface alteration or surface ablation.

Ethical Approval

Not applicable.

Funding

The project Ultra Performance Wing (UP Wing, project number: 101101974) is supported by the Clean Aviation Joint Undertaking and its members.

Co-Funded by the European Union. Views and opinions are however those of the author(s) only and do not necessarily reflect those of the European Union or Clean Aviation Joint Undertaking. Neither the European Union nor the granting authority can be held responsible for them.

Conflict of Interest

I declare that the authors have no competing interests as defined by Springer, or other interests that might be perceived to influence the results and/or discussion reported in this paper.

Availability of data and materials

Not applicable (this manuscript does not report data generation or analysis).

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No competing interests reported.

Laser cleaning of insect residue with a TEA-CO2 -laser in shockwave regime

Status:

Version 1

Abstract

Figures

Introduction

Methodology

2.1 Shockwave based laser cleaning

2.2 Generation of insect residue

2.3 Microscopy

2.4 Materials

Results

Discussion

Conclusion

Declarations

References

Additional Declarations

Status:

Version 1