The Influence of the Powder Characteristics on 316L Stainless Steel Coatings Sprayed by Cold Gas Spray
Round 1
Reviewer 1 Report
The paper presents results of the morphological, elemental, mechanical, and tribological analysis of four coatings made of 316L stainless steel deposited by cold gas spray (CGS) on a carbon steel substrate.
Two deposition powders are recommended for CGS process namely, 316L-CGS_1 (Ref: 316L from Daye, China) and 316L-CGS_2 (Ref: SS316L from Plasma Giken, Japan), and the other two are dedicated to conventional thermal spraying techniques: 316L-HVOF (Ref: DiammalloyTM 1003 from Oerlikon Metco, USA) and 316L-APS (Ref: 316L from Sandvik, Sweden).
The study reported good results from the viewpoint of porosity and corrosion resistance and almost similar hardness values for all the coatings, but better abrasion resistance for CGS coatings.
The paper is original and the amount of experimental research is impressive, with many novel results.
My concerns are related to frictional tests. I have some questions and suggestions for the authors.
The results of friction tests, carried out on a homemade ball-on-disc tribometer, are not following the normal trends. Usually, the friction force and the friction coefficient will increase by increasing the normal load on the contact.
Following this observation, we invite the authors to present:
- One functional sketch of the tribometer.
- The measuring principle and the data acquisition chain, including type of sensors, their characteristics, sensor calibration graph, data acquisition board type, data acquisition frequency.
- Any scientific explanation, accompanied by similar results found in literature would be welcome.
- Microscopy images of the wear trace on tested samples for friction tests, accompanied by EDS results and XRD analysis.
Taking into account that the paper already reported a lot of other interesting results, I recommend to keep only the abrasion test and to eliminate for the moment the friction tests, till the authors will have a possible explanation for the obtained results.
- The introduction section has to be enriched. In the Supplementary references section, there is some recent literature on CGS and APS coatings.
Supplementary references
- https://doi.org/10.3390/coatings10111079
- https://doi.org/10.3390/coatings10020123
- https://doi.org/10.3390/met9060678
- https://doi.org/10.3390/coatings10121186
- https://doi.org/10.3390/met10091195
- https://doi.org/10.3390/app10155153
Author Response
1. One functional sketch of the tribometer.
The authors inserted an explanation about the testing in the text and images of the tribometer scheme and real equipment, following the ASTM G99 standard.
2. The measuring principle and the data acquisition chain, including type of sensors, their characteristics, sensor calibration graph, data acquisition board type, data acquisition frequency.
The authors inserted the testing description in the text: “For the analysis of the sliding wear resistance of the 316L coatings, ball-on-disk tests were carried out, in accordance with ASTM G99-04 (2004) standard. For this test, the samples were previously prepared, by grinding and polishing up to the maximum roughness Ra 0.8 µm. The tests were performed at room temperature (27±2 ËšC) and maximum 20% moisture in dry conditions using a WC-Co ball (∅=11mm), with a sliding rate of 0.13 m.s-1 for a total sliding distance of 1000 m. During the test, the coefficient of friction (CoF) between surfaces was recorded and plotted for the load of 10 N with the acquisition rate of 1 value per lap, with a total of 22737 CoF values. The wear volume loss of the ball on disk samples was calculated by the equation (1), as recommended by the ASTM G99-04 (2004). Where R is the wear track radius, d is the wear track width, r is the ball radius. The friction wear rate is the disk volume loss divided by load and track length or sliding distance.”
3. Any scientific explanation, accompanied by similar results found in literature would be welcome.
The authors inserted a discussion: “The coefficient of friction (CoF) between a WC counterpart and the 316L CGS coatings was measured by means of ball-on-disk experiments. Figure 9 presents the evolution of the CoF obtained during the experiment for all the coatings and a 316L bulk material. The CoF values calculated at the end of the experiment, when the system reaches a stationary behavior are included in Table 6. Similar CoF were measured for all the coatings prepared by CGS, independently on the powder used for their deposition. The 316L bulk CoF, 0.746, is close to results seen in literature with the same testing load, 0.7 [46] and 0.8 [47]; however, the CoF of the coatings were higher, with the highest CoF for the 316L-APS, 0.934.
The wear mechanism of the coating and bulk samples was the same, with partially abrasive type, with typical furrows in the direction of the ball relative movement, indicated as area (1) in Figure 10 (b). Some debris act as a third body during the sliding wear and these materials are extensively deformed and adhered to the worn surface of wear track, presented as area (2) in Figure 10 (b), which refers to adhesive type wear mechanism. The ratio of abrasive/adhesive type wear was higher for the bulk than for the coatings, prevailing the abrasive mode. The EDS mapping of the coating and bulk samples revealed the oxidation of the debris adhered to the wear track, as exemplified by the 316L bulk analysis in Figure 10 (c-f).”
4. Microscopy images of the wear trace on tested samples for friction tests, accompanied by EDS results and XRD analysis.
The authors inserted the worn samples images, CoF and wear rate results and discusions.
5. The introduction section has to be enriched. In the Supplementary references section, there is some recent literature on CGS and APS coatings.
Supplementary references
- https://doi.org/10.3390/coatings10111079
- https://doi.org/10.3390/coatings10020123
- https://doi.org/10.3390/met9060678
- https://doi.org/10.3390/coatings10121186
- https://doi.org/10.3390/met10091195
- https://doi.org/10.3390/app10155153
The authors thanks for the suggestions. They were carefully evaluated and inserted properly in the text.
Reviewer 2 Report
The article "The influence of the powder characteristics on 316L coatings sprayed by Cold Gas Spray” by Rodolpho F. Vaz, Alessio Silvello, Javier Sanchez, Vicente Albaladejo, and Irene García-Cano describes study on the different 316L powders, produced by different manufacturers, which were deposited by Cold Gas Spray (CGS) technique. Their microstructure, adherence, hardness, as well as the performance on corrosion and wear testing were evaluated. The influence of the characteristics of each feedstock powder on the behavior of each coating was highlighted. Finally, to benchmark the performance of cold sprayed 316L, the testing of a 316L stainless steel bulk were also evaluated. In my opinion the work presents a scientific value but before publishing, Authors have to make some corrections.
My comments are following:
- In my opinion, the "Introduction" chapter is too extensive and should be shortened.
- page 4, line 158: The authors report the Vickers indenter load in hardness tests was 0.3 kgf. Kgf are not SI units. Enter the load in N.
- page 5, Table 2: The scientific article should precisely define the chemical composition of the materials used for the research. The chemical composition of 316L steel should be presented in detail.
- Page 8, line 257: The abbreviations used for the first time (in this case, OM) should be explained.
- Page 8, lines 258 – 259: There is “In these images, it can be observed that all the coatings show the densified and lamellae microstructure characteristic of CGS coatings,..” The lamellar structure of the coatings is not visible in Fig. 5.
- Page 8, lines 260 – 261: There is “In addition, the interface observed for all the coatings reveal that these are well adhered to the substrate.” Observation of the interface between the substrate and the coating cannot prove good adhesion of the coating. The results presented in Table 4 confirm the poor adhesion of the APS coating.
- Page 11: There is no caption under Fig. 7.
- Please see the units in table 5 for potential of corrosion.
- Page 15, lines 385 -387: There is “All the 316L coatings are good choices to recover and protect carbon steel parts, because, in comparison to carbon steel bulk, they have higher corrosion resistance and lower abrasion rate. However, regarding the CoF, the carbon steel bulk has lower CoF than all 316L coatings evaluated.” The authors did not investigate this, so such a provision cannot be included in the "Conclusions" chapter.
Author Response
1. In my opinion, the "Introduction" chapter is too extensive and should be shortened.
The authors edited the text for shorting.
2. page 4, line 158: The authors report the Vickers indenter load in hardness tests was 0.3 kgf. Kgf are not SI units. Enter the load in N.
The authors inserted the SI unit for load in the text.
3. page 5, Table 2: The scientific article should precisely define the chemical composition of the materials used for the research. The chemical composition of 316L steel should be presented in detail.
The authors edited the Table. There were inserted all the elements measured by the ICP technique.
4. Page 8, line 257: The abbreviations used for the first time (in this case, OM) should be explained.
The authors inserted in page 4 the description optical microscopy (OM).
5. Page 8, lines 258 – 259: There is “In these images, it can be observed that all the coatings show the densified and lamellae microstructure characteristic of CGS coatings,..” The lamellar structure of the coatings is not visible in Fig. 5.
The authors edited the text: In these images, it can be observed that all the coatings show the densified and typical structure of CGS coatings.
6. Page 8, lines 260 – 261: There is “In addition, the interface observed for all the coatings reveal that these are well adhered to the substrate.” Observation of the interface between the substrate and the coating cannot prove good adhesion of the coating. The results presented in Table 4 confirm the poor adhesion of the APS coating.
The authors edited the text: In addition, the interface substrate/coating observed for all the coatings reveal inexistence of cracks, detachment, or incrusted alumina from the sandblasting substrate surface preparation, denoting these coatings are properly sprayed for the performance testing.
7. Page 11: There is no caption under Fig. 7.
The authors inserted the caption: Figure 7: Tafel slopes of 316L coatings and bulk.
8. Please see the units in table 5 for potential of corrosion.
The authors changed the unit. The correct is mV.
9. Page 15, lines 385-387: There is “All the 316L coatings are good choices to recover and protect carbon steel parts, because, in comparison to carbon steel bulk, they have higher corrosion resistance and lower abrasion rate. However, regarding the CoF, the carbon steel bulk has lower CoF than all 316L coatings evaluated.” The authors did not investigate this, so such a provision cannot be included in the "Conclusions" chapter.
The authors edited this conclusion, focusing on the comparison between the 316L coatings and bulk, eliminating the carbon steel, which was not evaluated in this work.
Reviewer 3 Report
Dear Authors,
I think that the manuscript (MS) must be necessarily improved to be publishable.
The MS is now in form of report, and the results are not particularly significant, because the 4 different powders resulting in 4 very similar coatings.
Some English wording errors can be found in the text together with a lot of useless repetitions, please revise it.
The Authors must revise the MS to significantly improve the scientific soundness.
In the following, some suggestions to improve the MS.
In the title you must add stainless steel.
Abstract
Please shorten the abstract up to 200 words.
The novelty of the present study should be clarified.
Introduction
The whole introduction must be shortened particularly between 70-99 lines and the novelty of the present study should be clarified also in this section.
Materials and Methods
Table 1: please correct putting in the 3 column label Particle Size Distribution. Please add the morphology in the table.
Results and Discussions
Fig 2: please make visible markers.
Line 191: double 1…
Line 199: substitute “In Figure 2 (a-d) are shown the SEM images of the feedstock powders used in this study.”
Lines 201-203: please correct the description, the first 2 powders are not well-rounded, as well as the second 2 powders are not perfectly spherical.
Figure 2d: your SEM image shows the presence of very small particles together with particles with a diameter of about 60 μm. Please add an inset and describe better this morphology.
Figure 3 shows the XRD patterns or diffractograms.
Figure 4: please check the sample 4, by observing the SEM image where is the contribution of small particles? Please use the same samples’ name.
Line 233: you have measured the powders flowability with one method. Please check your sentence.
Line 234: Table 3 is repeated.
Lines 246-249: please reconsider this point. Those, that you name big particles, are the biggest particles among your samples.
Lines 250-253: please rewrite sentence in a more clear and scientific way.
Line 257: OM is not defined in the text.
Figure 5: please better describe the images in the text.
Please rewrite in a more clear way the sentences in lines 293-302.
Figure 7: there is Fig 7 2 times.
Conclusions
Lines 375-377: please clarify the sentence
Lines 383-384: Substitute with: There is no particular advantage to use gas atomized 316L powders over water atomized 316L powders.
Best regards
Author Response
1. In the title you must add stainless steel.
The authors edited the title.
2. Please shorten the abstract up to 200 words.
The authors reviewed the abstract: “Thermally sprayed 316L stainless steel coatings are commonly used on metallic structures due to their corrosion resistance and wear resistance when compared to carbon Steel. Cold Gas Spray (CGS) is a convenient thermal spray process to deposit 316L coatings, producing thick and very dense coatings, with low deleterious changings on the feedstock properties to the coating condition. The powder characteristics influence the microstructure of the coating, such as porosity and oxide contents, which alters its corrosion and wear behavior. CGS is an efficient technique to reduce the problems associated with material melting commonly found in other conventional thermal spray methods. In this work, different 316L powders, produced by different manufacturers, were deposited by CGS, with the same equipment and parameters, with the objective to evaluate the relation between the powders’ characteristics and coating properties. Their microstructure, adherence, hardness, as well as performance on corrosion and wear testing were evaluated. The water atomized powders presented better results than gas atomized powders.”
3. The novelty of the present study should be clarified.
The authors inserted in the text: “The objectives for this work are to study the effect of the differences between several commercially available 316L stainless steel powders on the properties of the resulting coatings and to assess the viability of cold sprayed 316L coatings for wear and corrosion protection of carbon steel and for repairing of stainless steel structures. The evaluation of the coatings obtained from different powders, but with the same CGS equipment and parameters makes possible a better understanding of the effect of the powder characteristics on the coating properties, different to the observed in the literature, which presents properties of distinct powders sprayed in different conditions. To accomplish these objectives, fully dense 316L coatings were prepared via CGS using four different commercial starting powders. The microstructure and mechanical properties of each coating, such as wear and corrosion resistance, were investigated, as well as the powders characteristics.”
4. The whole introduction must be shortened particularly between 70-99 lines and the novelty of the present study should be clarified also in this section.
The authors reviewed the introduction to become more concise.
5. Table 1: please correct putting in the 3 column label Particle Size Distribution. Please add the morphology in the table.
The authors edited the Table.
6. Fig 2: please make visible markers.
The authors edited and reviewed all the Figures.
7. Line 191: double 1…
The authors edited the text.
8. Line 199: substitute “In Figure 2 (a-d) are shown the SEM images of the feedstock powders used in this study.”
The authors edited the text.
9. Lines 201-203: please correct the description, the first 2 powders are not well-rounded, as well as the second 2 powders are not perfectly spherical.
The authors edited the text. Changed the “well-rounded” to “irregular”. Changed “spherical” to “quasi-spherical” and “spherical with satellite particle”.
10. Figure 2d: your SEM image shows the presence of very small particles together with particles with a diameter of about 60 μm. Please add an inset and describe better this morphology.
The authors inserted an explanation for the formation of the satellite particles in gas atomized powders: “The amount of satellite particles in gas atomized powders is attributed to the fabrication process’ parameters by Beckers et al. [17], since the satellite are particles previously solidified that collide and adhere to larger particles, and their formation is influenced by the particle concentration in a atomization flow, the direction of atomizing gases, the design of the atomizing chamber, and other parameters [17].”
11. Figure 3 shows the XRD patterns or diffractograms.
The authors edited the text: “XRD patterns of the 316L feedstock powders.”
12. Figure 4: please check the sample 4, by observing the SEM image where is the contribution of small particles? Please use the same samples’ name.
The authors edited the Figure and changed the labels.
Laser Scattering determines the relative volume distribution of the measured sample and the volume amount of small particles is not sufficient to be represented in the 316L-APS histogram.
13. Line 233: you have measured the powders flowability with one method. Please check your sentence.
The authors inserted in the text the standard for this test: ASTM B213-03, which is properly indicated in the Materials and Methods section too.
14. Line 234: Table 3 is repeated.
The authors edited the text.
15. Lines 246-249: please reconsider this point. Those, that you name big particles, are the biggest particles among your samples.
The authors edited the text, in accordance with the suggestion.
16. Lines 250-253: please rewrite sentence in a more clear and scientific way.
The authors eliminated the sentence and discussed in the previous paragraph: “Taking into account that all the powders have been sprayed under identical spraying conditions, feed rate may show a significant effect on the deposition efficiency and coating characteristics, as it will determine the amount of particles introduced into the powder laden jet, and for hence, will affect to the final particle temperature and/or velocity. The lower values presented in Table 3 for the water atomized powders, 316L-CGS_1 and 316L-CGS_2, are related to the shape of particles, since the spherical shape tends to have higher flowability, and to the d90 value of particle size distribution, which is 60 μm for the water atomized powders and 47 μm for the gas atomized powders.”
17. Line 257: OM is not defined in the text.
The authors inserted in page 4 the description optical microscopy (OM).
18. Figure 5: please better describe the images in the text.
The authors evaluated the images contents and morphologies, inserting the discussion: “The OM cross section images recorded for the CGS coatings deposited in this study are shown in Figure 5. In these images, it can be observed that all the coatings show the densified and typical structure of CGS coatings, independently of the feedstock material used for the deposition. In addition, the interface substrate/coating observed for all the coatings reveal inexistence of cracks, detachment, or incrusted alumina from the sandblasting substrate surface preparation, denoting these coatings were properly sprayed for the performance testing and further application.
For comparative reasons, the porosity of all the coatings was calculated by means of image analysis and the values obtained are included in Table 4. It is worth indicating that these porosity percentages are calculated as a mean value of ten images for each material, and for this reason, the OM images shown in Figure 5 does not necessarily represent this mean value. It is accepted that the density of the coating and deposition efficiency are directly related to the particle size distribution, shape of the feedstock particles, and CGS parameters [32], as the amount of small particle composing the size distribution had severe influence on the porosity of CGS coatings, as related by Spencer and Zhang [33], presenting increasing of porosity with the powder d10 value. In this respect, coatings showing high density and deposition efficiency would be expected when the material used has a smaller d10 value in the particle size distribution and spherical morphology, favoring their flattening and homogeneity of splats phase in the coating, when compared to coatings obtained by irregular shape feedstock powders [32]. All the coatings prepared in this study show porosity percentages lower than 0.5%, independently on the raw material features. These values of porosity are even lower than the values presented by other authors: 1.9% [29], 2.2% [34], 3.3% [9], and 1.86% [5], for identical coating materials. Even the differences between coatings are not significant, the porosity results suggests that for those 316L powders, higher coating density may be achieved when water atomized powders (irregular morphology), with lower d10 value than the spheroidal, are used for CGS deposition. The powder 316L-APS, even with higher powder feed rate and apparent density, 0.50 g.s-1 and 3.73±0.01 g.cm-3, presented the lowest deposition efficiency, 80%, which resulted on the thinnest coating, 381±12 μm. Justified by the particle distribution and highest d10 value, 29 μm, that influences reducing the particle energy at the impact on the substrate, and consequently the particle flattening and anchoring.”
19. Please rewrite in a more clear way the sentences in lines 293-302.
The authors edited the text: “As mentioned the adhesion strength of the coatings was also measured and the data collected are included in Table 4. In addition, all the coating show adhesive failure between coating and substrate, since all of the coatings were completely detached from the substrate. The adherence of 316L CGS coatings is presented with a wide range of values in the literature: > 53 MPa [34], > 60 MPa [40], 80 MPa [13], and 13 MPa [41]. Comparing these references with the Table 4, all of the studied 316L CGS coatings presented relative low values, even lower to 5 MPa for the 316L-APS coating. In spite of this, significant differences are shown between coatings, which can be ordered in terms of adherence as follows 316L-CGS_1 > 316L-CGS_2 > 316L-HVOF > 316L-APS. These differences between all the coatings might indicate that better adhesion of particles to the substrate is achieved for the water atomized 316L-CGS_1 powder. This material also showed the higher deposition efficiency, which had been related with a higher particle velocity. Thus, the adhesion results will suggest that this powder would reach the higher particle temperature, favoring the particle plastic deformation during the impact and as a result, the anchoring of the coating to the substrate.”
20. Figure 7: there is Fig 7 2 times.
The authors edited the text.
21. Lines 375-377: please clarify the sentence
The authors edited the text: “Four different 316L water and gas atomized powders were successfully deposited by CGS. Thick and dense coatings were obtained. The 316L-HVOF powder has better starting characteristics and flow performance than the other evaluated powders (apparent density, flow rate, and deposition feed rate); although, it does not have the best performance in terms of coating properties, the water atomized powders present lower abrasion rate and potential of corrosion. The water atomized powders 316L-CGS_1 and 316L-CGS_2 shown electrochemical properties, measured by the corrosion testing, closer to the 316L reference bulk.”
22. Lines 383-384: Substitute with: There is no particular advantage to use gas atomized 316L powders over water atomized 316L powders.
The authors edited the text.
Reviewer 4 Report
Review of manuscript
“The influence of the powder characteristics on 316L coatings sprayed by Cold Gas Spray”
The paper topic is actual. Authors tried to characterize and compare 316L coatings sprayed by Cold Spraying (CS) of different 316L powders. Unfortunately, authors demonstrate simplified analysis of experimental data. Some additional results are required to increase manuscript performance. An extensive rewriting and edition is needed to make better English.
Some comments are below.
Abstract
- 60% of Abstract (Lines9-20) is about well-known features of CS. This text needs to be eliminated
-
- A statement (Line 14-15) “…no significant changes in the characteristics of the material in the powder form to the material in the coating” is not correct. It is well known (see some CS monographs) occurrence of various structure formation processes during CS which change the particle structure
- Abstract does not reflect the real experimental results; some general sentences are available. Please, rewrite abstract
- Introduction
- Line 42-43; Sentence “…A very important feature of this repair process is to guarantee that the characteristics and properties of the repaired area are similar or better than the worn material, preventing premature failure or wear and/or improving the life of the component [4]” – not proper English
- Lines 75-80: Authors state “… About the selection of specific shape powders for use in CGS, for Jeandin et [21], it depends on the answering of two main questions: first, how the particles impinge during the process and then, the nature of the bonding between two adjacent splats.” – This statement is not completely correct because the answer for the first question includes the explanation for second question. A plastic deformation is the main result of impingement, and localized plastic deformation is the main process resulting in interparticle bonding. But authors do not even mention the strain localization processes. Please, rewrite.
Materials and Method
- Lines 107-108: the sentence “To benchmark the wear and corrosion behavior of CGS 316L, the wear and corrosion behavior of 316L bulk were also evaluated.” needs to be shifted to chapter Materials and Methods
-
- Line 133: Authors state: “ For these tests, powder-feeding rate was kept constant during the whole experiment.” What is the reason of such a statement? Table 3 provides comparison with powder feed rate parameters. Thus powder feed rate is not constant. Please, correct.
- Lines 148: “For detailed inspection of coatings microstructure was used a Thermo Fisher Phenom Pro Desktop SEM (Eindhoven, Netherlands) Scanning Electron Microscopy (SEM).” – not proper English
- Lines 150-151: Authors state that “…porosity of coatings was analyzed with 10 measures per coating with the software ImageJ from images at 200x magnification taken with the optical microscope, according to standard ASTM E2109-01 (2014)”. However, authors do not prove the accuracy of the measurements in spite the obtained porosity data are in the range of 0.5% (Table 4). In accordance with Table 4, porosity varies in the range of 0.04%-0.19%. It is very precise measurements which seem to be difficult to achieve by image analysis, and 10 images seem to be not enough for this purpose.
- Lines 162-163: “The coating sample was adhered to a non-sprayed counterpart with the adhesive HTK Ultrabond 100 with maximum adhesion strength of 70 MPa.” – not proper English
- Line 195: Figure 1 gives well known information and pictures. So, there is no need to present it in the paper.
- Results and Discussion
- Line 198: The main drawback of the chapter 3.1. “Characterization of Powders” is absence of particle microstructure characterization results. Authors need to add these data and more fully characterize the powders and their possible difference.
- Line 205: “….atomized powders is a result of the used of inert gas in the atomization process” – not proper English
- Line 234: “….of every powder is indicated in Table 3Table 3.” – to edit
- Line 236: “…that may be achieved into the CGS equipment.” – to edit
- Lines 239-241: Authors state:” Taking into account that all the powders have been sprayed under identical spraying conditions, feeding rate may show a significant effect on the correct powder deposition, as it will determine the number of particles introduced into the hot pressurized flow of gases, and for hence, will affect to the final particle temperature and/or velocity.” The authors’ statements are unclear. What does it mean “…correct powder deposition”? The term “ hot pressurized flow of gases” is unknown. Is it “powder laden jet”? Authors obtained an influence of variation powder flow rate in the range of 8.21-17.61 g.s-1 on powder feed rate (0.41-0.55 g.s-1). It is about 20%. More detailed analysis is needed
- Line 254: Table 3: There is no parameter “Deposition feed rate”. CS experts use the parameters “powder feed rate” and “deposition rate”. Please, correct
- Lines 269-271: Authors state: “In this respect, coatings showing high density and low porosity would be expected when the material used has a narrow particle size distribution and spherical morphology favoring their compactness when impinging the substrate.” This statement is not clear. What is the parameter “compactness”? There is no such a parameter in CS terminology. From other side, authors need to consider particle deformation features.
- Line 221: The XRD diffractograms corresponding to all the 316L powders shown in Figure 3 indicate the presence of austenite. However, authors do not show any microstructures and XRD diffractograms of the 316L coatings which are needed to define particle deformation and phase transformations due to particle impact.
- Lines 257-260: Authors state “The OM cross section images recorded for the CGS coatings deposited in this study …. show that all the coatings demonstrate … lamellae microstructure characteristic of CGS coatings, independently of the feedstock material used for the deposition”. How is it possible to see lamellar structure on the Fig.5? Authors did not etch the surface. Authors need to present more detailed microstructure examination results.
- Line 278: Table 4 shows the coating thickness and deposition efficiency. Why are coating thickness and deposition efficiency of 316L-APS coatings are smaller than those of other coatings whilst powder feed rate (Table 3 ) is bigger? Authors need to explain.
- Lines 278, 291-302: Table 4 shows small adhesion strength. Authors need to compare these data with literature. Authors did not analyse the interface structure. For this reason, explanations (lines 291-302) are not sufficient. There are a lot of interface examination results in CS literature. Looks like this description is not at proper scientific level
- Lines 280-290: Microhardness of stainless steel coatings depends on microstructure because of processes such as recrystallization, martensite formation due to plastic deformation, etc. Authors even did not mention about this. Adiabatic shear band formation as the strain localization process is known to influence both hardness, bonding and adhesion strength. However, authors did not mention about this as well.
- Line 305: 3. “Corrosion Performance Testing”. Analysis of Tafel curves made by authors does not consider the coatings microstructure features which may define such a corrosion behavior. So, it looks reasonable to make the microstructure examination before and after corrosion tests, and assemble these data
- Lines 329- 332: Authors found that “the differences in Ecorr indicate that the incorporation of 316L layer onto a 316L surface may generate galvanic couples which will worsen the resistance of this part to the corrosive media, accelerating its degradation and mass loss [6,29].” However, authors did not examine the structure features of this effect in detail. The results of such an examination will considerably improve the paper performance.
- Line 341: There is lack of surface topography and structure examination results of the worn samples in p. 3.5. Tribological Behavior Testing. It makes impossible to appropriately analyse the friction and wear results presented by authors
- Lines 350-352: The authors’ assumption “ …. that in the 316L CGS coatings the wear mechanism taking place during friction is similar to that in the 316L bulk material” looks to be not true because the structure of bulk and deposited materials differs considerably. For the same reason, authors’ statement “… that the wear behavior of a material is directly related with its hardness and toughness” is very simplified, and authors need to analyze the wear behavior on the base of advanced state-of-art data.
- Conclusions
Authors need to make conclusion based on additional data of coating structure examination
Author Response
1. 60% of Abstract (Lines 9-20) is about well-known features of CS. This text needs to be eliminated.
The authors edited the abstract: “Thermally sprayed 316L stainless steel coatings are commonly used on metallic structures due to their corrosion resistance and wear resistance when compared to carbon Steel. Cold Gas Spray (CGS) is a convenient thermal spray process to deposit 316L coatings, producing thick and very dense coatings, with low deleterious changings on the feedstock properties to the coating condition. The powder characteristics influence the microstructure of the coating, such as porosity and oxide contents, which alters its corrosion and wear behavior. CGS is an efficient technique to reduce the problems associated with material melting commonly found in other conventional thermal spray methods. In this work, different 316L powders, produced by different manufacturers, were deposited by CGS, with the same equipment and parameters, with the objective to evaluate the relation between the powders’ characteristics and coating properties. Their microstructure, adherence, hardness, as well as performance on corrosion and wear testing were evaluated. The water atomized powders presented better results than gas atomized powders.”
2. A statement (Line 14-15) “…no significant changes in the characteristics of the material in the powder form to the material in the coating” is not correct. It is well known (see some CS monographs) occurrence of various structure formation processes during CS which change the particle structure.
The authors edited the abstract. This specific sentence was changed to: “…low deleterious changings on the feedstock properties to the coating condition.”
3. Abstract does not reflect the real experimental results; some general sentences are available. Please, rewrite abstract
The authors edited the abstract: ““Thermally sprayed 316L stainless steel coatings are commonly used on metallic structures due to their corrosion resistance and wear resistance when compared to carbon Steel. Cold Gas Spray (CGS) is a convenient thermal spray process to deposit 316L coatings, producing thick and very dense coatings, with low deleterious changings on the feedstock properties to the coating condition. The powder characteristics influence the microstructure of the coating, such as porosity and oxide contents, which alters its corrosion and wear behavior. CGS is an efficient technique to reduce the problems associated with material melting commonly found in other conventional thermal spray methods. In this work, different 316L powders, produced by different manufacturers, were deposited by CGS, with the same equipment and parameters, with the objective to evaluate the relation between the powders’ characteristics and coating properties. Their microstructure, adherence, hardness, as well as performance on corrosion and wear testing were evaluated. The water atomized powders presented better results than gas atomized powders.”
4. Line 42-43; Sentence “…A very important feature of this repair process is to guarantee that the characteristics and properties of the repaired area are similar or better than the worn material, preventing premature failure or wear and/or improving the life of the component [4]” – not proper English
The authors eliminated this specific sentence and revised the English.
5. Lines 75-80: Authors state “… About the selection of specific shape powders for use in CGS, for Jeandin et [21], it depends on the answering of two main questions: first, how the particles impinge during the process and then, the nature of the bonding between two adjacent splats.”
This statement is not completely correct because the answer for the first question includes the explanation for second question. A plastic deformation is the main result of impingement, and localized plastic deformation is the main process resulting in interparticle bonding. But authors do not even mention the strain localization processes. Please, rewrite.
The authors inserted mechanisms of adhesion in the text: “Some mechanisms collaborate to the adhesion of the particles to the substrate as listed by Sun et al. [15]: the adiabatic shear instability (ASI), which proposes the high velocity of a particle breaks the natural oxide film on the particle and substrate surface and the progressive plastic flow of the materials enables metallic bonding at atomic scale through direct contact between the adjacent fresh metal surfaces; mechanical interlocking, interfacial mixing, local melting and diffusion [20-23].”
6. Lines 107-108: the sentence “To benchmark the wear and corrosion behavior of CGS 316L, the wear and corrosion behavior of 316L bulk were also evaluated.” needs to be shifted to chapter Materials and Methods.
The authors moved the text to Materials and Methods.
7. Line 133: Authors state: “ For these tests, powder-feeding rate was kept constant during the whole experiment.” What is the reason of such a statement? Table 3 provides comparison with powder feed rate parameters. Thus powder feed rate is not constant. Please, correct.
The authors eliminated this phrase from the text.
8. Lines 148: “For detailed inspection of coatings microstructure was used a Thermo Fisher Phenom Pro Desktop SEM (Eindhoven, Netherlands) Scanning Electron Microscopy (SEM)” – not proper English
The authors edited the text: For Scanning Electron Microscopy (SEM) of the coatings’ cross-section was used a Thermo Fisher Phenom Pro Desktop SEM equipment.
9. Lines 150-151: Authors state that “…porosity of coatings was analyzed with 10 measures per coating with the software ImageJ from images at 200x magnification taken with the optical microscope, according to standard ASTM E2109-01 (2014)”. However, authors do not prove the accuracy of the measurements in spite the obtained porosity data are in the range of 0.5% (Table 4). In accordance with Table 4, porosity varies in the range of 0.04%-0.19%. It is very precise measurements which seem to be difficult to achieve by image analysis, and 10 images seem to be not enough for this purpose.
The authors edited the table. Reduced the precision to 1 decimal.
10. Lines 162-163: “The coating sample was adhered to a non-sprayed counterpart with the adhesive HTK Ultrabond 100 with maximum adhesion strength of 70 MPa.” – not proper English
The authors edited the text: “The adhesion strength of the coatings was measured using a tensile test, in accordance with the ASTM C633-13 standard, mounting the coated sample to the uncoated counterpart using the epoxy resin adhesive HTK Ultrabond 100 (Hamburg, Germany) cured at 180 ËšC for 1h, with traction-adhesive strength of 70 MPa.”
11. Line 195: Figure 1 gives well known information and pictures. So, there is no need to present it in the paper.
The authors eliminated the images.
12. Line 198: The main drawback of the chapter 3.1. “Characterization of Powders” is absence of particle microstructure characterization results. Authors need to add these data and more fully characterize the powders and their possible difference.
The authors inserted microstructure of the particles cross-section and discussed in the text.
13. Line 205: “….atomized powders is a result of the used of inert gas in the atomization process” – not proper English
The authors edited the text: atomized powders is a result of using an inert gas in the atomization process
14. Line 234: “….of every powder is indicated in Table 3Table 3.” – to edit
The authors edited the text.
15. Line 236: “…that may be achieved into the CGS equipment.” – to edit
The authors edited the text: that may be achieved in the CGS equipment.
16. Lines 239-241: Authors state: “Taking into account that all the powders have been sprayed under identical spraying conditions, feeding rate may show a significant effect on the correct powder deposition, as it will determine the number of particles introduced into the hot pressurized flow of gases, and for hence, will affect to the final particle temperature and/or velocity.” The authors’ statements are unclear. What does it mean “…correct powder deposition”? The term “hot pressurized flow of gases” is unknown. Is it “powder laden jet”? Authors obtained an influence of variation powder flow rate in the range of 8.21-17.61 g.s-1 on powder feed rate (0.41-0.55 g.s-1). It is about 20%. More detailed analysis is needed.
The authors re-written the paragraph: “The flowability of the powders was also measured by means of a Hall funnel and the flow rate (in g.s-1) of every powder is indicated in Table 3. Powder flowability is an important characteristic of powders for CGS as it influences the powder feed rate that may be achieved in the CGS equipment. In this respect, powder with low flow rate will show a low feeding rate into the CGS gun, and consequently, an increase of the flow rate of the powder would lead to an increase of the powder feed rate on the same spraying conditions, as confirmed in Table 3, with the highest value of apparent density, flow rate, and powder feed rate seen for the same powder, 316L-HVOF. Taking into account that all the powders have been sprayed under identical spraying conditions, feed rate may show a significant effect on the deposition efficiency and coating characteristics, as it will determine the amount of particles introduced into the powder laden jet, and for hence, will affect to the final particle temperature and/or velocity. The lower values presented in Table 3 for the water atomized powders, 316L-CGS_1 and 316L-CGS_2, are related to the shape of particles, since the spherical shape tends to have higher flowability, and to the d90 value of particle size distribution, which is 60 μm for the water atomized powders and 47 μm for the gas atomized powders.”
17. Line 254: Table 3: There is no parameter “Deposition feed rate”. CS experts use the parameters “powder feed rate” and “deposition rate”. Please, correct
The authors edited the table: Powder feed rate.
18. Lines 269-271: Authors state: “In this respect, coatings showing high density and low porosity would be expected when the material used has a narrow particle size distribution and spherical morphology favoring their compactness when impinging the substrate.” This statement is not clear. What is the parameter “compactness”? There is no such a parameter in CS terminology. From other side, authors need to consider particle deformation features.
The authors edited text. Substituted the compactness to density.
The authors edited text: “In this respect, coatings showing high density and low porosity would be expected when the material used has a narrow particle size distribution and spherical morphology favoring their flattening and homogeneity of splats when impinging the substrate, when compared to irregular shape feedstock powders [31].”
19. Line 221: The XRD diffractograms corresponding to all the 316L powders shown in Figure 3 indicate the presence of austenite. However, authors do not show any microstructures and XRD diffractograms of the 316L coatings which are needed to define particle deformation and phase transformations due to particle impact.
The XRD patterns indicated by the authors in Figure 2 objectives compare the different feedstock powders themselves and not the changing from the feedstock condition to coating condition. In this way, the comparison between water atomized and gas atomized powders is fully attempt with this presented XRD characterization.
The authors agree that the XRD of the coatings should improve the understanding of the coatings properties; however, an etching was done and revealed the particles deformation and the coating microstructure, which is base for the discussions in the text.
20. Lines 257-260: Authors state “The OM cross section images recorded for the CGS coatings deposited in this study …. show that all the coatings demonstrate … lamellae microstructure characteristic of CGS coatings, independently of the feedstock material used for the deposition”. How is it possible to see lamellar structure on the Fig.5? Authors did not etch the surface. Authors need to present more detailed microstructure examination results.
The authors re-written the paragraph: “The OM cross section images recorded for the CGS coatings deposited in this study are shown in Figure 5. In these images, it can be observed that all the coatings show the densified and typical structure of CGS coatings, independently of the feedstock material used for the deposition. In addition, the interface substrate/coating observed for all the coatings reveal inexistence of cracks, detachment, or incrusted alumina from the sandblasting substrate surface preparation, denoting these coatings were properly sprayed for the performance testing and further application.”
21. Line 278: Table 4 shows the coating thickness and deposition efficiency. Why are coating thickness and deposition efficiency of 316L-APS coatings are smaller than those of other coatings whilst powder feed rate (Table 3) is bigger? Authors need to explain.
The authors discussed the porosity, deposition efficiency, and coating thickness: “For comparative reasons, the porosity of all the coatings was calculated by means of image analysis and the values obtained are included in Table 4. It is worth indicating that these porosity percentages are calculated as a mean value of ten images for each material, and for this reason, the OM images shown in Figure 5 does not necessarily represent this mean value. It is accepted that the density of the coating and deposition efficiency are directly related to the particle size distribution, shape of the feedstock particles, and CGS parameters [31], as the amount of small particle composing the size distribution had severe influence on the porosity of CGS coatings, as related by Spencer and Zhang [32], presenting increasing of porosity with the powder d10 value. In this respect, coatings showing high density and deposition efficiency would be expected when the material used has a smaller d10 value in the particle size distribution and spherical morphology, favoring their flattening and homogeneity of splats phase in the coating, when compared to coatings obtained by irregular shape feedstock powders [31]. All the coatings prepared in this study show porosity percentages lower than 0.5%, independently on the raw material features. These values of porosity are even lower than the values presented by other authors: 1.9% [28], 2.2% [33], 3.3% [9], and 1.86% [5], for identical coating materials. Even the differences between coatings are not significant, the porosity results suggests that for those 316L powders, higher coating density may be achieved when water atomized powders (irregular morphology), with lower d10 value than the spheroidal, are used for CGS deposition. The powder 316L-APS, even with higher powder feed rate and apparent density, 0.50 g.s-1 and 3.73±0.01 g.cm-3, presented the lowest deposition efficiency, 80%, which resulted on the thinnest coating, 381±12 μm. Justified by the particle distribution and highest d10 value, 29 μm, that influences reducing the particle energy at the impact on the substrate, and consequently the particle flattening and anchoring.”
22. Lines 278, 291-302: Table 4 shows small adhesion strength. Authors need to compare these data with literature. Authors did not analyse the interface structure. For this reason, explanations (lines 291-302) are not sufficient. There are a lot of interface examination results in CS literature. Looks like this description is not at proper scientific level
The authors discussed the adhesion values: “As mentioned the adhesion strength of the coatings was also measured and the data collected are included in Table 4. In addition, all the coating show adhesive failure between coating and substrate, since all of the coatings were completely detached from the substrate. The adherence of 316L CGS coatings is presented with a wide range of values in the literature: > 53 MPa [33], > 60 MPa [39], 80 MPa [13], and 13 MPa [40]. Comparing these references with the Table 4, all of the studied 316L CGS coatings presented relative low values, even lower to 5 MPa for the 316L-APS coating. In spite of this, significant differences are shown between coatings, which can be ordered in terms of adherence as follows 316L-CGS_1 > 316L-CGS_2 > 316L-HVOF > 316L-APS. These differences between all the coatings might indicate that better adhesion of particles to the substrate is achieved for the water atomized 316L-CGS_1 powder. This material also showed the higher deposition efficiency, which had been related with a higher particle velocity. Thus, the adhesion results will suggest that this powder would reach the higher particle temperature, favoring the particle plastic deformation during the impact and as a result, the anchoring of the coating to the substrate.”
23. Lines 280-290: Microhardness of stainless steel coatings depends on microstructure because of processes such as recrystallization, martensite formation due to plastic deformation, etc. Authors even did not mention about this. Adiabatic shear band formation as the strain localization process is known to influence both hardness, bonding and adhesion strength. However, authors did not mention about this as well.
The authors discussed the microhardness values: “To complete the characterization of the coatings, the microhardness and adhesion strength of these 316L CGS coatings were also analyzed. The average values of microhardness measured for the coatings are shown in Table 4, and do not reveal significant differences among the different coatings, and are close to values obtained by other authors: 358±36 HV [6] and 325±41 HV [32]. The CGS hardness values should be compared to other thermal spraying process 316L coatings: 190±58 HV [33] for flame spraying, 270±30 HV [33] and 312 HV [34] for HVOF, 325 HV for arc spraying [35], and 262 HV for APS [34]. The CGS process characteristics of lower temperature and higher velocity of particles than other processes justify its higher coating hardness, since its higher particles’ velocity promotes the increasing in hardness due to their plastic strain hardening [36], and the relative low temperature prevents the material recrystallization, the ASI mechanism, as explained by Sun et al. [15].”
24. Line 305: 3. “Corrosion Performance Testing”. Analysis of Tafel curves made by authors does not consider the coatings microstructure features which may define such a corrosion behavior. So, it looks reasonable to make the microstructure examination before and after corrosion tests, and assemble these data
The authors agree with the reviewer that for a complete evaluation of the corrosion resistance, it might be interesting a comparison of coating microstructures before and after corrosion texting; however, for the scope of this work it was considered sufficient the tests carried out in order to find the differences between different feedstock materials. Of course, it is projected to carry out these tests in 316L CGS coatings in order to deeply evaluate their behavior against corrosion.
25. Lines 329- 332: Authors found that “the differences in Ecorr indicate that the incorporation of 316L layer onto a 316L surface may generate galvanic couples which will worsen the resistance of this part to the corrosive media, accelerating its degradation and mass loss [6,29].” However, authors did not examine the structure features of this effect in detail. The results of such an examination will considerably improve the paper performance.
In this section of the manuscript, the results obtained clearly show that 316L CGS coatings are less noble and less passively protected than a 316L substrate. Considering that these coatings are addressed to the restoration of stainless steel components, the results here shown pointed out that the incorporation of CGS coating may generate certain issues in the performance of the restored zones when exposed to corrosive media. As indicated in previous comments, deep examination of the microstructure features of these coatings and substrate before and after testing would be carried out in a future work focused on this specific property.
26. Line 341: There is lack of surface topography and structure examination results of the worn samples in p. 3.5. Tribological Behavior Testing. It makes impossible to appropriately analyse the friction and wear results presented by authors
The authors evaluated the wear surfaces by SEM images and EDS. The mappings, images and discussions were inserted in the text.
27. Lines 350-352: The authors’ assumption “ …. that in the 316L CGS coatings the wear mechanism taking place during friction is similar to that in the 316L bulk material” looks to be not true because the structure of bulk and deposited materials differs considerably. For the same reason, authors’ statement “… that the wear behavior of a material is directly related with its hardness and toughness” is very simplified, and authors need to analyze the wear behavior on the base of advanced state-of-art data.
The authors inserted SEM images and EDS mapping of the worn surfaces and samples wear tracks, and discussed the differences between the bulk and coating performance in the text.
28. Authors need to make conclusion based on additional data of coating structure examination
The authors considered the new data and results inserted in the text to restructure the Conclusion section.
Round 2
Reviewer 1 Report
The paper presents results on both CGS and APS powders, deposited by CGS (Cold Gas Spray) process. The subject is innovative, proving that APS powders could be deposited also by CGS. The results indicated the superiority regarding the better wear resistance of the CGS deposited powders when employing the recommended powders for CGS process.
All the requested amendments were accomplished. I recommend this paper for publication in the current form.
Author Response
The authors thank for the reviewing.
Reviewer 3 Report
Dear Authors,
I think that the manuscript (MS) must be still improved to be publishable.
- The English wording has still to be revised, as previously requested.
- The whole introduction was not significantly improved and shortened, as requested
- Line 109 and lines 170-174: your sentences are unclear.
- Table 1: please add the term “Distribution” after Particle Size in the text. Please add the morphology near the fabrication process.
- Line 356: “whilst for the rest of the powders” please substitute for the other three powders…
- Conclusions are written in a very bad English.
Best regards
Reviewer 4 Report
N/A
Author Response
The authors thanks for the reviewing.
