Effect of Shroud in Plasma Spraying on Chemical Composition and Thickness of Titanium Coatings

Round 1

Reviewer 1 Report

1. It is recommended to add a figure with a sketch of the Shrouded plasma spraying process in the introduction.
2. How is the data in table 1 obtained?
3. Correct the word 'degraded' in line 79.
4. It is recommended to add a photo of the experimental setup in section 2. Experimental Details.
5. It is recommended to add some numerical values of the results of the SEM images analysis of samples (porosity, pore size).
6. Show in Figure 3: lamellar structure, tiny splats, un-melted titanium particles, pores.
7. How will the research continue? For example, does Shroud affect the capacity of Plasma Spraying and the complexity of the processed products? Conducting mechanical tests of the obtained coatings.

Author Response

Dear reviewer

 

Thank you very much for your constructive comments and helping improve the quality of the manuscript.

 Please see below our response, which are highlighted in blue

 

1. It is recommended to add a figure with a sketch of the Shrouded plasma spraying process in the introduction.

The figure 1 has been added in the instruction, page 2, to show the shrouded plasma spraying process.  

 

2. How is the data in table 1 obtained?

The data was provided by the company, Xi'an Lilin International Trade Co., Ltd. (Xi’an, China). We have also sent out the titanium powders for chemical composition testing, and got a quite similar result. 

3. Correct the word 'degraded' in line 79.

It is ‘degreased’ (to remove excess grease or fat from)

4. It is recommended to add a photo of the experimental setup in section 2. Experimental Details.

The figure has been added in the experimental details, page 4, to show the shrouded plasma spraying process. 

5. It is recommended to add some numerical values of the results of the SEM images analysis of samples (porosity, pore size).

Yes, analyses on porosity and pore size are important part of the research project. It will be carried out later.

6. Show in Figure 3: lamellar structure, tiny splats, un-melted titanium particles, pores.

These features have been labelled in the figure, in the page 5.

7. How will the research continue? For example, does Shroud affect the capacity of Plasma Spraying and the complexity of the processed products? Conducting mechanical tests of the obtained coatings.

We will test the bond strength for the titanium coatings and 3 point bending test as well. Porosity analyses will be conducted. Optimum of design and operating parameters for coating’s deposition will be also taken into account.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors have written a manuscript with potentially interesting topic. Titanium coatings, and optimization of the methods for coating deposition are timely research topics.

However, the manuscript is written using extremely poor English with lots of grammar and style errors. Some sentences lose their meaning. There are not enough references after the Introduction section. In my opinion, the manuscript does not possess the novelty necessary to be published in Q2 journal. Methodology and used methods of measurements are at a basic level, and the statements in the discussion are not backed up by references.

I therefore do not recommend the publication of this manuscript.

Author Response

No response here

Reviewer 3 Report

Authors are presenting a demonstrative work on the effect of shroud in plasma spraying. They give a through characterization into the thickness and morphology for the coatings deposited with the shroud technique and compares the outcomes with respect to the standard plasma sprayed coatings.

Yet, some statements should be better expressed and further discussed within the text.  Some plots given are unnecessary. Moreover some statements and phrases are excessively repeated throughout the text. Some sections and plots can be merged together for an easier visualization to the readers.

The demonstrated work would find use for the scientific and industrial community; I would like to reconsider after a major revision.

I would like to thank all the Authors for their efforts and I kindly ask them to address my comments and suggestions below:

1. The text is clearly written and well presented. The English language and style are fine still an overall check is required.
2. In lines45-46: “…because titanium is very reactive at high temperatures to oxygen and nitrogen.” Reference needed.
3. In line 51-55:” In shrouded plasma spraying, the torch is modified with an attachment. The attachment physically shields … oxygen from the surrounding environment” suggest the authors to further explain the technique with a scheme.
4. In line 70-72 and table 1: “The chemical composition of the titanium powder”. The authors must include how this characterization is done. Else reference to the tabulated specs must be given. If the LECO is used as noted in line 87. This section must be expressed at the previous section to help readers.
5. In line 84... “Removing the mild steel substrates off physically.” And “The interior side of the titanium coatings was then polished” The post processing procedures applied will create effects on the microstructure density and low porosity and thicknesses mentioned throughout the text. I kindly ask the authors to comment on the possible artifacts created due to the post processing procedures mainly on the thickness, density and porosity. Moreover it is important to note if the later given thickness measurements were conducted prior and or after these processes and also the SEM images of polished cross sections please see comment 8.
6. Line 107 “result in a sharp drop of velocity and temperature.” Reference needed!
7. “the shielding effect of shroud brought along a better heating of titanium particles during plasma spraying which was expected to have more powders in a molten state” no evidence shown to prove this hypothesis. i kindly ask the authors to clarify and better explain.
8. In line97 “Figure 2 shows the SEM images of polished cross sections of the titanium coatings at low magnifications. It indicates from those images that the titanium coating plasma sprayed with the shroud is much dense in microstructure and has a low porosity; whereas the atmospheric plasma sprayed titanium coating without the shroud possesses a relatively high porosity.” In conjunction with the comment 5. This is misleading. As shown in the SEM images, the bottom portions of the coatings (where a polishing procedure is applied!) show a clearly denser structure. For this reason it seems like no density related conclusion can be done from post treated surfaces! I ask the authors to comment on that and clarify the concept.

8. In line 127 “The particles struck on the substrate, spread and quickly solidified. This built up the titanium coatings and formed the lamellar structure.” This statement must be strengthened with references. In a common sense the quick solidification must lead to a porous structure?! This must be better explained! The lamellar structure and or the unification during the solidification are occurring at longer time durations, contrary to the quick solidification which may lead to particulate like structures... This phenomenon is generally observed at nanoparticle formation/deposition assisted with plasma generation techniques.
9. In line 139 “Titanium like many other metal powders exhibits a surface passive oxide film” reference needed. further explanation and  comments suggested.
10. In line 159-160 “In the plasma spray process, the titanium particles were accelerated and then fully or partially melted before impacting onto the substrate where they got flatten, formed splats and built up a lamellar structure.” It is repetitive; I suggest the authors to synthesis the statement and merge together at the previous section.
11. In line 164:”turbulent shear at the periphery would produce large vortices” reference needed.
    I suggest the authors to further explain the dynamics to better show proof their hypothesis
12. The given explanation line 164…which lead to a sudden cooling and porous structure!!! Please see the comment 8. It is contradicting with the explanation given in line 127” The particles struck on the substrate, spread and quickly solidified. This built up the titanium coatings and formed the lamellar structure”

This is a sensitive concept, I kindly ask the authors to better express the dynamics.

14. In line 167 “It resulted in that oxidation was rather high in plasma spraying and took the priority over nitridation of titanium during the particles’ flight in the plasma jet since titanium nitride might oxidize at high temperatures, and then turned into titanium oxide”
15. Table 3 and figure 4 are repetitive presentations. I would suggest the authors to exclude the plot.
16. Like vise the previous comment, the thickness values already given previously in the text. The given figure no 5. Does not add any value. I would suggest the authors to embed the values in a single table only.
17. Same with the comment 13. In line 185 “The high-volume flow of external shroud argon would be able to cool down effectively the surface temperature of the as-sprayed titanium coatings when the shroud passed over the deposited materials.” This hypothesis is contradicting with the explanation given in line 127!
18. In line “When the powder feedstock was sprayed under the same spraying parameters, the coating thickness could act as an indicator for spray deposition efficiency. The thicker the plasma sprayed coating, the higher deposition efficiency is obtained. This result reveals that the deposition efficiency with a shroud was higher than that sprayed in air.” Under same deposition conditions, same amount of material is ejected in to the system chamber. Such that a denser coating should have led to a thinner deposition. It is not clear, how the authors achieve denser and thicker coating at the same time for the same deposition parameters? I kindly ask the authors to better clarify and explain this.
19. In line 210 “Thus, titanium particles in flight could stay at higher temperatures, and more particles would be melted, which consequently increased the deposition efficiency when particles impacting upon the substrate.”
    
    This statement is contradicting with the explanation given “Molecules of nitrogen and oxygen entrained into the plasma jet from the surrounding air were subjected to thermal dissociation into highly reactive monotonic atoms when heated by plasma. It resulted in that oxidation was rather high in plasma spraying and took the priority over nitridation of titanium during the particles’ flight in the plasma jet since titanium nitride might oxidize at high temperatures, and then turned into titanium oxide.” moreover, accordingly to the explanation given, higher oxidation is expected for denser media regarding to the higher temperature vs porosity vs oxidation. I kindly ask the authors to better explain the correlation and the dynamics in to the given parameters.

Thank you,

With my best regards.

Author Response

Dear reviewer

 

Thank you very much for your constructive comments and helping improve the quality of the manuscript.

 Please see below our response, which are highlighted in blue

-------------

1. The text is clearly written and well presented. The English language and style are fine still an overall check is required.

 

The overall check for the paper with regards to the English language and style has been carried out. The revised wordings have been highlighted in yellow in the text.

 

1. In lines45-46: “…because titanium is very reactive at high temperatures to oxygen and nitrogen.” Reference needed.

 

This has been referenced. The reference has been added at the end of the sentence in page 2.

 

1. In line 51-55:” In shrouded plasma spraying, the torch is modified with an attachment. The attachment physically shields … oxygen from the surrounding environment” suggest the authors to further explain the technique with a scheme.

 

The figure 1 has been added in the instruction, page 2, to show the shrouded plasma spraying process.  

 

1. In line 70-72 and table 1: “The chemical composition of the titanium powder”. The authors must include how this characterization is done. Else reference to the tabulated specs must be given. If the LECO is used as noted in line 87. This section must be expressed at the previous section to help readers.

 

The data was provided by the company, Xi'an Lilin International Trade Co., Ltd. (Xi’an, China). We have also sent out the titanium powders for chemical composition testing and got a quite similar result. 

 

1. In line 84... “Removing the mild steel substrates off physically.” And “The interior side of the titanium coatings was then polished” The post processing procedures applied will create effects on the microstructure density and low porosity and thicknesses mentioned throughout the text. I kindly ask the authors to comment on the possible artifacts created due to the post processing procedures mainly on the thickness, density and porosity. Moreover it is important to note if the later given thickness measurements were conducted prior and or after these processes and also the SEM images of polished cross sections please see comment 8.

 

Free stand titanium coating specimens were obtained by removing the mild steel substrates off physically, which is only for chemical composition testing. The microstructure observation and coating thickness measurement were conducted by the samples with mild steel substrates. the sentence has been revised by adding ‘for chemical composition testing’ at the end, in page 3.

 

1. Line 107 “result in a sharp drop of velocity and temperature.” Reference needed!

 

This has been referenced now. The reference [26] is added as shown in page 4.

 

1. “the shielding effect of shroud brought along a better heating of titanium particles during plasma spraying which was expected to have more powders in a molten state” no evidence shown to prove this hypothesis. i kindly ask the authors to clarify and better explain.

 

The evidence is from the microstructure observation (figure 4 and 5) . It indicates from those images that the titanium coating plasma sprayed with the shroud is much dense in microstructure and has a low porosity; whereas the atmospheric plasma sprayed titanium coating without the shroud possesses a relatively high porosity.

 

1. In line97 “Figure 2 shows the SEM images of polished cross sections of the titanium coatings at low magnifications. It indicates from those images that the titanium coating plasma sprayed with the shroud is much dense in microstructure and has a low porosity; whereas the atmospheric plasma sprayed titanium coating without the shroud possesses a relatively high porosity.” In conjunction with the comment 5. This is misleading. As shown in the SEM images, the bottom portions of the coatings (where a polishing procedure is applied!) show a clearly denser structure. For this reason it seems like no density related conclusion can be done from post treated surfaces! I ask the authors to comment on that and clarify the concept.

 

With comparison of the SEM images in figure 2, it is clear to see that the polished cross section of the shrouded titanium coatings present a smooth surface which indicates a low porosity in the shrouded coating;  and the air plasma-sprayed titanium coating has a relative rough surface with many voids which increase the porosity of the coating. The interface of the coating and the substrate is located at the bottom. There are normally some voids at the interfaces for both titanium coatings. The overall porosity inside of the shrouded titanium coating is lower than that for the unshrouded coatings. 

 

1. In line 127 “The particles struck on the substrate, spread and quickly solidified. This built up the titanium coatings and formed the lamellar structure.” This statement must be strengthened with references. In a common sense the quick solidification must lead to a porous structure?! This must be better explained! The lamellar structure and or the unification during the solidification are occurring at longer time durations, contrary to the quick solidification which may lead to particulate like structures... This phenomenon is generally observed at nanoparticle formation/deposition assisted with plasma generation techniques.

 

It has been referenced, as shown in page 5. For the plasma sprayed coatings, they belong to the porous structure when compared with bulk materials. The formation and development of porosity on plasma-sprayed coatings are governed by set spraying parameters. Optimized set spraying parameters have been used to achieve the most favorable coatings with minimum defects. Even with the optimized set spraying parameters, defects like porosity still occur.

 

1. In line 139 “Titanium like many other metal powders exhibits a surface passive oxide film” reference needed. further explanation and comments suggested.

 

It has been referenced, as shown in page 5. Titanium and its alloys have a strong affinity of oxygen and other gases. This leads to a dense self-sealing oxide layer formed immediately when exposed to an oxygen containing atmosphere. Explanation has been added and highlighted in yellow in the text, page 5.

 

1. In line 159-160 “In the plasma spray process, the titanium particles were accelerated and then fully or partially melted before impacting onto the substrate where they got flatten, formed splats and built up a lamellar structure.” It is repetitive; I suggest the authors to synthesis the statement and merge together at the previous section.

 

The sentence has been merged into the previous section, which is highlighted in yellow in the text (page 5).

 

 

1. In line 164:”turbulent shear at the periphery would produce large vortices” reference needed.
   I suggest the authors to further explain the dynamics to better show proof their hypothesis

 

The references have been added, which is highlighted in yellow in the text (page 6).

 

 

1. The given explanation line 164…which lead to a sudden cooling and porous structure!!! Please see the comment 8. It is contradicting with the explanation given in line 127” The particles struck on the substrate, spread and quickly solidified. This built up the titanium coatings and formed the lamellar structure”

 

For the plasma sprayed coatings, they belong to the porous structure when compared with bulk materials. The formation and development of porosity on plasma-sprayed coatings are governed by set spraying parameters. Optimized set spraying parameters have been used to achieve the most favourable coatings with minimum defects. Even with the optimized set spraying parameters, defects like porosity still occur.

 

1. In line 167 “It resulted in that oxidation was rather high in plasma spraying and took the priority over nitridation of titanium during the particles’ flight in the plasma jet since titanium nitride might oxidize at high temperatures, and then turned into titanium oxide”

 

Yes

 

1. Table 3 and figure 4 are repetitive presentations. I would suggest the authors to exclude the plot.

 

We hope to keep the plot, please, because graphs are a common method to visually illustrate relationships in the data. The purpose of a graph is to present data that are too numerous or complicated to be described adequately in the text. If the data shows pronounced trends or reveals relations between variables, a graph should be used.

 

1. Like vise the previous comment, the thickness values already given previously in the text. The given figure no 5. Does not add any value. I would suggest the authors to embed the values in a single table only.

 

We hope to keep the plot, please, because graphs are a common method to visually illustrate relationships in the data. The purpose of a graph is to present data that are too numerous or complicated to be described adequately in the text. If the data shows pronounced trends or reveals relations between variables, a graph should be used.

 

1. Same with the comment 13. In line 185 “The high-volume flow of external shroud argon would be able to cool down effectively the surface temperature of the as-sprayed titanium coatings when the shroud passed over the deposited materials.” This hypothesis is contradicting with the explanation given in line 127!

 

Plasma spraying is a coating process in which powders of the coating materials are fed into the plasma jet at around 10 000K, at which the coating materials melt and are sprayed over the substrate to be coated. For the plasma sprayed coatings, they belong to the porous structure when compared with bulk materials. The formation and development of porosity on plasma-sprayed coatings are governed by set spraying parameters. Optimized set spraying parameters have been used to achieve the most favourable coatings with minimum defects. Even with the optimized set spraying parameters, defects like porosity still occur.

 

1. In line “When the powder feedstock was sprayed under the same spraying parameters, the coating thickness could act as an indicator for spray deposition efficiency. The thicker the plasma sprayed coating, the higher deposition efficiency is obtained. This result reveals that the deposition efficiency with a shroud was higher than that sprayed in air.” Under same deposition conditions, same amount of material is ejected in to the system chamber. Such that a denser coating should have led to a thinner deposition. It is not clear, how the authors achieve denser and thicker coating at the same time for the same deposition parameters? I kindly ask the authors to better clarify and explain this.

 

In plasma spraying, deposition efficiency is defined as the ratio of the coating weight to the weight of total feedstock. Not all the feedstock powders could be heated to a fully or partially molten state and form the coatings. Because of air entrainment or the trajectories of the particles in the plasma flame, a lot of feedstock powder are not able to be accelerated, fully heated and impact onto the substrate to build up the coatings.

 

 

1. In line 210 “Thus, titanium particles in flight could stay at higher temperatures, and more particles would be melted, which consequently increased the deposition efficiency when particles impacting upon the substrate.”
   
   This statement is contradicting with the explanation given “Molecules of nitrogen and oxygen entrained into the plasma jet from the surrounding air were subjected to thermal dissociation into highly reactive monotonic atoms when heated by plasma. It resulted in that oxidation was rather high in plasma spraying and took the priority over nitridation of titanium during the particles’ flight in the plasma jet since titanium nitride might oxidize at high temperatures, and then turned into titanium oxide.” moreover, accordingly to the explanation given, higher oxidation is expected for denser media regarding to the higher temperature vs porosity vs oxidation. I kindly ask the authors to better explain the correlation and the dynamics in to the given parameters.

 

To answer this question, firstly let us give a brief introduction on the plasma spraying. The plasma spraying process involves the generation of a plasma jet, the injection and treatment of particles within the plasma jet and finally the formation of the coating, as shown below.

https://www.sciencelearn.org.nz/images/250-plasma-spray-process

To generate the plasma jet, a working gas such as an argon/hydrogen mixture is passed through a powerful electric arc discharge formed in the gap between a cathode and anode. The energy released rapidly heats up the gas mixture, converting it to high-temperature plasma at about 14 000 K. Rapid expansion occurs, lifting the speed of the jet, giving it a very high nozzle speed of up to 800 m/s. The coating material in a fine powder form (within the range 20–90 mm) is then injected into the plasma jet. Molten droplets form, which are propelled at high speed towards the object to be coated (substrate).

 

The figure below shows that the air entrainment in the plasma spraying process. In the typical plasma spray process operated under an atmospheric-pressure condition, the entrainment, of ambient air into the thermal plasma jet is inevitable. An air inflow to the flame alters chemical compositions of the plasma species, cools the thermal plasma flame, and decreases the jet velocity. Furthermore, the dissociation of entrained air enhances the specific heat of plasma, and then the plasma temperatures decrease even when operation power level of the plasma torch is not changed. Consequently, the gradients of temperature and velocity in the plasma jet increase with the degree of air entrainment.

 

 

https://users.encs.concordia.ca/~dolat/Research-TS-Processes.html

 

Therefore, based on our experiment, we found that less surrounding air would be entrapped into the plasma jet for the shrouded plasma spraying process because of the shielding of the shroud. In principle, the reduction in air entrainment with the shroud would result in a better heating of the particles. This has been explained in the context that less heat energy would be taken away from the plasma since less thermal dissociation of entrapped air would take place. Thus, titanium particles in flight could stay at higher temperatures, and more particles would be melted, which consequently increased the deposition efficiency when particles impacting upon the substrate. On the other hand, air entrainment and the friction with the surrounding air would also remove kinetic energy from the plasma when spraying without the shroud attachment, led to a sharp drop in velocity.

 

 

 

 

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

I would like to thank all the Authors for their efforts. The authors have answered my comments satisfactorily. I would recommend publication in its present form.

Yours Sincerely,

With my best regards,