Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The submitted manuscript discusses the mechanical and fractural analysis of borided AISI 9254 steel. The boriding process was performed using a used boron paste. The topic is attractive to the readers and authors tried their best to present their work in understandable way. However, some comments have been raised to elevate the quality of the current submission. The detailed comments are annotated in teh attached PDF file. Authors are recommended to go through the entire document and indicate each comment in their response.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Some grammatical problems must be solved.

Author Response

Comment 1

The title mentions the term “sustainable”; however, the entire paper does not focus on this term. The authors only reused boron paste, which is what indicates sustainability. This issue must be clarified in the revised manuscript.

Response 1
Title modified:
Characterization of the Morphology of Fe₂B Borided Layers on AISI 9254 Steel Using Reused Boron Paste: A Classical and Fractal Approach

Comment 2

“... were estimated.” (Estimated to be how much?) — Line 9

Response 2
Title Abstract

Boriding is a widely used thermochemical treatment to improve surface hardness and wear resistance in steels used in demanding mechanical applications. However, boriding processes using new boron paste increase costs and generate waste, creating a need for more sustainable alternatives. In this context, the reuse of dehydrated boron paste has proven effective in the formation of Fe₂B layers on AISI 9254 steel. In this study, AISI 9254 steel was borided using reused dehydrated boron paste at 1173 K, 1223 K, and 1273 K for 3600, 7200, 10800, and 14400 s. Optical microscopy revealed layer thicknesses ranging from 16.07 µm to 69.35 µm. X-ray diffraction confirmed the formation of single-phase Fe₂B, while EDS indicated elemental redistribution within the layer. The Vickers microhardness profile characterized the mechanical behavior, and the adhesion force showed HF1–HF2 ratings. The activation energy for boron diffusion in Fe₂B was calculated as 106.567 kJ·mol⁻¹. Auto-affine analysis verified the fractal nature of interface growth, with a scale ω(δ) according to ω(δ) ~ δᴴ. These results confirm that reused paste allows the formation of Fe₂B layers, supporting sustainable boriding strategies with controlled interfacial evolution.

Comment 3

The literature review must be enriched with information from similar works.

Response 3
Additional literature has been integrated.

[7] Murathan, Ö.F.; Davut, K.; Kilicli, V. Effect of austenitizing temperatures on the microstructure and mechanical properties of AISI 9254 steel. Materials Testing 2021, 63, 48–54. https://doi.org/10.1515/mt-2020-0007
[8] Charee, W.; Tangwarodomnukun, V. Experimental investigation and modeling of laser surface melting process for AISI 9254 commercially high silicon spring steel. Optics & Laser Technology 2019, 115, 109–117. https://doi.org/10.1016/j.optlastec.2019.02.013
[31] Sánchez-Fuentes, Y.; Linares-Duarte, L.A.; Balderas-López, J.A. et al. Effect of boriding time on the effective thermal diffusivity of borided AISI 1018 steel. Sci Rep 2025, 15, 19125. https://doi.org/10.1038/s41598-025-04105-1
[32] Góral, M.; Kościelniak, B.; Ochał, K.; Kubaszek, T.; Jopek, J.; Drajewicz, M. The structure of boride diffusion coatings produced on selected grades of structural steels. Solid State Phenom. 2024, 355, 95–100. https://doi.org/10.4028/p-hvSj08
[33] Omar, N.H.; Hasan, R.; Masripan, N.A.B. Kinetic study of boronized AISI 304 ball bearing using new and used boronizing powder. Proc. Mech. Eng. Res. Day 2017, 338–339.
[34] Sánchez Fuentes, L.; López Perrusquia, N.; Elías Espinosa, M.C.; Melo Máximo, D.V.; De la Mora Ramírez, T.; Olmos Domínguez, V.H.; Doñu Ruiz, M.A. Adhesion characterization on AISI 9254 steel boriding. Microsc. Microanal. 2024, 30 (Suppl. 1), ozae044.629. https://doi.org/10.1093/mam/ozae044.629
[35] Erdemir, A.; Eryilmaz, O.; Sista, V. Ultra-fast boriding for improved efficiency and reduced emissions in materials processing industries. Argonne National Lab., IL, USA, 2012.
[36] Daas, A.; Allaoui, L.A.; Zidelmel, S.; Allaoui, O. Paste borided layers produced on XC38 steel using a new activator. Mater. Perform. Charact. 2020, 9 (3), 392–399.

Comment 4

“Boriding with reused paste constitutes a thermochemical treatment capable of producing Fe₂B monolayers with enhanced wear resistance, corrosion resistance, adhesion, and other properties.”
Reference(s) needed — Line 37.

Response 4 (Line 41)
References integrated:

[5] Campos-Silva, I.E.; Rodríguez-Castro, G.A. Boriding to improve the mechanical properties and corrosion resistance of steels. In: Thermochemical Surface Engineering of Steels; Mittemeijer, E.J., Somers, M.A.J. (Eds.); Woodhead Publishing, 2015, 651–702. https://doi.org/10.1533/9780857096524.5.651
[12] Kulka, M. Trends in Physical Techniques of Boriding. In: Current Trends in Boriding. Engineering Materials; Springer, Cham, 2019. https://doi.org/10.1007/978-3-030-06782-3_5

Comment 5

“In steels or on steels?” — Line 40

Response 5 (Line 42)
Integrated correction.

Comment 6

On page 2, lines 69–73.

Response 6
Integrated correction.
This work describes the materials and methods applied to AISI 9254 steel, including the boriding treatment, characterization, and layer formation. It also presents the mechanical properties of AISI 9254 steel, estimates of diffusivities and activation energy Q, and morphological characterization of the borided layers.

Comment 7

On page 3, line 75.

Response 7 (Line 78)
Integrated correction — Mechanical characterization.

Comment 8

Who determined this composition? If it was determined in this work, authors must mention how. If taken from elsewhere, reference is needed — Line 77.

Response 8
Integrated correction.
Commercial AISI 9254 steel was used, with a chemical composition of 0.51–0.59% C, 1.20–1.60% Si, 0.60–0.80% Mn, max. 0.035% P, max. 0.040% S, and 0.60–0.80% Cr. (Line 80)

Comment 9

Figure 1 demonstrates the units in mm. Why are the units in cm in the text? Authors are recommended to use a consistent writing style. — Line 79.

Response 9 (Line 82)
Integrated correction.

Comment 10

What type of furnace was used? How was the specimen cooled? Rapidly or quenched? Several details are missing. — Line 80.

Response 10 (Line 85)
Integrated correction.
The container was cooled in a conventional muffle furnace without inert gas, and after the treatment time, the container was removed for air cooling.

Comment 11

What is the chemical composition of the dehydrated boron paste?

Response 11 (Line 84)
Integrated correction.
A commercial boron paste was used [56–57] by powder-pack process with reused dehydrated boron paste.

Comment 12

Was the process done at atmospheric pressure? Was there any inert gas or open-air environment?

Response 12 (Line 85)
Integrated correction.
The container was cooled in a conventional muffle furnace without inert gas, and after the treatment time, the container was removed for air cooling.

Comment 13

The specimen must be presented in a different color. The current schematic shows the specimen and surrounding material with similar coloring (Fig. 1).

Response 13
Integrated correction.

Comment 14

See the comment in line 79 (Fig. 1).

Response 14 (Line 82)
Integrated correction.

Comment 15

On page 3, line 83.

Response 15 (Line 91)
Integrated correction.

Comment 16

How was the specimen prepared for examination (sectioning, grinding, polishing, etching)? Authors must improve this part of the methodology. — Line 84.

Response 16 (Line 89)
Integrated correction.
The borided specimens were sectioned transversely for conventional metallographic processing and chemically etched with 2% nital.

Comment 17

1. How was the specimen prepared for this examination (polished surfaces)?

2. What were the accelerating voltage and current used in the XRD test?

3. Between which angles was the test performed?
   Authors must improve this part of the methodology.

Response 17 (Line 92)
Integrated correction.
To determine the phases, the borided specimens in each condition were subjected to X-ray diffraction (XRD) analysis using a Bruker D8 Advance diffractometer. CuKα radiation with a wavelength of 1.54 Å was used in a 2θ range of 20°–90°, operating at 35 kV and 25 mA.

Comment 18

1. Which hardness scale was used in this experiment (Brinell, Vickers, Rockwell)?

2. How was the test performed?
   Authors must improve this part of the methodology.

Response 18 (Line 95)
Integrated correction.
Vickers microhardness tests were performed with a load of 50 gf and an indentation time of 10 s. Five measurements were taken for each condition using Wilson® Hardness Tukon™ 1102 equipment.

Comment 19

On page 3, line 88.

Response 19
Integrated correction.

Comment 20

What does “HF” stand for in this figure? — Line 101.

Response 20
Integrated correction.
The adhesion of the boron layers was evaluated using the Daimler-Benz Rockwell C adhesion test prescribed by the VDI 3198 standard. A CV-700™ durometer was used. Figure 2 shows the adhesion classifications HF types according to the VDI 3198 standard [12].

Comment 21

Authors must be aware of copyright rules when presenting figures from other sources.

Response 21
Integrated correction (based on [15]).

Comment 22

Authors must elaborate more on this figure. How should the reader interpret the values?

Response 22
Integrated correction.

Comment 23

This table must be presented in the Results section.

Response 23
Integrated correction.

Comment 24

None of the experimental temperatures has a value of 1273.15 K.

Response 24
Integrated correction.

Comment 25

This term must be defined when first mentioned — Line 132.

Response 25 (Line 145)
Integrated correction — Energy Dispersive Spectroscopy (EDS).

Comment 26

This is expected in such experiments; however, authors must justify these findings. What should the reader understand from this result? — Line 133.

Response 26 (Line 147)
Integrated correction.
The profiles obtained by EDS show how boron and some alloying elements diffuse in the Fe₂B coating.

Comment 27

The X-ray results are poorly discussed. Authors are strongly recommended to improve the discussion — Lines 139–149.

Response 27 (Lines 154–175)
The X-ray diffraction (XRD) patterns of the boriding samples at 1173, 1223, and 1273 K for different treatment times are shown in Figure 5. In all cases, the main peaks correspond to the Fe₂B phase, confirming the formation of a monolayer of iron boride on AISI 9254 steel. No signals associated with FeB were detected, suggesting that the treatment conditions and the use of reused dehydrated boron paste favored controlled boron diffusion, limiting supersaturation on the surface. The elimination of the FeB phase is important, as this phase usually appears under conditions of high boron concentration or prolonged exposure times, and its presence can induce brittleness due to its thermal expansion coefficient differing from that of the substrate. Secondary peaks of Mn₂B and CrB were also observed, consistent with the chemical composition of AISI 9254 steel, rich in manganese and chromium. These secondary borides originate from the competitive diffusion of alloying elements and may contribute to increased surface hardness, as reported in Cr- and Mn-alloyed steels [5,22,24]. Likewise, the increasing intensity of the Fe₂B peaks with boriding temperature indicates a higher degree of crystallization of the coating, reflecting greater structural homogeneity and preferential growth in the (002) and (211) planes, characteristic of columnar Fe₂B. This behavior has been reported in other works by Campos-Silva et al. [10] and Ortiz-Domínguez et al. [17], where it is associated with an improvement in the mechanical integrity of the coating. X-ray diffraction analysis confirms that the reuse of dehydrated boron paste allows the production of single-phase Fe₂B layers, free of FeB, with secondary Mn₂B and CrB.

Comment 28–32

Minor editorial corrections.

Responses 28–32
Integrated corrections.

Comment 33

Authors must explain what the HF code implies. When there is HF1, what does that mean, and how is it different from HF2 in terms of adhesion (not only morphology)? — Line 168.

Response 33 (Line 209)
Integrated correction.
The HF code corresponds to the adhesion classification defined by the VDI 3198 standard using the Rockwell-C indentation test. An HF1 rating indicates excellent adhesion, with cracks or fissures around the indentation, while HF2 shows slightly lower adhesion characterized by radial cracks or localized delamination. Therefore, HF1 coatings have a stronger interfacial bond compared to HF2 coatings, not only in morphological terms but also in mechanical integrity and resistance to coating failure.

Comment 34

Authors must explain how these diffusivities were obtained — Line 191.

Response 34 (Line 227)
The diffusivities are represented by the slopes in Figure 8. They are obtained by plotting the squared layer thickness as a function of the treatment time.

Comment 35

Reference is needed — Line 194.

Response 35 (Line 230)
[30] Laidler, K.J. The development of the Arrhenius equation. J. Chem. Educ. 1984, 61(6), 494.

Comment 36

Which reference supports the methodology used (Lines 210–214)?

Response 36 (Lines 256–261)
Integrated references.

Comment 37

“The results of what?” — Line 210.

Response 37 (Line 249)
The results of the fractal characterization of the morphology of the borided layers.

Comment 38

We do not see figures describing morphology in this section — Line 217.

Response 38
It is not possible to visualize morphology directly from the Hurst exponent; a theoretical interpretation is provided instead.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Authors have studied  boriding process on AISI 9254 steel using reused dehydrated boron paste. They varied different temperature and time, and found the fluctuation associated with the layer thicknesses follows power-law scaling behavior described by Hurst exponents.

Please answer following questions:

1) On page 1, Line 53:  "However, it is important to evaluate the diffusivity of this process ...."

-- Can you explain the meaning of the term "evaluate the diffusivity of the process"?

2) On page 1, Line 55: "....investigating the microstructural, mechanical, and diffusion characteristics.." 
-- Diffusion characteristics of what?

3) a) You have selected AISI 9254 steel, but no explanation was given for this selection. Authors may be aware of the industrial use of this steel, but some readers may not know the context. 
b) Is this study valid for any type steel?

4) On Page 4, Table 1: Average borided layer thicknesses is given. Can you provide standard deviation for these measurements? This will be helpful for other researchers. 

5) Page 5: For figure 3 a-h, length scale is not mentioned on images or units for the area is not mentioned in the caption.
Fig. 5i does not have legends.

6)  On Page 9: Line 191 - units of k1, k2 and k3 are missing. Also, these values are not that different if you factor in scatter in the experimental data. Can you comment on this?

7) In Figure 8, data does not look linear for any given temperature, but you are trying to fit straight line. What could be the reason?

8) Page 10, Line 202-203 "the thicknesses and diffusivities are lower compared with those of other steels when non-reused paste is employed."
-- Can you cite data from literature for this?

Author Response

Comment 1

On page 1, line 53: “However, it is important to evaluate the diffusivity of this process ….”
Can you explain the meaning of the term “evaluate the diffusivity of the process”?

Response 1 (line 57)
In addition, evaluation of diffusivity permits the determination and analysis of the diffusion rate of boron atoms during the phenomenon, with the aim of comprehending, controlling, and optimizing the process, as reported in the literature.

Comment 2

On page 1, line 55: “... investigating the microstructural, mechanical, and diffusion characteristics …”
Diffusion characteristics of what?

Response 2 (line 59)
In this context, research into the microstructural and mechanical characteristics and diffusion of the boron paste treatment is essential because this process depends directly on the ability of boron to penetrate the surface of the material to form Fe₂B monolayers, offering technological alternatives while contributing to the preservation of natural resources.

Response 3
Considering the industrial relevance of AISI 9254 steel and the need to optimize its properties under a boriding treatment using reused dehydrated paste, this study is of great scientific and technological interest, providing valuable information for its implementation in manufacturing sectors. There are production areas that face the need to improve material performance and to optimize materials to extend their service life through the formation of monolayers on metallic substrates [5–8].

[7] Murathan, Ö.F.; Davut, K.; Kilicli, V. Effect of austenitizing temperatures on the microstructure and mechanical properties of AISI 9254 steel. Materials Testing 2021, 63, 48–54. https://doi.org/10.1515/mt-2020-0007
[8] Charee, W.; Tangwarodomnukun, V. Experimental investigation and modeling of laser surface melting process for AISI 9254 commercially high silicon spring steel. Optics & Laser Technology 2019, 115, 109–117. https://doi.org/10.1016/j.optlastec.2019.02.013

Comment 4

On page 4, Table 1: Average borided layer thicknesses are given. Can you provide the standard deviation for these measurements? This will be helpful for other researchers.

Response 4
A column with the standard deviation has been added to Table 1.

Comment 5

Page 5: For Figure 3 (a–h), the length scale is not mentioned on the images, and the units for the area are not specified in the caption. Figure 5i does not have legends.

Response 5
Integrated correction.

Comment 6

On page 9, line 191: The units of k₁, k₂, and k₃ are missing. Also, these values are not very different if one considers the scatter in the experimental data. Can you comment on this?

Response 6 (line 227)
Integrated correction.

If there is data dispersion but the data follows a rate of change according to the diffusion coefficients, that is, they are different magnitudes

Comment 7

In Figure 8, data do not look linear for any given temperature, yet a straight-line fit is applied. What could be the reason?

Response 7
The Pearson correlation coefficient for the fit in Figure 8 is 0.98; therefore, a linear model was selected.

Comment 8

Page 10, lines 202–203: “The thicknesses and diffusivities are lower compared with those of other steels when non-reused paste is employed.”
Can you cite data from the literature for this?

Response 8
Different research reports significant variations in layer thickness and boron diffusivity depending on the type of steel and the reuse status of the boronizing process [31–36]. References have been integrated into the article.

[31] Sánchez-Fuentes, Y.; Linares-Duarte, L.A.; Balderas-López, J.A. et al. Effect of boriding time on the effective thermal diffusivity of borided AISI 1018 steel. Sci Rep 2025, 15, 19125. https://doi.org/10.1038/s41598-025-04105-1
[32] Góral, M.; Kościelniak, B.; Ochał, K.; Kubaszek, T.; Jopek, J.; Drajewicz, M. The structure of boride diffusion coatings produced on selected grades of structural steels. Solid State Phenom. 2024, 355, 95–100. https://doi.org/10.4028/p-hvSj08
[33] Omar, N.H.; Hasan, R.; Masripan, N.A.B. Kinetic study of boronized AISI 304 ball bearing using new and used boronizing powder. Proc. Mech. Eng. Res. Day 2017, 338–339.
[34] Sánchez Fuentes, L.; López Perrusquia, N.; Elías Espinosa, M.C.; Melo Máximo, D.V.; De la Mora Ramírez, T.; Olmos Domínguez, V.H.; Doñu Ruiz, M.A. Adhesion characterization on AISI 9254 steel boriding. Microsc. Microanal. 2024, 30 (Suppl. 1), ozae044.629. https://doi.org/10.1093/mam/ozae044.629
[35] Erdemir, A.; Eryilmaz, O.; Sista, V. Ultra-fast boriding for improved efficiency and reduced emissions in materials processing industries. Argonne National Lab., IL, USA, 2012.
[36] Daas, A.; Allaoui, L.A.; Zidelmel, S.; Allaoui, O. Paste borided layers produced on XC38 steel using a new activator. Mater. Perform. Charact. 2020, 9 (3), 392–399.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This paper conducted boronizing treatment on AISI 9254 steel using reusable dehydrated boron paste. The treatment was carried out at 1173 K, 1223 K, and 1273 K for 3600 s, 7200 s, 10800 s, and 14400 s, respectively. The morphology of the layer was observed, the thickness of the layer and the formed Fe₂B compound were measured, and the microhardness of Fe₂B and the bonding strength with the substrate were studied. The activation energy of boron in Fe₂B was also determined. Finally, the interface growth was studied using the self-affine method. The results showed that the interface width ω followed the power-law relationship ω(δ)∼δH, and the fluctuations related to the layer thickness also followed the power-law scaling. This research has certain innovation and is worth recommending for publication, but there are still some problems that need to be modified.

1. The title of this paper needs to be modified. The main focus of this paper is not on mechanics. It only involves a little microhardness, and is more about the interface layer and the formed Fe₂B compound. Therefore, the title needs to be revised.
2. The beginning of the abstract should introduce the research background and explain why such work is necessary. The end of the abstract should introduce the significance of this research, which needs to be supplemented.
3. In the introduction section, the author has referenced many related literature, but there are few references to this journal. It is suggested that the author appropriately increase the references to related literature, especially those from recent years.
4. This paper mainly studies the boronizing treatment of AISI 9254 steel. Please explain what gaps this paper fills and what innovative work has been done.
5. The a-h figures in Figure 3 should be supplemented with scales. Also, the references for Equations 1 and 2 should be indicated. In addition, the formulas in Section 3.5 should be numbered.
6. What factors mainly affect the bonding strength between the Fe₂B layer and the metal substrate, and why are there different bonding grades?
7. Why did the author apply the self-affine method to study the interface growth, and how was the research conducted?

Author Response

Comment 1

The title of this paper needs to be modified. The main focus of this paper is not on mechanics. It only involves a little microhardness and is more about the interface layer and the formed Fe₂B compound. Therefore, the title needs to be revised.

Response 1

Morphological Characterization of Fe₂B Borided Layers on AISI 9254 Steel Using Reused Boron Paste: A Classical and Fractal Approach

Comment 2

The beginning of the abstract should introduce the research background and explain why such work is necessary. The end of the abstract should introduce the significance of this research, which needs to be supplemented.

Response 2
The recommendation has been followed and applied.

Boriding is a widely used thermochemical treatment to improve surface hardness and wear resistance in steels used in demanding mechanical applications. However, boronizing processes using new boron paste increase costs and generate waste, creating a need for more sustainable alternatives. In this context, the reuse of dehydrated boron paste has proven effective in the formation of Fe₂B layers on AISI 9254 steel. In this study, AISI 9254 steel was boronized using reused dehydrated boron paste at 1173 K, 1223 K, and 1273 K for 3600, 7200, 10800, and 14400 s. Optical microscopy revealed layer thicknesses ranging from 16.07 µm to 69.35 µm. X-ray diffraction confirmed the formation of single-phase Fe₂B, while EDS indicated elemental redistribution within the layer. The Vickers microhardness profile characterized the mechanical behavior, and the adhesion force showed HF1–HF2 ratings. The activation energy for boron diffusion in Fe₂B was calculated as 106.567 kJ·mol⁻¹. Auto-affine analysis verified the fractal nature of interface growth, with a scale ω(δ) according to ω(δ) ~ δᴴ. These results confirm that reused paste allows the formation of Fe₂B layers, supporting sustainable boronizing strategies with controlled interfacial evolution.

Comment 3

In the introduction section, the authors have referenced many related studies, but there are few references to this journal. It is suggested that the authors appropriately increase the references to related literature, especially those from recent years.

Response 3
Integrated additional literature from recent MDPI publications.

Comment 4

This paper mainly studies the boronizing treatment of AISI 9254 steel. Please explain what gaps this paper fills and what innovative work has been done.

Response 4

• The limited literature on boronizing AISI 9254 steel regarding the morphology and interfacial behavior of the Fe₂B layer using the reused dehydrated boron-paste process is addressed.

• The scarcity of studies applying fractal or self-similar methods to understand the growth of the Fe₂B interface using reused dehydrated boron paste is filled.

• This work demonstrates that it is possible to obtain a single-phase Fe₂B layer using reused dehydrated boron paste. It determines the activation energy, achieves optimal adhesion (HF1–HF2), and applies fractal and self-similar methods to study interfacial growth.

Comment 5

The a–h figures in Figure 3 should be supplemented with scales. Also, the references for Equations (1) and (2) should be indicated. In addition, the formulas in Section 3.5 should be numbered.

Response 5
The observation has been followed and applied accordingly.

Comment 6

What factors mainly affect the bonding strength between the Fe₂B layer and the metal substrate, and why are there different bonding grades?

Response 6
The adhesion of the Fe₂B/substrate bond depends largely on the controlled diffusion of boron, the composition of the steel, the morphology of the layer, and the residual stresses generated during treatment, as mentioned in various studies [1–3]. The different combinations of these factors give rise to varying degrees of bonding, ranging from highly adhered and resistant interfaces to weak bonds susceptible to delamination or fractures, as reported in the literature [4].

Comment 7

Why did the author apply the self-affine method to study the interface growth, and how was the research conducted?

Response 7
The self-affine method was applied because the morphology exhibits behavior with auto-affine tendencies, meaning it maintains statistical persistence and long-term memory in its roughness correlations. It is interesting to analyze non-persistent or short-range behaviors; however, for the morphologies studied in this steel, such behavior was not captured.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The reviewer thanks the authors of this manuscript for responding to all comments, one-by-one. All the missing informatinon have been incorporated in the revised version of this research paper.