Plasma Spraying with Wire Feeding: A Facile Route to Enhance the Coating/Substrate Interfacial Metallurgical Bonding
Round 1
Reviewer 1 Report
Submited manuscript presents the plasma spraying with wire feeding was adopted to increase the size of sprayed particles. The authors stated that the metallurgical bonding was clearly formed between deposited particles and the substrate. They showed that the adhesive strength of the coating was improved without surface roughening pretreatment.
The article is interesting, but these issues are not new. The manuscript requires more extensive supplementation. Please consider the following comments:
- Abstract is too general. It should contain relevant information from the research conducted by the authors.
- What publication is Figure 2 from?
- In Figure 7 it is not possible to determine the size of the Ni5Al particles. There's no scale.
- I understand that Figures 8 and 9 are related, but the exact designation of the curves in Figures 8d and 8f is missing. The reader must guess what color corresponds to which EDS curve of the element.
- The scales in Figure 8 are unreadable. The scale and magnification are not clearly visible.
- The subject of thermal spraying is well known. Much of the publication analyzed by the authors mostly comes from the beginning of 2000. Please justify it.
- The mechanism of changes on the border of interface metallurgical bonding is too weak described. Maybe it would be worth doing EBSD research? If the authors have access to the apparatus? Then they will know what phases are formed at the border.
- How does the microhardness change on the boundary of the metallurgical bonding? This is very important.
- The literature is not structured according to the journal's requirements.
- Conclusions are too general.
Author Response
Response to Reviewer 1 Comments
Dear Reviewer,
Thanks a lot for your thoughtful comments and helpful suggestions that have helped us improve this manuscript substantially.
Point 1: Abstract is too general. It should contain relevant information from the research conducted by the authors.
Response 1: Thanks for your suggestion. More general discription was added into the abstract, including coating (Ni5Al)/substrate (304 stainless-steel & 7075 aluminum alloy) materials, metallurgical bonding form (Melting & new interfacial phase by reaction), the method of adhesion strength test (tensile adhesive test), and the bonding strength result (82.67 ± 3.96 MPa and 64.45 ± 2.84 MPa respectively for NiAl coatings on 304 and 7075 sub-strates).
Point 2: What publication is Figure 2 from?
Response 2: Figure 2 is of the result of equation (3), which help us to understand the equations better. As it’s our work instead of a citation, those paragraghs has been moved into Materials and Methods as Governing Model.
Point 3: In Figure 7 it is not possible to determine the size of the Ni5Al particles. There's no scale.
Response 3: Thank you for your suggestion. The quality of the figure was improved and scales have been added to all SEM photos.
Point 4: I understand that Figures 8 and 9 are related, but the exact designation of the curves in Figures 8d and 8f is missing. The reader must guess what color corresponds to which EDS curve of the element.
Response 4: New EDS line analysis result is shown in Figure 13, and the explanation was added as: EDS line scan proved the melting and mixing at the interface.
Point 5: The scales in Figure 8 are unreadable. The scale and magnification are not clearly visible.
Response 5: Scales was added to the corresponding figures.
Point 6: The subject of thermal spraying is well known. Much of the publication analyzed by the authors mostly comes from the beginning of 2000. Please justify it.
Response 6: A paragragh in Introduction was re-structured and more references later than 2010 was cited.
Point 7: The mechanism of changes on the border of interface metallurgical bonding is too weak described. Maybe it would be worth doing EBSD research? If the authors have access to the apparatus? Then they will know what phases are formed at the border.
Response 7: We really appriciate your advise and it really opened up our mind. But it’s not accessble to apply this experiment successfully in a short time. We’ll do an EBSD analysis in future study. Thanks a lot for your comment.
Point 8: How does the microhardness change on the boundary of the metallurgical bonding? This is very important.
Response 8: Microhardness was tested by Nano-indentation, as shown in 3.5. Microhardness of the Reaction Zone/Phase. The hardness of the new phase (Al-Al3Ni Eutectic Phase as analysed) at the interface of NiAl/7075 is harder than both NiAl coating and 7075 alloy. The hardness of the metallurgical layer at the interface of NiAl/304 is not hard as NiAl coating and 304 alloy.
Point 9: The literature is not structured according to the journal's requirements.
Response 9: Corrected.
Point 10: Conclusions are too general.
Response 10: Some details were added into the Conclusion, including the average particle size (D50: 129.29 μm), particle temperature (2130K), bonding strength (82.67 ± 3.96 MPa and 64.45 ± 2.84 MPa respectively), and types of metallurgical bonding (Diffusion & new interfacial phase by reaction).
Reviewer 2 Report
Dear Authors,
Congratulations on your work, which is focused on a very interesting issue.
Trying to contribute to the paper improvement, please pay attention to my comments and suggestions below:
- Please try to clarify the first sentence of your abstract: "The thermal spraying coating and substrate is usually combined by mechanical seizure effect". As contextualization, this sentence seems too strong to start an abstract.
- Please try to improve the abstract referring how did you measure the adhesion of the coating to the substrate, as well as if there was diffusion, and quantifying the main effects in order to capture the attention of the reader.
- Part of the Introduction is describing the formula and mathematical models governing the particles impingement. This should be sent to the Materials and Methods section, as "Governing model".
- On the other hand, Introoduction doesn't refer previous attempts to solve the main problem you intend to solve through this work. Thus, this should be reorganized, and ,uch more attention should be done to the techniques and results previously achieved by other Researchers in order to overcome this problem.
- In 2.1, you use "1.6mm". Please always insert a blank space between values and units. Scientific documents are as here described. Rules are to comply with them.
- Please improve the sentence "...thus atomized more fully to achieve higher temperature and better sphericity.", for clarity.
- How did you ensure that the wire feeding speed is adequate to the energy emitted by the nozzle? Please explain.
- In 2.2 there are a series of assumptions without reference to the criteria used to select them. You must use references and justify your options. Otherwise, you paper cannot be as useful as you intend.
- Moreover, in 2.2. there are a series of formulas without any reference. Are they deveoped by you? You need to point out how they are appearing.
- I disagree with your approach regarding the surface preparation. You refer that: "...all the substrate surfaces were grounded by 800# SiC sandpaper.". Any reader would like to know the roughness obtained and the type of surface profile obtained (weak...robust), because this is what really interests to the reader. The use of a 800# sandpaper say nothing about the surface preparatiom, because we don't know if the orientation of the movement was always the same, how many time lasted the sanding process, and so on. Thus, please provide assertive data, allowing the reader easily understand what is going on with your experimental.
- The bonding force seems to be measured through a peel-off test. Is this the best way to test the adhesion of the coating? I'm not sure at all.
- Quality of Figure 7 is very poor. Please improve it.
- After what I have previously referred, it is difficult to trust in your results. You need to emphasize your work in a credible way, because I don't believe in some of the results presented.
- Regarding the sentence: "The bonding strength of coating on 7075 substrate was 64.45 ± 2.84 MPa, most of the fractured zone was on the glue, less than 10% area was found at the coating/substrate interface.", how do you believe that your results are credible?
- Figure 8 shows a lot of defective coatings. Are they good enough to allow a perfect analysis?
- The conclusions allow to understand that if the substrate is previously heated at a convenient temperature, the coating projection with conveniente energy could present similar results.
- No quantification of any of the measured parameters are referred in the literature.
Thus, I cannot suggest that your paper can be considered for publication. I think you need to pay attention to the topics I'm referring above and make another version of this work, but using reliable data, completing all the gaps identified in this version.
Good luck for the rework.
Kind regards.
Author Response
Response to Reviewer 2 Comments
Dear Reviewer,
Thanks a lot for your thoughtful comments and helpful suggestions that have helped us improve this manuscript substantially. The revised version is in the following document.
Point 1: Please try to clarify the first sentence of your abstract: "The thermal spraying coating and substrate is usually combined by mechanical seizure effect". As contextualization, this sentence seems too strong to start an abstract.
Response 1: Thank you for your advise. These sentences were added: Thermal spray coatings are widely used in many applications, the adhesion effect at the coat-ing/substrate interface plays an important role during the service life. The thermal spraying coating and substrate is primarily combined by mechanical seizure effect. In this work, a strategy to generate interfacial metallurgical bonding is proposed.
Point 2: Please try to improve the abstract referring how did you measure the adhesion of the coating to the substrate, as well as if there was diffusion, and quantifying the main effects in order to capture the attention of the reader.
Response 2: The melting and reaction can be found at the interface but no clear diffusion with the current results. Some experimental details and results were added, which edited as: Interface reaction can be found at both NiAl/7075 and NiAl/304 interface. Typical Al-Al3Ni eutectic phase with higher microhardness was formed at NiAl/7075 interface. The adhesive strength of the coatings was significantly improved to 82.67±3.96 MPa and 64.45±2.84 MPa respectively for NiAl coating on 304 and 7075 substrates through tensile adhesion tests (TAT) without surface rough-ening pretreatment.
Point 3: Part of the Introduction is describing the formula and mathematical models governing the particles impingement. This should be sent to the Materials and Methods section, as "Governing model".
Response 3: Thanks a lot for your advise, we have moved that part into the Materials and Methods section, as "2.1. Governing model".
Point 4: On the other hand, Introoduction doesn't refer previous attempts to solve the main problem you intend to solve through this work. Thus, this should be reorganized, and ,uch more attention should be done to the techniques and results previously achieved by other Researchers in order to overcome this problem.
Response 4: More references was added to the Introduction about how to form metallurgical bonding, including improving spraying particle temperature, pre-heating substrate surface and other ways of surface pre-treatment.
Point 5: In 2.1, you use "1.6mm". Please always insert a blank space between values and units. Scientific documents are as here described. Rules are to comply with them.
Response 5: Corrected.
Point 6: Please improve the sentence "...thus atomized more fully to achieve higher temperature and better sphericity.", for clarity.
Response 6: It is changed to: Therefore, it could be delivered into the higher temperature zone of the plasma jet, producing droplet with higher temperature.
Point 7: How did you ensure that the wire feeding speed is adequate to the energy emitted by the nozzle? Please explain.
Response 7: Yes, this is very important issue regarding this process. A large amount of pre-experiments have been done to determine the wire feeding speed. When it’s too fast, the wire will not be fully melted and the drops will be scattered, sometime it may even damage the nozzle. When it’s too slow, the ‘particle stream’ will incline to the feed-in direction. The proper feeding speed is required for every parameters to ensure concentrated and centered ‘particle stream’.
Point 8: In 2.2 there are a series of assumptions without reference to the criteria used to select them. You must use references and justify your options. Otherwise, you paper cannot be as useful as you intend.
Response 8: Thank you for your comment. Reference 17 has been added, namely: Merz, R.; Prinz, F. B.; Ramaswami, K.; Terk, M.; Weiss, L. E.. Shape deposition manufacturing. Proc Solid Freeform Fabrication Symp eds 1994, 1-8.
Point 9: Moreover, in 2.2. there are a series of formulas without any reference. Are they deveoped by you? You need to point out how they are appearing.
Response 9: Also in Reference 17 the enthapy method and the basic formulas were proposed. The same mothod was used in our work.
Point 10: I disagree with your approach regarding the surface preparation. You refer that: "...all the substrate surfaces were grounded by 800# SiC sandpaper.". Any reader would like to know the roughness obtained and the type of surface profile obtained (weak...robust), because this is what really interests to the reader. The use of a 800# sandpaper say nothing about the surface preparatiom, because we don't know if the orientation of the movement was always the same, how many time lasted the sanding process, and so on. Thus, please provide assertive data, allowing the reader easily understand what is going on with your experimental.
Response 10: We really appreciate your suggestions and it definitely improved our work. The surface roughness was characterised in the updated version by laser confocal scanning microscopy (Olympus OLS4100) and Ra was 0.038 ± 0.001 μm and 0.331 ± 0.023 μm respectively for 304 and 7075 substrate, which could hardly cause mechanical bonding to take place according to Reference 1&2, namely:
- Wang, Y.; Li, C. J.; Ohmori, A.. Influence of substrate roughness on the bonding mechanisms of high velocity oxy-fuel sprayed coatings. Thin Solid Films 2005, 485(1-2), 141-147.
- Bahbou, F.; Nylen, P.. Relationship between surface topography parameters and adhesion strength for plasma spraying. International Thermal Spray Conference, Düsseldorf, Germany, 2005.
Point 11: The bonding force seems to be measured through a peel-off test. Is this the best way to test the adhesion of the coating? I'm not sure at all.
Response 11: The tensile adhesion test (TAT) instead of the peel-off test were applied to measure coating’s adhesive strength. This adhesion test is believed to be a most widely used method to test metallic thermal spray coatings.
Ang A S M , Berndt C C . A review of testing methods for thermal spray coatings[J]. International Materials Reviews, 2014, 59(4):1743280414Y.0000000029.
Point 12: Quality of Figure 7 is very poor. Please improve it.
Response 12: The quality of Figures has been improved.
Point 13: After what I have previously referred, it is difficult to trust in your results. You need to emphasize your work in a credible way, because I don't believe in some of the results presented.
Response 13: More experimental details and results were added, including view of the equipments, surface roughness measurement, data of particle temperature experiment, TAT tested surfaces, Microhardness at the interfaces. The view of homemade equipments and the experimental process were added. Hopefully these additional details can reassure you. This is the first time that we literally report this new method in the journal. This proprietary method obtained two invention patent and has been successfully applied in industry.
Point 14: Regarding the sentence: "The bonding strength of coating on 7075 substrate was 64.45 ± 2.84 MPa, most of the fractured zone was on the glue, less than 10% area was found at the coating/substrate interface.", how do you believe that your results are credible?
Response 14: The surfaces after tensile test were added. And the expression “less than 10% area” has been modified into “a small part of area”.
Point 15: Figure 8 shows a lot of defective coatings. Are they good enough to allow a perfect analysis?
Response 15: In the former version, the defective area are the splats’ boundry and oxide etched. The plasma spray coatings has the typical lamellar-look structure. Some of the black zones inside the NiAl coatings is oxide that formed during the flight of particle. We didn’t metion clearly in the former version.
Point 16: The conclusions allow to understand that if the substrate is previously heated at a convenient temperature, the coating projection with conveniente energy could present similar results.
Response 16: Thanks for the comment and we do agree with this. Preheating substrate does improve the bonding strength as expected. And in my opinion, our work does provide a new way to improve coating’s adhesion strength, which is much easier to operate in real industrial application. Moreover, without any grit-blasting process, the deformation and the residual stress can be reduced.
Point 17: No quantification of any of the measured parameters are referred in the literature.
Response 17: The results in the literatures were cited and concluded as Table 5. Adhesive strength results of NiAl coatings on several kind of substrates with different thermal spraying methods were added.
Reviewer 3 Report
page 2, first paragraph
‘...is quite rapid that lasts only ....’
page 3, second paragraph and check throughout the text
This slang ‘atomization’ somehow hurts my hearing. In some places it sounds terribly funny.
Please do not use terms atomization or atomized to your process even if this is a technical slang!
To be precise, atomization of metal is its vaporization. In your conditions vaporization does really the case but formation of drops is the main process. May be it is better called spraying or spattering by plasma jet etc.
page 3, 2. Materials and Methods 2.1 Plasma spraying of wire and coating preparation
first paragraph:
Please correct this sentence: ‘To remove surface adsorption, substrates were heated to 350C and cooled to room temperature under argon atmosphere before spraying. ‘
To remove surface contamination, substrates were heated to 350C and cooled to room temperature under argon atmosphere before spraying.
second paragraph:
Please correct this sentence: ‘Therefore, it could be delivered into the higher temperature zone of the plasma jet, thus atomized more fully (how else can it be?) to achieve higher temperature and better sphericity. ‘
Therefore, it could be delivered into the higher temperature zone of the plasma jet, producing drops with higher temperature and better sphericity.
page 4,
here again: ‘atomized Ni5Al particles’ is gas phase of Ni and Al atoms.
Your ‘particles’ are drops and that is the limit of the detailing in your process.
page 5, 2.2. Particles Temperature Measurement
I don't understand how you measured ??3=?3*(??3Θ−?298?Θ)
??3Θ is standard molar enthalpy of what? what is ?298?Θ? How both of them were determined and related to the particles temperature? How was water evaporation taken into account?
To my opinion ??3= ?3*(r+ cpm*(T-Tm)+cps*(Tm-T1)), where r is latent heat of fusion, cpm is specific heat capacity of liquid metal; cps is specific heat capacity of solid metal, T is temperature of particles, Tm is melting point of particles.
Please give more adequate explanation and experimental details.
Author Response
Dear Reviewer,
Thanks a lot for your thoughtful comments and helpful suggestions that have helped us improve this manuscript substantially. The revised version is in the following document.
Point 1: page 2, first paragraph
‘...is quite rapid that lasts only ....’
Response 1: Modified.
Point 2: page 3, second paragraph and check throughout the text
This slang ‘atomization’ somehow hurts my hearing. In some places it sounds terribly funny.
Please do not use terms atomization or atomized to your process even if this is a technical slang!
To be precise, atomization of metal is its vaporization. In your conditions vaporization does really the case but formation of drops is the main process. May be it is better called spraying or spattering by plasma jet etc.
Response 2: Thanks a lot for your correction. All the words including “Atomized” and “Atomization” were changed into other expressions, such as “sprayed by the jet”, “wire spraying”, etc.. The words “Atomized” and “Atomization” were widely used in powder production industry and might not be very proper in this work.
Point 3: page 3, 2. Materials and Methods 2.1 Plasma spraying of wire and coating preparation
first paragraph:
Please correct this sentence: ‘To remove surface adsorption, substrates were heated to 350C and cooled to room temperature under argon atmosphere before spraying. ‘
To remove surface contamination, substrates were heated to 350C and cooled to room temperature under argon atmosphere before spraying.
Response 3: The sentence was corrected. This operation is to reduce the influence of surface absorbates and to avoid splashing.
Point 4: second paragraph:
Please correct this sentence: ‘Therefore, it could be delivered into the higher temperature zone of the plasma jet, thus atomized more fully (how else can it be?) to achieve higher temperature and better sphericity. ‘
Therefore, it could be delivered into the higher temperature zone of the plasma jet, producing drops with higher temperature and better sphericity.
Response 4: The sentence was corrected, thanks a lot.
Point 5: page 4,
here again: ‘atomized Ni5Al particles’ is gas phase of Ni and Al atoms.
Your ‘particles’ are drops and that is the limit of the detailing in your process.
Response 5: The word “atomized” was deleted.
Point 6: page 5, 2.2. Particles Temperature Measurement
I don't understand how you measured ??3=?3*(??3Θ−?298?Θ)
??3Θ is standard molar enthalpy of what? what is ?298?Θ? How both of them were determined and related to the particles temperature? How was water evaporation taken into account?
To my opinion ??3= ?3*(r+ cpm*(T-Tm)+cps*(Tm-T1)), where r is latent heat of fusion, cpm is specific heat capacity of liquid metal; cps is specific heat capacity of solid metal, T is temperature of particles, Tm is melting point of particles.
Response 6: The equation and the following explaination has been modified and re-written.
Point 7: Please give more adequate explanation and experimental details.
Response 7: More experimental details and results were added, including surface roughness measurement, composition (wt. %) of substrates, corresponding data in particle temperature experiment, fractured surfaces after tensile test, chemical composition analysis and microhardness of NiAl coating, 7075 substrate and the inter-phase. The introduction was reorganized and more explanation and experimental results were added.
Round 2
Reviewer 1 Report
The manuscript has been sufficiently improved. I recommend publication in Coatings journal.
Author Response
Thank you for your positive comment. Really appreciate your suggestions and kindness during the revising process.
Reviewer 2 Report
Dear Authors,
Congratulations and thank you for your willingness to revise the paper following my suggestions.
However, I would like to ask you to point out the units for each variable describe after each equation. There are some equations and variables which can assume different units, resulting in different values. Thus, please complete this part of your work.
Kind regards.
Author Response
Thank you for your suggestion. The units were added after the variables.
Reviewer 3 Report
Page 3 Lines 101-102
This sentence is senseless :
"As the sprayed particle will get vaporized ones it reaches its boiling point, the more effective way of melting substrate surface is to increase the particle’s diameter."
Please express your idea correctly and in a few simple sentences.
Author Response
Thank you for your suggestion. This sentence has been revised as: When the sprayed particle reaches its melting point, it will vaporize and less energy will be carried to the substrate surface. Therefore, the more efficient way of melting substrate surface is to increase the particle’s diameter.
