FIB-SEM Investigation of Laser-Induced Periodic Surface Structures and Conical Surface Microstructures on D16T (AA2024-T4) Alloy

Round 1

Reviewer 1 Report

Line 24: Yt:YAG has to be changed to Yb:YAG ?

Line 40: No mass citations; please assign a specific reference to a statement; therefore either split the statements and assign references 2 and 3, or take one of the two out

Line 44: see comment on line 40 regarding mass citation

Line 45: see comment to line 40 regarding mass citation

Line 46: see comment to line 40 regarding mass citation

Line 49: see comment to line 40 regarding mass citation

Line 287: see comment to line 40 regarding mass citation

Line 290: see comment to line 40 regarding mass citation

State of research for LIPSS on (other) aluminum alloys missing

Line 98: content for experimentals chapter
line 99/100: content of introduction/state of the art chapter

The chapter 5 is more a summary, not a conclusion

Author Response

Dear Reviewer!

Thank you for your valuable feedback, which has helped significantly to improve the article.

Here is our response to your comments:

Yt:Yag had indeed needed to be changed. This typing mistake was fixed. All multiple citations were removed, either by assigning a specific statement to a reference or by removing unneeded references.  

An overview of existing works about LIPSS on aluminium and its alloys was added to the introduction and the discussion sections. The added text is the following “
Femtosecond laser treatment was also shown to lead to an increase in nanohardness of Al both under vacuum and in ambient environment. For ambient environment, oxidation leads to formation of oxide phases. The concentration of oxygen may reach 16% [3]. Elemental changes in AA2024 with picosecond laser ablation were observed by EDS by Ahuir-Torres et al. [4].
This type of artificially aged Al-Cu alloy is used in aviation because of their attractive mechanical properties (e.g. strength-to-weight ratio) and resistance to mechanical damage.

Near-subwavelength periodic ripples (LIPSS, laser-induced periodic surface structures) are commonly referred to as surface self-assembled structures.  LIPSS are formed upon the interaction of linearly polarized laser irradiation and solid matter. The incident irradiation is scattered upon the random roughness features of the surface, and its interaction with surface-scattered electromagnetic waves and, in some cases, with surface plasmon polaritons leads to inhomogeneous energy absorption. This inhomogeneous energy deposition exhibits strong peaks in the Fourier space and so results in the creation of periodic and quasiperiodic structures. Radiation is absorbed by the electronic system, then energy is transferred to the lattice [19].

Various phenomena of thermal, chemical or hydrodynamical nature occur, leading to spatially modulated removal and redeposition of material. In the case of pulsed incident radiation, the ripples require multiple pulses to fully form, and the pattern of ultimate structure depends on the number of pulses. Thus, the importance of feedback phenomena arises. Several aspects of LIPSS formation are still controversially discussed in the literature in general [20].  

LIPSS formation on aluminium was reported in the literature reported for high [3] and low [21] fluences, and specifically for aluminium alloy AA2024 [22].

The formation of conical microstructures in metals at higher fluences is attributed to the contribution of two mechanisms, namely preferential ablation and redeposition of nanoparticles stimulated by ablation. Initially impurities or defects lead to the formation of small (several microns sized) precursor cones in places where the material is less ablated. Then, the precursor cones grow due to the preferential ablation of areas around the precursor cones. This preferential ablation is caused by two factors. First, the sloped cone edges (forming the angle with inciting beam) have larger area subjected to the same irradiation fluence and therefore ablate less. Second, some of the fluence is reflected from the sloped cone edges to the surrounding areas [18]. This explains the growth of the precursor cones into larger (tens of microns) conical microstructures. The microstructures can even grow higher than the level of the initial surface due to the re-deposition of nanoparticles ablated from furrows between the cones. In the case of scanning ablation regime, multiple layers of nanoparticles redeposit when the laser beam is away and then are melted when the beam returns [18]. Such structures grow both in height and in width, until two cones meet. Then the cones become linked and grow only upwards, while the furrows between them grow deeper downwards [23].

The detailed coverage of conical microstructures growth on aluminium and other metals at 800 nm 130 fs laser pulses is provided by Nayak et al. [24]. More specifically, bump-like microstructures along with maze-like microstructures were observed on AA2024 [25], and the the effect of laser fluence and number of passes on the roughness and reflectivity of ablated areas is examined [26].

One can conclude that the complex character of matter-radiation interaction over wide range of fluences causes versatile surface topological and microchemical modification for commonly used engineering alloys is a promising dedicated instrument for advanced surface enhancement. This makes rigorous advanced characterization of surface structures (equipped with the most modern techniques such as FIB milling and lamella cutting) of great importance.
The present work is devoted to the focused investigation of the D16T alloy, the Russian equivalent of the AA2024-T4 alloy. Using non-destructive AFM and confocal laser profilometry, high resolution SEM and FIB milling we systematically investigate the formation of LIPSS and conical microstructures, reaching 50 µm in size, covered by nanoparticles, that were created as a result of laser treatment using 250 fs pulses of 1033 nm Yb:YAG laser and discuss how laser scan patterns can lead to non-uniform profiles and surface structure variations. Internal non-uniformity of the micrometer sized cones is likely firstly witnessed at nanometer scale due to the fine FIB-SEM observations.”

Content regarding methods was moves to the Methods section, as indicated on the manuscript, see line 149. Sentence about laser ablation was removed from the profilometry results section. Conclusion section was modified to comply with the requirements set by the reviewer.

Best regards,

Igor Salimon

Reviewer 2 Report

The introduction section should be supplemented with more information about the state of the art (finding that there are other works isn't enough - the discussion about their results would be valuable).

Both, the confocal microscopy, as well as atomic force microscopy, allows calculating many useful surface topography parameters, which are more clear and readable than the profile charts. The charts in figure 3 should be bigger or removed because they currently are barely readable. 

Author Response

Dear Reviewer!

Thank you for your valuable feedback, which has helped significantly to improve the article.

Here is our response to your comments:

An overview of existing works about LIPSS and conical structures on aluminium and its alloys was added to the introduction and the discussion sections. The following text was added in the introduction to comply with the reviewer’s recommendations: “
Femtosecond laser treatment was also shown to lead to an increase in nanohardness of Al both under vacuum and in ambient environment. For ambient environment, oxidation leads to formation of oxide phases. The concentration of oxygen may reach 16% [3]. Elemental changes in AA2024 with picosecond laser ablation were observed by EDS by Ahuir-Torres et al. [4].
This type of artificially aged Al-Cu alloy is used in aviation because of their attractive mechanical properties (e.g. strength-to-weight ratio) and resistance to mechanical damage. Near-subwavelength periodic ripples (LIPSS, laser-induced periodic surface structures) are commonly referred to as surface self-assembled structures. LIPSS are formed upon the interaction of linearly polarized laser irradiation and solid matter. The incident irradiation is scattered upon the random roughness features of the surface, and its interaction with surface-scattered electromagnetic waves and, in some cases, with surface plasmon polaritons leads to inhomogeneous energy absorption. This inhomogeneous energy deposition exhibits strong peaks in the Fourier space and so results in the creation of periodic and quasiperiodic structures. Radiation is absorbed by the electronic system, then energy is transferred to the lattice [19].Various phenomena of thermal, chemical or hydrodynamical nature occur, leading to spatially modulated removal and redeposition of material. In the case of pulsed incident radiation, the ripples require multiple pulses to fully form, and the pattern of ultimate structure depends on the number of pulses. Thus, the importance of feedback phenomena arises. Several aspects of LIPSS formation are still controversially discussed in the literature in general [20].  LIPSS formation on aluminium was reported in the literature reported for high [3] and low [21] fluences, and specifically for aluminium alloy AA2024 [22].The formation of conical microstructures in metals at higher fluences is attributed to the contribution of two mechanisms, namely preferential ablation and redeposition of nanoparticles stimulated by ablation. Initially impurities or defects lead to the formation of small (several microns sized) precursor cones in places where the material is less ablated. Then, the precursor cones grow due to the preferential ablation of areas around the precursor cones. This preferential ablation is caused by two factors. First, the sloped cone edges (forming the angle with inciting beam) have larger area subjected to the same irradiation fluence and therefore ablate less. Second, some of the fluence is reflected from the sloped cone edges to the surrounding areas [18]. This explains the growth of the precursor cones into larger (tens of microns) conical microstructures. The microstructures can even grow higher than the level of the initial surface due to the re-deposition of nanoparticles ablated from furrows between the cones. In the case of scanning ablation regime, multiple layers of nanoparticles redeposit when the laser beam is away and then are melted when the beam returns [18]. Such structures grow both in height and in width, until two cones meet. Then the cones become linked and grow only upwards, while the furrows between them grow deeper downwards [23].The detailed coverage of conical microstructures growth on aluminium and other metals at 800 nm 130 fs laser pulses is provided by Nayak et al. [24]. More specifically, bump-like microstructures along with maze-like microstructures were observed on AA2024 [25], and the the effect of laser fluence and number of passes on the roughness and reflectivity of ablated areas is examined [26].

”.

The following text was added to the discussion section to comply with the reviewer’s recommendations: “
AA2024 that is ablated to the point of conical structures growth is demonstrated to be a broadband absorber [I]. We could easily observe this in our work. The low reflectivity can be observed by the naked eye, as the treated areas look black (Figure 1 a). The decrease in reflectivity was approximately evaluated by an integrating sphere measurement. The sphere PVE300 illuminated a spot having 1 mm diameter that obviously was bigger than the width of the treated area, therefore, the signal was collected from both treated area and surrounding untreated surface. The surface of untreated alloy showed reflectivity of 79±5% in the visible light (300-900 nm) wavelength range, while the spot that was positioned in the center of laser treated area illuminating both laser treated area and untreated surrounding alloy had integral reflectivity of 42±2% within the same wavelength range. Applying the linear rule of mixture to the contributions of treated and untreated areas, it was possible to estimate the reflectivity of treated area as 21±2%.

LIPSS with a 703 nm period were observed on AA2024 in a very recent work for a scan with 800 fs pulse 1030 nm laser [E]. Similarly, LIPSS on aluminium are mentioned in the work of by Umm-i-Kalsoom et al. [C]. Those LIPSS are observed for at much higher fluences for the treatment conducted using 30 fs pulse duration and an incident laser wavelength of 800 nm.

Bashir et al. observed LIPSS on Al for low energy fluences, 25 fs pulses and 800 nm incident wavelength. The authors conclude that the LIPSS observed are due to the excitation of surface plasmon polaritons and the period of the structures does sensitively depend on the material selected and the laser fluence [D].

A detailed coverage of conical microstructures growth on aluminium and other metals at 800 nm 130 fs laser pulses is provided by Nayak et al. [G]. A focused ion beam cutting of a conical structure grown on titanium is provided, with no internal structural change observed. Conical structures on aluminium exhibited less regularity than the ones created on stainless steel and titanium.

Bump-like microstructures along with maze-like microstructures were observed on AA2024 [H]. Models were proposed allowing to finely tune the laser irradiation parameters for the better reproduction of a desired type of microstructures. The differences in the surface structures observed on different metals are explained by differences in the strength of electron-phonon coupling and the thermal conductivities.

 Another work providing significant insight on the properties of microstructures grown on AA2024 via femtosecond ablation examines the effect of laser fluence and number of passes on the roughness and reflectivity of ablated areas [26].

Finally, it is worth to mention a work revealing the contents of an of onion-like aggregated nanoparticle sphere on AA2024 via focused ion beam milling [27].

In our case, we suppose that the observed conical structures and deep valleys surrounding them are mostly caused by preferential ablation of lower areas, as described in [18]. However, while this does mostly define the high aspect ratio of the conical structures, this is not the only mechanism of structures growth, as profilometry reveals presence of microstructures higher than the initial surface level. This is most likely caused by the re-deposition of ablated material on the cone tops, where less ablation occurs compared to other areas.

While detailed research of conical microstructures growth on AA2024 was conducted already, to the best of our knowledge, there were no attempts to study the internal contents of the conical microstructures by methods of FIB-SEM and conduct an EDX study of a lamella cut from the tip of a conical structure. A study conducts EDX study of the surface of ablated AA2024 [4]. Yet, there are significant differences between this work and ours. Exposures were static (no scanning was conducted). In that work, it was shown that more significant elemental change and oxidation was observed at the rim of laser impact and not in the center [4].  The section 3.4. provides insight into the structure of the tip of conical microstructure, grown above the initial surface by redeposition. EDX data reveals oxidization of treated areas as deep as 3 µm inside the conical microstructures (Figure 11 c). Areas of increased concentration of O and Cu are noticed with respective depletion of Al concentration (Figure 11 b, f), and vice versa. Inhomogeneous distribution of O, Cu and Al inside the tip can be caused by a number of overlapping possible mechanisms. Stain-like patterns visible inside the lamella cannot be easily explained by changes in elemental concentration or a change in density.”.

The surface roughness of the ablated areas after laser treatment was characterized using the Ra and Rz roughness parameters that were calculated adhering to the “Modified-ISO” standard. However, we are reluctant to directly calculate surface topography parameters from the AFM results, due to the fact that not only LIPSS, but precursors of growing conical structures, furrows and cracks are convolved in the complex surface topography, which becomes more obvious when sampling larger areas. Figure 3 has been modified. A color-coded chart is provided to demonstrate the type of surface modification as a function of the irradiation conditions. Examples of the type of surface modifications are also shown in the figure for clarity. The original figure 3 was moved to “supplementary materials” section, where it can be accessed in a larger size.

 

Best regards,

Igor Salimon

 

 

Reviewer 3 Report

The manuscript is devoted to laser treatment of aluminum alloy surface. The authors are recognized researchers in this field. However, minor improvements are required.
1. The content of the article as a whole does not raise questions. At the same time, the figures look careless. Use the best quality figures if possible. Figures 7 and 11 must necessarily be corrected. It is necessary to structure images more compactly and presentatively.
2. The Introduction and Conclusion should be written more strongly and clearly. It is necessary to clearly identify the problem of the study, and then give conclusions about the proposed solutions.

Author Response

Dear Reviewer!

Thank you for your valuable feedback, which has helped significantly to improve the article.

Here is our response to your comments:

All the figures in the text have been revised. In particular, figures 7 and 11 were modified to comply with the reviewer’s recommendations. An overview of existing works about LIPSS and conical structures on aluminium and its alloys was added to the introduction and the discussion sections. The following text was added to the introduction: “
Femtosecond laser treatment was also shown to lead to an increase in nanohardness of Al both under vacuum and in ambient environment. For ambient environment, oxidation leads to formation of oxide phases. The concentration of oxygen may reach 16% [3]. Elemental changes in AA2024 with picosecond laser ablation were observed by EDS by Ahuir-Torres et al. [4].
This type of artificially aged Al-Cu alloy is used in aviation because of their attractive mechanical properties (e.g. strength-to-weight ratio) and resistance to mechanical damage. Near-subwavelength periodic ripples (LIPSS, laser-induced periodic surface structures) are commonly referred to as surface self-assembled structures. LIPSS are formed upon the interaction of linearly polarized laser irradiation and solid matter. The incident irradiation is scattered upon the random roughness features of the surface, and its interaction with surface-scattered electromagnetic waves and, in some cases, with surface plasmon polaritons leads to inhomogeneous energy absorption. This inhomogeneous energy deposition exhibits strong peaks in the Fourier space and so results in the creation of periodic and quasiperiodic structures. Radiation is absorbed by the electronic system, then energy is transferred to the lattice [19].Various phenomena of thermal, chemical or hydrodynamical nature occur, leading to spatially modulated removal and redeposition of material. In the case of pulsed incident radiation, the ripples require multiple pulses to fully form, and the pattern of ultimate structure depends on the number of pulses. Thus, the importance of feedback phenomena arises. Several aspects of LIPSS formation are still controversially discussed in the literature in general [20].  LIPSS formation on aluminium was reported in the literature reported for high [3] and low [21] fluences, and specifically for aluminium alloy AA2024 [22].The formation of conical microstructures in metals at higher fluences is attributed to the contribution of two mechanisms, namely preferential ablation and redeposition of nanoparticles stimulated by ablation. Initially impurities or defects lead to the formation of small (several microns sized) precursor cones in places where the material is less ablated. Then, the precursor cones grow due to the preferential ablation of areas around the precursor cones. This preferential ablation is caused by two factors. First, the sloped cone edges (forming the angle with inciting beam) have larger area subjected to the same irradiation fluence and therefore ablate less. Second, some of the fluence is reflected from the sloped cone edges to the surrounding areas [18]. This explains the growth of the precursor cones into larger (tens of microns) conical microstructures. The microstructures can even grow higher than the level of the initial surface due to the re-deposition of nanoparticles ablated from furrows between the cones. In the case of scanning ablation regime, multiple layers of nanoparticles redeposit when the laser beam is away and then are melted when the beam returns [18]. Such structures grow both in height and in width, until two cones meet. Then the cones become linked and grow only upwards, while the furrows between them grow deeper downwards [23].The detailed coverage of conical microstructures growth on aluminium and other metals at 800 nm 130 fs laser pulses is provided by Nayak et al. [24]. More specifically, bump-like microstructures along with maze-like microstructures were observed on AA2024 [25], and the the effect of laser fluence and number of passes on the roughness and reflectivity of ablated areas is examined [26].

”.
The following text was added to the discussion section:”
Femtosecond laser treatment was also shown to lead to an increase in nanohardness of Al both under vacuum and in ambient environment. For ambient environment, oxidation leads to formation of oxide phases. The concentration of oxygen may reach 16% [3]. Elemental changes in AA2024 with picosecond laser ablation were observed by EDS by Ahuir-Torres et al. [4].
This type of artificially aged Al-Cu alloy is used in aviation because of their attractive mechanical properties (e.g. strength-to-weight ratio) and resistance to mechanical damage. Near-subwavelength periodic ripples (LIPSS, laser-induced periodic surface structures) are commonly referred to as surface self-assembled structures.  LIPSS are formed upon the interaction of linearly polarized laser irradiation and solid matter. The incident irradiation is scattered upon the random roughness features of the surface, and its interaction with surface-scattered electromagnetic waves and, in some cases, with surface plasmon polaritons leads to inhomogeneous energy absorption. This inhomogeneous energy deposition exhibits strong peaks in the Fourier space and so results in the creation of periodic and quasiperiodic structures. Radiation is absorbed by the electronic system, then energy is transferred to the lattice [19].Various phenomena of thermal, chemical or hydrodynamical nature occur, leading to spatially modulated removal and redeposition of material. In the case of pulsed incident radiation, the ripples require multiple pulses to fully form, and the pattern of ultimate structure depends on the number of pulses. Thus, the importance of feedback phenomena arises. Several aspects of LIPSS formation are still controversially discussed in the literature in general [20].  LIPSS formation on aluminium was reported in the literature reported for high [3] and low [21] fluences, and specifically for aluminium alloy AA2024 [22].The formation of conical microstructures in metals at higher fluences is attributed to the contribution of two mechanisms, namely preferential ablation and redeposition of nanoparticles stimulated by ablation. Initially impurities or defects lead to the formation of small (several microns sized) precursor cones in places where the material is less ablated. Then, the precursor cones grow due to the preferential ablation of areas around the precursor cones. This preferential ablation is caused by two factors. First, the sloped cone edges (forming the angle with inciting beam) have larger area subjected to the same irradiation fluence and therefore ablate less. Second, some of the fluence is reflected from the sloped cone edges to the surrounding areas [18]. This explains the growth of the precursor cones into larger (tens of microns) conical microstructures. The microstructures can even grow higher than the level of the initial surface due to the re-deposition of nanoparticles ablated from furrows between the cones. In the case of scanning ablation regime, multiple layers of nanoparticles redeposit when the laser beam is away and then are melted when the beam returns [18]. Such structures grow both in height and in width, until two cones meet. Then the cones become linked and grow only upwards, while the furrows between them grow deeper downwards [23].The detailed coverage of conical microstructures growth on aluminium and other metals at 800 nm 130 fs laser pulses is provided by Nayak et al. [24]. More specifically, bump-like microstructures along with maze-like microstructures were observed on AA2024 [25], and the the effect of laser fluence and number of passes on the roughness and reflectivity of ablated areas is examined [26].
“
Conclusion section was modified to comply with the requirements set by the reviewer.

 

Best regards,

Igor Salimon

 

Reviewer 4 Report

The paper presents experimental results on the generation of different quasi-periodic surface structures after irradiation of a specific aluminum alloy by multiple fs-laser pulses and a subsequent qualification of these nano- and microstructured surfaces by (FIB-)SEM, EDX, and AFM. The paper is written in a clear way using a good English language. Unfortunately, the scientific/technical novelty contained in this manuscript is very limited since all of these morphological structures were investigated and described in detail already for aluminum alloys in the scientific literature. I cannot see that this work provides some new and relevant information for the scientific communities working in its field of Laser Induced Periodic Surface Structures (LIPSS). Hence, I must recommend the rejection of this work. List of issues to be improved for a potential future submission: (1) New relevant research must be added. (2) The introduction section must be improved. The authors reference some papers on laser generated structures on metals and metal alloys, but they widely ignore the literature available for aluminum and aluminum alloys. (3) The introduction would benefit from a discussion of the formation mechanisms of the laser generated structures. (4) LIPPS should read LIPSS (several times). (5) The authors should provide laser fluences instead of laser pulse energies to allow a comparison to the scientific literature. (6) Section 3.2 is too short. It should be extended or completely removed. Note that the optical properties of fs-laser processed aluminum was already studied in detail in the literature. (7) I wonder why the ~100-200 nm large spherical nanostructures visible in Fig. 7(e) are not visible in the Fourier transform of Fig.7(c)? (8) How was the cleaning of the samples in acetone performed? Wiping? In an ultrasonic bath? (9) The dot is the decimal separator and not the comma (see text and figures). (10) The last paragraph of Section 4 on page 14 (lines 311-316) is too speculative and not supported by evidence. It should be removed.

Author Response

Dear Reviewer!

Thank you for your valuable feedback, which has helped significantly to improve the article.

Here is our response to your comments:

 

Laser-matter interaction involving ultrafast laser pulses is a well-studied subject in metals, including aluminium and aluminium alloys. However, the majority of reports in the literature involve surface topography modifications. Here, we report not only on the surface modifications which involve LIPSS and conical microstructures, but also on the compositional and structural changes that occur, such as porosity and Cu rich regions in the tips of the conical structures formed by redeposition of ablated material. To the best of our knowledge, no investigations of internal content of the conical microstructures with FIB-prepared lamellae revealing the depth of the structural changes were conducted yet. An overview of existing works about LIPSS and conical structures on aluminium and its alloys was added to the introduction and the discussion sections The following text was added to the introduction: “
Femtosecond laser treatment was also shown to lead to an increase in nanohardness of Al both under vacuum and in ambient environment. For ambient environment, oxidation leads to formation of oxide phases. The concentration of oxygen may reach 16% [3]. Elemental changes in AA2024 with picosecond laser ablation were observed by EDS by Ahuir-Torres et al. [4].
This type of artificially aged Al-Cu alloy is used in aviation because of their attractive mechanical properties (e.g. strength-to-weight ratio) and resistance to mechanical damage. Near-subwavelength periodic ripples (LIPSS, laser-induced periodic surface structures) are commonly referred to as surface self-assembled structures. LIPSS are formed upon the interaction of linearly polarized laser irradiation and solid matter. The incident irradiation is scattered upon the random roughness features of the surface, and its interaction with surface-scattered electromagnetic waves and, in some cases, with surface plasmon polaritons leads to inhomogeneous energy absorption. This inhomogeneous energy deposition exhibits strong peaks in the Fourier space and so results in the creation of periodic and quasiperiodic structures. Radiation is absorbed by the electronic system, then energy is transferred to the lattice [19].Various phenomena of thermal, chemical or hydrodynamical nature occur, leading to spatially modulated removal and redeposition of material. In the case of pulsed incident radiation, the ripples require multiple pulses to fully form, and the pattern of ultimate structure depends on the number of pulses. Thus, the importance of feedback phenomena arises. Several aspects of LIPSS formation are still controversially discussed in the literature in general [20].  LIPSS formation on aluminium was reported in the literature reported for high [3] and low [21] fluences, and specifically for aluminium alloy AA2024 [22].The formation of conical microstructures in metals at higher fluences is attributed to the contribution of two mechanisms, namely preferential ablation and redeposition of nanoparticles stimulated by ablation. Initially impurities or defects lead to the formation of small (several microns sized) precursor cones in places where the material is less ablated. Then, the precursor cones grow due to the preferential ablation of areas around the precursor cones. This preferential ablation is caused by two factors. First, the sloped cone edges (forming the angle with inciting beam) have larger area subjected to the same irradiation fluence and therefore ablate less. Second, some of the fluence is reflected from the sloped cone edges to the surrounding areas [18]. This explains the growth of the precursor cones into larger (tens of microns) conical microstructures. The microstructures can even grow higher than the level of the initial surface due to the re-deposition of nanoparticles ablated from furrows between the cones. In the case of scanning ablation regime, multiple layers of nanoparticles redeposit when the laser beam is away and then are melted when the beam returns [18]. Such structures grow both in height and in width, until two cones meet. Then the cones become linked and grow only upwards, while the furrows between them grow deeper downwards [23].The detailed coverage of conical microstructures growth on aluminium and other metals at 800 nm 130 fs laser pulses is provided by Nayak et al. [24]. More specifically, bump-like microstructures along with maze-like microstructures were observed on AA2024 [25], and the the effect of laser fluence and number of passes on the roughness and reflectivity of ablated areas is examined [26].

”.
The following text was added to the discussion section:”
Femtosecond laser treatment was also shown to lead to an increase in nanohardness of Al both under vacuum and in ambient environment. For ambient environment, oxidation leads to formation of oxide phases. The concentration of oxygen may reach 16% [3]. Elemental changes in AA2024 with picosecond laser ablation were observed by EDS by Ahuir-Torres et al. [4].
This type of artificially aged Al-Cu alloy is used in aviation because of their attractive mechanical properties (e.g. strength-to-weight ratio) and resistance to mechanical damage. Near-subwavelength periodic ripples (LIPSS, laser-induced periodic surface structures) are commonly referred to as surface self-assembled structures.  LIPSS are formed upon the interaction of linearly polarized laser irradiation and solid matter. The incident irradiation is scattered upon the random roughness features of the surface, and its interaction with surface-scattered electromagnetic waves and, in some cases, with surface plasmon polaritons leads to inhomogeneous energy absorption. This inhomogeneous energy deposition exhibits strong peaks in the Fourier space and so results in the creation of periodic and quasiperiodic structures. Radiation is absorbed by the electronic system, then energy is transferred to the lattice [19].Various phenomena of thermal, chemical or hydrodynamical nature occur, leading to spatially modulated removal and redeposition of material. In the case of pulsed incident radiation, the ripples require multiple pulses to fully form, and the pattern of ultimate structure depends on the number of pulses. Thus, the importance of feedback phenomena arises. Several aspects of LIPSS formation are still controversially discussed in the literature in general [20].  LIPSS formation on aluminium was reported in the literature reported for high [3] and low [21] fluences, and specifically for aluminium alloy AA2024 [22].The formation of conical microstructures in metals at higher fluences is attributed to the contribution of two mechanisms, namely preferential ablation and redeposition of nanoparticles stimulated by ablation. Initially impurities or defects lead to the formation of small (several microns sized) precursor cones in places where the material is less ablated. Then, the precursor cones grow due to the preferential ablation of areas around the precursor cones. This preferential ablation is caused by two factors. First, the sloped cone edges (forming the angle with inciting beam) have larger area subjected to the same irradiation fluence and therefore ablate less. Second, some of the fluence is reflected from the sloped cone edges to the surrounding areas [18]. This explains the growth of the precursor cones into larger (tens of microns) conical microstructures. The microstructures can even grow higher than the level of the initial surface due to the re-deposition of nanoparticles ablated from furrows between the cones. In the case of scanning ablation regime, multiple layers of nanoparticles redeposit when the laser beam is away and then are melted when the beam returns [18]. Such structures grow both in height and in width, until two cones meet. Then the cones become linked and grow only upwards, while the furrows between them grow deeper downwards [23].The detailed coverage of conical microstructures growth on aluminium and other metals at 800 nm 130 fs laser pulses is provided by Nayak et al. [24]. More specifically, bump-like microstructures along with maze-like microstructures were observed on AA2024 [25], and the the effect of laser fluence and number of passes on the roughness and reflectivity of ablated areas is examined [26].
“
A paragraph about the mechanisms of LIPSS formation was added to the introduction (lines 63-75). All instances of this and other typos were corrected. We agree with the reviewer, this is a fair comment. The pulse energy was recalculated into fluence to comply with the requirements set by the reviewer’s request. Section 3.2. has been removed. The relevant content (reflectivity) is mentioned in the discussion section instead. We agree with the reviewer the round features should register in the FFT. However, the size distribution of these globules ranges from 100 to 300 nm and their distribution is not regular. Thus, resulting in poor registration, below the noise level of the FFT. The cleaning was performed in an ultrasonic bath. We have added this information in the text: “lines 313, 407” The decimal separator was changed to dot throughout the text. The paragraph was removed.

 

Best regards,

Igor Salimon

 

Round 2

Reviewer 4 Report

The authors have performed the requested revisions and technically improved their manuscript. Unfortunately, they included new small typing mistakes, which must be corrected in the production stage of the manuscript. Line 24: "The energy of the pulses varied from.." should read "The fluence of the pulses varied from .." (the author refer now to fluences=energy densities). Line 49: "silicone" should read "silicon" as otherwise it refers to a wrong material. Line 109: "The Roman numerals correspond to the energy of the pulses" should read "The Roman numerals correspond to the fluence of the pulses" (the author refer now to fluences=energy densities). Line 289-290, Table 5: "Pulse energy, J/cm2 " should read "Fluence, J/cm2 ". (the author refer now to fluences=energy densities). Lines 345 - 361: The references [A] ... [H] should be all correctly linked to references provided in the references list. Line 410: "0,73" should read "0.73". (wrong decimal separator).

Author Response

Dear Reviewer!

Thank you very much for your valuable feedback.

Here are the minor improvements that were made to comply with your requests.

On Line 53, silicone was changed to silicon to refer to the correct material. On Lines 27, 112 and 282-283, mentions of pulse energy were corrected to refer to fluences. Throughout the article, all instances of this typo were corrected. Mentions of pulse energy now only remain in the Table 1, providing an equivalence between pulse energy and fluences, and in the conclusion on Line 400. On Lines 373-380, the references were corrected. On Line 438: the wrong decimal separator was corrected. The same mistake was corrected on Lines 382, 384 and 284. No instances of wrong decimal separation remain.

Best regards,

Igor Salimon