Influence of Carbon Diffusion and the Presence of Oxygen on the Microstructure of Molybdenum Powders Densified by SPS
Round 1
Reviewer 1 Report
The effect of carbon is an important issue in Spark Plasma Sintering technology with clear practical importance. Despite Spark Plasma Sintering is usually realized in vacuum or in an inert atmosphere, the use of graphite tool results in carbonization of sintering environment. This can be crucial for certain materials reacting with carbon. One of such materials is Molybdenum which is prone to carbide building. This important topic is rarely discussed in the literature. Therefore, the subject of presented paper is very interesting for both scientists and practical engineers working with SPS technology. The authors clearly showed the influence of carbon interaction with Mo on grain growth and resulting mechanical properties. The decision to lower the sintering temperature below 1500°C is absolutely correct and well justified. The proposed solution with activation of Mo powder by mechanical milling is original and logical. The paper must be definitely published. However, certain improvements related to presentation style are necessary. Some suggestions for that are summarized in comments below.
- General comments
0.1. The manuscript must be edited by an experienced researcher. Many sentences are not concrete or even wrong formulated. For example in lines 13 and 14 it is written that “Molybdenum, due to its physical, chemical and mechanical properties, is a material that usually be used in industry”. Not all industries use molybdenum. Another example is the sentence in lines 15-16: “Dense molybdenum parts are usually shaped by powder metallurgy process “. This is not true. Pressing and sintering of Mo powder is only a part in technological chain at manufacturing of Mo parts. Sintered Mo preforms are usually forged and rolled for shaping and improvement in properties. The production route of Mo parts is described in many literature sources.
0.2. The manuscript also must be also additionally edited in respect to English language. For example in lines 19-20 it is written that “The formation of carbide is usually expected for a sintering temperature up to 1500°C…” Probably, “over 1500°C” is correct?
- Introduction.
1.1. Please, check the presented area of Mo application. Not all of them are appropriate (tooling, automotive industry) and other important areas are missed (e.g. military armor, metallurgy, electric contacts etc.).
1.2. The increase in density by hot working (forging, rolling) definitively must be mentioned besides such exotic processes as explosive consolidation or microwave energy (is it the right name?)
1.3. Lines 55-56. Please, replace the expression “.. the implementation of molybdenum powder by SPS” by more suitable one.
- Experimental procedure
2.1. The standard characteristic of Mo powder (d10, d50, d90) must be provided. Please, remove the value for Mo density (10.2 g/cm^3) in line 64. The readers can be confused.
2.2. Line 67. Was the powder annealed at 800°C before SPS or this was done only for XPS measurement? Please make this clear.
2.3. Please, make the scale bare in Fig. 1a better visible. The same for the axis labels and numbers in Fig. 2b.
2.4. The results in Fig. 2b must be explained in more detail. What it the meaning of presented peaks?
2.5. Why do you call Mo powder Mo-45? Please, explain this.
2.6. What thickness of graphite felt was used? Why the thermal gradients were not prevented? Please, add just a couple of words about.
2.7. Why different types of cooling (so called natural and controlled) were used? What was their influence on microstructure or properties? Please, explain in detail.
2.8. What pressure profile (pressure versus time profile) was applied?
2.9. What does it mean “…3 points Archimedes technique” (line 92)? Please, correct this term or explain in detail.
2.10. Please, refer standards for mechanical sampling and testing (lines 105-106).
- Experimental results
3.1. Lines 112-113. Grain growth cannot be dependent directly on applied pressure, only density. Please, correct or explain.
3.2. Line 122. Please, indicate and justify somewhere the required density. Why a value of 98.4% is not sufficient, for instance? Nearly the same value (98.7%) was discussed later as acceptable (line 319).
3.3. Line 219. What kind of commercial Mo powder was additionally used? Please give more detail.
3.4. Line 233. What is the meaning of “a high oxidized molybdenum powder”? Please, provide the manufacturer or details of in-house oxidation.
3.5. Lines 256-259. This sentence must be edited. Gaseous phases cannot be observed on fracture surface.
3.6. Figs. 12 and 14b. Please, make the scale bars visible.
- Conclusion.
4.1. It will be very reasonable to compare achieved mechanical properties (in particular, percent elongation) with properties of commercially available molybdenum.
Author Response
Dear colleague, I would first like to thank the reviewers for their in-depth review of our paper. Their various suggestions were very useful for us to improve our article. All their comments have been taken into account and have been added to the revised document attached. Best Regards F. bernard
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This manuscripts deals with the production of Mo parts and possible solutions to attain dense and ductile parts. It is a complete study of different experimental parameters that however requires too many clarifications before it can be consider valid to be published. This makes the manuscript being near to be rejected, but in case the authors get giving an answer and clarify in the text all the different comments it could be considered valid to be published.
Several points should be considered:
1. English should be revised by a native speaker before the manuscript can be accepted. Authors should revise some typos in the text, i.e. line 119, and some others.
2. I guess in Metals references should not be numbered in the text according to the initial letter of the author’s surname, but according to the appearance order in the text.
3. Use of Mo is not very extended in industry, although authors indicate in line 29 some of the industries making use of this material, some specific uses in these industries could be of interests for readers.
4. Many of the Figures in the manuscript have a very low resolution. Authors should include higher resolution images before the manuscript can be published.
5. Figure 1a should include another at higher magnification to show the two different kinds of surfaces in the powder particles, and maybe give an explanation for this.
6. Line 70. A first reference is made to Mo-45. This designation is not included in lines 62-63.
7. Lines 73-77. The experimental arrangement for the die-powders is not clear. An image/scheme of this should be included. Please clarify how the current passage through the sample is avoided.
8. Line 78. Please indicate the values or range of the direct current (maybe in Table 1).
9. Line 80. 1150 ºC does not appear in table 1.
10. Line 81. 7 Mpa does not appear in Table 1.
11. Line 83. How is the pressure decrease during cooling?
12. Line 83-84. Please include in the die image how are the massive Ta discs placed
13. Line 87. Indicate in the text how many samples of Number 1, 2… were prepared and studied. Also include in Table 1 whether the cooling process was natural or controlled for each specimen. Specimen 8 was protected with Ta (layer deposition or disc?).
14. Line 102. As for the SEM, include data of the optical microscope used.
15. Line 104-105. It is not clear where the tensile specimens from the sample were obtained. A scheme would be useful to see which part of the specimen is actually being tested. Did you obtain the specimens and conducted the test according to any standard?
16. Line 115. How was the initial grain size of 25 microns measured? This value should be included when describing the powder. Please check throughout the manuscript that there are no confusions between particle and grain (crystallite) when referring to measured sizes.
17. Line 126. Not being and expert in experimental SPS, if heat is generated in the die, the compact external boundaries will be in contact with the heat die, is it usual a clearly higher temperature in the compact core to make grain growth more than in the periphery? Note that thermal gradients are avoided according to lines 75-76.
18. Line 132. Why 15 min? You have data for 10 and 30 min. Similarly, in line 133, why 2 or 3 min?
19. Line 122. 90 Mpa 1900 ºC 97.8% density, Line 138 70 Mpa 1900 ºC 98.2%, Why higher density with lower pressure? How many measured were carried out for each value? Error values should be added throughout the text in the different values (both coming from different measurements of the same property of coming from instruments uncertainties).
20. Line 139. Value of 99.9 and still seem to appear some black point in the micrograph. Is it porosity? Image quality should be improved.
21. Line 141. Using the same magnification in images makes easier the comparison among them. Please, check this in the whole manuscript, for instance for comparison of Fig 8 and 9.
22. Line 153. Data at 28 and 70 Mpa, why 50 Mpa?
23. Line 169. Indicate the approx. thicknesses found.
24. Line 195. Why using in a) the 500 microns scale? Higher magnification shows better the presence or absence of carbides, as in b). Please indicate in Fig caption whether the image comes from the core or periphery of the sample.
25. Line 209-211. How is avoided C from the die with discs in contact with the punches?
26. Line 220-221. How was this carbide containing powder obtained? Indicate in the Exp Procedure. Line 233. Same for oxidized powder.
27. Line 229. Thin layer, please quantify.
28. Line 233-234. Grain size lower. How was measured? please indicate value. Again, do you refer to grain or particle?
29. Line 235. Ta barrier, layer or disc?
30. Line 247. Very low T (1450 ºC) and no grain growth. It could be thought that SPS is not making its work, what about the obtained density? This data for sure clarifies this.
31. Line 251. Please indicate in caption the SPS condition for a, b and c.
32. Line 252. Temperature does not coincide with that of the text.
33. Line 261. Please indicate in caption the SPS condition.
34. Line 271. Sintering under 1500 ºC is the solution, what about the obtained densities?
35. Lines 285-295. This should be in the Exp. Procedure. Clarify that 250 / -250 refers to omega… Clarify whether the ranges 10-30 and 10-75 correspond to the initial and milled powder.
36. Line 301. Words unmilled and milled appear in this line, ??
37. Line 320. How was C avoided?
38. Line 323. Is it transgranular fracture characteristic of ductile fracture? Brittle fracture can be divided into intergranular and transgranular. Ductile fracture shows dimples and deformation but it is not possible to differentiate the different grains because of direction changes in the advance of the crack. 13% is a quite high elongation, but the image in Figure 14 looks more like a transgranular brittle fracture. Please check and comment.
40. Line 329. There is a big difference in the elastic limit of the unmilled powder (near 500 MPa) and milled powder (about 300 MPa) compacts. Please comment on this.
41. Line 347. Temperature 800 ºC was not commented before.
Author Response
Dear colleague, I would first like to thank the reviewers for their in-depth review of our paper. Their various suggestions were very useful for us to improve our article. All their comments have been taken into account and have been added to the revised document attached. Best regards F. Bernard
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Dear authors, thank you for the efforts to improve the quality of the manuscript. I would have however liked finding and answer to my comments instead of just the text changed, this makes easier following whether you have or not given an answer to my doubts. In this sense, I have not found in the text an answer/change or argument against to several of my comments in the previous review. Comments 1, 4, 7, 8, 13, 16, 17, 18, 19, 20, 24, 27, 28, 29, 33, 34, 37, 38 and 40 should be reviewed again and included if neccessary in the manuscript. Please also give in a separate document and explanation to make easier following your arguments.
Author Response
Dear Reviewer 1,
Thanks again for your interesting and constructive comments. You will find two files : the first concerns the revised paper after a reading by an english colleague for Cambridge University, the second concerns the answer to each point mentioned in your report.
Best regards
Frédéric Bernard
Author Response File: Author Response.pdf
Round 3
Reviewer 2 Report
Authors have now given an adequate answer and included the appropriate comments in the text, according to my suggestions. I consider that the manuscript can now be pubished.
Please, just check that there is a mistake in the references, maybe just in the last one, but please check numbering.
