Erosion Wear Behavior of HVAF-Sprayed WC/Cr₃C₂-Based Cermet and Martensitic Stainless Steel Coatings on AlSi7Mg0.3 Alloy: A Comparative Study

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this paper, the erosion wear resistance of WC-10Co4Cr, Cr₃C₂-25NiCr and martensitic SS steel coatings on AlSi7Mg0.3 substrate sparyed by was studied. The results show that the HVAF-sprayed WC-10Co4Cr coating has good erosion wear resistance. This work has some research value, but there are still some problems, and the following recommendations are as follows:

1. The presentation of the current state of national and international research is poor in logic, and it is suggested that the presentation be revised with additional information to optimize the logic.

2. The analysis of the experimental phenomenon in the figure is mostly citing the previous findings to verify the results, and lack of innovative analysis and discussion, so the innovativeness of the article needs to be improved.

3. The results of this paper are presented in a large number of group diagrams, and there are unclear information in the pictures, such as “Al cellular dendrites” in Figure 2b, and the font size in the diagrams is not uniform.

4. Section 2.7 duplicates the title of section 3.2, please revise.

5. In equation (1) of the text, variables and constants are not distinguished in the expression, please revise.

Comments on the Quality of English Language

Moderate English editing required. 

Author Response

Dear Reviewer,

Thank you very much for your helpful comments. Below you will find responses to all your recommendations.

1. The presentation of the current state of national and international research is poor in logic, and it is suggested that the presentation be revised with additional information to optimize the logic.

Response 1: Thank you for your constructive feedback. We’ve completely revised the Introduction section with additional information to optimize the logic as shown below.

1. Introduction

Aluminum alloys, favored for their high strength-to-weight ratio compared to conventional steels [1,2], are widely utilized in aerospace, automotive, and polymer extrusion industries [3–5]. Despite their strength, aluminum materials, particularly the widely used AlSiMg0.3 alloys, exhibit poor wear resistance, necessitating the use of protective coatings [6]. These coatings must be sufficiently thick to withstand impact-abrasive wear and high contact loads, preventing deformation of the ductile aluminum substrate. Dissimilar joining of iron subgroup-based materials (Fe, Ni, Co) to aluminum alloys is often problematic due to the formation of brittle intermetallic phases, which can lead to cracking under high contact pressures [7,8].

The microstructure and mechanical performance of coatings are significantly influenced by both their chemical composition and the deposition technique employed. Electrochemical methods such as electroplating [9,10] and anodizing [11], laser cladding [12–16], and thermal spraying [17,18] represent some of the most widely used coating technologies. Electrochemical coating methods, while offering certain advantages, are constrained by their limited ability to produce coatings with substantial thickness and are also subject to several other drawbacks. While conventional anodizing of aluminum alloys provides hard coatings with a thickness of 10–50 µm, their inherent brittleness makes them prone to cracking and delamination under demanding conditions [19,20]. Furthermore, the composition of anodized films can vary depending on the specific aluminum alloy used [21]. Micro-arc oxidation offers a compelling alternative to traditional anodizing, with significantly enhanced hardness (5–10 GPa) and thicker coatings (up to 150 µm) [22,23], but it also faces inherent limitations. Such coatings are vulnerable to failure under high contact pressures, and their production costs escalate considerably when dealing with large or intricately shaped components. Despite such advantages of laser cladding as a low dilution rate, a minimized heat-affected zone, and a robust interfacial metallurgical bonding, its cost also remains a significant barrier to wider adoption [12].

Given these findings, thermal spray methods may be advantageous in many applications due to their versatility in depositing a wide range of materials with satisfactory performance. Recent advancements in arc spraying have enabled the deposition of steel coatings up to 4 mm thick, utilizing a modified nichrome sublayer. These coatings, applied to Al-based parts, exhibit enhanced adhesion strength (up to 80 MPa), reaching microhardness values of 800 HV and demonstrating the ability to withstand high contact loads of up to 100 MPa [24]. Thermal sprayed coatings, deposited onto Al-based alloys with a thickness of 100–400 μm, have proven advantageous in reducing the probability of coating failure during adhesive wear. These coatings have demonstrated high wear resistance in both lubricated and dry conditions, showing effectiveness in applications such as textile equipment [25], polymer extrusion tools [26,27], cylinder blocks, and car brake discs [28].

To enhance the wear resistance of aluminum alloys susceptible to adhesive wear, high-velocity oxygen/air-fuel (HVOF/HVAF) spraying is used to deposit metal-ceramic coatings (cermets) onto the aluminum substrate. This process, characterized by high particle velocities (over 500 m/s) and low flame temperatures (below 3000 °C), results in coatings with superior adhesion, low porosity, reduced decarburization, and improved wear resistance compared to other thermal spray methods [29].

Bolelli et al. [30] studied the performance of WC-CoCr cermet coatings, up to 150 μm thick, deposited onto aluminum alloys using HVOF spraying under ball-on-disk loading conditions up to 2600 MPa. Their findings revealed several key features:

• A graded transition zone forms between the WC-CoCr coating and the aluminum substrate due to cermet particle penetration, dramatically enhancing wear resistance compared to hard anodized film;
• Thicker WC-CoCr cermet coatings exhibit reduced wear due to a decrease in surface layer peeling;
• Cyclic impact testing resulted in localized transverse cracks within the coating, negatively affecting performance but occurring less frequently than in WC-CoCr cermets deposited on a less ductile steel base.

Aluminum blades, enhanced with HVAF-sprayed wear-resistant coatings, offer advantages in industrial fans used for ventilation in subway tunnels, coal and ore mines. Their reduced weight improves performance compared to steel blades, and they can operate at temperatures up to 400 °C [31], suitable for many industrial applications. However, the presence of fine solid particles in fan operating environments poses a challenge, as aluminum blades exhibit poor erosion resistance.

Erosion wear resistance is influenced by factors such as impact angle, velocity, solid particle size, concentration, and hardness of the erodent [32]. Additionally, the material’s ductility and strength play a crucial role. Ductile metals, such as austenitic steel, aluminum, and gray cast iron, generally exhibit the highest erosion rates at low attack angles (15° to 60°). Aluminum alloys specifically show maximum wear at a 15° angle, where microcutting is the dominant wear mechanism. This wear rate decreases by 4–5 times as the attack angle increases to 60°, where plastic deformation becomes the primary wear mechanism [33]. Brittle materials experience maximum erosion at high attack angles (60° or more) due to the primary erosion mechanism of spalling caused by cracking [34]. HVOF-sprayed coatings have proven effective in extending the operating life of components exposed to erosion [35]. A comparative study demonstrated that HVOF-sprayed coatings exhibit higher hardness (~9%) and superior erosion resistance compared to coatings produced by spark plasma sintering (SPS) [36].

Comparative studies have shown that HVAF Cr₃C₂-based coatings on steel substrates exhibit superior resistance to dry, gas-abrasive, and cavitation wear, along with enhanced microhardness, lower porosity, and smoother surfaces compared to HVOF coatings [37]. Similar properties have been observed for WC-10Co4Cr coatings [38], while the HVAF spraying process itself is considered more technologically advanced [39]. HVOF-sprayed WC-10Co4Cr coatings deposited onto steel substrates demonstrate approximately twice the erosion resistance of similar Cr₃C₂-25NiCr coatings [40,41]. While cheaper steel coatings are commonly used, their wear resistance is significantly lower than cermets.

This study investigates the erosion wear resistance of HVAF-sprayed WC-10Co4Cr, Cr₃C₂-25NiCr, and martensitic SS steel coatings on an AlSi7Mg0.3 alloy under dry erosion conditions, mimicking the demanding environment of fan blades. By systematically examining the influence of abrasive type (quartz sand and granite gravel), erodent attack angle, coating thickness, and microhardness, the research provides valuable insights into optimizing wear resistance for these critical components. The comprehensive analysis, utilizing optical, electron microscopic, and X-ray diffraction techniques, deepens our understanding of erosion wear mechanisms and offers valuable data for informed material selection and design in fan blade applications.

2. The analysis of the experimental phenomenon in the figure is mostly citing the previous findings to verify the results, and lack of innovative analysis and discussion, so the innovativeness of the article needs to be improved.

Response 2: Thank you for your valuable comment. You are partly right. As you know, for this study, we focused on the 3 most commonly used WC-10Co4Cr, Cr₃C₂-25NiCr and SS steel materials for coatings, and compared their wear resistance under identical conditions. Firstly, since there are not many such systematic studies, this gap needs to be eliminated. And secondly, in the study we use optimized HVAF-spraying modes, which allowed us to obtain significantly better values ​​for abrasive wear compared to Bolelli et al. Thus, the paper contains both scientific novelty and useful data for readers.

3. The results of this paper are presented in a large number of group diagrams, and there is unclear information in the pictures, such as “Al cellular dendrites” in Figure 2b, and the font size in the diagrams is not uniform.

Response 3: Thank you for your valuable comment. Since our manuscript is devoted to a comparative analysis of microstructure and volumetric wear after abrasive impact, the results presented in the form of collages in Figures 2, 4 and 5 provide the most visual information to the reader. We hope that the above figures can be left in their original state. Minor adjustments, including color correction and font size modifications, were made to Figure 2. In addition, we’ve removed the inscription “Al cellular dendrites” in Figure 2b because, as you rightly noted, they really are not visible, and brought the font size to a uniform size. Following the recommendation of another reviewer, Figure 4a,b have been removed. A detailed description and schematic representation of the abrasive wear experimental procedure can be found in the references provided in the "Erosion Wear Test" section. Additionally, we’ve added the error bars for Figures 2l, 4l and 5d.

4. Section 2.7 duplicates the title of section 3.2, please revise.

Response 4: We really appreciate your valuable feedback. You are absolutely right. We’ve changed the subtitles to 2.7. X-ray diffraction analysis and 3.2. Phase composition.

5. In equation (1) of the text, variables and constants are not distinguished in the expression, please revise.

Response 5: We’re very grateful for your feedback. You are absolutely right. We’ve revised this.

Thank you for your inestimable contribution to improving our manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

1. This paper claims that “The influence of the abrasive type (quartz sand and granite gravel), erodent attack angle, thickness and microhardness of the coatings on their and Al substrate’s wear resistance has been systematically investigated under dry erosion conditions typical for fan blades.” However, only one attack angle of 60°was studied, and only one thickness was studied for the Cr3C2-25NiCr and SS steel coatings. Therefore,  it cannot be regarded as a systematic study.

 2. Figure 4 presents the main results of several coatings after erosion wear tests; however, the specific experimental parameters are missing, as is also the case for Figure 5. To which conditions in Figure 4 do they correspond, respectively? The resolution of the SEM images is insufficient, resulting in blurred details.

 3. Many descriptions of the results in the manuscript do not provide sufficient evidence. For example, on Page 9, lines 306-307, How to determine the thickness of the surface layer range of 10-17 um?

 4. In Figure 5, it is suggested that cross-section images of coatings with thicknesses of 100 and 200 µm be provided for comparison.

 5. Error bars are suggested to be added in Figure 5d.

 6. Figure 4a b should be introduced in the “Erosion wear tests” section.

 7. There are some logical issues with the results and discussion regarding Figure 4.

 

Comments on the Quality of English Language

Minor editing of English language required

Author Response

Dear Reviewer,

Thank you very much for your helpful comments. Below you will find responses to all your recommendations.

1. This paper claims that “The influence of the abrasive type (quartz sand and granite gravel), erodent attack angle, thickness and microhardness of the coatings on their and Al substrate’s wear resistance has been systematically investigated under dry erosion conditions typical for fan blades.” However, only one attack angle of 60° was studied, and only one thickness was studied for the Cr₃C₂-25NiCr and SS steel coatings. Therefore, it cannot be regarded as a systematic study.

Response 1: Thank you for your constructive comment. We’ve removed the word ‘systematic’ from the Abstract to avoid misleading the reader.

Abstract: The paper presents a comparative study of the erosive wear resistance of WC-10Co4Cr, Cr₃C₂-25NiCr and martensitic stainless steel (SS) coatings deposited onto an AlSi7Mg0.3 (Al) alloy substrate by high-velocity air-fuel (HVAF) spraying. The influence of the abrasive type (quartz sand and granite gravel), erodent attack angle, thickness and microhardness of the coatings on their and Al substrate’s wear resistance was investigated under dry erosion conditions typical for fan blades. The HVAF-spraying process did not affect the Al substrate’s structure, except for when the near-surface layer was 20‒40 μm thick. This was attributed to the formation of a modified Al-Si eutectic with enhanced microhardness and strength in the near-substrate area. Mechanical characterization revealed significantly higher microhardness values for the cermet WC-10Co4Cr (~12 GPa) and Cr₃C₂-25NiCr (~9 GPa) coatings, while for the SS steel coating, the value was ~5.7 GPa. Erosion wear tests established that while Cr₃C₂-25NiCr and SS steel coatings were more sensitive to abrasive type, the WC-10Co4Cr coating exhibited significantly higher wear resistance, outperforming the alternatives by 2‒17 times under high abrasive intensity. These findings highlight the potential of HVAF-sprayed WC-10Co4Cr coatings for extending the service life of AlSi7Mg0.3-based fan blades exposed to erosion wear at normal temperatures.

2. Figure 4 presents the main results of several coatings after erosion wear tests; however, the specific experimental parameters are missing, as is also the case for Figure 5. To which conditions in Figure 4 do they correspond, respectively? The resolution of the SEM images is insufficient, resulting in blurred details.

Response 2: Thank you for your comment. We’ve provided a full explanation regarding the wear test parameters in 2.3. Erosion wear tests.

The following designations of wear testing modes were accepted: index S ‒ abrasive quartz sand, index G ‒ abrasive granite gravel. These letters are followed by indices of abrasive jet velocity, m/s, and jet loading with abrasive, kg. For example, S 50-2 means that an abrasive material is quartz sand impacting the sample at an abrasive jet velocity of 50 m/s and with a jet loading of 2 kg.

Unfortunately, the quality of SEM images is limited by the capabilities of the microscope at high magnifications. However, the quality of the presented images allows us to highlight the features of the coating structure.

3. Many descriptions of the results in the manuscript do not provide sufficient evidence. For example, on Page 9, lines 306−307, How to determine the thickness of the surface layer range of 10−17 um?

Response 3: Thank you for your valuable comment. One of the co-authors gave some hypothetical values, but this does not make sense, because you are right, in this particular case no clear evidence was provided. Therefore, we’ve removed this statement from the manuscript (see Response 7).

4. In Figure 5, it is suggested that cross-section images of coatings with thicknesses of 100 and 200 µm be provided for comparison.

Response 4: We really appreciate your valuable feedback. You are absolutely right − this would also be informative. Unfortunately, we no longer have this opportunity due to lack of any samples.

5. Error bars are suggested to be added in Figure 5d.

Response 5: Thank you for your constructive feedback. We’ve added the error bars for Figures 2l, 4l and 5d.

6. Figure 4a,b should be introduced in the “Erosion wear tests” section.

Response 6: Thank you for your valuable comment. We’ve removed Figure 4a,b. A detailed description and schematic representation of the abrasive wear experimental procedure can be found in the references provided in the "Erosion Wear Test" section.

7. There are some logical issues with the results and discussion regarding Figure 4.

Response 7: Thank you for your comment. You are absolutely right. We’ve completely revised the 3.3. Wear resistance highlights section.

3.3. Wear resistance highlights

Metallographic analysis of cross-sections (Figure 4a‒k) revealed distinct wear characteristics among the 200 μm thick coatings when subjected to abrasive impact with quartz sand and granite gravel.

Erosion wear studies on uncoated AlSi7Mg0.3 alloy blades indicated a characteristic abrasive-dependent layer on the surface, regardless of the abrasive type (Figure 4a,b). This layer exhibited numerous pits, microcracks, and embedded erodent particles. Figure 4a and 4b illustrate the abrasive layer areas of the uncoated AlSi7Mg0.3 alloy after wear tests with quartz sand and granite gravel, respectively. In both cases, the Al alloy exhibits deep cavities and cracks, extending both along and across the wear surface. Crack formation was predominantly observed along the Al-Si eutectic, which runs along the grain boundaries of the Al solid solution matrix (Figure 4a,b).

Abrasive wear testing demonstrated that SS steel coatings exhibited lower wear resistance compared to cermet coatings (Figure 4c,d). The SS steel coating thickness decreased by ~25 µm after a wear test using the harder granite gravel abrasive.

Figure 4e-k displays the cross-sectional macrostructure of the cermet Cr₃C₂-25NiCr and WC-10Co4Cr coatings after wear tests with quartz sand and granite gravel. Notably, these coatings demonstrated greater resistance to abrasives of varying hardnesses compared to the SS steel coatings. The primary defects observed were thin microcracks oriented perpendicular to the wear surface (Figure 4f,k). Comparing wear characteristics based on erodent type, exposure to quartz sand resulted in fewer deformation areas. In contrast, granite gravel, a harder erodent, led to significantly uneven surface wear and increased surface layer defectiveness.

Wear analysis of the 200 µm thick coatings (Figure 4l) yielded insightful observations. Notably, the WC-10Co4Cr coating exhibited the highest erosion wear resistance, with its performance differing significantly from other materials based on the abrasive type and jet loading. It demonstrated 2‒3 times higher resistance compared to the Cr₃C₂-25NiCr coating and 2‒17 times higher resistance compared to the SS steel coating and the AlSi7Mg0.3 substrate.

The use of fragmented abrasive (granite gravel) resulted in increased wear on uncoated AlSi7Mg0.3 alloy, which, in turn, led to a corresponding increase in the wear resistance of the coatings (Figure 4l). At low abrasive jet loading and velocity, the AlSi7Mg0.3 base alloy exhibited relatively high wear resistance. This resulted in a moderate increase in the relative wear resistance of the coatings (most notably for the WC-10Co4Cr coating), with a 1.4-fold increase for spherical abrasive and a 2.8-fold increase for fragmented abrasive.

The obtained data indicated that the WC-10Co4Cr coatings maintained consistent thickness and microhardness during wear tests, regardless of whether quartz sand or granite gravel was used as the abrasive. The surface layer also displayed a smooth appearance, devoid of cavities or cracks. These observations suggest that the wear of the SS steel and Cr₃C₂-25NiCr coatings is more strongly influenced by the abrasive type compared to the WC-10Co4Cr coating.

Thank you for your inestimable contribution to improving our manuscript.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The paper presented a study on comparative erosion wear of HVAF-sprayed WC/Cr3C2-based cermet and martensitic stainless-steel coatings on AlSi7Mg0.3 alloy. The paper seems well presented. However, there are a few suggestions as follows:

1, Some pictures seem containing quite many pictures such as Fig. 2, 4, and 5. Sometimes, it would be fine but sometimes it is not adequate to present so much information in one picture. Fig. 2 is not too bad, which is just a bit small. Fig. 4 would be great to separate it into 3 pictures as test equipment (a)-(b), SEMs (c)-(m) and results (n). Caption of Fig.4 (n) is too simple to understand. This is a two-results according to two abrasives. Legends is a bit confusing as with coatings and without coatings. Similarly, Fig. 5 is a bit too small.

2, Title is a bit unclear. It is suggested ‘Comparative study on erosion wear of HVAF-sprayed coatings of WC/Cr3C2-based cermet and martensitic stainless-steel on AlSi7Mg0.3 alloy’  

3, HVAF: a full name should be given at the first-time use and should not be used in the title. However it is given in the abstract, it would be fine to use it in the title.

4, L45, ‘severe operating conditions’: is there any examples on which operating conditions?    

5, L49: Reference?  

6, L97, ‘The maximum erosion of brittle materials occurs at high attack angles (60° or more)’: for a brittle material, the maximum erosion used to be at 90^o impact angle.

7, L121: what are the mechanical properties of the coating materials?  

8, L132: the particle sizes of the coating powders need to give.

9, L140: how are the thicknesses of the coatings measured?

10, L148: the test equipment needs to refer to the figure.

11, L334, ‘relative wear resistance’: a clear definition of the term is needed.

Comments on the Quality of English Language

Overall is fine. Just need to double check. 

Author Response

Dear Reviewer,

Thank you very much for your helpful comments. Below you will find responses to all your recommendations.

1. Some pictures seem containing quite many pictures such as Fig. 2, 4, and 5. Sometimes, it would be fine but sometimes it is not adequate to present so much information in one picture. Fig. 2 is not too bad, which is just a bit small. Fig. 4 would be great to separate it into 3 pictures as test equipment (a)−(b), SEMs (c)−(m) and results (n). Caption of Fig.4 (n) is too simple to understand. This is a two-results according to two abrasives. Legends is a bit confusing as with coatings and without coatings. Similarly, Fig. 5 is a bit too small.

Response 1: We appreciate your constructive feedback. It’s valuable to us. Since our manuscript is devoted to a comparative analysis of microstructure and volumetric wear after abrasive impact, the results presented in the form of collages in Figures 2, 4 and 5 provide the most visual information to the reader. We hope that the above figures can be left in their original state. Following the recommendation of another reviewer, Figure 4a,b have been removed. A detailed description and schematic representation of the abrasive wear experimental procedure can be found in the references provided in the "Erosion Wear Test" section. Figures 2, 4, and 5 have undergone color correction and font alignment adjustments. Additionally, we’ve added the error bars for Figures 2l, 4l and 5d.

2. Title is a bit unclear. It is suggested ‘Comparative study on erosion wear of HVAF-sprayed coatings of WC/Cr₃C₂-based cermet and martensitic stainless-steel on AlSi7Mg0.3 alloy’.

Response 2: Thank you for your valuable recommendation. We’ve changed the manuscript title to ‘Erosion Wear Behavior of HVAF-Sprayed WC/Cr₃C₂-Based Cermet and Martensitic Stainless-Steel Coatings on AlSi7Mg0.3 Alloy: A Comparative Study’.

3. HVAF: a full name should be given at the first-time use and should not be used in the title. However, it is given in the abstract, it would be fine to use it in the title.

Response 3: We really appreciate your valuable feedback. You are absolutely right. In the abstract, we used the abbreviation extension ‘high-velocity air-fuel (HVAF)’ to allow the use of the abbreviation HVAF for the manuscript title.

4. L45, ‘severe operating conditions’: is there any examples on which operating conditions?

Response 4: Thank you for your interest comment. Here are some examples of ‘severe operating conditions’ that could cause anodized films on aluminum alloys to crack and peel:

Mechanical Stress:

• Impact loading: Strong impacts, like those experienced in a collision or drop, can generate stress concentrations that exceed the film’s strength.
• Abrasive wear: Friction from sand, grit, or other abrasive materials can cause scratches and wear on the surface, leading to cracking.
• Bending or flexing: Repeated bending or flexing of the aluminum substrate can cause the brittle film to crack or peel.

Environmental Factors:

• Temperature extremes: Rapid changes in temperature or prolonged exposure to high temperatures can cause the film to expand and contract, leading to stress and potential cracking.
• Chemical attack: Exposure to aggressive chemicals, like strong acids or bases, can degrade the anodized film, making it more susceptible to cracking and peeling.

5. L49: Reference?

Response 5: Thank you for your comment. Below is the reference for [13].

[13] Zhang, J.; Dai, W.; Wang, X.; et al. Micro-arc oxidation of Al alloys: mechanism, microstructure, surface properties, and fatigue damage behavior. J. Mater. Res. Technol. 2023, 23, 4307–4333. [CrossRef]

6. L97, ‘The maximum erosion of brittle materials occurs at high attack angles (60° or more)’: for a brittle material, the maximum erosion used to be at 90° impact angle.

Response 6: We’re very grateful for your valuable feedback. The traditional understanding was that maximum erosion for brittle materials typically occurred at a 90° impact angle.

Here’s why the understanding has shifted:

• Material Behavior: While brittle materials are generally characterized by their low tensile strength and propensity for fracture, the exact mechanism of erosion can be more complex.
• Erosion Mechanisms: Erosion in brittle materials is often driven by a combination of factors, including:
• Spalling: The breaking off of small pieces of material due to impact.
• Microcracking: The formation of cracks that propagate under repeated impacts.
• Fissuring: The formation of larger cracks that can lead to material failure.
• Angle and Energy Transfer: At very high impact angles (close to 90°), the energy transfer from the erodent particle is more likely to be directed towards fracturing the material directly. This can lead to significant spalling and material loss. However, at lower angles, the impact energy is more spread out, potentially leading to a greater number of microcracks and fissures that can weaken the material over time.

Important Note:

• Material Properties: The specific erosion behavior of a brittle material can vary depending on its composition, microstructure, and the nature of the erodent.
• Research: Ongoing research is constantly refining our understanding of erosion mechanisms in different materials.

In summary: While a 90° impact angle can lead to significant material loss in brittle materials, recent research suggests that maximum erosion can occur at lower angles (around 60° or more) depending on the specific material and erosion conditions. This is because the erosion process is complex and involves multiple mechanisms that are influenced by the impact angle and other factors.

7. L121: what are the mechanical properties of the coating materials?

Response 7: Thank you for your comment. Here’s a breakdown of their typical mechanical properties relevant to wear resistance:

WC-10Co4Cr:

• Hardness: Very high hardness, typically around 1400–1600 HV (Vickers Hardness).
• Toughness: Good toughness due to the cobalt binder phase, making it resistant to chipping and cracking.
• Wear Resistance: Excellent wear resistance, particularly against abrasive wear.

Cr₃C₂-25NiCr:

• Hardness: High hardness, typically around 1000–1200 HV.
• Toughness: Moderate toughness, but still good resistance to chipping and cracking.
• Wear Resistance: Excellent wear resistance, especially against abrasive and erosive wear.

Martensitic SS Steel:

• Hardness: Can be hardened to high levels, typically around 400–500 HV, depending on heat treatment.
• Toughness: Good toughness, providing resistance to impact and shock loads.
• Wear Resistance: Moderate wear resistance, better than standard stainless steel but lower than WC-Co or Cr₃C₂-based coatings.

8. L132: the particle sizes of the coating powders need to give.

Response 8: Thank you for your comment. Fractional and chemical composition of the feedstock powders is given in Table 1.

9. L140: how are the thicknesses of the coatings measured?

Response 9: Thank you for your comment. The thickness of our coatings was determined through both (1) optical and (2) scanning electron microscopy (SEM) analysis, as illustrated in Figure 2.

(1) In particular, the thickness of the coatings was measured by examining a polished cross-section under an optical microscope equipped with a calibrated scale, measuring the distance between the coating and substrate interface.

(2) In addition, the thickness of the coatings was measured by analyzing cross-sectional SEM images, which provided a high-resolution view of the interface between the coating and the substrate, allowing for precise measurements.

10. L148: the test equipment needs to refer to the figure.

Response 10: Thank you for your comment. We’ve removed Figure 4a,b. A detailed description and schematic representation of the abrasive wear experimental procedure can be found in the references provided in the "Erosion Wear Test" section.

11. L334, ‘relative wear resistance’: a clear definition of the term is needed.

Response 11: We’re very grateful for your feedback. You are absolutely right. Here we mean the relative wear resistance of the WC-10Co4Cr coating depending on its thickness. In our case, the WC-10Co4Cr coating thickness values are 50, 100 and 200 µm. We’ve changed the subtitle to something more appropriate ‘Thickness-Dependent Wear Resistance of the WC-10Co4Cr Coatings’.

Thank you for your inestimable contribution to improving our manuscript.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The manuscript investigates the effect of HVOF coatings on the AlSi7Mg0.3 alloy. Tribological and several mechanical properties are investigated. The quality of the manuscript is suitable for this journal. While reading, I had a few suggestions that I hope will help shape the final version of the manuscript. Some questions need to be addressed and corrections made as follows:

1. Introduction, it will be better to focus specifically on the aluminum cast alloy AlSiMg0.3. Currently, the authors only discuss aluminum alloys in general. Additionally, the authors should mention some disadvantages of the AlSiMg0.3 alloy and how coatings might address these issues.

 

2. Introduction, the authors claim that “The selection of coating material depends on both its wear resistance and cost.” It would be better to avoid such statements.

 

3. Subsection 2.1, Table 1: Please check the numbers such as ≤0,10, which should be written as ≤0.10.

 

4. Subsection 2.2, Table 2 presents the parameters adopted for the HVAF-spraying process. It is important to explain the criteria behind these choices.

 

5. Subsection 2.2, the authors state that “After deposition, all samples were ground manually to a roughness of Ra = 0.32 µm using a diamond grinding wheel.” The authors should justify the choice of this specific surface roughness (Ra = 0.32 µm) for this work.

 

6. Additionally, if possible, I recommend including the roughness results of the samples, as these findings may interest potential readers.

 

7. Subsection 3.1, the hardness results of the AlSi7Mg0.3 alloy should be included in Figure 2(l) and discussed.

 

8. Lines 247-249, the authors claim that “Most likely, this is due to the lower kinetic….” This statement must be supported by references.

 

9. Subsection 3.1, the authors state that “and their porosity values do not exceed 3.5%.” How was this determined? Please clarify.

 

10. Lines 258-260, the authors state that “Obviously, this is due to increased heat removal….” This statement must be supported by references, and the main mechanisms should be clarified.

 

Author Response

Dear Reviewer,

Thank you very much for your helpful comments. Below you will find responses to all your recommendations.

1. Introduction, it will be better to focus specifically on the aluminum cast alloy AlSiMg0.3. Currently, the authors only discuss aluminum alloys in general. Additionally, the authors should mention some disadvantages of the AlSiMg0.3 alloy and how coatings might address these issues.

Response 1: We appreciate your constructive feedback. It’s valuable to us. The main focus of the Introduction was the selection of coating materials and deposition technologies. In the Introduction we discuss the low wear resistance of aluminum materials, in particular the widely used AlSiMg0.3 alloy. We’ve completely revised the Introduction section with additional information as shown below.

1. Introduction

Aluminum alloys, favored for their high strength-to-weight ratio compared to conventional steels [1,2], are widely utilized in aerospace, automotive, and polymer extrusion industries [3–5]. Despite their strength, aluminum materials, particularly the widely used AlSiMg0.3 alloys, exhibit poor wear resistance, necessitating the use of protective coatings [6]. These coatings must be sufficiently thick to withstand impact-abrasive wear and high contact loads, preventing deformation of the ductile aluminum substrate. Dissimilar joining of iron subgroup-based materials (Fe, Ni, Co) to aluminum alloys is often problematic due to the formation of brittle intermetallic phases, which can lead to cracking under high contact pressures [7,8].

The microstructure and mechanical performance of coatings are significantly influenced by both their chemical composition and the deposition technique employed. Electrochemical methods such as electroplating [9,10] and anodizing [11], laser cladding [12–16], and thermal spraying [17,18] represent some of the most widely used coating technologies. Electrochemical coating methods, while offering certain advantages, are constrained by their limited ability to produce coatings with substantial thickness and are also subject to several other drawbacks. While conventional anodizing of aluminum alloys provides hard coatings with a thickness of 10–50 µm, their inherent brittleness makes them prone to cracking and delamination under demanding conditions [19,20]. Furthermore, the composition of anodized films can vary depending on the specific aluminum alloy used [21]. Micro-arc oxidation offers a compelling alternative to traditional anodizing, with significantly enhanced hardness (5–10 GPa) and thicker coatings (up to 150 µm) [22,23], but it also faces inherent limitations. Such coatings are vulnerable to failure under high contact pressures, and their production costs escalate considerably when dealing with large or intricately shaped components. Despite such advantages of laser cladding as a low dilution rate, a minimized heat-affected zone, and a robust interfacial metallurgical bonding, its cost also remains a significant barrier to wider adoption [12].

Given these findings, thermal spray methods may be advantageous in many applications due to their versatility in depositing a wide range of materials with satisfactory performance. Recent advancements in arc spraying have enabled the deposition of steel coatings up to 4 mm thick, utilizing a modified nichrome sublayer. These coatings, applied to Al-based parts, exhibit enhanced adhesion strength (up to 80 MPa), reaching microhardness values of 800 HV and demonstrating the ability to withstand high contact loads of up to 100 MPa [24]. Thermal sprayed coatings, deposited onto Al-based alloys with a thickness of 100–400 μm, have proven advantageous in reducing the probability of coating failure during adhesive wear. These coatings have demonstrated high wear resistance in both lubricated and dry conditions, showing effectiveness in applications such as textile equipment [25], polymer extrusion tools [26,27], cylinder blocks, and car brake discs [28].

To enhance the wear resistance of aluminum alloys susceptible to adhesive wear, high-velocity oxygen/air-fuel (HVOF/HVAF) spraying is used to deposit metal-ceramic coatings (cermets) onto the aluminum substrate. This process, characterized by high particle velocities (over 500 m/s) and low flame temperatures (below 3000 °C), results in coatings with superior adhesion, low porosity, reduced decarburization, and improved wear resistance compared to other thermal spray methods [29].

Bolelli et al. [30] studied the performance of WC-CoCr cermet coatings, up to 150 μm thick, deposited onto aluminum alloys using HVOF spraying under ball-on-disk loading conditions up to 2600 MPa. Their findings revealed several key features:

• A graded transition zone forms between the WC-CoCr coating and the aluminum substrate due to cermet particle penetration, dramatically enhancing wear resistance compared to hard anodized film;
• Thicker WC-CoCr cermet coatings exhibit reduced wear due to a decrease in surface layer peeling;
• Cyclic impact testing resulted in localized transverse cracks within the coating, negatively affecting performance but occurring less frequently than in WC-CoCr cermets deposited on a less ductile steel base.

Aluminum blades, enhanced with HVAF-sprayed wear-resistant coatings, offer advantages in industrial fans used for ventilation in subway tunnels, coal and ore mines. Their reduced weight improves performance compared to steel blades, and they can operate at temperatures up to 400 °C [31], suitable for many industrial applications. However, the presence of fine solid particles in fan operating environments poses a challenge, as aluminum blades exhibit poor erosion resistance.

Erosion wear resistance is influenced by factors such as impact angle, velocity, solid particle size, concentration, and hardness of the erodent [32]. Additionally, the material’s ductility and strength play a crucial role. Ductile metals, such as austenitic steel, aluminum, and gray cast iron, generally exhibit the highest erosion rates at low attack angles (15° to 60°). Aluminum alloys specifically show maximum wear at a 15° angle, where microcutting is the dominant wear mechanism. This wear rate decreases by 4–5 times as the attack angle increases to 60°, where plastic deformation becomes the primary wear mechanism [33]. Brittle materials experience maximum erosion at high attack angles (60° or more) due to the primary erosion mechanism of spalling caused by cracking [34]. HVOF-sprayed coatings have proven effective in extending the operating life of components exposed to erosion [35]. A comparative study demonstrated that HVOF-sprayed coatings exhibit higher hardness (~9%) and superior erosion resistance compared to coatings produced by spark plasma sintering (SPS) [36].

Comparative studies have shown that HVAF Cr₃C₂-based coatings on steel substrates exhibit superior resistance to dry, gas-abrasive, and cavitation wear, along with enhanced microhardness, lower porosity, and smoother surfaces compared to HVOF coatings [37]. Similar properties have been observed for WC-10Co4Cr coatings [38], while the HVAF spraying process itself is considered more technologically advanced [39]. HVOF-sprayed WC-10Co4Cr coatings deposited onto steel substrates demonstrate approximately twice the erosion resistance of similar Cr₃C₂-25NiCr coatings [40,41]. While cheaper steel coatings are commonly used, their wear resistance is significantly lower than cermets.

This study investigates the erosion wear resistance of HVAF-sprayed WC-10Co4Cr, Cr₃C₂-25NiCr, and martensitic SS steel coatings on an AlSi7Mg0.3 alloy under dry erosion conditions, mimicking the demanding environment of fan blades. By systematically examining the influence of abrasive type (quartz sand and granite gravel), erodent attack angle, coating thickness, and microhardness, the research provides valuable insights into optimizing wear resistance for these critical components. The comprehensive analysis, utilizing optical, electron microscopic, and X-ray diffraction techniques, deepens our understanding of erosion wear mechanisms and offers valuable data for informed material selection and design in fan blade applications.

2. Introduction, the authors claim that “The selection of coating material depends on both its wear resistance and cost.” It would be better to avoid such statements.

Response 2: Thank you for your valuable recommendation. We’ve removed this statement.

3. Subsection 2.1, Table 1: Please check the numbers such as ≤0,10, which should be written as ≤0.10.

Response 3: Thank you for your valuable recommendation. We’ve corrected it as ≤0.10.

4. Subsection 2.2, Table 2 presents the parameters adopted for the HVAF-spraying process. It is important to explain the criteria behind these choices.

Response 4: We appreciate your constructive feedback. We’ve added a detailed explanation in the section 2.2. HVAF-spraying process parameters.

The parameters listed in Table 2 for the HVAF spraying process were carefully chosen to achieve a balance between deposition efficiency, coating thickness, and desired microstructural and mechanical performance. The spray distance of 180 mm was selected to balance particle velocity and ensure adequate kinetic energy for proper bonding while minimizing the risk of particle rebound. The gas pressures of 0.61 MPa for air, 0.58 MPa for propane 1 (base fuel), and 0.45 MPa for propane 2 (secondary fuel) were optimized to achieve a stable combustion process and provide sufficient energy for particle acceleration. The carrier gas flow rate of 68 l/min was set to ensure efficient particle transport and prevent excessive powder agglomeration. The powder feed rate of 200 g/min was chosen to maintain a consistent coating deposition rate and avoid excessive build-up. The spray gun movement speed of 1.0 m/s was selected to control the coating thickness per pass at 40 microns while ensuring smooth and uniform layers.

5. Subsection 2.2, the authors state that “After deposition, all samples were ground manually to a roughness of Ra = 0.32 µm using a diamond grinding wheel.” The authors should justify the choice of this specific surface roughness (Ra = 0.32 µm) for this work.

Response 5: Thank you for your comment. This roughness (Ra = 0.32 µm) is within the acceptable range for wear resistance testing in accordance with GOST 23.201-78.

6. Additionally, if possible, I recommend including the roughness results of the samples, as these findings may interest potential readers.

Response 6: We really appreciate your valuable feedback. You are absolutely right − this would also be informative. Unfortunately, we no longer have this opportunity due to lack of any samples.

7. Subsection 3.1, the hardness results of the AlSi7Mg0.3 alloy should be included in Figure 2(l) and discussed.

Response 7: Thank you for your valuable recommendation. In fact, we used a common commercial as-cast AlSi7Mg0.3 alloy with a Vickers hardness (H_V) in the range of 0.7−0.9 GPa. Unfortunately, we no longer have this opportunity due to lack of any samples.

8. Lines 247-249, the authors claim that “Most likely, this is due to the lower kinetic….” This statement must be supported by references.

Response 8: Thank you for your comment. We do not claim, we just assume it. In this case, no reference is required. When we can't refer to something, we make an assumption.

9. Subsection 3.1, the authors state that “and their porosity values do not exceed 3.5%.” How was this determined? Please clarify.

Response 9: Thank you for your comment. You can find this information in the section 2.6. Microhardness and porosity characterization.

The porosity of the coatings was determined by quantitative analysis of grinding surface images captured using a light microscope. ImageJ analysis software (LOCI, University of Wisconsin, USA) was employed to calculate the percentage of porosity by determining the ratio of pore area to the total area of the grinding surface.

10. Lines 258-260, the authors state that “Obviously, this is due to increased heat removal….” This statement must be supported by references, and the main mechanisms should be clarified.

Response 10: We really appreciate your valuable feedback. You are absolutely right. We have replaced the above statement with an assumption “This is probably due to increased heat removal…”, so a reference is not necessary. When HVAF spraying occurs, the cermet particles impact the substrate, transferring heat. The heat transfer rate is influenced by the thermal conductivities of both materials. The higher thermal conductivity of the AlSi7Mg0.3 substrate allows it to absorb and dissipate heat more effectively than the cermet coating, leading to a temperature gradient within the coating.

Thank you for your inestimable contribution to improving our manuscript.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have made revisions responding the major comments of the previous review, but there are still some problems:

1. The small words beneath the scales in Figures 1(a), (b), and (c) is unclear. 

2. In Section 2.1, "…which is widely used for fan blade manufacturing due to its high casting and mechanical characteristics." should be supplemented with a relevant reference.

3. It is advisable to use italics for variables throughout the text to distinguish them from constants.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

 Minor editing of English language required.

Author Response

Dear Reviewer,

Thank you very much for your helpful comments. Below you will find responses to all your recommendations.

1. The small words beneath the scales in Figures 1(a), (b), and (c) is unclear.

Response 1: Thank you for your valuable comment. These small words under the scales in Figures 1(a), (b) and (c) represent the name of the Neophot optical microscope. We’ve removed this to avoid misleading the reader.

2. In Section 2.1, "…which is widely used for fan blade manufacturing due to its high casting and mechanical characteristics." should be supplemented with a relevant reference.

Response 2: Thank you for your valuable comment. We’ve supplemented the above statement with the relevant reference.

The substrate was 25 × 15 × 4 mm blanks of ENAC-AlSi7Mg0.3 (ENAC-42100) alloy, which is widely used for fan blade manufacturing due to its high casting and mechanical characteristics [31].

3. It is advisable to use italics for variables throughout the text to distinguish them from constants.

Response 3: Thank you for your constructive feedback. We’ve completely revised the text of our manuscript and applied italics to variables where applicable.

Thank you for your inestimable contribution to improving our manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

It is suggested that Figure 4 a b in the original version should be kept to help readers understand the erosion wear testing process more clearly and quickly.

Author Response

Dear Reviewer,

Thank you very much for your helpful comments. Below you will find responses to all your recommendations.

1. It is suggested that Figure 4a,b in the original version should be kept to help readers understand the erosion wear testing process more clearly and quickly.

Response 1: Thank you for your constructive comment. We’ve added Figure 2a,b to the section 2.3. Erosion wear tests according to your and other reviewers' recommendations.

Thank you for your inestimable contribution to improving our manuscript.

Author Response File: Author Response.pdf