Self-Lubricating Property of TiB2-Ni Coating in the Hot Forging Die of Aluminum Alloy
Round 1
Reviewer 1 Report
The authors have carried out an interesting work but I am afraid that the manuscript missing few fundamental logics. The authors have shown that the thermal cycle at 600 deg C did not have a significant effect on hardness variation, the variation shown by the authors is insignificant. This means the reinforced TiB2 particles are very much stable at the testing temperature. Whereas author claims that the B form oxides, which act as the solid lubricant during the wear test. The authors' claims are contradictory.
Author Response
Response to Reviewer 1 Comments
On behalf of all the contributing authors, I would like to express our sincere appreciations for your constructive comments concerning our article entitled “Self-lubricating property of TiB2-Ni coating in the hot forging die of aluminum alloy”. These comments are all valuable and helpful for improving our article. According to your comments, we have made corrections and modifications (in blue color) in the revised manuscript. Point-by-point responses to the reviewer are listed below this letter.
I look forward to hearing from you soon.
Sincerely Yours,
Huajun Wang, Prof.
School of Materials Science and Engineering
Wuhan University of Technology, Wuhan
China
- The authors have carried out an interesting work but I am afraid that the manuscript missing few fundamental logics. The authors have shown that the thermal cycle at 600 deg C did not have a significant effect on hardness variation, the variation shown by the authors is insignificant. This means the reinforced TiB2 particles are very much stable at the testing temperature. Whereas author claims that the B form oxides, which act as the solid lubricant during the wear test. The authors' claims are contradictory.
Response:
Thank you very much for this comment. TiB2 is not stable during the plasma melt injection process, but reacts with nickel-based alloys to generate new phases such as TiB2, TiB, TiC and CrB, which are considered as one of the significant reasons to the increased hardness of composite coatings, and also the maintained high hardness of composite coatings during the thermal fatigue cycles. Therefore, we considered that the small fluctuation of hardness during sliding process did not mean the TiB2 inside coating was stable.
In addition, it was reported that at high temperature, when the oxide layer is depleted by wear, boron has a tendency to diffuse to surfaces with higher oxygen potential and oxidizes again to replenish the depleted lubricant layer. We analyzed the chemical composition of the debris on the disc (TN15) after sliding by EDS, and found that it contained a large amount of B without Ti, which helped to confirm the tendency of boron to diffuse to the friction surface.
Line 194, Page 6:
Combined with the XRD spectrum of TN15 (Figure 6) and previous studies[27], the black blocky phase are mainly TiB2, TiB, TiC, and CrB; the white matrix is the γ-(Fe, Ni) solid solution phase; and the gray strip-shaped phase is rich in Fe, Cr, and Ni.
Line 332, Page 12:
It was reported that the boron inside the coating tends to diffuse to surfaces with higher oxygen potential and oxidize to replenish the depleted lubricant layer[32].
Changes made on Line 373, Page 14:
The chemical composition of randomly selected debris was shown in Table 5. The debris on discs of TN5 and TN10 (Figure 15(b)(d)) mainly contained aluminum alloys, coating composition, oxides, carbides and little boron. A large amount of boron and no titanium was detected in the debris on the disc of TN15 (Figure 15(f)), which confirmed that the boron had a tendency to replenish the boron-rich lubricant consumed on the coating surface, while titanium tended to retain inside the coating.
Table 5 EDS results of the debris on different aluminum alloy discs.
( Please read the attachment )
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This manuscript is a detailed description of the investigation of the self-lubricating property of TiB2-Ni coating in the hot forging die of aluminum alloy. This topic is really current and the area is of great industrial interest. The paper is well drafted, understandable. The Figures and illustrations are all clear and they support the text in manuscript. The characterizations and measurements, such as morphology, XRD, microhardness, fatigue and wear behaviour are well described and satisfactorily evaluated.
Overall, I can recommend this manuscript for publication.
Author Response
Response to Reviewer 2 Comments
On behalf of all the contributing authors, I would like to express our sincere appreciations for your constructive comments concerning our article entitled “Self-lubricating property of TiB2-Ni coating in the hot forging die of aluminum alloy”. These comments are all valuable and helpful for improving our article. According to your comments, we have made corrections and modifications (in blue color) in the revised manuscript. Point-by-point responses to the reviewer are listed below this letter.
I look forward to hearing from you soon.
Sincerely Yours,
Huajun Wang, Prof.
School of Materials Science and Engineering
Wuhan University of Technology, Wuhan
China
- This manuscript is a detailed description of the investigation of the self-lubricating property of TiB2-Ni coating in the hot forging die of aluminum alloy. This topic is really current and the area is of great industrial interest. The paper is well drafted, understandable. The Figures and illustrations are all clear and they support the text in manuscript. The characterizations and measurements, such as morphology, XRD, microhardness, fatigue and wear behaviour are well described and satisfactorily evaluated.
Response:
Thanks for the reviewer’s positive comment.
Author Response File: Author Response.pdf
Reviewer 3 Report
The subject matter is very interesting, important, and has a special value considering practical applications. The paper is clearly presented and well organized. There are still some things that could be improved, and a few questions that have to be answered before publication. Therefore, I suggest a mandatory revision of the following points to increase the quality of the paper:
1. The authors should provide detailed data on the devices used for testing, i.e. device name, manufacturer, and country of manufacture.
2. In addition, the parameters for which the tests were performed, ie SEM, XRD, and other methods, should be provided.
3. In Figs. 6a and 6b the authors should show where the SEM pictures are shown in Figures 6c and 6d were taken.
4. Moreover, the zone of the coating at the interface with the substrate should be shown with higher magnification.
5. No information on the HAZ.
6. Why was H13 steel chosen as the substrate?
7. In lines 268-269, the authors write: It can be seen that a large number of grooves filled by bright white substance, which was proved to be aluminum alloy and its oxides by EDS, are distributed on TN5.
Information on EDS testing is not available in the manuscript.
Line 281-282: The transferred aluminum alloy is also mixed with debris generated by the composite coating, as well as oxides and carbides.
Line 288: At the early stage of dry sliding, there were less oxide and boric acid generated on the surface of coatings. The transferred aluminum alloy was extruded ....
Line 300: Multi-layer transferred aluminum alloy was observed on TN10 ....
Line 330-338: The debris on discs of TN5 and TN10 (Figure 14 (b) (d)) mainly contains aluminum alloys, oxides and carbides, while a large amount of boron and less Ti is detected in the debris on discs of TN15 (Figure 14 (f)). The boron-rich products formed on the surface of coatings increased with the TiB2, which reacted with water and oxygen in the air to generate boric acid, an efficient solid lubricant, to avoid the metal-to-matel contact of friction pairs, thereby not only reduce friction coefficient but also reduce the transferred aluminum alloy on the coating surface and the abrasive wear of counterpart [21.33]. In the friction process, the debris mixed with the boric acid stripped from the lubricant film can serve as lubricant particles and move between the friction surfaces.
line 343: This boron-rich debris ...
You can read similar information in the conclusions.
On what basis did the authors make these descriptions?
The authors should provide the results of the research on the chemical composition of EDS. In line 270 they have mentioned EDS but have not shown the results.
8. In addition, XRD tests of the worn coating surface should be performed.
9. Authors should show SEM images of the surface of specimens and counter-specimens prior to friction wear testing.
11. If possible, the authors should perform EDS tests of the coating on a metallographic specimen in the form of linear distributions of element concentrations and analyses in selected areas.
12. I would highly appreciate it if the authors added in literature (introduction):
https://doi.org/10.1016/j.wear.2021.203663 and
https://doi.org/10.1016/j.wear.2018.12.026
Recommendation:
This manuscript in the presented form is not acceptable for publication in the Coatings. The major revision is necessary.
Author Response
Response to Reviewer 3 Comments
(Please read the pictures in the attachment)
On behalf of all the contributing authors, I would like to express our sincere appreciations for your constructive comments concerning our article entitled “Self-lubricating property of TiB2-Ni coating in the hot forging die of aluminum alloy”. These comments are all valuable and helpful for improving our article. According to your comments, we have made corrections and modifications (in blue color) in the revised manuscript. Point-by-point responses to the reviewer are listed below this letter.
I look forward to hearing from you soon.
Sincerely Yours,
Huajun Wang, Prof.
School of Materials Science and Engineering
Wuhan University of Technology, Wuhan
China
- The authors should provide detailed data on the devices used for testing, i.e. device name, manufacturer, and country of manufacture.
Response:
Thank you very much for this comment. We have completed the information.
Changes made on Line 133, Page 4:
The composition of the specimen was analyzed by X-ray diffractometer (XRD, D8 Advance, Bruker, Germany) with Cu Kα radiation (λ=1.54056 Å) and the scanning speed was 1°/min and the scanning angle was 30° to 100°. The microstructure and composition of the specimen were performed by sanning electron microscopy (SEM, JXA-8230/INCAX-ACT, JEOL, Japan) with energy dispersive spectrometer (EDS).
Changes made on Line 138, Page 4:
Microhardness was tested on coatings surface by microhardness tester (HMAS-C1000SZA, FULEY, China) with a load of 0.1 Kg for a 10 s dwell time.
Changes made on Line 142, Page 4:
The self-made thermal fatigue apparatus was shown in Figure 4.
Changes made on Line 161, Page 5:
The wear tests of coatings sliding against the 7075-T6 aluminium alloy (the chemical composition is shown in Table 3) disc were conducted by a self-made pin-on-disk tribometer, as shown in Figure 5, lasted 1h in ambient air.
- In addition, the parameters for which the tests were performed, ie SEM, XRD, and other methods, should be provided.
Response:
Thank you very much for this comment. We have completed the information.
Changes made on Line 133, Page 4:
The composition of the specimen was analyzed by X-ray diffractometer (XRD, D8 Advance, Bruker, Germany) with Cu Kα radiation (λ=1.54056 Å) and the scanning speed was 1°/min and the scanning angle was 30° to 100°.
- In Figs. 6a and 6b the authors should show where the SEM pictures are shown in Figures 6c and 6d were taken.
Response:
Thank you very much for this comment. Figure 6(a)(b) showed the microstructure of the bottom of the composite coating. The microstructure of the upper-middle part of the coating was uniform, so Figure 6(c)(d) were randomly selected and photographed in the upper-middle part of the coating to provide a clear view of the phases . We have added a detailed description in the article.
Changes made on Line 188, Page 6:
The addition of TiB2 inhibited the growth of dendrites at the interfacial transition zone, as shown in Figure 6(a)(b), indicating that the temperature gradient of the melt pool was disturbed.
Changes made on Line 191, Page 6:
Grey strip-shaped phase, flower-like eutectic structure, acicular phase and black blocky phase were observed to be uniformly distributed in the upper-middle part of the composite coatings, as shown in Figure 6(c)(d).
Changes made on Line 205, Page 6:
Figure 6. SEM images of the coatings from the cross section: (a) the interface between TN10 and H13 substrate; (b) the interface between TN15 and H13 substrate;(c) the magnified image of the uniform microstructure in the upper-middle part of TN10; (d) the upper-middle part of TN15 with uniform microstructure.
- Moreover, the zone of the coating at the interface with the substrate should be shown with higher magnification.
Response:
Thank you very much for this comment. We have supplemented the magnified images of the interface between the coating and substrate.
Changes made on Line 205, Page 6:
Figure 6. SEM images of the coatings from the cross section: (a) the interface between TN10 and H13 substrate; (b) the interface between TN15 and H13 substrate; (c) the magnified image of the uniform microstructure in the upper-middle part of TN10; (d) the upper-middle part of TN15 with uniform microstructure.
- No information on the HAZ.
Response:
Thank you very much for this comment. We have supplemented the description of HAZ.
Changes made on Line 101, Page 3:
As shown in Figure 2 and Figure 3, the Ni60A powders were ejected from the powder feeding port and then melted in the plasma arc and deposited on the H13 steel plate in protection of argon gas, while the TiB2 powders were sent into the molten pool from outer powder injection nozzle. The heat affected zone (HAZ) of the molten pool of plasma melt injection is similar to that of conventional welding, which is not conducive to the spread of ceramic powder for its lower temperature than transferred arc zone (TAZ). The non-transferred arc zone (NTAZ), on the other hand, is located in the center of the heat source and has the highest temperature, which tends to cause burnout of the ceramic powder. Therefore, the PTA+PMI method was chosen instead of the mixed powder method to minimize the burnout of TiB2 powder, thus improving the utilization.
- Why was H13 steel chosen as the substrate?
Response:
H13 steel is a widely used hot work die steel with high wear resistance, toughness and resistance to thermal shock, which is suitable as a hot forging die material for aluminum alloy. We have added the reasons for using H13 steel as the substrate.
Changes made on Line 91, Page 2:
H13 steel is a widely used hot work die steel with high wear resistance, toughness and resistance to thermal shock, which is suitable as a hot forging die material for aluminum alloy. The H13 steel plate of 160 ×110 ×15 mm3 was used as the substrate (C 0.37, Cr 5.13, Mo 1.43, V 1.00, Si 1.00, Mn 0.34, Fe balance).
- In lines 268-269, the authors write: It can be seen that a large number of grooves filled by bright white substance, which was proved to be aluminum alloy and its oxides by EDS, are distributed on TN5.
Information on EDS testing is not available in the manuscript.
Line 281-282: The transferred aluminum alloy is also mixed with debris generated by the composite coating, as well as oxides and carbides.
Line 288: At the early stage of dry sliding, there were less oxide and boric acid generated on the surface of coatings. The transferred aluminum alloy was extruded ....
Line 300: Multi-layer transferred aluminum alloy was observed on TN10 ....
Line 330-338: The debris on discs of TN5 and TN10 (Figure 14 (b) (d)) mainly contains aluminum alloys, oxides and carbides, while a large amount of boron and less Ti is detected in the debris on discs of TN15 (Figure 14 (f)). The boron-rich products formed on the surface of coatings increased with the TiB2, which reacted with water and oxygen in the air to generate boric acid, an efficient solid lubricant, to avoid the metal-to-matel contact of friction pairs, thereby not only reduce friction coefficient but also reduce the transferred aluminum alloy on the coating surface and the abrasive wear of counterpart [21.33]. In the friction process, the debris mixed with the boric acid stripped from the lubricant film can serve as lubricant particles and move between the friction surfaces.
line 343: This boron-rich debris ...
You can read similar information in the conclusions.
On what basis did the authors make these descriptions?
The authors should provide the results of the research on the chemical composition of EDS. In line 270 they have mentioned EDS but have not shown the results.
Response:
Thank you very much for these comments.
The bright white substance filling in the grooves on TN5 was proved to be mainly aluminum alloy and its oxides by EDS planar scanning, and we have added Figure 14 and corresponding descriptions in the paper.
Changes made on Line 301, Page 10:
It can be seen that a large number of grooves filled by bright white substance, and the bright white substance was proved to be mainly from aluminum alloy disc by EDS planar scanning, as shown in Figure 14, are distributed on TN5;
Figure 13. SEM micrographs of the worn surface of pin: (a) TN5. (b) TN10. (c) TN15. (d) The etch pits filled with aluminum alloy on TN10.
Figure 14. EDS planar scanning results of Figure 13(a).
Response:
Since not only 7075 aluminum alloy but also O, B, C, Si, Mn, Fe and other chemical compositions from the composite coating were detected in the debris left on the friction surface, we inferred that this debris would likely to be embedded in the softer transferred aluminum alloy during the friction process. We have performed chemical composition analysis of the debris found on the aluminum alloy disc and added the results in the paper.
Changes made on Line 372, Page 14:
One obvious phenomenon is that the debris on discs increase with the increase of TiB2 in coatings. The chemical composition of randomly selected debris was shown in Table 5. The debris on discs of TN5 and TN10 (Figure 15(b)(d)) mainly contained aluminum alloys, coating composition, oxides, carbides and little boron. A large amount of boron and no titanium was detected in the debris on the disc of TN15 (Figure 15(f)), which confirmed that the boron had a tendency to replenish the boron-rich lubricant consumed on the coating surface, while titanium tended to retain inside the coating.
Table 5 EDS results of the debris on different aluminum alloy discs.
Response:
At the early stage of dry sliding, the coating was less oxidized. Boric acid is generated by the following reaction:
TiB2+O2→TiO2+B2O3
B2O3+H2O →H3BO3
Therefore, we inferred that less oxide and boric acid was generated during the early stage of sliding.
Response:
Regarding the morphology of the transferred aluminum alloy, we have added a clear photo of the multi-layer transferred aluminum alloy on the surface of TN10 and EDS planar scanning results to prove its formation of multi-layer structure on TN10.
Changes made on Line 337, Page 12:
Multi-layer transferred aluminum alloy was observed on TN10, as shown in Figure 15, which because the lubricant film was gradually formed on the surface of coating in the continuous wear process,thus the aluminum alloy was likely to adhere on the already formed transferred aluminum alloy layers.
Figure 15. Multi-layer transferred aluminum alloy on the TN10.
- In addition, XRD tests of the worn coating surface should be performed.
Response:
Thank you very much for this comment. We conducted conducted XRD test on the TN15 pin after sliding, and the phase component of the coating was mainly Al, Cr, Fe, Ni, C and so on, while B was almost undetectable. In our opinions, there were three main reasons:
(1) Low content of TiB2. In the literature, boric acid was detected on the surface of composites with 50 wt% TiB2 after wear test, while the TiB2 in our prepared composite coating was only 15 wt% at most.
(2) Below the favored temperature of boric acid. In the literature, boron-rich materials exhibited better lubricating property at 700-900℃ than 300-500℃, while a temperature of 430℃ was used in the wear test in this article to fit the hot die forging conditions of aluminum alloys.
(3) The effect of transferred aluminum alloy. During the sliding process, the aluminum alloy transferred to the coating surface affected the detection of boron.
Therefore, we performed EDS test on the debris and used it as an important evidence of the generation of boron oxide and boric acid on coating surface, and the chemical composition of debris has been added in the article.
Reference: Rahul, K.; Maksim, A.; Le, L.; Irina, H. Sliding wear performance of in-situ spark plasma sintered Ti-TiBw composite at temperature up to 900℃. Wear, 2021, 476, 203663.
- Authors should show SEM images of the surface of specimens and counter-specimens prior to friction wear testing.
Response:
Thank you very much for this comment. Both the pins and the aluminum discs were polished by emery paper prior to the wear test. We have added a description of this in the article.
Changes made on Line 161, Page 5:
The wear tests of coatings sliding against the 7075-T6 aluminium alloy (the chemical composition is shown in Table 3) disc were conducted by a self-made pin-on-disk tribometer, as shown in Figure 5, lasted 1h in ambient air. The temperature of 430℃, the loads of 12N and the speed of 100r/min (0.21m/s) in the test were referred to the actual hot die forging conditions of aluminum alloy[22,23]. In addition, both aluminum alloy discs and pins were polished by emery paper (#80-#1500).
- If possible, the authors should perform EDS tests of the coating on a metallographic specimen in the form of linear distributions of element concentrations and analyses in selected areas.
Response:
Thank you very much for the suggestion. We have supplemented the EDS planar scanning images of the composite coatings to analyze the transfer of the aluminum alloy, as stated earlier.
- I would highly appreciate it if the authors added in literature (introduction):
https://doi.org/10.1016/j.wear.2021.203663 and
https://doi.org/10.1016/j.wear.2018.12.026
Response:
Thank you very much for the suggestions. We have added them in literature (introduction).
Changes made on Line 45, Page 1:
Common solid lubricant materials include MoS2, graphite, boric acid, oxides (B2O3, TiO2), fluorides (CaF2[11], BaF2, SrF2) and so on, which are usually limited by working conditions, cost and physicochemical property.
Changes made on Line 66, Page 2:
Kumar et al. prepared 50wt%Ti-50wt%TiBw composite containing TiB, TiB2 and Ti by SPS, and boric acid films were found generated on the surface of the composite during dry sliding process at 700-900℃, resulting in a reduction of COF.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The article may be accepted in its present form.
Reviewer 3 Report
I am impressed. The authors have done a lot of work to improve the manuscript. The article is very good. The manuscript is well organized and well written. All experiments are clearly described, and the results are adequately discussed and linked to the conclusion. The authors have considered the reviewer's comments and made corrections to this article. They have responded to all comments from the reviewer. The authors have made significant positive changes to the manuscript.
Recommendation: This manuscript in the presented form is acceptable for publication in the Coatings.
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
