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Microstructuring with ultrashort-pulse lasers: Faster or smoother?

High-power picosecond lasers ablate, clean, and polish metal surfaces better than a nanosecond laser.

FIGURE 1. A 2 ps laser (blue) removed about as much steel 1.2738 as the 400 ns benchmark laser in the single pulse regime; a 10 ps laser (red) never achieved comparable ablation results.
FIGURE 1. A 2 ps laser (blue) removed about as much steel 1.2738 as the 400 ns benchmark laser in the single pulse regime; a 10 ps laser (red) never achieved comparable ablation results.

ANDREAS BRENNER and ANDREAS THOSS

LOLS10在线直播下注So far, it is well known that ultrashort-pulse (LOLS10在线直播下注P) lasers can reach highest precision in microstructuring, but they are too expensive, too slow, or just too exotic. That may change with the new generation of high-power LOLS10在线直播下注P lasers. With average powers of 100 W or more, they enable more throughput while maintaining their precision. That is good news for the tooling and mold industry, where steel tools have to be processed with very high precision.

LOLS10在线直播下注Still, finding the right settings for LOLS10在线直播下注P lasers requires some understanding of processes and laser parameters. With the right parameters, you can turn more LOLS10在线直播下注P power into higher efficiency until they ablate even more than a nanosecond laser does. To find out more about optimum settings for LOLS10在线直播下注P laser processes, experts at the Fraunhofer Institute for Laser Technology (Fraunhofer ILT; Aachen, Germany) compared microstructuring results from nanosecond lasers with those from picosecond lasers. They found that the LOLS10在线直播下注P lasers can be 4X faster or they can reach a 4X smaller surface roughness with similar efficiency, if the right process parameters are chosen.

Further, the experts developed optimized parameters to use LOLS10在线直播下注P lasers for different processes such as cleaning and polishing. That allows for a dedicated photonic process chain where the same laser system can be used for subsequent tasks such as ablation, cleaning, and polishing. This way, the LOLS10在线直播下注P laser leaves its niche and becomes a very handy tool for various tasks in modern industry—it finally arrives on the shop floor.

Comparing picosecond and nanosecond pulses

It seems obvious that higher average power means higher ablation rate. For LOLS10在线直播下注P lasers, this is only partially true. As discussed earlier in this magazine,1 there is an optimum fluence (pulse energy per spot area) at about 7X the threshold fluence. Higher energies per pulse do not lead to higher ablation rates per second, and they may generate rough surfaces with cone-like protrusions. A viable solution for this problem can be the application of multibeam optics, directing the energy of one pulse into many similar beamlets.2

The other way to exploit higher average power would be higher repetition rates with moderate pulse energies—this poses challenges for the scanner technology, which have yet to be mastered. A much simpler solution for many purposes is the use of bursts, which uses conventional galvo scanners. A burst is a sequence of several equal laser pulses, separated by a few nanoseconds. The power of a burst is then the sum of the single pulses’ powers. Bursts are repeated at the regular repetition rate of the laser of typically some 100 kHz. More details of LOLS10在线直播下注P bursts can be found elsewhere.3,4

LOLS10在线直播下注The actual advantage of LOLS10在线直播下注P lasers is their processing precision. They can ablate with submicron accuracy and almost without heat-affected zones. Once higher average power LOLS10在线直播下注P lasers are available, the crucial question is, can these advantages be maintained at higher throughput rates? To answer this question, scientists at Fraunhofer ILT have compared the ablation processes of LOLS10在线直播下注P lasers and of nanosecond lasers.

For benchmarking, a fiber laser with 400 ns pulse duration and variable pulse energies was used. It was compared with a LOLS10在线直播下注P laser with pulse lengths of 2 ps and 10 ps in single-pulse mode (FIGURE 1). By reducing the pulse duration from 10 ps to 2 ps, the efficiency increases by a factor of about 2.5. With this improvement, the LOLS10在线直播下注P laser is able to ablate as many volumes per minute and watt as the 400 ns laser. The 2 ps laser ablates more than 40 mm3 steel per minute at an average power of 312 W. Interestingly, the 10 ps pulses reached only about 30% of the ablation per watt and minute. This demonstrates that LOLS10在线直播下注P is not equal to LOLS10在线直播下注P—after all, FIGURE 1 shows that a LOLS10在线直播下注P laser with pulses as short as 2 ps can ablate as much or more material than a nanosecond laser.

Now, what happens if bursts of laser pulses are applied? FIGURE 2 shows how the ablation rate increases with the number of pulses per burst and with the applied fluence per pulse. All pulses in a burst contribute to ablation, so more pulses per burst lead to a higher ablation per burst.FIGURE 2. A 2 ps laser used with a different number of pulses per burst ablates more material than a comparable 400 ns benchmark laser.FIGURE 2. A 2 ps laser used with a different number of pulses per burst ablates more material than a comparable 400 ns benchmark laser.

For example, in single pulse operation with a fluence of 1 J/cm2, the LOLS10在线直播下注P laser (2 ps) achieved an ablation rate of 1.49 mm3/min. With 10 pulses per burst (PpB), the same laser achieved 10.15 mm3/min. With a fluence of 4 J/cm2, the system even reached 42 mm3/min. The maximum applied power has been 312 W at a repetition rate of 500 kHz.

Finding the sweet spot

LOLS10在线直播下注There is an obvious question resulting from these observations: What happens to the surface quality if the number of pulses is increased? The short answer is, it depends. The surface quality improves when we go from one to five pulses per burst, but it gets worse for 10 pulses per burst. 

FIGURE 3 shows measurements of the surface quality in ablation experiments on a steel surface using a 2 ps laser. Again, the fluence has been increased stepwise and data has been taken for different bursts and compared to the roughness achieved with a nanosecond laser. It should be noted that the fluence here refers to every single pulse. Accordingly, much more energy is deposited during a burst.FIGURE 3. The surface roughness has been measured for bursts with an increasing number of 2 ps pulses per burst (PpB) and compared with ablation results from a 400 ns laser (benchmark area).FIGURE 3. The surface roughness has been measured for bursts with an increasing number of 2 ps pulses per burst (PpB) and compared with ablation results from a 400 ns laser (benchmark area).

It turns out that the roughness from the LOLS10在线直播下注P ablation is larger than from the nanosecond laser for small and large fluences for very low and high numbers of pulses per burst. In between, there is a sweet spot, where the surface roughness for the LOLS10在线直播下注P laser ablation goes down to 0.5 µm.

LOLS10在线直播下注After all, these results show that the LOLS10在线直播下注P laser allows for two different processing strategies:

High ablation rate. With bursts of up to 10 pulses and fluences of up to 3 J/cm2, the LOLS10在线直播下注P laser can achieve an ablation rate that is 3–4X as high as from the nanosecond laser. The surface roughness can be kept at similar levels.

Low roughness. With fluences between 1 and 2 J/cm2 and five pulses per burst, the surfaces can be made much smoother (down to 0.5 µm) while retaining the same ablation rate as with nanosecond lasers.

Of course, this depends on the actual material used; these tests were done on steel 1.2738, a material that is often used for mold tools as applied in the fabrication of plastic parts. FIGURE 4 shows several examples, where the LOLS10在线直播下注P laser has been used for microstructuring of steel tool surfaces. The ablation rate for the average power of 90 W (5 PpB) was similar to the ablation rate of a nanosecond laser. At the same time, the surface was much smoother.FIGURE 4. Three different textures made on a steel surface using 2 ps pulse bursts with 90 W/average power; the ablation rate was about 11 mm3/min and the roughness at about 0.6 µm.FIGURE 4. Three different textures made on a steel surface using 2 ps pulse bursts with 90 W/average power; the ablation rate was about 11 mm3/min and the roughness at about 0.6 µm.

For higher average power of 165 W and similar burst settings, the ablation rate doubles to about 20 mm3/min, while the surface roughness doubles to 1.3 µm (pyramid structure) or 1.5 µm (carbon-tex structure). A typical industrial nanosecond laser achieves a roughness of 1.5 µm at 5 mm3/min or 2.5 µm at 10 mm3/min. So, the LOLS10在线直播下注P laser at 165 W can ablate 4X as much material as the nanosecond laser at similar surface qualities. Or, with lower power, it achieves a 4X lower surface roughness at similar ablation rates.

Cleaning and polishing with LOLS10在线直播下注P lasers

Microstructuring is a process that benefits from the precision of LOLS10在线直播下注P lasers. The processes rely on the extremely short interaction of these lasers. In cleaning processes, for example, ultrashort pulses blast away oxide layers from surfaces. With proper settings, this cleaning process happens almost instantaneously without melting the actual surface.

Polishing is a different process where the surface is locally molten to flatten the surface on that spot. Depending on the size of the unevenness, the surface can be heated to different degrees. Obviously, cleaning and polishing require different sets of process parameters. In the polishing process, for example, these parameters must not exceed the fluence threshold for ablation.

Understanding the processes helps to find appropriate parameter spaces. For cleaning, a rapid process without heat accumulation is needed. Thus, single pulses with a fluence just above the ablation threshold will do the job.

For polishing, heat accumulation is desired. Therefore, a long burst with low energy pulses will be well suited. Their added fluence must not exceed the ablation threshold. So, the energy of the individual pulse within the burst is kept much lower than for cleaning. The scanning rate also has to be adjusted, as it may need several bursts on one spot to get the surface molten.FIGURE 5. Result of a LOLS10在线直播下注P polished surface that leads to lower the initial surface roughness by a factor of two.FIGURE 5. Result of a LOLS10在线直播下注P polished surface that leads to lower the initial surface roughness by a factor of two.

Different surface structures need different polishing processes. For example, a roughness of about 1.0 µm may need another treatment than a roughness of about 0.5 µm. FIGURE 5 shows the success of a LOLS10在线直播下注P laser polishing process (a) where the initial grinded surface is smoothed from initially 0.6 µm to 0.3 µm. A more detailed and scientific view on the quite-new approach of LOLS10在线直播下注P polishing is available.5

Fast processing of smooth surfaces with a photonic process chain

So far, it has been shown that on the one hand, LOLS10在线直播下注P lasers can ablate faster or smoother than nanosecond lasers. On the other hand, LOLS10在线直播下注P lasers can be used for cleaning and polishing surfaces. Together, this knowledge can be used to establish a fully photonic process chain where rapid LOLS10在线直播下注P ablation is followed by a cleaning step and, finally, a LOLS10在线直播下注P polishing procedure. Researchers at Fraunhofer ILT found that the total LOLS10在线直播下注P process is still 14–59% faster than a comparable nanosecond laser ablation. At the same time, the surface roughness is about 0.17 µm, which is about 90% better than the surface from the nanosecond process.FIGURE 6. A steel tool surface processed with LOLS10在线直播下注P lasers in a three-step process chain—structuring, cleaning, and selective polishing—is shown.FIGURE 6. A steel tool surface processed with LOLS10在线直播下注P lasers in a three-step process chain—structuring, cleaning, and selective polishing—is shown.

This integrated photonic process chain offers various benefits for the tool and mold industry—for example, it can be implemented in five-axis machines. Typical application cases are in the automotive industry, where tools for plastic parts in a car interior can be processed with this method. As a test, a sample tool for Volkswagen has been processed with LOLS10在线直播下注P lasers in a three-step process chain consisting of structuring, cleaning, and polishing (FIGURE 6). The sample has been made in one machine with one clamping. In similar processes with nanosecond structuring, the workpiece has to be reworked. The processing time is kept constant with LOLS10在线直播下注P lasers while achieving a surface roughness of 0.5 µm on the non-polished areas (4X lower than with the nanosecond process).

LOLS10在线直播下注This research has been executed within the project eVerest. It has been funded by the German Federal Ministry of Research and Education (BMBF).

REFERENCES

1. N. Hodgson et al., “Industrial femtosecond lasers and material processing,” Industrial Laser Solutions LOLS10在线直播下注online (Jan. 2019); .

2. J. Finger and A. Thoss, “High-throughput ultrashort-pulse laser micromachining,” Industrial Laser Solutions, 21-23 (Jan/Feb 2020); .   

3. J. Finger et al., Adv. Opt. Technol., 7, 3 (May 2018); .

4. A. Brenner, B. Bornschlegel, and J. Finger, J. Laser Micro. Nanoen., 14, 1, 100–107 (2019); doi:10.2961/jlmn.2019.01.0017.

5. A. Brenner, L. Röther, M. Osbild, and J. Finger, Proc. SPIELOLS10在线直播下注, 11268, 112680P (Mar. 2, 2020); doi:10.1117/12.2551481.

ANDREAS BRENNER is Team Manager Thin Film Structuring at the Fraunhofer Institute for Laser Technology (Fraunhofer ILT), Aachen, Germany (), while ANDREAS THOSS (th@thoss-media.de) is CEO of THOSS Media and a member of the Industrial Laser Solutions Editorial Advisory Board.

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