Duplex Treatment of AISI 420 Steel by RF-ICP Nitriding and CrAlN Coating Deposition: The Role of Nitriding Duration
Round 1
Reviewer 1 Report
The manuscript provides some interesting results and can be considered after some minor revisions:
1- Please provide SEM or AFM image of the surface morphology of deposited films.
2- Figure 6, please add the error bars for AISI 420 and WN samples.
Author Response
Dear Reviewer,
many thanks for your remarks. We improved our manuscript. All changes are highlighted by green in the re-submitted article.
Note #1: Please provide SEM or AFM image of the surface morphology of deposited films.
Response #1: Thank you for your remark. We added images of surface morphology (Figure 5) and some discussion about this data (the lines 191-204).
Note #2: Figure 6, please add the error bars for AISI 420 and WN samples.
Response #2: Thank you for your remark. We modified Fig. 7 to better view error bars.
Best wishes,
Dmitrii V. Sidelev
PhD., Associate Prof.
Tomsk Polytechnic University
phone: +7-3822-70-17-77 (add. 2518)
mob. phone: +7-983-238-71-79
Author Response File: 
Author Response.docx
Reviewer 2 Report
The manuscript reports duplex treatment involving magnetron sputtered CrAlN coating on nitrided AISi420 steel, in comparison with CrAlN coating on un-nitrided steel. But authors failed to compare the duplex treated steel with nitrided-alone steel. Thus, it not clear whether the benefits came from the duplex treatment or from nitriding alone. Considering the wear results in Table 1, the thin CrAlN coating was worn through in all cases, it is not clear at what stage the coating was worn through? It is possible that nitriding alone can provide similar or even better wear resistance than the duplex treated samples.
Other comments are as follows:
(1) The chemical composition of the steel should be given.
(2) The heat treatment history of the steel substrate and its hardness should be given. From the hardness measurements in Fig. 7, it seems that the substrate has a low hardness of about 2 GPa, corresponding to the annealed state. This is quite odd because nitriding of alloy steels is normally carried out in the hardened and tempered state.
(3) section 2.2: wear test ... the counter-body ball material and hardness should ben given. Sliding duration/distance should also be given.
(4) Fig. 3: The optical images are of poor resolution. Is it possible to replace them with high resolution SEM images?
(5) section 3.3: It is necessary to show high resolution images to show the surface morphology. Surface roughness parameters do not given a complete picture of surface morphology.
(6) Discussion: The smaller nitrided case depth of the N60 sample was not explained convincingly. What is the sputtering rate? Can it sputtered away 13 micron thick material in the extra 20 minutes? How could sputtering affect the diffusion zone? Where is the extra thickness already developed during the first 40 minutes? The explanation given in the section is not logic. The formation of Fe2-3N could reduce the growth of the compound layer, but the diffusion zone (developed in 40 minutes) should not be reduced.
(7) Some of the conclusions (related to Fe2-3N...) are not valid.
(8) English language needs some corrections.
Author Response
Dear Reviewer,
many thanks for your remarks. We improved our manuscript. All changes are highlighted by green in the re-submitted article.
Note 1: The manuscript reports duplex treatment involving magnetron sputtered CrAlN coating on nitrided AISi420 steel, in comparison with CrAlN coating on un-nitrided steel. But authors failed to compare the duplex treated steel with nitrided-alone steel. Thus, it not clear whether the benefits came from the duplex treatment or from nitriding alone. Considering the wear results in Table 1, the thin CrAlN coating was worn through in all cases, it is not clear at what stage the coating was worn through? It is possible that nitriding alone can provide similar or even better wear resistance than the duplex treated samples.
Response 1: Thank you for your remark. Indeed, nitrided-alone steel can have better wear resistance than that of duplex-treated samples. We additionally prepared the samples using only PN. The specific wear rate of nitride-alone steel was 1.56·10-6 mm3/(m·N), which is in 4 times less than the specific wear of the duplex-treated sample N40 (4·10-6 mm3/(m·N)). However, the duplex treatment is usually applied to modify both mechanical properties and corrosion resistance. Thus, we slightly modified the text in the part “Introduction” (the lines 30-32) and added new results about corrosion resistance of samples in part 3.7 “Corrosion resistance” and “Discussion” (the lines 287-314 and 401-418). The description of corrosion tests was added in part 2.2 (the lines 136-144).
Note #2: The chemical composition of the steel should be given.
Response #2: Thank you for the remark. We added the chemical composition of the AISI 420 steel (lines 79-80).
Note #3: The heat treatment history of the steel substrate and its hardness should be given. From the hardness measurements in Fig. 7, it seems that the substrate has a low hardness of about 2 GPa, corresponding to the annealed state. This is quite odd because nitriding of alloy steels is normally carried out in the hardened and tempered state.
Response #3: Thank you for the remark. The as-received AISI steel substrates were not exposed by any type of thermal treatment before duplex treatment. We added it in the text (the lines 84-86).
Note #4: section 2.2: wear test ... the counter-body ball material and hardness should be given. Sliding duration/distance should also be given.
Response #4: Thank you for the remark. We have added information about counter-body ball material (line 132) and sliding distance (lines 133-135).
Note #5: Fig. 3: The optical images are of poor resolution. Is it possible to replace them with high resolution SEM images?
Response #5: Thank you for the remark. Cross-section optical images provided correct identification of the compound zone unlike SEM images. This is clearly observed by comparing optical and SEM images of the N40 sample. Therefore, we propose to keep optical (Figure 3) images for all samples and SEM (Figure 4) only for the N40 sample.
Note #6: section 3.3: It is necessary to show high resolution images to show the surface morphology. Surface roughness parameters do not given a complete picture of surface morphology.
Response #6: Thank you for your remark. We added high-resolution images of sample surface (Fig. 5) and some discussion about this data (the lines 191-204).
Note #7: Discussion: The smaller nitrided case depth of the N60 sample was not explained convincingly. What is the sputtering rate? Can it sputtered away 13 micron thick material in the extra 20 minutes? How could sputtering affect the diffusion zone? Where is the extra thickness already developed during the first 40 minutes? The explanation given in the section is not logic. The formation of Fe2-3N could reduce the growth of the compound layer, but the diffusion zone (developed in 40 minutes) should not be reduced.
Response #7:
Thank you for your comment. The change of the thickness of the compound layer (CL) and the diffusion zone (DZ) are associated with two processes – nitrogen diffusion into the substrate and surface sputtering by argon and nitrogen ions. We compared the diffusion rates through Fe4N and F2-3N solid solutions with the sputtering rates of these phases for the experimental conditions.
7.1) Diffusion rate calculations
In order to calculate the nitrogen diffusion rate it is necessary to obtain the diffusion coefficient for Fe4N and F2-3N solid solutions. The diffusion coefficients should not be assumed as a constant value to the phases α-Fe, γ′ and ε, instead they should be implemented as a function of temperature. From the literature [10.1016/j.surfcoat.2022.128813; 10.1016/j.apsusc.2004.11.029], we obtained the temperature-dependent diffusion coefficients of nitrogen (N) in iron nitride phases γ′ and ε.
Fig. 1. Diagram of the temperature-dependent diffusion coefficients of nitrogen (N) in the nitride phases γ′ (A) and ε (B) based on mathematical calculations from different sources. For A) both curves are very close to each other compared to B), which shows a larger dispersion [10.1016/j.surfcoat.2022.128813].
Since the plasma nitriding process was carried out at 470 oC, diffusion coefficients in Fe4N and F2-3N are presented in Table 1.
Table 1 - Diffusion coefficients for Fe4N and F2-3N solid solutions.
|
|
Diffusion coefficient, mm2/s |
|
Fe4N |
5⋅10-8 |
|
F2-3N |
9⋅10-10 – 4⋅10-9 |
The average distance that a diffusing particle passes during time t is of the order of , and called the characteristic diffusion path. For calculations of the diffusion rate of Fe2-3N, the diffusion coefficient is assumed to be equal to 4⋅10-9 mm2/s. Hence, after 20 minutes, the particle will diffuse 7.7 µm and 2 µm for Fe4N and F2-3N respectively.
7.2) Sputtering rate calculations
Empiric equation of sputtering rate can be used to estimate:
,
where – sputtering yield, – substrate current density (А/µm2), – molar weight (g/mol), – Avogadro's number (mol-1), – substrate density (g/ µm3), – electronic charge.
Sputtering yield was obtained using the TRIM software, the input data was followed:
- argon ions with energy 80 eV (substrate bias equals -80 V);
- primary beam angle – 00;
- target data – Fe4N and F2-3N (we used EDS data for these calculations).
Results of calculations of sputtering rate is shown in Table 2.
Table 2 - Results of calculations of sputtering rate.
|
|
Fe4N |
F2-3N |
|
Sputtering yield |
0.332 |
0.317 |
|
Sputtering rate, µm/h |
140 |
105 |
Hence, ~47 µm of Fe4N phase and 35 µm Fe2-3N phase can be sputtered by argon ions in 20 minutes. Nitrogen ions are also involved in the sputtering process, however, for comparative analysis, we neglect this component.
The present results show that the transition from Fe4N phase to Fe2-3N, the decrease in both, diffusion and sputtering rates can be. However, diffusion rate decrease by 3.5, while sputtering rate decreases by 1.3.
So, we added some information about calculations of sputtering rate and data of nitrogen diffusion coefficients in the paper (the lines 327-341).
Note #8: Some of the conclusions (related to Fe2-3N...) are not valid.
Response #8: Thank you for your remark. We added discussion about non-linear dependence of nitriding rate of AISI 420 steel samples. So, we believed that conclusions are satisfactory.
Note #9: English language needs some corrections.
Response #9: Thank you for your remark. We improved English.
Best wishes,
Dmitrii V. Sidelev
PhD., Associate Prof.
Tomsk Polytechnic University
phone: +7-3822-70-17-77 (add. 2518)
mob. phone: +7-983-238-71-79
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Please include a unit for the sputtering yield values 0.332 and 0.317, line 338, page 12.
Author Response
Dear Reviewer,
many thanks for your note “Please include a unit for the sputtering yield values 0.332 and 0.317, line 338, page 12”.
The sputtering yield is a dimensionless quantity. We forgot to indicate the dimension of qe in the paper.
The sputtering rate of γ'-Fe4N and ε-Fe2-3N was calculated using the following formula:
, (4)
where K – sputtering yield, j – ion current density to a substrate (А/µm2), M2 – molar weight of substrate material (g/mol), NA – Avogadro's number (mol-1), ρ – density of substrate material (g/µm3), qe – elemental charge (С).
So, we corrected the text in the article (the line 336).
Best wishes,
Dmitrii V. Sidelev
PhD., Associate Prof.
Tomsk Polytechnic University
phone: +7-3822-70-17-77 (add. 2518)
mob. phone: +7-983-238-71-79
Author Response File: Author Response.docx
