Effect of Modulation Period on the Microstructure and Tribological Properties of AlCrTiVNbN/TiSiN Nano Multilayer Films

Comments 1: “the interplanar spacing raises declines” what it means is not clear

Response 1: Thanks for your careful work. According to the advice, we revised the manuscript. The revision is as follows in Page 9 line 232.

All prepared films displayed an FCC structure with a preferred planar direction of (200). As λ rises, the interplanar distance declines and the surface roughness raises.

Comments 2: The title is technically correct, but could be clearer. For example: Effect of Modulation Period on the Microstructure and Tribological Properties of AlCrTiVNbN/TiSiN Nanomultilayer Films

Response 2: ：Thanks for your advice. According to the advice, we revised the title.

Effect of Modulation Period on the Microstructure and Tribological Properties of AlCrTiVNbN/TiSiN nano multilayer films

Comments 3: Abstract：The expression in the last sentence: “coherent growth and alternative stress filed” “field” will be, also “alternating stress field” should be preferred instead of “alternative stress”.

Response 3：According to the advice, we changed the expression. The revision is as follows in Page 1 line 20.

The improvement in the properties of the film was ascribed to the coherent growth and alternating stress field between the AlCrTiVNbN and the TiSiN layers.

Comments 4: What are the total film thicknesses? (Has it been measured in SEM?)

Response 4：Figure 3 presents the cross-sectional images of AlCrTiVNbN/TiSiN nano-multilayer films with various modulation periods labelled. The total film thickness was shown in Figure 3. We revised the manuscript as follows in Page 4 line 135-137.

The surface and cross-sectional images of the AlCrTiVNbN/TiSiN nano multilayer films presented in Fig. 3 revealed a columnar crystal, oriented perpendicular to the substrate surface. The total film thickness declines from 944 nm to 659 nm with the raising λ.

Comments 5: Has the substrate been preheated? (It may have an effect on stress formation.)

Response 5：The substrate was not preheated. We revised the manuscript as follows in Page 2 line 79.

The film deposition is carried out at room temperature. The substrate was not preheated.

Comments 6: The repeatability of the measurements (n ≥ 3?) is not mentioned.

Response 6：Thanks for your careful work. According to the advice, we revised the manuscript. The revision is as follows in Page 3 line 99-108.

It should be indicated that hardness and elastic modulus were measured via an Anton Paar GmbH nano-indenter from Austria, equipped with a Berkovich tip. The unloading and loading rates were set to 20 mN⸱min-1, with a maximum applied load equal to 20 mN. For each sample, five measurement Commentss were selected, and the average of these values was explored as the final result. The tribological performance was analyzed using a CFT-I material surface performance tester (Zhongke Co., China). A GCr15 ball with a diameter equal to 6 mm served as the counter material. The applied load (F) was 100g, and the reciprocating frequency (f) was 200 C⸱min-1, the reciprocating sliding distance was 5 mm, and the total test duration was 20 min. Each friction test was performed three times for every sample.

Comments 7: Stress estimates were made in XRD analysis but not calculated. Quantitative data such as lattice strain or peak broadening would be supportive.

Response 7：According to the advice, we added the FWHM (Full Width at Half Maximum) values in Table 2 and revised the manuscript as follows in Page 3-4.

Fig. 2 illustrates the XRD patterns, showing an FCC structure with a preferred planar direction of (200). Moreover, Table 2 provides the diffraction angle 2θ, interplanar distance and the full width at half maximum (FWHM), revealing a positive correlation between λ and 2θ, while the interplanar distance exhibits a negative correlation.

The 2θ value corresponding to the (200) plane of AlCrTiVNbN mono film is lower than those observed for all AlCrTiVNbN/TiSiN nano multilayer films. It is found that as λ increases, 2θ values and the FWHM rise in the (200) plane, while the interplanar distance decreases. Further analysis based on Scherrer’s formula revealed an increase in the FWHM values of the XRD peaks, indicating a reduction in the interplanar distance and decreased crystallinity [24]. This observation is consistent with the 2θ results for the (200) plane. Based on the template effect [25-27], the TiSiN layer follows the coherent lattice structure of the AlCrTiVNbN layer during the deposition process. The lattice constant of the TiSiN layer is smaller than that of the AlCrTiVNbN layer, so the TiSiN layer bears the tensile stress and the AlCrTiVNbN layer bears the compressive stress. A tensile and compressive stress field was formed in the AlCrTiVNbN/TiSiN nano multilayer films, which enhanced the characteristics of the AlCrTiVNbN/TiSiN nano multilayer films.

Table 2. The values of 2θ, interplanar distance, and FWMH were obtained for the films.

Comments 8: The term “broccoli shape” in SEM images is not scientific. Instead, “cauliflower-like morphology” or “nodular structure” could be preferred.

Response 8：According to the advice, we revised the description of the surface morphology of the film in Page 4 line 142.

The surfaces of the films look like cauliflower-like morphology and have crakes.

Comments 9: H/E and H³/E² comments are good but why are these parameters directly related to tribological performance? It should be supported with more sources.

Response 9：According to the advice, we added explanation and the revision is as follow in page 6 line 177-182.

H/E reflects the fracture roughness of the films and the H3/E2 represents the resistance ability to plastic deformation of the films [29-30]. The higher H/E and H3/E2 means the better the tribological properties of the films. The M4 has the highest H/E and H3/E2, which tends to improve the tribological properties of films [30-31].

The reasons of mechanical properties enhancement are the alternating stress field between AlCrTiVNbN layer and TiSiN layer, solution strengthening. Firstly, when λ < 4 nm, the TiSiN layer grew epitaxially along the AlCrTiVNbN layer, which improves the mutual growth of the layers. The interfaces between the TiSiN and the AlCrTiVNbN layers can inhibit the dislocation motion, and an alternative stress field was formed between the layers. Secondly, the AlCrTiVNbN layer has solution strengthening because it has 7 elements like the HEAS [22].

Comments 10: Not all figures are labeled; in particular, the captions for Fig. 3 and Fig. 4 are missing or inadequate.

Response 10：According to the advice, we added a description of the Figure 3 and Figure 4 as follows in Page 4 and 5, respectively.

Figure 3. Surface and cross-sectional images obtained for the AlCrTiVNbN/TiSiN nano multilayer films: (a) 4nm, (b) 5nm, (c) 6nm, and (d) 7nm.

Figure 4. Three-dimension images of AlCrTiVNbN/TiSiN nano multilayer films: (a) 4nm; (b) 5nm; (c) 6nm; (d) 7nm;(e)0nm.

Comments 11: The tables are missing units. For example: "Diffraction angle 2θ/(°)" is correct, but nm should be written for "interplanar spacing".

Response 11：Thanks for your advice. We added the unit in the Table 2 in Page 4.

Table 2. The values of 2θ, interplanar distance, and FWMH were obtained for the films.

Comments 12: Check the citations. Is citation 22 in the correct place?

Response 12：Thanks for your careful work. According to the advice, we have adjusted the position of [22] in the article, The revision is as follow in page 6.

Comments 13: How did the film thickness change with the modulation period? Were the total thicknesses kept equal?

Response 13: According to the advice, we added the film thickness in Page 4 line 135-137.

The surface and cross-sectional images of the AlCrTiVNbN/TiSiN nano multilayer films presented in Fig. 3 revealed a columnar crystal, oriented perpendicular to the substrate surface. The total film thickness declines from 944 nm to 659 nm with the raisingλ.

Comments 14: How many replicates were used to measure? Why are standard deviation data not presented?

Response 14：Based on your suggestion, we have added an error value to describe the specific performance values of the AlCrTiVNbN/TiSiN nanolayer films. All tests were performed with a minimum of three replicates (n≥3), and measurement errors are represented by error bars in the corresponding figures/graphs.

Comments 15: Why was only 4 modulation periods chosen? Couldn’t thinner structures such as 1–3 nm be tested?

Response 15：The 1-3 nm modulation period requires a shorter deposition time for per target material, which demands higher control precision for the equipment. Our current system cannot achieve the film with smaller modulation periods. If conditions permit in the future, we will additionally fabricated smaller modulation periods.

Comments 16: Should the tribological contributions of oxidation products be discussed more openly?

Response 16：According to the advice, we revised the manuscript as follows in Page 8.

Fig. 10 depicts the element distribution images of the M4 film. The Al, Ti, V, Nb, N elements disappear on the wear track, but O elements on the wear track are obviously more than the deposition surface of the M4 film. This indicates that oxidation takes place during the friction process. The oxidation products generated play a dual role: They not only decrease surface abrasion and spalling but also form protective layers which acts as solid lubricants like Magnéli phases to reduce adhesion [32]. The oxides Al₂O₃, Cr₂O₃, TiO₂ and V₂O₅ are formed during the friction process. The oxides Al₂O₃ and Cr₂O₃ exhibited high hardness, whereas the oxides TiO₂ and V₂O₅ of the Magnéli phase displayed self-lubrication, which is helpful to improve the tribological characteristics of the films [33]. The element Cr on the wear track is more than the deposition surface of the M4 film, which mean the M4 film is worn through and the substrate appears.

Comments 17: Why are all EDS analyses limited to the M4 sample only?

Response 17：We conducted EDS analysis on all samples, but due to space constraints, not all EDS spectra could be included in the manuscript. Sample M4 demonstrated the best performance, thus it was subjected to targeted analysis.

4. Response to Comments on the Quality of English Language

Point 1: The English grammar is weak throughout the article, creating confusion in places. Some sentences give the impression of direct translation.

Response 1: Thanks for your advice. Based on them, we have corrected the grammatical errors and will enhance our English skills in our future work

Comments 1: Introduction: This sentence should be corrected: “The thickness of ZrO₂ layer is 0.6 nm, The (AlCrTiZrMo)N/ZrO₂ film has the maximum hardness and elastic modulus because of the epitaxial growth interface[23].

Response 1: Thanks for your advice. According to the advice, we have modified this sentence. The revision is as follow in page 2 line 49-52.

The (AlCrTiZrMo)N/ZrO2 film displayed the highest values of elastic modulus and hardness for a ZrO2 layer thickness equal to 0.6 nm. This observation may be attributed to the formation of an epitaxial growth interface that strengthens the film structure [23].

Comments 2: Introduction: “The above literature analysis showed that HENs are very promising materials, which motivated us to undertake this research” before the sentence “Our team studied”

Response 2: According to the advice, we revised the Introduction. The revision is as follow in page 2 line 54-60.

The literature review indicates that HENs possess significant potential for advanced applications. Based on these findings, an investigation was conducted on the cavitation erosion resistance of TiSiN/NiTiAlCoCrN nano multilayer films with various λ values. The results reveal that as λ increases, the mass loss from cavitation erosion initially declines and subsequently rises. The optimal cavitation erosion resistance is achieved in the TiSiN/NiTiAlCoCrN nano multilayer film with λ=11 nm

Comments 3: Add the methods used for mechanical and tribological performance analysis (nanoindentation).

Response 3: According to the advice, we revised 2.2 Characterization of film properties as follows in Page 3.

It should be indicated that hardness and elastic modulus were measured via an Anton Paar GmbH nano-indenter from Austria, equipped with a Berkovich tip. The unloading and loading rates were set to 20 mN⸱min-1, with a maximum applied load equal to 20 mN. For each sample, five measurement points were selected, and the average of these values was explored as the final result. The tribological performance was analyzed using a CFT-I material surface performance tester (Zhongke Co., China). A GCr15 ball with a diameter equal to 6 mm served as the counter material. The applied load (F) was 100g, and the reciprocating frequency (f) was 200 C⸱min-1, the reciprocating sliding distance was 5 mm, and the total test duration was 20 min. Each friction test was performed three times for every sample. The wear track volume (V) was quantified utilizing a laser scanning confocal microscope, whereas the wear rate was determined mathematically as follows:

Comments 4: Figure 3: I suggest enlarging the font when describing the arrows, because is illegible.

Response 4: According to the advice, we enlarged the font in the image.

Comments 5: Figure 3: There is no description of what a, b, c, d mean.

Response 5: According to the advice, we added a description of the Figure 3 as follows in Page 4.

Figure 3. Surface and cross-sectional images obtained for the AlCrTiVNbN/TiSiN nano multilayer films: (a) 4nm, (b) 5nm, (c) 6nm, and (d) 7nm.

Comments 6: Figure 4: There is no description of what a, b, c, d and e mean.

Response 6: According to the advice, we added a description of the Figure 4 as follows in Page 5.

Figure 4. Three-dimension images of AlCrTiVNbN/TiSiN nano multilayer films: (a) 4nm; (b) 5nm; (c) 6nm; (d) 7nm;(e)0nm.

Comments 7: Figures 5,7,8, and 10: I suggest enlarging the font, which describes the axis.

Response 7: Based on your suggestion, we have enlarged the axis label fonts in the figures to enhance readability.

Figure 5. Surface roughness of AlCrTiVNbN/TiSiN nano multilayer films.

Figure 6. Mechanical properties of AlCrTiVNbN/TiSiN nano multilayer films: (a) Hardness and elastic modulus values; (b) H/E and H³/E²

Figure 7. (a)Friction curve; (b)Average coefficient of friction of AlCrTiVNbN/TiSiN nano multilayer films.

Comments 8: There should be more values of hardness and Young’s modulus in the text of the article. In line 152 the authors mentioned the highest value of hardness was 15.51GPa (there is no value of Young’s modulus). I suggest adding, for comparison the lowest value of H and E (Please add for which sample is it). The same situation for H/E and H³/E². It should be mentioned about the highest and lowest value (which sample). The authors can (not obligatory) also create the Table with these values.

Response 8: According to the advice, we added the lowest values of H and E in the sample for comparison in the original text, tables of average hardness and average modulus of elasticity corresponding to different modulation cycles were made.

Fig. 6 and Table 3 depict the hardness, elastic modulus, H/E and H³/E² of the nano multilayer films. The hardness of M4 film is higher than the AlCrTiVNbN film and the other nano multilayer films, and reaches to 15.51±0.16GPa, simultaneously, the elastic modulus of M4 peaks at 182.89±2.38GPa, representing the maximum value observed across all samples. As λ increases, the hardness, elastic modulus, H/E, and H³/E² decrease. For the M7 film, the hardness and elastic modulus drop to 9.472±0.39GPa and 163.35±3.69GPa, respectively, along with the lowest H/E and H³/E² ratios.

Table 3. Mechanical properties of the films.

Comments 9:  Figure 10 is a question of location in the article.

Response 9: According to the advice, we have adjusted the position of Figure 10 in the article.

The revision is as follow in page 9.

Comments 10: Conclusion: Please add some sentences about mechanical properties and values of H, E and H/E and H³/E² in the Conclusion part.

Response 10: According to the advice, we have added references to H/E and H³/E² to our conclusions as follows. The revision is as follow in page 9.

(2) The nano multilayer film with λ=4 nm (M4) shows a smooth surface with small particles. It also demonstrates the highest hardness of 15.51±0.16 GPa and elastic modulus of 182.89±2.38 GPa, along with the peak H/E (0.084) and H³/E² (0.111) ratios. As λ raises, the hardness and elastic modulus of the films decline. This was ascribed to the solution strengthening and the formation of a compression and tensile alternating stress field between the TiSiN and the AlCrTiVNbN layers.

(3) As λ raises, the COF of the films and the wear rate of the films raises. The film with λ = 4 nm(M4) shows the lowest COF of 0.73 and the wear rate equal to (8.29±0.18)×10⁻⁸ mm³·N⁻¹·m⁻¹.The oxidation products decrease surface abrasion and display self-lubrication during the tribological process. The wear mechanism includes abrasive wear, adhesive wear, and oxidation wear.

Comments 11: Please write in the Conclusion which sample (M4, M5, M6 or M7) showed the best properties.

Response 11: According to the advice. we revised the Conclusion. The revision is as follow in page 9.

(2) The nano multilayer film with λ=4 nm (M4) shows a smooth surface with small particles. It also demonstrates the highest hardness of 15.51±0.16 GPa and elastic modulus of 182.89±2.38 GPa, along with the peak H/E (0.084) and H³/E² (0.111) ratios. As λ raises, the hardness and elastic modulus of the films decline. This was ascribed to the solution strengthening and the formation of a compression and tensile alternating stress field between the TiSiN and the AlCrTiVNbN layers.

(3) As λ raises, the COF of the films and the wear rate of the films raises. The film with λ = 4 nm(M4) shows the lowest COF of 0.73 and the wear rate equal to (8.29±0.18)×10⁻⁸ mm³·N⁻¹·m⁻¹.The oxidation products decrease surface abrasion and display self-lubrication during the tribological process. The wear mechanism includes abrasive wear, adhesive wear, and oxidation wear.