Plasma Spraying NiCoCrAlY-Cr2O3-AgMo Coatings: Fabrication and Tribological Mechanisms
Round 1
Reviewer 1 Report
Comments and Suggestions for Authors1) You claim that this coating may be used for sealing in aircraft engines. Are you referring to abradable coatings? You should expand what makes this coating a good candidate for such applications.
2) The deposition method was plasma spray. You are most likely referring to atmospheric plasma spray. If this is the case why did you select APS rather than vacuum plasma spray? What is the effect on the oxidation of the coating?
3) Did you heat-treat the coating to inter-diffuse the substrate/coating and improve adhesion? If not why?
4) Fig 1c,d images need to be replaced with BE SEM images. This will make easier to assess the microstructure.
5) Please add a higher magnification SEM image and the associate elemental map of the cross section of the coating. Focus on the coating/surface interface area, especially adhesion. Expand the discussion on the element distribution and how plasma spray affected the coating outcome.
6) How does the wear performance of the studied coating compare to widely used coatings in similar applications like WC-Co, YSZ, MCrAlY? You may compare your results with similar studies from literature.
7) Did you analyse the coating debris (SEM, XRD) after wear tests? This would help expand the discussion on the wear mechanisms.
My closing comment is that this is an interesting work and after a few changes it should be published in Coatings.
Author Response
Reviewer 1:
- You claim that this coating may be used for sealing in aircraft engines. Are you referring to abradable coatings? You should expand what makes this coating a good candidate for such applications.
Response: Thanks for the suggestions.
The service environment of aero-engine sealing coating is harsh, which needs to withstand the test of high-speed friction and wear, high temperature oxidation and alternating hot and cold heat cycle. This requires that the sealing coating should have excellent high temperature self-lubrication and wear resistance. Ag can reduce the friction coefficient of the material at low temperature, improve the toughness of the coating, and improve the matching between the coating and the substrate. Therefore, this paper uses Ag and AG-Mo composite solid lubricant as the lubrication phase, Cr2O3 as the wear-enhancing phase, Ni base and Co base alloy as the metal phase, in order to prepare a high temperature solid white lubrication wear-resistant coating material that can be used for 800°C, has the effect of temperature free lubrication and wear resistance, and is well matched with the matrix. After reading a lot of literature on fingertip seals and wear-resistant coatings, NiCoCrAIY-Cr2O3-AgMo coating has both excellent wear performance and better friction performance under conditions ranging from 0°C to 800°C, and nickel molybdate and silver molybdate co-lubricate the friction surface above 600°C to reduce the wear of the coating at high temperatures. The wear rate and friction coefficient are both low when the main working temperature of the seal is 600°C. In this paper, wide-temperature tribological experiments are carried out for the coating to explore the friction and wear conditions of the self-lubricating coating at various temperatures, providing technical support for the design of the seal friction pair.
- The deposition method was plasma spray. You are most likely referring to atmospheric plasma spray. If this is the case, why did you select APS rather than vacuum plasma spray? What is the effect on the oxidation of the coating?
Response: Thanks for the suggestions.
APS has the advantages of simple operation, low energy consumption and low cost, but in the process of atmospheric spraying, the spraying particles interact with the surrounding gas, which will cause oxidation on the surface of the spraying particles, so the coating will inevitably include oxygen impurities. The rapid cooling of flying particles will produce a large temperature gradient stress on the surface, and when the stress is large enough, the surface will appear micro-cracks. For APS, the oxidation probability of molten particles is reduced; Particle vacuum flight, the resistance is small, the flight speed will not change greatly, the impact force of the previous layer of coating is uniform, so the coating density uniformity is better. In addition, the spraying process is not subject to the effect of air cooling, the melting state of molten particles is better before deposition, and the coating density is high.
Therefore, compared with the APS coating, the VPS coating has more superior physical properties: low porosity, low impurity content, high heat transfer performance, and high bonding strength. In this paper, we mainly focused on the influence of coating formulation on coating properties, and we can pay more attention to the influence of preparation process scheme and preparation parameters on coating properties in the later stage. Many thanks to the reviewers for pointing out the future research direction for us.
- Did you heat-treat the coating to inter-diffuse the substrate/coating and improve adhesion? If not why?
Response: Thanks for the suggestions.
A large number of studies have shown that heat treatment can significantly reduce the number of microcracks on the surface of ion sprayed coatings. We conducted a cyclic thermal shock test on the coating. After rigorous thermal shock test, no cracks appeared on the surface of the coating except for the original pores, so no heat treatment was performed. Subsequently, heat treatment strengthening could be considered to change the coating performance.
- Fig 1c,d images need to be replaced with BE SEM images. This will make easier to assess the microstructure.
Response: Thanks for the suggestions. We have added some BE SEM images of this section in the article
- Please add a higher magnification SEM image and the associate elemental map of the cross section of the coating. Focus on the coating/surface interface area, especially adhesion. Expand the discussion on the element distribution and how plasma spray affected the coating outcome.
Response: Thanks for the suggestions. We have added higher magnification SEM in the article.
We added the distribution of elements at the joint of the coating and the substrate surface, and it can be seen from the distribution diagram that the combination of the substrate and the coating is better. The duct ability and toughness of NiCoCrAlY metal bonding phase can absorb and disperse external loads, prevent stress concentration between hard phases, improve the cracking resistance and overall hardness of the coating, and improve the bonding strength between the coating and the substrate, reducing the peel and damage of the coating under heavy load.
- How does the wear performance of the studied coating compare to widely used coatings in similar applications like WC-Co, YSZ, MCrAlY? You may compare your results with similar studies from literature.
Response: Thanks for the suggestions.
Chen [1] prepared the WC-Cu/Co composite coating on Inconel 718 superalloy by laser cladding technology, and the microhardness of the composite coating was increased to more than 1.7 times that of the substrate. The Co-5wt%WC-5wt.%Cu coating had excellent wear resistance at 600 °C, with a wear rate of 4.50 × 10−5 mm3/N·m. Co-5wt.%WC-10wt.%Cu coating and Co-5wt.%WC-15wt.%Cu coating (600 °C) had excellent anti-friction properties, and their friction coefficients were 0.589 and 0.482, respectively. It is found that the average friction coefficients of NiCoCrAlY-Cr2O3-AgMo prepared in our manuscript at 400℃, 500℃ and 600℃ were 0.438, 0.410 and 0.268. When the temperature raised to 600℃, NiMoO4 film was generated on the surface of NiCoCrAlY-Cr2O3-AgMo, which reduced the wear rate of GH605 cylindrical pin to 2.78×10-6mm3/N·m, and the friction coefficient and wear rate were lower than that of the WC-Cu/Co based composite coating.
Bai [2] used plasma spraying to deposit YSZ composite coating on Inconel 718 alloy matrix. The coefficient of friction of the YSZ coating was between 0.55 and 0.80 during 25-600oC, and the lowest friction coefficient was 0.41 at 800 °C. The average friction coefficients of NiCoCrAlY-Cr2O3-AgMo coatings prepared in this paper are 0.438, 0.410 and 0.268 at 400℃, 500℃ and 600℃. The friction coefficients of NiCoCrAlY-Cr2O3-AgMo coatings show advantages at various25-600℃.
Pereira [3] prepared MCrAlY (NiCoCrAlY and CoNiCrAlY) coatings on stainless steel by laser cladding technology. The friction coefficient of NiCoCrAlY coating decreased from 0.49 to 0.45 from 25℃ to 500℃, and the friction coefficient of CoNiCrAlY coating increased from 0.46 to 1.0 from 25℃ to 500℃. The average friction coefficients of NiCoCrAlY-Cr2O3-AgMo coatings at 400℃, 500℃ and 600℃ are 0.438, 0.410 and 0.268, which were superior to MCrAlY coatings.
- Did you analyze the coating debris (SEM, XRD) after wear tests? This would help expand the discussion on the wear mechanisms.
Response: Thanks for the suggestions.
SEM and XRD analysis are complementary in the study of wear debris after friction and wear experiments. SEM provides information on the morphology and surface characteristics of the wear debris, while XRD reveals the phase composition and possible phase transitions or chemical reactions of the wear debris. The integration of these two analytical methods facilitates a more comprehensive understanding of the mechanisms and processes of friction and wear.
It is a great pity that we did not have an effective analysis of wear particles, and we will consider conducting research in this area in the future.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe paper presents a novel plasma-sprayed coating that is self-lubricating and wear-resistant. The data presented were discussed clearly and the paper in general was well-written. Hence, I recommend to accept the manuscript for publication after minor revisions. Below are some questions and comments that must be considered by the authors to improve the readability of the paper:
1. How long is the coating time for the CoNiCrAlY bond coat to achieve 100 microns? How long is the coating time for the NiCoCrAlY-Cr2O3-AgMo coating to achive 100 microns?
2. Merging Tables 2 and 3 could help the readers to better compare the chemical compositions of GH4169 and GH605.
3. There is a missing reference iin page 4 line 171.
4. Figure 1, please show the orientation of the cross-section in (c).
5. How many experimental replicates were performed for each sample for the friction and wear tests? Please show the uncertainties in the data presented.
Author Response
- How long is the coating time for the CoNiCrAlY bond coat to achieve 100 microns? How long is the coating time for the NiCoCrAlY-Cr2O3-AgMo coating to achieve 100 microns?
Response: Thanks for the suggestions. Under the spraying process parameters in this paper, it takes 30 min to prepare a 100 μm thickness coating on a size of 20 cm *20cm substrate. In addition, the preparation time of NiCoCrAlY-Cr2O3-AgMo is almost same as that of CoNiCrAlY.
- Merging Tables 2 and 3 could help the readers to better compare the chemical compositions of GH4169 and GH605.
Response: Thanks for the suggestions. We set Table 2 and Table 3 according to the order in which GH4169 and GH605 appeared in the paper, which was also convenient for readers to read and not easy to confuse. Thus. We did not merge the two tables.
- There is a missing reference in page 4 line 171. 3.
Response: Thanks for the suggestions. We have modified this reference in the manuscript.
- Figure 1, please show the orientation of the cross-section in (c).
Response: Thanks for the suggestions. We have modified the Fig.1 in the manuscript.
- How many experimental replicates were performed for each sample for the friction and wear tests? Please show the uncertainties in the data presented.
Response: Thanks for the suggestions. In order to reduce experimental errors, each group of friction experiments was repeated three times, in which the volume wear rate was averaged.
Author Response File: Author Response.pdf
