Advancements in Electrospark Deposition (ESD) Technique: A Short Review
Round 1
Reviewer 1 Report
This review aims to describe in detail some aspects of the Electro Spark Deposition technique to understand the Electro Spark Deposition 19 processing preparation of alloys ordinarily considered unweldable by other processes and to give 20 some important clues to the readers to contribute to the crack-free repair of surface defects and 21 coatings deposition.
Author Response
Thanks to the reviewer's comment
Reviewer 2 Report
The paper reviews the current progress in electrospark deposition technique. The review is well organized and refers to main aspects of the technique including the description of the physics and some latest achievements.
The paper gives some good good understanding of the subject, so I would recommend it for the publication with the minor improvements. The improvements are mostly related with correcting errors or misprints.
1. Line 163: "dielectric gas. that"
2. Line 362 "33.]."
3. Line 439 should be the header
4. Line 485 should be the header
5. Line 531 "2024 alloy" needs some additional specification
6. Line 552 "High velocity oxygen fuel spray" should not start from capital letter.
7. Line 679 "AA2024 alloy" also needs specification
Author Response
The authors would like to thank the reviewer for his valuable contribution and for providing useful suggestions for improving the work. They have done a careful revision of the text to eliminate errors.
- Line 163: "dielectric gas. that"
The correction was made: “…of the dielectric gas, which…”
- Line 362 "33.]."
The correction was made: “… solidification [20,30,33].”
- Line 439 should be the header
Thanks, yes it's the header. The correction was made: “2.7.1.1. Analysis of defectiveness within of deposits”
- Line 485 should be the header
Thanks, yes it's the header. The correction was made: “2.7.1.2. Substrate/coating interface behavior”
- Line 531 "2024 alloy" needs some additional specification
As the Reviewer suggest the authors added further information regarding 2024 alloy: “…. 2024 alloy (alloy based on Al-Cu-Mg system). This alloy was a forerunner of a variety of the 2xxx aluminum alloy series, is generally applied in T4 or T6 temper, as these treatments give good mechanical properties. Due to its superior properties, such as high strength-to-weight ratio, excellent resistance to fatigue crack growth, and good fracture toughness, it is widely used in the aerospace industry [7,21].”
- Line 552 "High velocity oxygen fuel spray" should not start from capital letter.
The correction was made: “… high velocity oxygen fuel spray”
- Line 679 "AA2024 alloy" also needs specification
As the Reviewer suggest the authors added further information regarding alloy 2024: ”2024 aluminium alloy's composition includes 3.8-4.9% Cu and 1.2-1.8% Mg as major alloying elements. The higher nominal Cu content of 1.5% increases the weld crack sensitivity in AA2024, which therefore hinders its weldability. It also has <0.5% Fe, <0.5% Cr, <0.5% Si, <0.25% Zi, <0.15% Ti, and 0.3-0.9% of Mn. The other trace elements that are present in the alloy is less than 0.15%.”
Author Response File: 
Author Response.docx
Reviewer 3 Report
Thank you for the review paper!
Author Response
The authors thank the reviewer for recommending the publication of the manuscript and for taking the time to review it
Reviewer 4 Report
Dear authors,
I am sending a review of the article with recommendations for improvement.
Considering that, the ESD method refers manly to metals, I suggest including the word metals or in the title of the article or between keywords.
All image descriptions (below the image) must contain information about the type of material coating and substrate material.
I am missing data about the maximum coating thickness for different parameters and materials. For example on line, 417, the maximum thickness is mentioned, but there is no information about the actual thickness.
Chapters detailing the microstructure of thin films (chapter 2.6. Microstructure morphology of electro spark coating and 2.7.1. Nano-crystalline deposits) are not sufficiently supported by high-resolution microscopic images (e.g. missing HRTEM analysis of thin films with very fine grains).
When describing the crystal grain size calculation (Scherrer equation), I suggest you include a XRD diffractogram to clearly show the calculation.
In Figure 15 a scale bar is missing.
I suggest that you include information about the mechanical properties of the layers in the article (especially about the hardness and wear resistance of the coating)
An extensive review of the literature on this topic is made, but only one source is cited for the period 2020 to 2022. I suggest a review of the literature for this period.
Author Response
The authors would like to thank the reviewer for his advice. The responses to requests are listed below.
- Considering that, the ESD method refers manly to metals, I suggest including the word metals or in the title of the article or between keywords.
We agree with the Reviewer that it is right to include the word metals among the keywords. In particular, the authors have added “worn metal components” and “metal substrates” among the keywords
- All image descriptions (below the image) must contain information about the type of material coating and substrate material.
As suggested by the reviewer, information on the material coating type and the substrate material was added in all image descriptions.
“Figure 5. a) SEM micrograph showing the deposition track of NI6625 (Inconel 625) on Inconel 738 substrate with….”
“Figure 8. Microstructural morphology in the Al-based coatings: (a) structure of Al–Si coating deposited on ZL101 aluminum alloy substrate [33], (b) Magnification of Al–Si coating deposited on ZL101 aluminum alloy substrate showing the microstructures to of two thin deposited layers and the layer–layer interface [33], (c) microstructure of Al 2024-T4 coating deposited upon homologue substrate (the circles A and B highlight the same cellular dendritic microstructure more or less fine in different layers)…”
“Figure 11. Cross-sectional images NiCrAlY coatings fabricated on superalloy GH4169 with Es of a) 0.2 J and b) 1.35J, as can be seen, the deposit has some defects [48]; c) SEM micrograph of multiple-layer deposition of 2024-T4 Al alloy coating upon homologous substrate showing the typical voids of the ESD coating (Es = 0.9J): small spherical voids (n°1), large, random shaped voids (n°2) and laminar porosity (n°3) [20].”
“Figure 12. Microstructure near the interface between the substrate and coating: a) OM and b) SEM micrographs of A357-T61 coating deposied upon homologous substrate [49], c) magnification of interface (as marked “A” in fig. b) and d) OM and e) SEM micrographs of WE43-T6 coating deposited upon homologous substrate, f) magnification of interface (as marked “B” in fig. c) [50].”
“Figure 13. Electrospark deposit in alloy 2024-T4 on homologous substrate. ..”
“Figure 14. FESEM micrograph for sample made by depositing a Fe48Cr18Mo7B16C4Nb7 crystalline alloy on a 316L stainless steel substrate at…”
“Figure 15. OM and SEM images of microstructure parallel to building direction: a) cross section macrograph of ESD deposit in AA2024-T4 on a homologous substrate…”
- I am missing data about the maximum coating thickness for different parameters and materials.
For example on line, 417, the maximum thickness is mentioned, but there is no information about the actual thickness.
We thank the Reviewer for pointing out this oversight. The value of the maximum thickness has been entered as well as thicknesses for other types of alloys:
“For example, it was found for the WE43 alloy deposited upon homologue substrate, that after 50 subsequent deposition passes the thickness changes from 82.6 μm for the deposit produced at the lowest energy (0.15 J) to 407.6 μm for the deposit produced at the highest Es (1.69 J) [50]. However, some authors show that after a given energy value or after a definite passes number the deposit quality is reduced. In particular, it was found for Al-Ni alloy (Al–3 wt% Ni) a maximum thickness ranging from ∼120 to 140 µm was achieve, with a normalized energy-density of ∼0.689 J/mm2. At that point, by further increasing the normalized energy density, the thickness no longer increases and/or the quality of the deposit decreases [76]. According to Johnson et al. this coating thickness limit occurs due to embrittlement, as a result of induced thermal strain, of the previously deposited layer during subsequent deposition passes [66]. In addition, it has also been found that for the same number of passes and the same level of energy used, the coatings thickness changes depending on the metal alloy deposited. Studies report that for several Al alloys, coating thicknesses between 30 to 50 microns are obtained following a single pass, using an energy of 0.55J/pulse [30].”
- Chapters detailing the microstructure of thin films (chapter 2.6. Microstructure morphology of electro spark coating and 2.7.1. Nano-crystalline deposits) are not sufficiently supported by high-resolution microscopic images (e.g. missing HRTEM analysis of thin films with very fine grains).
The authors apologize to the reviewer and a new study has been added to support the fine microstructure of the coatings:
“Wang Mao-cai et al [D] investigated by SEM and TEM the microstructural characteristics of a NiCoCrAlYTa coating epitaxially built-up on a directionally solidified (DS) Ni-based superalloy blade tip by ESD. In particular, they showed that the built-up coating is characterized by such features as multi-sublayered construction with transition zone and zones with microstructure of super-fine columnar dendrites and cellular dendrites. The layer consists of γ-dendrites and β-precipitate particles; and the precipitation and growth of the β-phase are along the γ-dendrite (Figure 10 a-c). In figure 10 c, it can be seen that the precipitates in front of the cellular columns make the columns growth stop (as marked “S”). The coarse precipitate can also lead to the growth direction changes from their original orientation (as marked “C”). Moreover, in TEM image (Fig. 10 d), four parallel strips of bamboo-like structure, which consists of rod-like particles with different lengths, can be observed. The intervals between strips are about 800-1200 nm. The electro-beam diffraction pattern from the bamboo-like dendrite (Fig. 10 e) shows characteristics of a BCC crystalline, i.e. β phase for the NiCoCrAlYTa ESD coating. The high magnification TEM images clearly highlight the existence of the nanoparticles between two bamboo-like dendrites (fig. 10 f, g). These nanoparticles have been identified as a BCC structure, or β phase (fig. 10 h). furthermore, in Fig.10g it was observed that the γ phase has a homogeneous cellular substructure of about 6 nm inside which some bright white particles are randomly distributed. These bright white particles can γ’ phases. While, from the diffraction pattern shown in Fig. 10i it was observed that the matrix has an FCC structure, that is a γ phase (zone B in Fig. 10 f).
Figure 10. NiCoCrAlYTa ESD built-up coating on Ni-based DS superalloy (DZ22) blade tip: a) laminate microstructure with super-fine column dendrite b) few precipitates in first thin de-posited layer, c) distinguished precipitates in fifth thin deposited layer, d) TEM images showing microstructures of sublayer near to interface between built-up coating and substrate: four parallel rows of bamboo-like dendrite; e) Diffraction pattern from bamboo-like dendrite (marked A in Fig. f); f) and g) Magnified images showing nano-particles existing in inter-dendrites; h) Diffraction pattern from nanoparticles (marked C in Fig. f) and i) Diffraction pattern from matrix (marked B in Fig. f).”
- When describing the crystal grain size calculation (Scherrer equation), I suggest you include a XRD diffractogram to clearly show the calculation.
As suggested by the reviewer, the XRD diffractogram was added. Furthermore, for completeness, the SEM image used for the determination of the minimum grain size of the Al3Ni phase has also been added.
“The XRD spectra of the for Al–Ni alloy deposit and the as-cast ingot as-cast ingot used to produce the electrode as well as the SEM micrograph used for the determination of the minimum grain size of the Al3Ni phase is shown in figure 9a and b, respectively.”
Figure 9. a) Comparison between the XRD spectra of the ESD deposit and the as-cast ingot for Al–Ni alloy and b) SEM micrograph of an Al–Ni coating produced using a pulse energy of 0.1 J.
- In Figure 15 a scale bar is missing.
We thank the Reviewer for pointing out this oversight. We added the scale bar in figure 15a.
- I suggest that you include information about the mechanical properties of the layers in the article (especially about the hardness and wear resistance of the coating)
As suggested by the reviewer the information about the mechanical properties of the coating has been added
“2.7.3. Mechanical and tribological properties of the deposits
Typically, the mechanical properties of ESD coatings were evaluated by making Vickers micro-hardness measurements on the coating cross-section. Specifically, some researches, in order to study the effect of mixed refined microstructure on the hardness values as well as the microstructural characteristic of the built-up coating were done. They performed Vickers indentations at the interfaces between the layers or in line arrangement and perpendicular to the coating/substrate interface. It is well known that hardness of a given alloy is directly influenced by solidification structure. According to the Hall-Petch equation, hardness is inversely proportional to the square root of grain size. In addition to grain refinement, extra fine particles can increase the grain-boundary activity which plays an important role on the load transfer in the strengthening of the coating. Also the wear-resistance of the coatings is related to the hardness. Moreover, the tribological performance of a ESD coating depends on the thickness of the coating, and the size, distribution and morphology of particles in the coating [92]. The wear mechanisms are mainly adhesive wear, abrasive wear, fatigue wear and oxidation wear under different conditions. Several authors, have shown that the hardness of the de-posits was generally higher than that of the substrate regardless of the depositing parameters. In particular, they reported that the micro-hardness of the coatings gradually decreased from the coating top surface to the substrate [49,92-94]. As shown above, the higher hardness value of the ESD coatings compared to that of the substrate is mainly due to the formation of the fine microstructure induced by rapid solidification. For example, W. Wang et al [93] found that the highest microhardness value of the Mo coating was 1369.5 HV0.1, which is about 6.7 times higher than that of the H13 steel substrate, and the wear resistance of the coating was about seven times higher than that of the substrate. The good wear resistance of the coating corresponds to the defect-free and high microhardness of the coating and the good metallurgical bonding between the substrate and coating. Furthermore, the presence of a large number of cemented carbides dispersed in the coating greatly improves the wear resistance of the coating. Similarly, J. S. Wang et al [92] found a microhardness value of WC-0.8Co coating much higher (1441±132 HV0.3) than the microhardness value of the substrate (320±10 HV). In addition, they showed that the coating has a good wear resistance and the average wear resistance was 3.3 time larger than of the cast steel roll substrate. The high hardness and wear resistance of the coating is determined by the dispersion of fine hard phases in the coating.
Although the fine microstructure present in all coatings confers a greater hardness, in some cases it has been observed an average hardness value of the coating equal to [79] or lower than that of the substrate [20,21,50,51]. In the latter case the hardness value of the coating is strongly influenced by the presence of widespread defects.”
- An extensive review of the literature on this topic is made, but only one source is cited for the period 2020 to 2022. I suggest a review of the literature for this period.
As suggested by the reviewer an review of the literature on this topic was carried out:
[96] Ph.V. Kiryukhantsev-Korneev, A.D. Sytchenko, V.A. Gorshkov, P.A. Loginov, A. N. Sheveyko, A.V. Nozhkina, E.A. Levashov. Complex study of protective Cr3C2–NiAl coatings deposited by vacuum electro-spark alloying, pulsed cathodic arc evaporation, magnetron sputtering, and hybrid technology. Ceramics International 48 (2022) 10921–10931
[97] K.A. Kuptsov, A.N. Sheveyko, D.A. Sidorenko, D.V. Shtansky. Electro-spark deposition in vacuum using graphite electrode at different electrode polarities: Peculiarities of microstructure, electrochemical and tribological properties. Applied Surface Science 566 (2021) 150722
[93] Wenquan Wang, Ming Du, Xinge Zhang, Chengqun Luan and Yingtao Tian. Preparation and Properties of Mo Coating on H13 Steel by Electro Spark Deposition Process. Materials 2021, 14, 3700. https://doi.org/10.3390/ma14133700
[94] De Wang, Junhao Gao, Rui Zhang, Shaojun Deng, Shuyuan Jiang, Donghai Cheng, Pin Liu, Zhenyu Xiong, Wenqin Wang. Effect of TaC particles on the microstructure and oxidation behavior of NiCoCrAlYTa coating prepared by electrospark deposition on single crystal superalloy. Surface & Coatings Technology 408 (2021) 126851
Author Response File: Author Response.docx
Round 2
Reviewer 4 Report
Dear Authors,
The changes to the article are extensive and my opinion is that they can publish the article. In the article, however, I am missing the diagram of the microhardness through the weld.
