Effect of Heat Treatment on the Mechanical and Tribological Properties of Dual-Reinforced Cold-Sprayed Al Coatings

Round 1

Reviewer 1 Report

1) Please specify the conditions of cryomilling.

2) Line 144-145: it is better to report the percentage of porosity of the coatings. Please, explain how the porosity was determined. If, porosity was determined by the metallographic method, please, specify the number of micrographs employed to measure porosity performed on each sample.

3) Please indicate the voids and pores in Figure 2.

4) Scale bars in optical images are hard to recognize; please revise.

5) Please correct Figure 3 and 4 for better clarification and readership (the scale of the main divisions on the y-axis).

Author Response

We want to thank the reviewer for their comments. We hope the following responses (in red) adequately address their concerns. 

1) Please specify the conditions of cryomilling.

Cryomilling conditions added to lines 75 and 76.

2) Line 144-145: it is better to report the percentage of porosity of the coatings. Please, explain how the porosity was determined. If, porosity was determined by the metallographic method, please, specify the number of micrographs employed to measure porosity performed on each sample.

This was a qualitative statement based on our review of the optical micrographs (Figure A1). We did not directly measure the pore sizes, so we will remove that statement. The coatings in generally were dense and we do not think the %porosity is going to be that different from sample to sample.

3) Please indicate the voids and pores in Figure 2.

This has been done.

4) Scale bars in optical images are hard to recognize; please revise.

This has been done.

5) Please correct Figure 3 and 4 for better clarification and readership (the scale of the main divisions on the y-axis).

The vertical scale in figure 3a has been adjusted so the both parts have the same scale. The numbers were adjusted in figure 4b to match the style in 4a.

Reviewer 2 Report

1) Novell's method. It needs analysis.
2) The processing of powers needs explanation.
3) What is the value and the importance of Feret's diameter?
4) The grain growth needs explanation.
5) What is the definition and the importance of Grubb's test?

 

Author Response

We want to thank the reviewer for their comments. We hope the following responses (in red) adequately address their concerns. 

1) Novell's method. It needs analysis.

Not sure what you mean by “Novell’s method.” Do you mean Norrell? In a previous study, T. Norrell had milled powders and our collaborators sprayed those powders. We then did the characterized those samples. We continue that characterization here.

2) The processing of powers needs explanation.

The method of powder processing can be found at “Norrell, T.; Ferguson, G.; Ansell, T.; Saladin, T.; Nardi, A.; Nieto, A. Synthesis and Corrosion Behavior of Cold Sprayed Dual Nanoparticle Reinforced Al Coatings. Surface and Coatings Technology 2020, 401, 126280.”

3) What is the value and the importance of Feret's diameter?

Feret’s diameter was used to estimate the average wear debris size, which is listed in Table 3.

4) The grain growth needs explanation.

Unfortunately, our EBSD system has been down for some time, and we do not expect it to be back for at least another month. Because of this, we have not been able to analyze grain orientation before or after heat treatment of the samples. We have found, e.g., in sample A1_C4, a grain structure within an individual splat (figure added in appendix). Because we are unable to really quantify this structure before and after heat treatment, however, we can only cite other works that shows similar behavior to our own work, which we have done.

We did add the following statements at the end of section 3.1. “Although grain structure was not investigated with electron backscatter diffraction in this work. Because the cold sprayed coatings were primarily Al and the reinforcing particles reside at the splat boundaries, changes to grain structure, i.e., recrystallization, would occur within a splat similar to that observed by Rokni et. al. and Hall et. al.”

5) What is the definition and the importance of Grubb's test?

Grubb’s test is used to detect and remove outliers in datasets assumed to be a normal distribution of the property being tested. It was used in this study to remove outliers associated with indents performed at splat boundaries.

Reviewer 3 Report

Review

of paper “Effect of Heat Treatment on the Mechanical and Tribological  Properties of Dual-Reinforced Cold-Sprayed Al Coatings”

Cold spraying is an additive manufacturing technology being developed for application of metal and metal-ceramic coatings for use in various aircraft and ship repair. From this viewpoint the topic of further development and examination of cold spraying and heat treatment of Al powder feedstocks is actual and hot. Authors show some interesting data. However, authors need to make additional friction tests, EBSD examination, correct some errors and clarify uncertainties for the article publication   

Some comments are below.

Introduction

Lines 26-48: Authors present general information about Al MMCs development. However, there is a lack of information about cold spraying of MMCs which is being applied from approximately 2004 (for example, take a look Proceeding of International Thermal Conference, 2004). Additionally, it looks to be reasonable to show in more detail some works of V. Champagne et al. and other authors in this area of application. 

Lines 49-50:  The authors citing review [10] in the sentence “Cold sprayed Al MMC coatings have shown improvements in the mechanical  and tribological properties over base Al cold-sprayed coatings; however, poor tensile strength limit their application [10]” is discussible because the review [10] only mentioned about this citing paper [S. R. Bakshi et al, Microstructure and wear properties of aluminum/aluminum–silicon composite coatings prepared by cold spraying, Surface & Coatings Technology 204 (2009) 503–510]. However, this paper presents the early results of examination of aluminum/aluminum–silicon composite coatings (not Al MMC coatings) prepared by cold spraying.              Authors need to make more detailed state-of-art analysis of as sprayed coatings mechanical properties to exactly define the paper goals

Lines 52-53:  Authors statement “…Annealing is often employed to change the grain size and improve the mechanical properties of metallic materials” is not clear. Annealing is used not only to change grain size. What’s about sintering processes and dissolution of alloying elements and their redistribution?  Please, correct and specify.

Lines 54-59: Authors mention the well known papers of Rokhni et al about Al and Al alloy coating structure and properties. However, there is no any  literature data about Al MMC coatings in as sprayed and post-spraying heat treated conditions for comparison. Please, add.

Lines 60-62: Based on previous state-of-art,  it is not clear how “…  to  improve the mechanical properties and wear resistance of cold sprayed Al MMCs” due to application of heat treated dual reinforced Al MMC. Please, prove the task statement in detail.

Procedure

Line 115: Additionally, the work of indentation (WOI) was obtained from the load-displacement curves. Please, describe the calculation procedure.

Line 137: It looks like 29.258 m travel distance of the wear ball is not correct. Calculation demonstrates the total distance of 28.260 m. Please, correct

Results

Line 146: Authors state:”… Splat sizes across all samples ranged from 40 to 70 μm.” However, authors do not describe how they determine these values. Please, specify

Lines 146-150: Authors state:”…Inter-splat grain bridging, and  growth was not observed after heat-treatment as was seen by the investigation of Hall  et. al. [16].” Citing [16] is wrong. It is [17] reference. How were parameters of inter-splat grain bridging and drain growth defined? What is the experimental and calculation procedure?

Line 149: The authors statement “…Still, grain growth across splat boundaries was not observed at any temperature or anneal time” cannot be seen on the microstructure images because of low magnification. EBSD analysis needs to be made. Sintering effect needs to be defined

Line 158: Authors’ explanation “… This can be attributed to changes in grain size (i.e., recrystallization) of the Al matrix” needs to be proved by EBDS examination of grain structure

Line 183: The authors statement “…The results of WOI mirrors the results of the elastic modulus and hardness” is not clear because WOI is the function of the plastic strains. So, the elastic module does not correlate with WOI.  Please, explain in detail

Lines 200-213: The COF dependences on sliding time shown on Fig. 5 are defined only in the range of sliding distance of about 28.26m (time 30 min). Usually, only running stage of COF diagram is seen at this case (take a look the paper [H. Ashrafi et al., Tribological Properties of Nanostructured Al/Al12(Fe,V)3Si Alloys, Acta Metall. Sin. (Engl. Lett.), 2015, 28(1), 83–92] as an example). The sliding distance of about 300-500m is needed to exactly define the real friction and wear behaviour of MMC. It means that authors showed fall of COF at the running stage and did not obtained the real friction coefficient after the running stage.

Line -241: The data shown in  Table 2. “Coefficient of friction (COF), mass loss, and specific wear rate of samples A1 – A5 heat treated with condition 4 (C4)’’ belong only for running stage. So, there is no the stable wear conditions during the wear test. Authors need to make the new wear tests at distance of about 500 m.

Discussion and conclusion

From my viewpoint the discussion and conclusion needs to be rewritten based on  the new friction and wear test results taking into account the above comments

Author Response

We want to thank the reviewer for their comments. We hope the following responses (in red) adequately address their concerns. 

 

Introduction

1. Lines 26-48: Authors present general information about Al MMCs development. However, there is a lack of information about cold spraying of MMCs which is being applied from approximately 2004 (for example, take a look Proceeding of International Thermal Conference, 2004). Additionally, it looks to be reasonable to show in more detail some works of V. Champagne et al. and other authors in this area of application. 

We added three citations to address this. We tried to keep this first paragraph broad and to quickly introduce the topic of cold spray. A good review paper on cold sprayed MMCs was added in the second paragraph to address that concern; He, L.; Hassani, M. A Review of the Mechanical and Tribological Behavior of Cold Spray Metal Matrix Composites. Journal of Thermal Spray Technology 2020, 29, 1565–1608, doi:10.1007/s11666-020-01091-w.

2. Lines 49-50: The authors citing review [10] in the sentence “Cold sprayed Al MMC coatings have shown improvements in the mechanical  and tribological properties over base Al cold-sprayed coatings; however, poor tensile strength limit their application [10]” is discussible because the review [10] only mentioned about this citing paper [S. R. Bakshi et al, Microstructure and wear properties of aluminum/aluminum–silicon composite coatings prepared by cold spraying, Surface & Coatings Technology 204 (2009) 503–510]. However, this paper presents the early results of examination of aluminum/aluminum–silicon composite coatings (not Al MMC coatings) prepared by cold spraying.              Authors need to make more detailed state-of-art analysis of as sprayed coatings mechanical properties to exactly define the paper goals

Authors understands what the reviewer is getting at in this comment. Yes, Bakshi et. al. researched the mechanical properties of cold sprayed Al composites with Si particles not fibers, whiskers, or nanoparticles. The term MMC, though is vague and broad and includes any metal with any secondary, non-metallic phase, typically ceramic phase.

Authors do agree that our review of the state-of-the-art may be a little lacking and so we added some more papers reinforcing the theme of Al-MMCs in terms of strengths and weaknesses.

3. Lines 52-53: Authors statement “…Annealing is often employed to change the grain size and improve the mechanical properties of metallic materials” is not clear. Annealing is used not only to change grain size. What’s about sintering processes and dissolution of alloying elements and their redistribution?  Please, correct and specify.

In metallurgy, sintering is specifically a fusion and densification process while annealing is a post fabrication heat treatment. Often, these two occur at the same time. If we take an example of a fabricated metal sheet (at full density) with undesirable properties like low ductility. This may be due to a course inhomogeneous grain structure, heat-treating said metal, i.e., annealing, will result in a fine grain structure due to recrystallization that will result in greater ductility. Because, in this example, full density is already achieved, sintering specifically will result in no change beyond the fact that annealing is occurring at the same time. The case where sintering is discussed, specifically, is in the consolidation of powders into a bulk and dense (ideally) form as is often the case in powder metallurgy.  

In our statement on lines 52 and 53, we aimed to show the goal of this work was to change the mechanical properties of the coating through both reinforcement and heat-treatment. This is stated explicitly on lines 60 – 63.

4. Lines 54-59: Authors mention the well known papers of Rokhni et al about Al and Al alloy coating structure and properties. However, there is no any literature data about Al MMC coatings in as sprayed and post-spraying heat treated conditions for comparison. Please, add.

Our previous work on cold sprayed, “Norrell et. al., Synthesis and Corrosion Behavior of Cold Sprayed Dual Nanoparticle Reinforced Al Coatings. Surface and Coatings Technology 2020, 401, 126280,” was on the same Al-MMC looked at here. This work is referenced. We have included another paper looking by “Loganathan, A.; Rengifo, S.; Hernandez, A.F.; Zhang, C.; Agarwal, A. Effect of Nanodiamond Reinforcement and Heat-Treatment on Microstructure, Mechanical and Tribological Properties of Cold Sprayed Aluminum Coating. Surface and Coatings Technology 2021, 412, 127037, doi:10.1016/j.surfcoat.2021.127037.”

5. Lines 60-62: Based on previous state-of-art, it is not clear how “…  to  improve the mechanical properties and wear resistance of cold sprayed Al MMCs” due to application of heat treated dual reinforced Al MMC. Please, prove the task statement in detail.

These lines were changed to the following, hopefully making our aims clearer. “To improve both the mechanical properties and wear resistance of Al MMCs that are cold sprayed, Al MMC coatings will be reinforced with two particles, sprayed onto an Al substrate, and then coated samples will be heat treated. It is the aim of this paper to show the synergistic effects of both dual-reinforcement and heat-treatment to the properties of Al-MMCs.”

 

Procedure

6. Line 115: Additionally, the work of indentation (WOI) was obtained from the load-displacement curves. Please, describe the calculation procedure.

We estimate the area under the load-displacement curve by using the trapezoidal rule of integration. This was performed in Matlab, which has a function designed specifically to perform this calculation called “trapz(x,y).” You pass values for x, y corresponding to your load-displacement curve into the function and it calculates the area under the curve. A brief explanation was added to section 2.2, lines 118 – 119.

7. Line 137: It looks like 29.258 m travel distance of the wear ball is not correct. Calculation demonstrates the total distance of 28.260 m. Please, correct

You are correct, our travel distance was incorrect. This is corrected.

 

Results

8. Line 146: Authors state:”… Splat sizes across all samples ranged from 40 to 70 μm.” However, authors do not describe how they determine these values. Please, specify

Splat sizes in the optical micrographs were measured using ImageJ. This statement was removed. It is difficult to get accurate measurements of the splat sizes with the optical images. Often splats will overlap each other and often, the boundary between the splats is not clear.

9. Lines 146-150: Authors state:”…Inter-splat grain bridging, and growth was not observed after heat-treatment as was seen by the investigation of Hall  al. [16].” Citing [16] is wrong. It is [17] reference. How were parameters of inter-splat grain bridging and drain growth defined? What is the experimental and calculation procedure?

We will address the questions first, we simply used our optical and SEM images to confirm that splats are not fusing into each other after heat treatment. The larger question was whether the grains within individual splats were changing size through recrystallization. The cited paper, Hall et. al., is the correct reference. They confirmed recrystallization occurred, which we suspect occurred with our samples.   

10. Line 149: The authors statement “…Still, grain growth across splat boundaries was not observed at any temperature or anneal time” cannot be seen on the microstructure images because of low magnification. EBSD analysis needs to be made. Sintering effect needs to be defined

Although we agree with the reviewers’ comments here, our EBSD system is down and has been throughout this study. So, we are unable to perform this analysis on our samples. We can only infer that splat boundaries serve a sufficient barrier to dislocation motion and grain growth from the literature.

11. Line 158: Authors’ explanation “… This can be attributed to changes in grain size (i.e., recrystallization) of the Al matrix” needs to be proved by EBDS examination of grain structure

Same response as with the pervious comments.

12. Line 183: The authors statement “…The results of WOI mirrors the results of the elastic modulus and hardness” is not clear because WOI is the function of the plastic strains. So, the elastic module does not correlate with WOI. Please, explain in detail

The statement has been removed.

13. Lines 200-213: The COF dependences on sliding time shown on Fig. 5 are defined only in the range of sliding distance of about 28.26m (time 30 min). Usually, only running stage of COF diagram is seen at this case (take a look the paper [H. Ashrafi et al., Tribological Properties of Nanostructured Al/Al12(Fe,V)3Si Alloys, Acta Metall. Sin. (Engl. Lett.), 2015, 28(1), 83–92] as an example). The sliding distance of about 300-500m is needed to exactly define the real friction and wear behaviour of MMC. It means that authors showed fall of COF at the running stage and did not obtained the real friction coefficient after the running stage.

Although we would like to run longer wear tests, one concern we had was the thickness of our coatings. Our concern was that longer tests would begin wearing into the substrate, which would invalidate our tests. We did provide a figure in the appendix of actual wear data for one run. All runs looked like this. We do feel that the data levels out after five minutes of testing and that this first five minutes is the running stage and the data where it levels out is real.

14. Line -241: The data shown in Table 2. “Coefficient of friction (COF), mass loss, and specific wear rate of samples A1 – A5 heat treated with condition 4 (C4)’’ belong only for running stage. So, there is no the stable wear conditions during the wear test. Authors need to make the new wear tests at distance of about 500 m.

Same response as with comment 13.

 

Discussion and conclusion

15. From my viewpoint the discussion and conclusion needs to be rewritten based on the new friction and wear test results taking into account the above comments

Thank you for your insightful comments. We have made some of the changes suggested and we think adequately addressed the other concerns.

Round 2

Reviewer 2 Report

After the corrections you have made, your manuscript is ready for publishing.

Reviewer 3 Report

I have no comments