Heat-Resistant Protective Coatings Applied to Aircraft Turbine Blades by Supersonic Thermal Spraying and Diffusion-Aluminizing

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The work focused on heat-resistant protective coatings applied to aircraft turbine blades by supersonic thermal spraying and diffusion aluminizing. The work used technique on running engine under test bench conditions, that is really different with conventional experimental work in lab. The figures are proper to express the research, and description is logic. There are still some misleading or omit can be improved as followings:

1.     Abstract: the title is focused on an advanced method by thermal spray… Some tests and conclusion can be added to the abstract, not all the background in the abstract. An abstract is an abbreviation on the manuscript including at most three parts: background, test and key conclusions.

2.     Introduction: monocrystalline is not frequently used in superalloys, usually used is single crystalline.

3.     Introduction: two-layer coating on turbine blade had been used, so some introduction on two-layer structure can be simplified. Your thermal spray method, however, is your highlight in this paper, more introduction can be added.

4.     Fig.7, there is a big gap after service in (c), so this method (HVOF + SLURRY) is not good? Or this gap doesn’t matter to the blade.

5.     Fig. 9, the pictures on the right column can’t be discerned.   

6.     Which method is best for application? It can be concluded in the conclusions.    

Author Response

Comments 1:

Due to the fact that the abstract can contain a maximum of 200 words, it is difficult to expand it. In accordance with the reviewer's suggestion, the abstract is divided into three parts: background, description of the research, and key conclusions.

 

Comments 2:

Thank you for your attention. This is how single crystalline is usually used. I propose to remove the word "mono-crystalline". This concept appeared in the introduction because the authors studied the blades used for applying protective coatings and their engine tests are made of a mono-crystalline nickel superalloy.  

 

Comments 3:

I propose adding the following information about the VPA and SLURRY aluminizing methods to the introduction.

The VPA method is a powder method. The coating production process is carried out in a chamber furnace in a protective atmosphere at a temperature of up to 1373 K. In the VPA method, the blade is placed in a container surrounded by a powder mixture consisting of: active metal powders and an activator. The mechanism of diffusion metallization in an environment consisting of a bed of active elements and an activator in the form of halides is based on the formation of a gas phase with a high partial pressure of the halides of the saturating element, and then on the transport of the coating element to the surface of the coated blade. The SLURRY suspension method consists in immersing the blade in a suspension and drying it. The suspension contains metal powders, an organic binder (water glass) and a flux (molten and powdered halides NaCl, KCl, NaF). After immersing the blade in the suspension, it is heated for the appropriate time and temperature in a furnace in an argon atmosphere.

 

Comments 4:

The vertical line in Figure 7 (c) is a crack. In coatings made by the HVOF+SLURRY method, suspension aluminization does not allow for controlling the Al content on the blade surface. This can cause high activity of the Al source in the aluminization process, which leads to a significant increase in the brittleness of the coating at temperatures below 750°C and, as a result, the formation of cracks in the coating applied to the blades already during their assembly. No cracks are allowed in the protective coatings, and it is only stated whether they have occurred. Coatings made by the HVOF+SLURRY method are not intended for use in turbine blades.

 

Comments 5:

In the studies of changes occurring in the microstructure, attention was paid to the assessment of the impact of the development of the precipitation process of secondary phases, assessing the changes occurring in the microstructure image based on their morphological features, location of occurrence and their chemical composition. The photos in Fig. 9 b), d), f) present the precipitations of secondary M₂₃C₆ carbides. These are electron diffraction patterns of the precipitation. Identification of these precipitations allows for the assessment of the degradation of the superalloy microstructure during the operation of the blade at high temperatures. Unfortunately, we only have this type of diffraction pattern. It should be added here that in the first stage of long-term impact of temperature and stress in the microstructure of the nickel superalloy, M₂₃C₆ carbide precipitations are formed, the number and size of which increase with the extension of the operation time.

 

Comments 6:

The best method is a protective coating composition obtained by supersonic spraying with high-velocity oxygen fuel (HVOF) of AMPERIT 405.001 coating powder and aluminizing with CrAl 70/30 coating material with AlF3 activator using the Vapour Phase Aluminizing (VPA) method. I propose to introduce such a provision into the Conclusions.

Reviewer 2 Report

Comments and Suggestions for Authors

scale can be given in the pictures.

 

Is the vertical line in figure 7c a crack? If it is a crack, is it mentioned in the text?

 

The paragraph on line 402 mentions the formation of many cracks. Information should be given about the depths of the cracks. This is a parameter that may be needed to determine the service life of the coating.

Comments on the Quality of English Language

I am not proficient in English.

Author Response

The vertical line in Figure 7c is a crack. In coatings made by the HVOF+SLURRY method, suspension aluminization does not allow for controlling the Al content on the blade surface. This can cause high Al source activity in the aluminization process and, as a result, cracking of the coating applied to the blades. At the end of Chapter 3.5 it is stated that "too high Al source activity in the aluminization process led to cracking and delamination of the coating applied to the blades". Also in the summary it is explained that "In the case of the protective coating made by the HVOF+SLURRY method, too high Al source activity in the SLURRY aluminization process leads to a significant increase in the brittleness of the coating at temperatures below 750°C, which is visible during its operation by numerous cracks". The depth of the cracks was not determined during the tests, because no cracks are allowed in the protective coatings. It is only stated whether they occurred.

Reviewer 3 Report

Comments and Suggestions for Authors

This paper is a valuable study that uses an actual gas turbine engine to test protective coatings. 

Please correct your manuscript according to the comments attached. 

 

 

 

Comments for author File: Comments.pdf

Author Response

The answers are in the file Word

Author Response File: Author Response.pdf