Influence of Cr-Al-Si-N and DLC-Si Thin Coatings on Wear Resistance of Titanium Alloy Samples with Different Surface Conditions

Round 1

Reviewer 1 Report

please find the attached file.

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 1 Comments

Dear reviewer,

Thank you so much for your kind evaluation of our work. We agree with all your proposals and comments and have modified the manuscript accordingly.

We hope the manuscript will be suitable for publishing in Coatings and attract many potential journal readers with your comments. The introduced corrections in the text of the manuscript are marked yellow.

Kind regards,

Authors.

Reviewer comments

Point 1: It is highly recommended to provide a graphical abstract, as it will increase the visibility of the work and make the manuscript more appealing.

Response 1: Thank you so much for your kind recommendation. The graphical abstract is attached.

Point 2: In the abstract please consider including specific quantitative results. Presenting key findings, such as wear rate reduction percentages, friction coefficient values, and any substantial improvement in wear resistance, would provide readers with a clearer understanding of the actual impact and significance of your study's outcomes.

Response 2: Thank you for your fair remark. The abstract is revised.

Point 3: Please, demonstrate the novelty and importance of this paper in the abstract and at the end of the introduction.

Response 3: Thank you, the abstract and the end of the introduction are revised.

Point 4: Provide a clear research gap that the study aims to fill.

Response 4: Thank you, it is added at the end of the introduction.

Point 5: As you elucidate the prevalent utilization of titanium alloys known for their distinctive properties, I recommend incorporating recent literature that explores an additional application of TI alloys in implants. This facet, which you have not yet addressed, has the potential to provide supplementary information to readers. This approach would further enrich the comprehension of your discourse:

https://link.springer.com/article/10.1007/s11665-023-07928-z

Response 5: Thank you for your kind advice. Due to the etic point of view, another link is added to complete the description of possible applications. However, we found the given reference useful and consider it for taking into account in further research.

Point 6: Could you elaborate on the methods used to evaluate the adhesion bond strength between the Cr-Al-Si-N and DLC-Si coatings and the titanium alloy substrate?

Response 6: Thank you for asking it. Scratch testing was performed according to the international standard ASTM C1624-05 "Standard Test Method for Adhesion Strength and Mechanical Failure Modes of Ceramic Coatings by Quantitative Single Point Scratch Testing".

According to the test standard (ASTM C1624-05), a multilevel system of failure and critical loads can be used to classify the damage mechanism, respectively, determining cohesive cracking/failure (LC1) processes directly in the coating layer; subsequent adhesive failure/peeling (LC2) between coating and substrate; the final stage of destruction at the critical load LC3, when the outer layer of the coating is completely peeled off. The impact of the indenter occurs with a gradual increase in load (up to 25 N). As the load increases along the scratching distance, the coating undergoes three indicated stages of destruction.

Point 7: In relation to chemical-thermal treatments and other coating deposition techniques mentioned in the introduction, could you briefly discuss how the performance of Cr-Al-Si-N/DLC-Si coatings compares? What advantages do these coatings offer over other methods in terms of wear resistance enhancement and potential limitations?

Response 7: Thank you for your question. Regarding the advantages of the selected Cr-Al-Si-N coatings over alternative nitride ceramic coatings such as Ti-Cr-N, Ti-Al-N, Ti-Zr-N, Ti-Al-Cr-N deposited by vacuum plasma methods, this composition is today recognized by various scientists as one of the most promising [1-3]. The results of well-known works show that in the deposited Cr-Al-Si-N film, a combined amorphous/nanocrystalline structure is observed containing CrN, AlN, and Si3N4 phases. These phases exhibit high hardness, sufficiently high crack resistance, and the best corrosion resistance required for parts made of titanium alloys used in aggressive environments. The only methods for the formation of such coatings are the methods of vacuum-arc evaporation of cathode materials (used in the present study) and magnetron sputtering of target materials (more expensive and less productive, but allowing to obtain coatings with a minimum content of microdroplets).

1. Yuwei Ye, Zhiyong Liu, Wei Liu, Dawei Zhang, Yongxin Wang, Haichao Zhao, Liping Wang, Xiaogang Li. Bias design of amorphous/nanocrystalline CrAlSiN films for remarkable anti-corrosion and anti-wear performances in seawater. Tribology International, 2018, 121, 410-419.
2. Linqing He, Li Chen, Yuxiang Xu. Interfacial structure, mechanical properties and thermal stability of CrAlSiN/CrAlN multilayer coatings. Materials Characterization, 2017, 125, 1-6.
3. Junkai Liu, Zhexin Cui, Dayan Ma, Junqiang Lu, Yanguang Cui, Chong Li, Wenbo Liu, Zhe Hao, Pengfei Hu, Meiyi Yao, Ping Huang, Guanghai Bai, Di Yun. Investigation of oxidation behaviors of coated Zircaloy as accident-tolerant fuel with CrAlN and CrAlSiN coatings in high-temperature steam. Corrosion Science, 2020, 175, 108896

Therefore, the combination of a promising Cr-Al-Si-N coating with well-proven antifriction DLC coatings allowed the authors of the work to count on a significant effect. As we pointed out in the introduction, we have already studied the effectiveness of such coatings when deposited on hard materials—tool ceramics and hard alloys. However, no combination of such coatings has been used before this study for relatively soft titanium alloys.

Let's consider alternative technologies of chemical-thermal treatment, which consist of diffusion saturation of the surface layer of parts made of titanium alloys. There are specific features and limitations. The nitrogen and oxygen diffusion rate in titanium alloys is much lower than that of hydrogen [4, 5]. The layer obtained from interaction with these gases is characterized by increased hardness and brittleness, and it has to be removed from the surface of titanium products by etching or additional machining. Thus, those media that are usually used in the processing of steels, especially hydrogen-containing gases and their mixtures, are unacceptable for the chemical-thermal treatment of titanium and its alloys. This is due to the significant hydrogenation of the metal to a level sufficient for the development of hydrogen embrittlement.

As an alternative process of chemical-thermal treatment, saturation of the surface layer of titanium alloys with boron carbide in borating media containing additional chromium and silicon can be used [6]. This treatment makes it possible to obtain a fairly hard surface layer (30–33 GPa) with a low coefficient of friction, but the saturation rate is meager, and the total treatment time is up to 8 hours.

4. Santiago Domínguez-Meister, Iñigo Ibáñez, Anastasia Dianova, Marta Brizuela, Iñigo Braceras. Nitriding of titanium by hollow cathode assisted active screen plasma and its electro-tribological properties. Surface and Coatings Technology, 2021, 411, 126998.
5. Puja Yadav, Kuldeep K. Saxena. Effect of heat-treatment on microstructure and mechanical properties of Ti alloys: An overview. Materials Today: Proceedings, 2020, 26 (2), 2546-2557.
6. Ivanov, S.G., Guriev, M.A., Loginova, M.V. et al. Boriding of titanium OT4 from powder saturating media. Russ. J. Non-ferrous Metals, 2017, 58, 244–249.

Today, according to authoritative scientific groups, one of the most promising processes for the surface modification of titanium alloys is the MAO microarc oxidation process (MAO is also known as plasma electrolytic oxidation). It does not require expensive technological equipment, is easy to implement, and allows coating deposition with a wide range of properties [7-9]. MAO is an electrochemical process of modification (oxidation) of the surface of titanium alloys in an electrolyte plasma. The process is carried out at a high voltage, due to which microarc discharges occur at the breakdown points of the barrier layer on the surface. In the breakdown region, the temperature and pressure increase sharply, and part of the metal goes into the solution, where it is present in the form of ions. The other part of the molten metal interacts with the electrolyte components and forms the MAO coating. The coating is formed on the surface and inside the sample. In addition, high temperatures in the breakdown zone lead to the formation of a gradient transition layer at the metal-coating interface. This layer provides strong adhesion of the coating to the substrate. However, MAO processes have a severe drawback - the surface roughness of the titanium alloy increases greatly during the formation of the coating. This is not so critical for products that need to be protected from corrosion, but it is critical for products that operate in tribo-connections and need to be wear-resistant. For such products, additional post-processing is required after MAO. Such shortcomings are lacking the technological approach proposed and studied in this paper.

7. Gangqiang Li, Fengcang Ma, Ping Liu, Shengcai Qi, Wei Li, Ke Zhang, Xiaohong Chen, Review of micro-arc oxidation of titanium alloys: Mechanism, properties and applications. Journal of Alloys and Compounds, 2023, 948, 2023, 169773.
8. Pedro Akira Bazaglia Kuroda, Felype Narciso de Mattos, Carlos Roberto Grandini, Conrado Ramos Moreira Afonso, Micro-abrasive wear behavior by ball cratering on MAO coating of Ti–25Ta alloy. Journal of Materials Research and Technology, 2023, 26, 1850-1855
9. Guoqiang Li, Yaping Wang, Liping Qiao, Rongfang Zhao, Shufang Zhang, Rongfa Zhang, Chunmei Chen, Xinyi Li, Ying Zhao. Preparation and formation mechanism of copper incorporated micro-arc oxidation coatings developed on Ti-6Al-4V alloys. Surface and Coatings Technology, 2019, 375, 74-85

Point 8: Can you elaborate on the rationale behind selecting specific deposition parameters for the Cr-Al-Si-N and DLC-Si coatings? Factors such as temperature, pressure, gas mixture composition, and bias voltage can significantly impact coating properties. How were these parameters optimized to ensure the desired coating characteristics and adhesion strength?

Response 8: Thank you for asking it. As noted in Section 2, the coatings on the studied samples of titanium alloys were deposited on a unit model π311 (Platit, Switzerland). Optimizing the deposition modes was not aimed at the presented study, and we used the technological cycles embedded in the control programs of the plant control system. Equipment manufacturer (Platit) declares these modes as rational. The relevant sentence is added to the text of the manuscript (Subsubsection 2.2.1).

Point 9: Regarding the wear resistance evaluation, what was the rationale behind selecting the specific load, frequency, and number of friction cycles for the fretting wear tests? Were these conditions chosen based on the anticipated real-world usage of the coated components? Additionally, did you consider the potential influence of surface roughness changes during fretting wear on the wear results?

Response 9: Thank you. The loads during tests on a model 1401 friction machine when implementing the “sphere-plane” scheme under fretting wear conditions were selected experimentally based on providing conditions for sufficiently intense wear of the surface layer of titanium alloy samples. The load itself was regulated by a combination of two parameters - the force (normal force in contact) and the relative displacement of the counterbodies. It is important to emphasize that the number of cycles in multi-cycle tests for fretting wear is essentially a time mode: the more cycles, the longer the test time. The number of cycles produced during the experiments was limited, i.e., the experiment was completed when the duration provided volumetric wear sufficient for instrumental measurement and comparative analysis.

For example, as can be seen from Table 7, when using ground samples with Cr-Al-Si-N / DLC-Si coating at load = 1 N, offset = 60 μm, and 100,000 cycles, they wore out very slowly. Therefore, it was necessary to increase the load and the number of cycles (test duration) up to load = 20 N, offset = 100 μm and 300,000 cycles, i.e., test them under more extreme conditions to compare their volumetric wear with DLC-Si coatings without a ceramic sublayer.

Separate studies on the effect of surface roughness on the quantitative values of volumetric wear during fretting wear were not carried out in the work. The influence of such a parameter as roughness needs a separate study. It should be noted the authors have differing opinions about the nature of the influence of this parameter. For example, the data in the works below are ambiguous.

10. Wang, N., Zhu, J., Liu, B. et al. Influence of Ultrasonic Surface Rolling Process and Shot Peening on Fretting Fatigue Performance of Ti-6Al-4V. Chin. J. Mech. Eng. 34, 90 (2021). https://doi.org/10.1186/s10033-021-00611-1
11. Qureshi, W., Cura, F. & Mura, A. Experimental characterization of roughness parameters for fretting wear in spline couplings. Meccanica 52, 1975–1984 (2017). https://doi.org/10.1007/s11012-016-0535-7

Researchers believe that when two surfaces come into contact, contact does not occur over the entire area but only on a relatively small number of roughness protrusions. As a result of surfaces sliding relative to each other, the irregularities of one surface erase the irregularities of the opposite surface, and a smooth track is formed, i.e., roughness is smoothed out. In another opinion, microcavities retain wear products, thereby increasing wear resistance.

Point 10: Can you provide more insight into how the microstructure of the coatings might contribute to their observed mechanical properties? Are there any specific features in the micrographs that could explain differences in hardness and elastic modulus?

Response 10: Thank you for asking. Separate microstructural studies of coatings using high-resolution SEM or TEM were not carried out in the work. The microstructures presented in Figure 6 are given for a better understanding of the object of study and for assessing the thickness of the coatings. If we talk in general about the effect of the microstructure of coatings on mechanical properties, then the main regularities are known. The presence of defects, such as pores and numerous microdrops, significantly affects the value of the critical load at which the coating is destroyed and its wear resistance. The fewer defects assist in forming the denser microstructure, and then a significant deformation of the crystal lattice is observed. That indicates the formation of compressive stresses, which provides the best combination of "microhardness - modulus of elasticity" of the coatings. The number (density) of defects depends on the deposition modes, such as pressure, bias voltage, and arc current. In our studies, we used the deposition modes recommended by the Platit plant manufacturer as rational.

Point 11: How did you differentiate between cohesive and adhesive failure in the scratch testing results? Could you provide additional details about the criteria used to identify these types of failure and how they were quantified?

Response 11: Thank you for asking. To respond to this fair remark, we need to go back to Point 6, where the method description was provided. To complete this response, we need to add that the load corresponding to LC1, LC2, and LC3 is determined quantitatively by the level of the acoustic emission signal. The acoustic emission signal is sensitive to ongoing damage. It gives an instantaneous response (amplitude bursts), which makes it possible to determine the quantitative values of LC1, LC2, and LC3 when correlated with the load acting at a particular moment. Thus, acoustic emission amplitude bursts allow us to determine these types of failure. The relevant sentences were highlighted in the text of the manuscript.

Point 12: Regarding the fretting wear tests, why did DLC-Si coatings exhibit an increase in friction coefficient after extended cycling under higher loads? What might be causing this behavior, and is it a common phenomenon in DLC coatings?

Response 12: Thank you for your comment on it. As seen in Figure 12, a sharp increase in the friction coefficient is observed for a single-layer DLC-Si coating after 100,000 cycles. It is related to local breakthroughs of the coating, loss of its lubricity, exposure of the base material (titanium alloy), and changes in the tribocontact zone. The Cr-Al-Si-N/DLC-Si coating, which has a higher adhesion strength and the best ratio of "hardness - modulus of elasticity", provides high lubricity and, as a result, stable conditions in the tribocontact zone for a longer time. The same trend is observed in Figure 13. Local failures occur in the DLC-Si coating, and the friction coefficient increases sharply for single-layer DLC coatings after 21,000 fretting cycles. The Cr-Al-Si-N/DLC-Si coating continues to perform antifriction functions at the same time.

It is consistent with the literature data we briefly listed in the Introduction. The data indicates that an increased level of residual stresses is observed when single-layer DLC coatings are applied to materials with low hardness. Then, the strength of the adhesive bond with the base decreases, and, as a result, the DLC layer does not effectively resist external mechanical loads. To eliminate the negative effect, the practice of forming various sublayers before applying DLC coatings has become widespread. Our Cr-Al-Si-N sublayer allows us to cope with this task in a certain way.

Point 13: The conclusion is concise and provides clear conclusions based on the results of the study. The points made are supported by the data. However, the conclusion could benefit from the following suggestions: -What are the main limitations of this study, and how could they be addressed in future research? Are there any additional tests or analyses that could provide deeper insights into the mechanisms at play.

Response 13: Thank you for pointing it out. First, the study's limitations are related to the fact that we still simulate operational loads (abrasive and fretting wear) on specially prepared experimental samples with coatings on laboratory equipment. Calowear Abrasion Tester was used to assess the resistance of the surface layer of the samples to abrasion, and friction machine model 1401 was used to assess the resistance of the surface layer of the samples to wear under fretting conditions. Testing experimental samples on laboratory equipment to imitate possible operational loads is an important and traditional study stage; there are reasons to proceed to the next stage only with the positive results of the laboratory condition testing. The next stage is testing the coated experimental PARTS on specialized STANDS. Of course, the next level will provide an even deeper understanding of the operating mechanisms since the acting loads and the configuration of the parts' contact surfaces are as close as possible to real operating conditions. The relevant sentence is added to the text of the manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

In this work, the sieries Cr-Al-Si-N and DLC-Si coatings were prepared on Ti-Al-Zr-Sn-Nb alloy. Two techniques of the finishing milling and lapping with micro-grained abrasive were used to study the influence of initial surface microrelief on deposition effect. Moreover, the mechanical properties, tribological behaviors and their scientific mechanisms of coating samples were also discussed. The results are interesting, and the research content is systematic and substantial. However, the language of the manuscript needs further polishing and a few explanations need to be added in order to meet the bar for publication. Specific comments are included below.

1. Figure 2 can be combined with Figure 7 to facilitate comparison of 3D surface profile and roughness changes before and after deposition.

2. Section “3.1. Microstructure and properties of coatings”, The results of nanoindentation hardness of the coating are related to the depth of indentation. Generally, the indentation depth is 1/10 of the coating thickness. The elastic modulus is an intrinsic property of the material. The outermost layers of the DLC-Si and Cr-Al-Si-N/DLC-Si samples are the same coating, but the results of the elastic modulus are quite different. Whether the indentation across the interface between Cr-Al-Si-N and DLC-Si coatings is important, please provide indentation depth.

3. It is recommended that the Figure 7 and 12 not be displayed across pages.

4. Since the full scratch morphology is not shown in Figure 9, the readers do not know which part of the scratch is shown. Did the delamination occur in Cr-Al-Si-N/DLC-Si sample after scratch test? It is recommended to supplement the whole scratch and local enlarged pictures.

5. EDS is a semi-quantitative analysis method with high error rate for elements with low atomic number. Please add the error rate in Table 5 to prove the validity of oxygen and nitrogen content.

6. Please explain why the single variable method is not used to set the test parameters of the friction and wear test, and why the load and the number of cycles are changed at the same time.

7. Lines 476-479: Whether the influence of the difference between the elastic modulus of the coating and the friction pair on the operational properties is considered.

The effect of the stress between the coating and the substrate on the properties is discussed extensively in this paper, and whether there is a method (such as film stress tester or XRD) to test the stress state and stress value, which can better interpret the research results.

Minor editing of English language required.

Author Response

Response to Reviewer 2 Comments

Dear reviewer,

Thank you so much for your kind evaluation of our work. We agree with all your proposals and comments and have modified the manuscript accordingly.

We hope the manuscript will be suitable for publishing in Coatings and attract many potential journal readers with your comments. The introduced corrections in the text of the manuscript are marked green.

Kind regards,

Authors.

Reviewer comments

Point 1: Figure 2 can be combined with Figure 7 to facilitate comparison of 3D surface profile and roughness changes before and after deposition.

Response 1: Thank you so much for your kind recommendation. Two figures are combined.

Point 2: Section “3.1. Microstructure and properties of coatings”, The results of nanoindentation hardness of the coating are related to the depth of indentation. Generally, the indentation depth is 1/10 of the coating thickness. The elastic modulus is an intrinsic property of the material. The outermost layers of the DLC-Si and Cr-Al-Si-N/DLC-Si samples are the same coating, but the results of the elastic modulus are quite different. Whether the indentation across the interface between Cr-Al-Si-N and DLC-Si coatings is important, please provide indentation depth.

Response 2: Thank you for asking it. In experiments, the value of the applied load and the corresponding indenter penetration depth were chosen based on the condition of about 15% of the coating thickness. The measurements were carried out at a load of 4.0 mN, and the corresponding indenter penetration depth was about 0.36 µm. Indeed, it was experimentally established that the same coating, but on different substrates - “soft” (titanium alloy) and “hard” (ceramic sublayer) has different elastic moduli. A similar pattern was established in work [49 in the text of the manuscript] when similar coatings were deposited on hard alloy substrates. The corresponding sentence is added to the text of the manuscript in Subsection 2.3.

[49] Grigoriev, S.N.; Volosova, M.A.; Fedorov, S.V.; Mosyanov, M. Influence of DLC Coatings Deposited by PECVD Technology on the Wear Resistance of Carbide End Mills and Surface Roughness of AlCuMg₂ and 41Cr4 Workpieces. Coatings 2020, 10, 1038. https://doi.org/10.3390/coatings10111038

Point 3: It is recommended that the Figure 7 and 12 not be displayed across pages.

Response 3: Thank you for pointing it out. We have tried to work out the wanted layout of Figures 7 and 12. We hope that they look better in the current version of the manuscript.

Point 4: Since the full scratch morphology is not shown in Figure 9, the readers do not know which part of the scratch is shown. Did the delamination occur in Cr-Al-Si-N/DLC-Si sample after scratch test? It is recommended to supplement the whole scratch and local enlarged pictures.

Response 4: Thank you for pointing this out. The SEM images that are presented in Figure 9 and the results of the EDS analysis presented in Table 5 were performed in the area of 6.0-6.25 mm from the start of the indenter scratching path. In this area, the critical load LC3 has already been recorded for the studied samples with coatings, which identifies the complete delamination of the coating from the sample. It was revised in the text of the manuscript.

Point 5: EDS is a semi-quantitative analysis method with high error rate for elements with low atomic number. Please add the error rate in Table 5 to prove the validity of oxygen and nitrogen content.

Response 5: Thank you for pointing it out. The remark is fair since calibration with standards is not appropriate for light elements such as oxygen and nitrogen (carbon and boron are in the same group of elements) and allows determining only their presence, but not accurately. There is also possible overlapping between nitrogen and other contributions (for example, Ti). Estimated measurement error is of 5 at% [1]. However, for the polished samples of ceramics it should not exceed 0.5% [2]. Table 5 is revised.

[1] Tessier, F. Determining the Nitrogen Content in (Oxy)Nitride Materials. Materials 2018, 11, 1331. https://doi.org/10.3390/ma11081331

[2] Miler, M.; Mirtič, B. Accuracy and precision of EDS analysis for identification of metal-bearing minerals in polished and rough particle samples. Geologija 2013, 56(1), 5–17. https://doi.org/10.5474/geologija.2013.001

Point 6: Please explain why the single variable method is not used to set the test parameters of the friction and wear test, and why the load and the number of cycles are changed at the same time.

Response 6: Thank you for asking it. We use two variants (combinations) of loads traditionally used in tests for fretting wear of hard films in the experiments: load = 1 N and offset = 60 μm. However, use a more intense load case (to reduce the test time) when we see that wear develops very slowly for some samples (with DLC-Si and Cr-Al-Si-N/DLC-Si coatings): load = 20 N and offset = 100 µm. For the number of cycles, wear is inherently not only a mode in multi-cycle fretting tests but the testing time in the physical sense, i.e., the number of cycles is proportional to the testing time. The relevant passage is added to the text of the manuscript.

Point 7: Lines 476-479: Whether the influence of the difference between the elastic modulus of the coating and the friction pair on the operational properties is considered.

Response 7: Thank you. In lines 476-479, we draw a conclusion based on the well-known study [72 in the text of the manuscript] and conclude that high values of coating hardness itself do not guarantee their best performance properties, and the modulus of elasticity is also very important. The best option is when its value is the closest to the value of the base material. This well-known position was also confirmed in our studies as follows: The Cr-Al-Si-N coating has the maximum hardness (30 GPa) and elastic modulus (370 GPa). However, this elastic modulus value is the most distant from the titanium alloy value. As expected, this coating did not show the best wear resistance. At the same time, the dependencies "modulus of elasticity - operational properties" were not drawn separately in the work because, nevertheless, the operational properties depend on the physical and mechanical properties of the coatings (first of all, hardness, modulus of elasticity and coefficient of friction). It is shown in Section 4 (highlighted green).

[72] Voronin, N.A. The Effective and True Adhesive Strength of Thin Protective Coatings. J. Mach. Manuf. Reliab. 2019, 48, 320–327. https://doi.org/10.3103/S1052618819040150

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The requested comments have been addressed accordingly, and the quality of the manuscript is now very good. Congratulations to all the authors!