Tribological Properties of Protic Ionic Liquid as an Additive in Aqueous Glycerol Solution for Ruby-Bearing Steel Tribo-Contact
, Milda Gumbytė, Artūras Kupčinskas, Jolanta Treinytė and Albinas Andriušis
Round 1
Reviewer 1 Report
The paper reported the tribological properties of protic ionic liquid as an additive in aqueous glycerol solution for ruby-bearing steel tribo-contact. The experiment is technical sound. I recommend it to publish after suitable revision.
1、EDS was used to analyze the element content on the surface of wear scars in Figure 11. More surface analysis techniques are proposed to use, such as XPS, etc., to analyze the surface chemistry. These techniques also merit the description of Figure 12.
2、Whether was there significant temperature change during test in Figure 4 and Figure 6? Whether does the temperature change have an effect on the variations of the COF?
Author Response
Title: Tribological properties of protic ionic liquid as an additive in aqueous glycerol solution for ruby-bearing steel tribo-contact
Reviewer's comments
EDS was used to analyze the element content on the surface of wear scars in Figure 11. More surface analysis techniques are proposed to use, such as XPS, etc., to analyze the surface chemistry. These techniques also merit the description of Figure 12.
Author's response
We agree with the reviewer. Moreover, we tried to do some inspections using XPS and Raman spectroscopy. Unfortunately, we have no answer because the wear scars are very small and have a curved surface (in the case of the ball). We plan to experiment on the larger scale specimens to get a reliable answer.
Reviewer's comments
Whether was there significant temperature change during test in Figure 4 and Figure 6? Whether does the temperature change have an effect on the variations of the COF?
Author's response
The temperature in the oil bath was kept stable at – 25 °C. However, we expect the temperature between the interacting surfaces (microcontact) may be much higher. So, in the discussion, we speculated that the temperature in the microcontact influences COF. However, the "macro" temperature was stable.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
1. The coefficient of friction (COF) variation during the tribo-test was continuously recorded. Please give the more explanation.
2. What are reasons behind for measuring the viscosity and density at the temperature range in between 20 to 70 °C, why not more differences?
3. In figure 3, seem that the temperatures were observed 20, 25 30 and so on, it will be finest if authors can keep 5 degrees Celsius gap from each temperature, onwards 30-degree Celsius also.
4. In figure 4, shown 30 minutes time observation, it should be 1 hour or need to give clarification for better understanding of readers.
5. Figure 6, seems that sudden increasing and decreasing of COF under higher remarks, why?
6. Figure 9, OM images of the wear scars on the plate observation scale are not visible, please revised.
7. In conclusion section "Significant friction and wear reduction were achieved using investigated PIL 272 additives. At the concentration of 0.5 %, 2.6- and 15.8 times, friction and wear 273 reduction were reached." Please check this sentence.
Author Response
Title: Tribological properties of protic ionic liquid as an additive in aqueous glycerol solution for ruby-bearing steel tribo-contact
Reviewer's comments
- The coefficient of friction (COF) variation during the tribo-test was continuously recorded. Please give the more explanation.
Author's response
The COF was recorded with a frequency of 2 Hz. The data from the last 10 min was used to calculate the mean COF. We supplemented the 2.3 section with this explanation.
Reviewer's comments
- What are reasons behind for measuring the viscosity and density at the temperature range in between 20 to 70 °C, why not more differences?
Author's response
The temperature range was selected according to the properties of investigated lubricating fluid and performed tribo-test. The minimum temperature of 20 °C was slightly lower than during the tests. Above the temperature of 70 °C intensive water evaporation occurs, and the results become unstable. Even though we kept the temperature in the oil bath stable at 25 °C, we expect the actual temperature in the microcontact may be higher. Therefore we wanted to know the behaviour of the fluid at higher temperatures.
Reviewer's comments
- In figure 3, seem that the temperatures were observed 20, 25 30 and so on, it will be finest if authors can keep 5 degrees Celsius gap from each temperature, onwards 30-degree Celsius also.
Author's response
Considering the comment of the reviewers, we supplemented Figure 3 with additional data. Due to increased data points, the line did not fill between the points. Therefore we used trendlines instead of lines between points.
Reviewer's comments
- In figure 4, shown 30 minutes time observation, it should be 1 hour or need to give clarification for better understanding of readers.
Author's response
There is no standard time duration for this kind of tribo-test. However, we agree that more extended experiments would be beneficial. Therefore if we find some patterns, we perform longer experiments similar to those shown in Figure 6.
Reviewer's comments
- Figure 6, seems that sudden increasing and decreasing of COF under higher remarks, why?
Author's response
The processes leading to COF's "sudden increase" and "slow decline" are explained in section 3.3, Page 9, Lines 250-262.
<< the layer grows and becomes thicker while shearing between its layers occurs. At this point, part of the layer is attached to the ball surface resulting in intense shearing (Figure 12 c). It implies that shearing within the layer requires more energy than sliding the ball on the layer. As a result, the friction is increased. The explained process could be distinguished in the extended experiments repetitions from 1 to 4 where sudden (duration ≈ 1 min) COF rise is observed (Figure 6). Although friction is higher in this period, the more intense wear does not occur because the layer separates interacting surfaces. This additional energy is consumed to shear the layer. Due to sharing, the contact temperature increases, leading to lower lubricating fluids' viscosity (Figure 3) and disturbing layer formation equilibrium. Thus, after some time, the layer vanishes from the ball and friction decreases. The friction slope, which takes about five minutes, was observed in the COF variation curves (Figure 6).>>
Reviewer's comments
- Figure 9, OM images of the wear scars on the plate observation scale are not visible, please revised.
Author's response
We have improved the visibility of the scales.
Reviewer's comments
- In conclusion section "Significant friction and wear reduction were achieved using investigated PIL 272 additives. At the concentration of 0.5 %, 2.6- and 15.8 times, friction and wear 273 reduction were reached." Please check this sentence.
Author's response
The COF and wear reduction are correct. The additive-free W:GL has a COF of 0.13 and a wear volume of 43.47 µm3. The additive-loaded W:GL+0.5% PIL has a COF of 0.05 and a wear volume of 2.75 µm3. This data is presented in Figures 5 and 7.
All the changes made in the manuscript are marked in red colour. The changes that occurred due to language editing were not highlighted.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The revised manuscript can be accepted now according to the repsonse to comments.
