In Situ Preparation of Nano-Cu/Microalloyed Gradient Coating with Improved Antifriction Properties
, Dongrui Wang 2, Jie Yang 2, Fujie Li 2, Lingli Zuo 1,3, Feng Ge 3 and Yunbo Chen 1,3,*
Round 1
Reviewer 1 Report
Targeting to improve the tribological properties of crankshaft and the connecting rod pair, this study reports on obtaining a nano-Cu/microalloyed gradient coating by CuO nanoparticles addition on top of surface modified 42CrMo steel, followed by in-situ gaseous nitriding process. The results section was focused mainly on the characterization of the mentioned coating from structural, chemical and mechanical point of view. Additionally, the friction and wear performance in dry and oil environment is presented.
A more scientific description of the results is needed, since most of comments sections are descriptive. In this context, multiple terms need to be replaced (resolved (line 149), stacked together (line 127), affect (line 178), rose (line 252), etc.); some incorrect phrases are present (most of the surface elements are copper (line 146)); repetitions (generally (lines 43-44)); Table 4 is mentioned in the text (line 146), however, only 3 tables are presented in the manuscript. Overall, English needs to be revised trough the manuscript.
1. The authors should detail the procedure of coating 42CrMo steel using CuO nanoparticles and how the thickness was controlled (section 2.2.).
2. The paragraph in which the parts of the nitriding furnace are described in the manuscript needs to be revised (line 83-90).
3. What was the initial roughness for each surface finish? What was the roughness after CuO nanoparticles deposition?
4. The authors should mention the JCDPS No. used in X-ray diffraction pattern for CuO identification in Fig. 3(c) and Cu, ε-Fe2-3N, γ’-Fe4N in Fig. 5(d), respectively. Also, the refinement phenomenon (with a further contribution on wear resistance) is not clearly demonstrated. The paragraph from lines 178-184 needs to be revised.
5. How the mentioned failure events were correlated with each critical load? Please describe more clearly. Was also the presence of the substrate detected or the bonding strength was evaluated just based on the characteristic load at which peeling off was observed? The time and distance for adhesion investigation should be added in the Experimental procedures section.
6. What was the reason authors chose to perform the wear test in dry environment with 2 N load, whereas under oil lubrication 10 N load was applied? The oil lubrication test was performed using 4 steel balls, however, only the untreated and treated steel balls are mentioned. No features are detected on wear morphologies presented for the coated samples (Fig. 10), although wear volume was calculated (Fig. 9). Some wear track cross-sections would be helpful to clarify the wear differences between samples. Also, debris were mentioned in the section related to friction coefficient evaluation, however, the SEM images (Fig. 10) did not show their presence.
7. The authors should describe the conventional process as compared with in-situ nitriding. It is not clear what sample (# 1, 2, 3) from in-situ gaseous treatment was chosen for oil friction test. The paragraph from lines 266-277 needs to be revised. The signal presented for friction coefficient (Fig. 12) needs to be smooth in order to see the differences explained in the text for certain samples behavior during oil friction test.
8. The References section needs to be carefully revised, since two different parts are present. Also, references in Chinese could not be addressed.
Author Response
- The authors should detail the procedure of coating 42CrMo steel using CuO nanoparticles and how the thickness was controlled (section 2.2.).
A: A certain amount of CuO nanoparticles was mixed evenly with the binder (Mixed reagent of 90% terpineol and 10% ethyl cellulose) and coated on the surface of the sample by manual brushing. The thickness of the coating is controlled by the number of times of brushing.
- The paragraph in which the parts of the nitriding furnace are described in the manuscript needs to be revised (line 83-90).
A: In the nitriding process, ammonia gas was used as the gas medium, and the positive pressure of 550 Pa was kept in the furnace by U-tube manometer. The ammonia decomposition rate was measured with an ammonia decomposition meter.
- What was the initial roughness for each surface finish? What was the roughness after CuO nanoparticles deposition?
A: The surface of crankshaft shall be subject to fine grinding before modification, and its roughness shall be less than 0.2um. The crankshaft is polished after surface modification, and the roughness is also controlled below 0.2um to meet the installation accuracy requirements.
- The authors should mention the JCDPS No. used in X-ray diffraction pattern for CuO identification in Fig. 3(c) and Cu, ε-Fe2-3N, γ’-Fe4N in Fig. 5(d), respectively. Also, the refinement phenomenon (with a further contribution on wear resistance) is not clearly demonstrated. The paragraph from lines 178-184 needs to be revised.
A: The interest of this paper is to prepare copper layer in situ by gas nitriding atmosphere. During gas nitriding surface treatment, the main phase of nitriding layer is ε- Fe2-3N and γ'- Fe4N, as shown in Fig. 5(d).
- How the mentioned failure events were correlated with each critical load? Please describe more clearly. Was also the presence of the substrate detected or the bonding strength was evaluated just based on the characteristic load at which peeling off was observed? The time and distance for adhesion investigation should be added in the Experimental procedures section.
A: Critical load, the bonding strength and the time and distance for adhesion investigation have been analyzed and discussed in detail in the previous research papers, see reference [22] for details.
[22] Green preparation of copper/microalloyed gradient coating on crankshaft surface with improved wear resistance, Mater. Res. Express, 2019, 6, p 116588.
- What was the reason authors chose to perform the wear test in dry environment with 2 N load, whereas under oil lubrication 10 N load was applied? The oil lubrication test was performed using 4 steel balls, however, only the untreated and treated steel balls are mentioned. No features are detected on wear morphologies presented for the coated samples (Fig. 10), although wear volume was calculated (Fig. 9). Some wear track cross-sections would be helpful to clarify the wear differences between samples. Also, debris were mentioned in the section related to friction coefficient evaluation, however, the SEM images (Fig. 10) did not show their presence.
A: The critical load of the contact specimen is calculated according to Hertz contact theory, so the contact critical load is different, which theoretical calculation of critical load was given in reference [22].
[22] Green preparation of copper/microalloyed gradient coating on crankshaft surface with improved wear resistance, Mater. Res. Express, 2019, 6, p 116588.
- The authors should describe the conventional process as compared with in-situ nitriding. It is not clear what sample (# 1, 2, 3) from in-situ gaseous treatment was chosen for oil friction test. The paragraph from lines 266-277 needs to be revised. The signal presented for friction coefficient (Fig. 12) needs to be smooth in order to see the differences explained in the text for certain samples behavior during oil friction test.
A: In order to better simulate the actual working environment, a four ball friction and wear tester was used to carry out the friction test under the condition of mineral oil lubrication, and the results are shown in Fig. 12. The friction coefficient has been smoothed in order to see the differences explained in the text for certain samples behavior during oil friction test.
- The References section needs to be carefully revised, since two different parts are present. Also, references in Chinese could not be addressed.
A: The References section has been carefully revised, respectively.
Reviewer 2 Report
The authors have discussed the improvement in anti-friction properties of in-situ prepared nano-Cu/microalloyed gradient coating. The experiments are adequate, flow is okay, but grammatical mistakes are noted. Also, a few symbols and SI units are written incorrectly, and some explanations are missing. The following comments should be addressed before considering for publication:
1. What is the novelty of the work?
2. Line 49: HV not hv.
3. Line 74-75: should be 400 and 2000 grit SiC abrasive paper, using # creates confusion with the coating codes (table 2).
4. Line 111: wear testing machine name should be in the capital.
5. Line 111: It should be 2 N, not 2n.
6. Line 120: it should be 'an hour-long friction and wear test', not 'one hour friction and wear test'.
7. Line 121: should be written as 1200 rpm, not 1200r/min.
8. Fig 7: Why the coatings #1, #2, #3 have similar hardness but different grain sizes? Why coating #0 has higher hardness at the surface?
9. Line 205: What is bonding strength? Is it the critical load value that the authors have reported? Is it LC2 or LC3? Again 'N', not 'n'.
10. Line 213-217: can the authors explain why the bonding strength is only 34 N for the crankshaft when coatings studied in this work have such a high bonding strength (107 - 150 N)?
11. Fig 9: if the hardness profiles of coatings #1, #2 and #3 are similar, how the wear volumes are so different? if the grain size refinement is a contributing factor to this as the authors mentioned in line 157, then how do coatings #2 and #3 show such noticeable differences in wear volume (1 and 0.5)?
12. Fig 12: there is almost no difference in friction curve or in the wear spot dia for in-situ and conventional gas-nitrided coating. So, what are the reasons that authors conclude that the in-situ process is better than the conventional one? Is the friction mechanism different from in-situ to conventional processes?
13. Re-write the conclusion section. It should be exact and precise based on the results, not a summary of what work has been done in this study or any previous study.
Author Response
- What is the novelty of the work?
A: Through the failure analysis of crankshaft connecting rod, the preparation process of copper/microalloy layer is designed and optimized, and the anti-wear and lubrication mechanism of copper/microalloy layer is revealed.
- Line 49: HV not hv.
A: The error has been corrected.
- Line 74-75: should be 400 and 2000 grit SiC abrasive paper, using # creates confusion with the coating codes (table 2).
A: The error has been corrected.
- Line 111: wear testing machine name should be in the capital.
A: The error has been corrected.
- Line 111: It should be 2 N, not 2n.
A: The error has been corrected.
- Line 120: it should be 'an hour-long friction and wear test', not 'one hour friction and wear test'.
A: The error has been corrected.
- Line 121: should be written as 1200 rpm, not 1200r/min.
A: The error has been corrected.
- Fig 7: Why the coatings #1, #2, #3 have similar hardness but different grain sizes? Why coating #0 has higher hardness at the surface?
A: The surface of samples 1 #, 2 #, and 3 # is a nano copper layer, and the copper layer is soft, so the hardness value is basically the same. At different sintering temperatures, the surface nano copper particles grow up with the increase of temperature, so the grain size increases with the increase of temperature. 0 # sample is not prepared with copper layer on the surface, but is only nitrided with the base sample. Iron nitride is formed on the surface layer during the nitriding process, which shows high hardness value.
- Line 205: What is bonding strength? Is it the critical load value that the authors have reported? Is it LC2 or LC3? Again 'N', not 'n'.
A: The critical load of the contact specimen is calculated according to Hertz contact theory, so the contact critical load is different, which theoretical calculation of critical load was given in reference [22].
[22] Green preparation of copper/microalloyed gradient coating on crankshaft surface with improved wear resistance, Mater. Res. Express, 2019, 6, p 116588.
The error has been corrected.
- Line 213-217: can the authors explain why the bonding strength is only 34 N for the crankshaft when coatings studied in this work have such a high bonding strength (107 - 150 N)?
A: Using the scratch method, literature [22] reported that the binding force of copper coating on QT500-7 surface was 43N. However, in the study of this paper, the bonding force of copper coating on the surface of 42CrMo steel is significantly improved, which may be related to the interface bonding between copper and alloy elements, which still needs further research.
- Fig 9: if the hardness profiles of coatings #1, #2 and #3 are similar, how the wear volumes are so different? if the grain size refinement is a contributing factor to this as the authors mentioned in line 157, then how do coatings #2 and #3 show such noticeable differences in wear volume (1 and 0.5)?
A: The wear resistance of the coating is not only related to its hardness, but also to its surface microstructure, as shown in the Figure 5. It is also closely related to the binding force of the coating, as shown in the Figure 8. Therefore, after different surface modification processes, the surface hardness value is similar, but the wear rate has a significant change.
- Fig 12: there is almost no difference in friction curve or in the wear spot dia for in-situ and conventional gas-nitrided coating. So, what are the reasons that authors conclude that the in-situ process is better than the conventional one? Is the friction mechanism different from in-situ to conventional processes?
A: Here is almost no difference in friction curve or in the wear spot dia for in-situ and conventional gas-nitrided coating. Although the difference of wear spot diameter is not significant, the wear rate of surface modification is significantly reduced, which can show the friction mechanism different from in-situ to conventional processes.
- Re-write the conclusion section. It should be exact and precise based on the results, not a summary of what work has been done in this study or any previous study.
A: The conclusion has been rewritten in order to better highlight the research conclusion and the innovation of the paper.
Reviewer 3 Report
The article deals with research on ensuring wear resistance by applying coatings with improved anti-friction properties. The presented research is topical and will be of interest to many researchers worldwide. The practical significance of the obtained results is in their application for improving the functional properties of highly loaded parts of compressors.
Despite the available strengths of the article, it has the following flaws that could be eliminated:
1. The article’s aim/goal/purpose should be highlighted in the text more transparently.
2. The authors previously published their paper on Cu/microalloyed gradient coating on crankshaft surface with improved wear resistance: doi: 10.1088/2053-1591/ab4aa7. In this regard, the scientific novelty of the submitted article should be thoroughly substantiated.
3. I recommend adding analytical expressions for analysis of the obtained graphical results.
4. Quality of Figs. 1, 6-b, 7, and 11 should be increased.
5. A discussion of the obtained results is not presented. In this regard, it is unclear how they correspond to recent studies in wear resistance, e.g., doi: 10.3390/met11081153 and many other research works worldwide.
6. The conclusions include declarative statements. Therefore, please add some quantitative indicators to highlight the aim’s achievement.
7. There is the improper numbering of the reference list (lines 388-391).
8. The number of self-citations of the 1st author [14, 18, 22] should be decreased.
Author Response
- The article’s aim/goal/purpose should be highlighted in the text more transparently.
A: The article’s aim/goal/purpose has been highlighted in the text more transparently.
- The authors previously published their paper on Cu/microalloyed gradient coating on crankshaft surface with improved wear resistance: doi: 10.1088/2053-1591/ab4aa7. In this regard, the scientific novelty of the submitted article should be thoroughly substantiated.
A: In this paper, the influence of different materials(QT500-7、42CrMo steel) on the preparation of copper/ microalloy coating was studied, and the anti-wear and lubrication mechanism of the coating was discussed.
- I recommend adding analytical expressions for analysis of the obtained graphical results.
A: This is a very good suggestion. We will add analytical expressions for analysis of the obtained graphical results later.
- Quality of Figs. 1, 6-b, 7, and 11 should be increased.
A: Quality of Figs has been increased as far as possible.
- A discussion of the obtained results is not presented. In this regard, it is unclear how they correspond to recent studies in wear resistance, e.g., doi: 10.3390/met11081153 and many other research works worldwide.
A: In this paper, the influence of different materials(QT500-7、42CrMo steel) on the preparation of copper/ microalloy coating was studied, and the anti-wear and lubrication mechanism of the coating was discussed.
- The conclusions include declarative statements. Therefore, please add some quantitative indicators to highlight the aim’s achievement.
A: The conclusion has been rewritten in order to better highlight the research conclusion and the innovation of the paper.
- There is the improper numbering of the reference list (lines 388-391).
A: The error has been corrected.
- The number of self-citations of the 1st author [14, 18, 22] should be decreased.
A: The number of self-citations of the 1st author [14] has been decreased. Literatures [18] and [22] are the theoretical and research basis of this paper, which is closely related to the content of this paper.
Round 2
Reviewer 1 Report
The authors revised the manuscript only partially according to the comments. The quality of the manuscript is not satisfactory and I did not recommend the publication in the present form.
- References in Chinese need to be replaced with proper alternatives.
- Some incorrect phrases are still present (,,most of the surface elements are copper’’ needs to be replaced with ,, as compared with the rest of the elements, higher amount of copper was detected on the surface’’).There is a repetition of the term ,,generally’’ in the same phrase and other terms that were already suggested to be replaced.
- By applying the mentioned surface treatments (400 grit SiC abrasive paper rough grinding, 2000 grit SiC abrasive paper fine grinding and polished surface) the roughness was modified and it was mentioned in the text that the effect of this surface roughness on coating performance was explored. What was the roughness after each surface treatment?
- Fig 4(d)- how can the authors distinguish which phase (ε-Fe2-3N, γ’-Fe4N) corresponds to each peak from the measured XRD pattern without a standard-based identification?
- Part of question no. 6 did not receive any response
- For Question no. 7 the authors didn't provide clarification related to the sample used for oil friction test (# 1, 2, 3)
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors have responded to most of the queries, but Q11 is partially answered and Q12 is not answered at all. The explanations are missing for both questions. Adding these explanations into the main text would have increased its merit.
Some incorrect phrases are still present, so it is recommended to check the grammatical mistakes again.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
The manuscript has been improved. It can be recommended for publication.
Author Response
Thank you for your advice and guidance.
Round 3
Reviewer 1 Report
I recommend publication in the present form.
Author Response
Thank you for your comments and suggestions, which are of great significance to the improvement of the paper.
Best wishes.
Reviewer 2 Report
The authors have partially answered the question but did not include the explanation in the main text. That explanation is supposed to improve the merit of the work, which has been advised previously. Thus minor revision is given and if that is done, then the manuscript can be accepted in the present form.
Author Response
Thank you for your valuable suggestions. In view of the problems raised, we have made further supplements and improvements in the paper.
