Tribology Properties of Synthesized Multiscale Lamellar WS2 and Their Synergistic Effect with Anti-Wear Agent ZDDP
Round 1
Reviewer 1 Report
Authors present a complete study, from synthesis to microscale tribological characterization, under ambient conditions, of WS2 particles with sizes of 2µm and 500nm and ZDDP used as additives in PAO lubricant.
The particle growth and their characterization, and in general the measurement procedures are very well done and all details are presented in the text very clearly with a correct and proper use of the English language.
For all that reasons the paper represent a small but solid step in the understanding and use of lamellar particles as additives in PAO lubricant .
The paper could be published in this Journal without significant modifications.
I like just to ask authors to clarify three simple questions :
1) Description of Synthesis procedure pag. 2. How authors differentiate the two particle size during the procedure ?
2) Description of tribological test page 3 . It is not completely clear if test were done in full lubrication regime or using droplet lubrication.
3) Fig. 6 panel B. Is the label correct ? It seems taht Cyan and brown color are inveterd.
Author Response
Response to Reviewer 1
Reviewer #1: Authors present a complete study, from synthesis to microscale tribological characterization, under ambient conditions, of WS2 particles with sizes of 2µm and 500nm and ZDDP used as additives in PAO lubricant.
The particle growth and their characterization, and in general the measurement procedures are very well done and all details are presented in the text very clearly with a correct and proper use of the English language.
For all that reasons the paper represent a small but solid step in the understanding and use of lamellar particles as additives in PAO lubricant.
The paper could be published in this Journal without significant modifications.
I like just to ask authors to clarify three simple questions:
Description of Synthesis procedure pag. 2. How authors differentiate the two particle size during the procedure?
Response: We appreciate your careful review on our manuscript. The two sizes of tungsten powder used in the experiment were purchased from China Metallurgical Research Institute. The synthesis of each WS2 is completed independently. For the synthetic WS2, it is saved separately and distinguished with labels.
Description of tribological test page 3. It is not completely clear if test were done in full lubrication regime or using droplet lubrication.
Response: Thanks for your careful reading and insightful suggestions. The lubrication method used in this experiment is droplet lubrication, which has been highlighted in this paper.
3) Fig. 6 panel B. Is the label correct? It seems taht Cyan and brown color are inveterd.
Response: Yes, it is. The cyan curve represents the better experimental results in Figure 5 (b). The brown curve is the actual experimental result of adding ZDDP, so this is correct.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
In the reciprocating motion, typically the frequency of oscillation is applied instead of sliding speed, because this speed is not constant during one cycle. What kind of speed have you used: average or maximum?
Please add the reason of selection of the normal load.
Please add information about the sliding distance which was higher for a larger speed.
In figure 4 the scale is difficult to read. Please improve this figure.
The roughness height was determined by what parameter?
The analysis of wear is only qualitative. Have you tried to obtain the amounts of wear? This can be done after measurement of surface topography of wear scars.
The authors claimed that roughness height was reduced during tests. Have you measured surface roughness after tests?
Please use disc or disk.
Page 2, line 46 „surface area is large” – what does it mean?
English should be polished.
Author Response
Response to Reviewer 2
In the reciprocating motion, typically the frequency of oscillation is applied instead of sliding speed, because this speed is not constant during one cycle. What kind of speed have you used: average or maximum?
Please add the reason of selection of the normal load.
Response: We chose the loading force according to the references. LI Zhipeng. Research on Tribological Properties of Cylinder Liner-piston Ring Friction Pair of Internal-combustion Engine. China: Jiangsu Normal University; 2016.
Please add information about the sliding distance which was higher for a larger speed.
Response: It can be seen from that the total sliding distance increases with the increase of velocity under the same experimental time . Therefore, the sliding distance is longer at a higher speed.
In figure 4 the scale is difficult to read. Please improve this figure.
Response: Thanks for your careful reading and insightful suggestions. This figure has been improved.
The roughness height was determined by what parameter?
Response: The surface of the steel disc prepared in this paper is polished to a mirror surface. When the surface of the material is mirror, it is generally considered that its roughness is less than 0.05µm. The roughness of the steel balls is provided by the manufacturer.
The analysis of wear is only qualitative. Have you tried to obtain the amounts of wear? This can be done after measurement of surface topography of wear scars.
Response: We didn't get the amount of wear, which is our weakness. For wear analysis, we used scanning electron microscopy (SEM) and electron-probe micro-analyzer (EPMA) to examine the surface morphology and chemical element distribution of wear scars.
6) The authors claimed that roughness height was reduced during tests. Have you measured surface roughness after tests?
Response: In fact, we did not use an equipment to measure the surface roughness after the tests, and we reached such a conclusion based on the experimental results and theoretical derivation.
7) Please use disc or disk.
Response: Thanks for your careful reading and insightful suggestions. It has been corrected and highlighted in the paper.
8) Page 2, line 46 „surface area is large” - what does it mean?
Response: The “surface area is large” here means that the contact area between the large-sized 2H-WS2 and the friction pair is larger.
9) English should be polished.
Response: We appreciate your careful review on our manuscript. We have used MDPI's Editing service to polish the English language of our manuscript. Attached below is the English editing certificate.
Author Response File: Author Response.pdf
Reviewer 3 Report
The authors described on the tribological properties of synthesized multiscale lamellar WS2 and their synergetic effect with anti-wear agent ZDDP. This research in terms of tribological suitability is not a novelty; only the evaluation of the synergic effect between 2H-WS2 and ZDDP can be an interesting research topic. The presented results seem to be interesting, but the authors should clarify the following questions.
The Experimental section is poorly written; please add more detail about the experimental procedure. The authors did not provide information about the purity of the used materials. What happened with suspension with 2H-WS2 in the time after the sonication – (for example, 1- 2- or 3-hours). Did the Authors not observed the sedimentation of solid materials of 2HWS2. Why did the Authors add ZDDP? Please explain? How do the Authors know that the ZDDP is the right anti-wear agent? Why is 2H-WS2 selected? It was possible to use, for example, MoS2. The graphs shown in Figure 6 are not complete, and it is not known what key the authors used to present them. The authors have noticed that the smaller the particle, the lower the coefficient of friction, please explain this relationship better?Author Response
Response to Reviewer 3
The authors described on the tribological properties of synthesized multiscale lamellar WS2 and their synergetic effect with anti-wear agent ZDDP. This research in terms of tribological suitability is not a novelty; only the evaluation of the synergic effect between 2H-WS2 and ZDDP can be an interesting research topic. The presented results seem to be interesting, but the authors should clarify the following questions.
The Experimental section is poorly written; please add more detail about the experimental procedure. The authors did not provide information about the purity of the used materials.
Response: Thanks for your careful reading and insightful suggestions. The experimental part is divided into three parts: WS2 synthesis, sample oil preparation and experimental environment. Some details have been added.
What happened with suspension with 2H-WS2 in the time after the sonication - (for example, 1- 2- or 3-hours). Did the Authors not observed the sedimentation of solid materials of 2H-WS2.
Response: In fact, solid materials of 2H-WS2 do sediment after the sonication. However, in order to ensure the consistency of the experiment, we used ultrasonic treatment before each test to ensure that 2H-WS2 is evenly dispersed in the oil. Thus, there is no careful observation of sedimentation 2H-WS2 in 1-3 hours.
Why did the Authors add ZDDP? Please explain? How do the Authors know that the ZDDP is the right anti-wear agent?
Response: ZDDP, as a commonly used lubricant anti-wear additive, has been used for more than 70 years and is still a key component of almost all modern gasoline and diesel engine lubricants. Therefore, it is meaningful to study the interaction between WS2 particles and ZDDP.
Why is 2H-WS2 selected? It was possible to use, for example, MoS2.
Response: Although the structures of WS2 and MoS2 are similar, some properties of WS2 are obviously due to MoS2. In addition, WS2 can play a more important role in extreme conditions. The reason for choosing lamellar WS2 is that the current research focuses on the fullerene structure of WS2, the purpose of this paper is to study the anti-wear and anti-wear effect of lamellar WS2 in lubricating oil.
The graphs shown in Figure 6 are not complete, and it is not known what key the authors used to present them.
Response: The cyan line in Fig. 6 indicates the sample oil with better anti-friction performance in Fig. 5, while the red and brown lines refer to the experimental results with both WS2 and ZDDP additives. The purpose of Fig. 6 is to show the changes in the friction coefficient after adding both WS2 and ZDDP compared to adding WS2 alone.
The authors have noticed that the smaller the particle, the lower the coefficient of friction, please explain this relationship better?
Response: We appreciate your careful review on our manuscript. The smaller the particle, the lower the coefficient of friction，probably because small-sized 2H-WS2 are more easily to be deposited on the concave surface, filling the surface, thereby reducing the relative surface roughness and the friction between sliding surface. In addition, small-sized 2H-WS2 are more easily embedded and repaired the metal surface after scratched.
Author Response File: Author Response.pdf
