Microstructure and Performance of High-Velocity Oxygen-Fuel Coupled Physical Vapor Deposition (HVOF-PVD) Duplex Protective Coatings: A Review

Round 1

Reviewer 1 Report

The HVOF-PVD is based on the acceleration and heating of particles of the sprayed material in a high-temperature high-velocity flow of hydrocarbon fuel combustion products in oxygen. Acceleration and heating of the sprayed particles by the gas flow occurs either in a free jet or in the accelerating channel of the burner. The temperature in the combustion chamber reaches 3300 K, the gas flow velocity exceeds 2300 m/s. Under these conditions, the method makes it possible to deposit coatings from most of the existing refractory materials, including such as cermets, oxide and oxide-free ceramics, on relatively expensive machine parts. A review of these technologies is very useful for science and practice. At the same time, here are some recommendations:

 1. I propose to supplement the introductory part of the article with an analysis of research on this topic:

 Brezinova, J.; Guzanová, A.; Draganovska, D.; Maruschak, P. O.; Landová, M. Study of selected properties of thermally sprayed coatings containing WC and WB hard particles. Acta Mech. Autom., 2016, 10, 296–299, DOI:10.1515/ama-2016-0046.

Brezinova, J.; Guzanová, A.; Tkáčová, J.; Brezina, J.; L'achova, K.; Draganovska, D.; Pastorek, F.; Maruschak, P.; Prentkovskis, O. High velocity oxygen liquid-fuel (HVOLF) spraying of WC-based coatings for transport industrial applications. Metals, 2020, 10, 1675, DOI:10.3390/met10121675.

 2. The properties of gas-thermal coatings can be represented as interconnected subsystems that determine their geometric parameters, thermophysical and structural characteristics, strength, residual stresses. These properties, in turn, depend on the characteristics of the carrier gas, the sprayed material, the energy parameters of the heating source, as well as on external influences on the process of thermal spraying. I propose to pay more attention to the systematization of these aspects.

 3. I propose to summarize and classify the main methods according to the temperature of the heat source, particle velocity, adhesive strength, content oxygen in the coating and its porosity, performance, relative cost of coating.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

This review paper discusses performance of bi-layer HVOF and PVD coatings, regarding mechanical, tribological, and corrosion behaviour. This paper is well written and can be suggested for publication after some minor changes:

- On page 4, line 134, the authors discuss that grinding is typically used as a pre-treatment before applying the PVD top-coat. Please make sure to discuss why this treatment is necessary, e.g. to reduce the surface roughness, since the PVD top-coat does not need a mechanical bonding (surface roughness would actually increase the local stresses at the interface)

- It is unclear, what combination of materials (HVOF and PVD coating) should be used - both materials are typically not identical. Please discuss why a certain material was used, e.g. the combination of CrN / Cr3C2-NiCr on page 5, line 151 (and other places in the manuscript)

- In the manuscript, the authors are referring multiple times to a 'soft' substrate. Please add more details in these cases what kind of material system you are discussing.

- Please give more details of the simulation shown in Fig. 6, e.g. what is used material model, what are the loading conditions, and so on.

- The text in Figure 13 is too pixelated - please enhance the picture quality. Furthermore, it is a bit unclear which EDX scan is linked to which microstructure.

- The format of all references should be carefully checked - inconsistency could be found in Ref. 29, 60, 67, and 77.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

The prepared review is of a rather high level, but it seems to me that it is necessary to structure the literature data.

1) Section 2 should be renamed "Microstructure and morphology of coatings". Here it is necessary to clearly indicate, perhaps in the form of a Table, that the microstructure of the HVOF coating is layered, wavy, etc. And the microstructure of the PDV coating is completely different. Therefore, they perfectly complement each other.

2) In Section 2, it is also necessary to pay great attention to the problem of HVOF coating porosity.

3) Section 3 also lacks the organization of the data presented. Data are better presented in Tables. So it will be clearer.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Accept.

Reviewer 3 Report

The manuscript can be accepted for publication.