Wear and Lubrication Performance of Different Reticular-Textured TiN-Coated Surfaces under Lubricated Conditions

Round 1

Reviewer 1 Report

In general, paper is well written and the authors present a texture geometric study on brass with TiN coating under lubricated rotating condition. However, paper needs to be revised before accepting it for publication, see further details below:

1. While the author clearly conveyed the objective and systematic study on the parameters, the application background and relative research need to be introduced in introduction. This is important for the usage of brass as substrate.  Also, the reason for selecting stainless steels as friction pair also has been ignored by authors.

2. Were brass pins flat-ended or spherical? What was hardness of pins.

3. The thickness, morphologies, and compositions of TiN coating were not given in the paper.

4. What was the accuracy of the gap measurement? It should be added to the part of tribological test.

5. In the lubrication test, why this lubricant was used and which type of the oil it was?

6. In the lubrication test, it was found that the load-carrying capacity was strongly affected by the operation condition, i.e. by both speed and gap. However, how to ensure that the two mating surfaces were parallel to each other?

7. Although lubrication test results showed improvement, but the order of improvement was so small. It will be most probably lost in the uncertainty of a real application. Authors should clearly mention that these improvements are purely observed under laboratory conditions.  

8. In general, the authors did not explain or discuss the origin of the load carrying capacity. In literature there were a number of explanations to load carrying capacity in parallel film bearings. One was thermal wedge. But can it be expected here? One was cavitation in the dimple and pressure generation at the edge of the dimple when the film converges again. Inlet suction played a role in that case. Can this effect be expected here? A third one was that the thin film theory didn’t hold and additional terms in the Navier-Stokes equations must be considered, the inertia terms for example. Was that effect to be expected here? The authors should study their setup and try to illustrate if any of these effects are important in their experiment.

9. Fig 11 should be revised to present these smaller wear loss of

Author Response

Thank you very much for your careful review and constructive suggestions with regard to our manuscript entitled " Wear and lubrication performance of different reticular-textured TiN-coated surfaces under lubricated conditions" (ID: processes-2040686). Those comments are helpful for authors to revise and improve our paper. We have studied comments carefully and tried our best to revise and improve the manuscript and made great changes in the manuscript according to the referees′ good comments. Revised portion is marked in red in the paper. We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the corrections will meet with approval. Please feel free to contact us with any questions and we are looking forward to your consideration. The main corrections in the paper and the responses to the reviewer’s comments are as following：

1. While the author clearly conveyed the objective and systematic study on the parameters, the application background and relative research need to be introduced in introduction. This is important for the usage of brass as substrate.  Also, the reason for selecting stainless steels as friction pair also has been ignored by authors.

Reply: We thank you very much for your careful review and constructive suggestions. Your suggestions makes our manuscript more read-friendly. 

The purpose of choosing brass substrate was that many mechanical components were made of brass, such as slippery boots in the hydraulic pump. And the majority of researches focused on the steel substrate. So choosing brass as the substrate materials would perfect shortcomings of current research to some degree. (Line37-line40)

2. Were brass pins flat-ended or spherical? What was hardness of pins.

Reply: We are very sorry for our negligence of the explanation. Brass pins were flat-ended. And the brass nominal hardness was 105-175HV. (Line97)

3. The thickness, morphologies, and compositions of TiN coating were not given in the paper.

Reply: Thanks for your kind suggestions. The thickness of TiN coating was about 6. The corresponding description was added in the manuscript.(Table 2). (Line112-line114)

4. What was the accuracy of the gap measurement? It should be added to the part of tribological test.

Reply: Thanks for your kind suggestions. The gap between the upper sample and the lower sample was 5μm. The accuracy of position sensor is 0.01m, friction test machine(Bruker-3, USA) control this accuracy. (Line136-line137)

5. In the lubricationtest, why this lubricant was used and which type of the oil it was?

Reply: Thanks for your kind suggestions. This lubricant was used because it was often used in actual working conditions. Mobil 15W-40 was lubricant type.

6. In the lubrication test, it was found that the load-carrying capacity was strongly affected by the operation condition, i.e. by both speed and gap. However, how to ensure that the two mating surfaces were parallel to each other?

Reply: Thanks for your kind suggestions, which is valuable for improving the accuracy of the manuscript. In order to ensure two mating surface were parallel, high precision parallelism of texture samples has been ensured during sample preparation. And in the experimental process of the experimental equipment of high precision and the use of special fixture to ensure the parallel position of the contact surface. In the experiment, references[19-21] were used for reference to adjust the parallelism. (Line135-line136)

7. Although lubrication test results showed improvement, but the order of improvement was so small. It will be most probably lost in the uncertainty of a real application. Authors should clearly mention that these improvements are purely observed under laboratory conditions.  

Reply: Thanks for your constructive suggestion. The corresponding description was added in the manuscript. (Line197)

8. In general, the authors did not explain or discuss the origin of the load carryingcapacity. In literature there were a number of explanations to load carrying capacity in parallel film bearings. One was thermal wedge. But can it be expected here? One was cavitation in the dimple and pressure generation at the edge of the dimple when the film converges again. Inlet suction played a role in that case. Can this effect be expected here? A third one was that the thin film theory didn’t hold and additional terms in theNavier-Stokes equations must be considered, the inertia terms for example. Was that effect to be expected here? The authors should study their setup and try to illustrate if any of these effects are important in their experiment.

Reply: Thanks for your kind suggestions, which is valuable for improving the accuracy of the manuscript. While considering additional terms in the Navier-Stokes equations, both convergent wedges and cavitation have an impact on the generation of load carrying capacity. From the comparison of simulation phenomena in the paper, it can be seen that due to the existence of reticular texture, when the lubricating oil flows through the reticular texture grooves, the pressure will first decrease and then increase, and the grooves are accompanied by cavitation phenomenon, so the convergence wedge and cavitation are the main reasons for the improvement of load carrying capacity. (Line196-line198)

9. Fig 11 should be revised to present these smaller wear loss of(line313-line314)

Reply: Thanks for your constructive suggestion. The corresponding revision was added in the manuscript. (Line326-line327)

Thank you for your comments that the information of our manuscript is more complete and we hope the corrections will meet with approval.

Reviewer 2 Report

The manuscript “Wear and lubrication performance of different reticular-textured TiN-coated surfaces under lubricated conditions”, is written well and contains some interesting results. However, I think that this manuscript requires some improvements in following areas:

• Add some quantitative finding in abstract.
• Include: wear and lubrication as keyword
• Referencing style is not as per the requirement. Correct it.
• Section 2: Add chemical compositions of used materials
• Explain sub figures of fig. 2 and 3.
• Include the actual photograph of experimental setup employed in study.
• Error bar can be added in Figure 4.
• What is the reason for such substantial increment of load carrying capacity in case of TiN-Ang-120. Explain this behaviour with results of past studies as well.  
• Figure 6 and 7: Pressure and velocity scale bars are not visible.
• Discussion: Compare your findings with similar studies.
• Mention the limitations and further scope of improvement in last section.

Author Response

Thank you very much for your careful review and constructive suggestions with regard to our manuscript entitled " Wear and lubrication performance of different reticular-textured TiN-coated surfaces under lubricated conditions" (ID: processes-2040686). Those comments are helpful for authors to revise and improve our paper. We have studied comments carefully and tried our best to revise and improve the manuscript and made great changes in the manuscript according to the referees′ good comments. Revised portion is marked in red in the paper. We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the corrections will meet with approval. Please feel free to contact us with any questions and we are looking forward to your consideration. The main corrections in the paper and the responses to the reviewer’s comments are as following：

1.Add some quantitative finding in abstract.

Include: wear and lubrication as keyword

Reply: Thanks for your constructive suggestion. Some quantitative finding and keywords were added in the manuscript.

Referencing style is not as per the requirement. Correct it.

Reply: We are very sorry for our incorrect reference. We have made correction about reference style according to the requirement.

2.Section 2: Add chemical compositions of used materials

Reply: We are very sorry for our negligence of the explanation. In this work, brass was consist of copper (nominal composition:0.68 wt%) and zinc (nominal composition: 0.32 wt%) was selected to pins. The 304 stainless steel was consist of Fe(67-71.5%), Cr(17.5-19.5%),Ni(8-10.5%).The compositions description was added in the manuscript. (Line98-line101)

3.Explain sub figures of fig. 2 and 3.

Include the actual photograph of experimental setup employed in study.

Reply: We are very sorry for our negligence of the explanation. The corresponding description was added in the manuscript. (Line116-line117, line130-line131)

4.Error bar can be added in Figure 4.

Reply: Thanks for your constructive suggestion. The error bar was added in the manuscript. (Line179-line180)

5.What is the reason for such substantial increment of load carrying capacity in case of TiN-Ang-120. Explain this behaviour with results of past studies as well.  

Reply: We are very sorry for our negligence of the explanation. The reason for such substantial increment of load carrying capacity in case of TiN-Ang-120°, on the one hand, was the effect of velocity gradients[23]. The change of groove angle made the flow of fluid different. It was in a certain angle with the direction of motion velocity, it had better flow when the angle was smaller. On the other hand, The increase of groove angle had a wireless relationship with the increase of bearing capacity. The maximum bearing capacity exists only when the groove angle was 120°. (Line186-line191)

6.Figure 6 and 7: Pressure and velocity scale bars are not visible.

Reply: Thanks for your constructive suggestion. The corresponding revision was added in the manuscript. (Line235-line239)

7.Discussion: Compare your findings with similar studies.

Mention the limitations and further scope of improvement in last section.

Reply: Thanks for your constructive suggestion. About compare with similar studies, it has been mentioned in the introduction. The limitations and further scope were added in the manuscript. (Line329-line332)

Thank you for your comments that the information of our manuscript is more complete and we hope the corrections will meet with approval.