Microstructure and Wear Behavior of Plasma-Sprayed TiO₂–SiAlON Ceramic Coating

Round 1

Reviewer 1 Report

The authors should consider the following aspects.

1. The introduction is poorly developed,
2. The authors added pictures with results (SEM micrographs of as-received powders), but SEM micrographs analysis alone are insufficient to identify the specific composition of the powders. I would suggest at least applying XRD to assist the identification,
3. What are the adhesion properties of the coating before tests?
4. Authors tried to argue that:” The wear mechanisms of the substrate were mainly severe abrasive wear and adhesive wear, accompanied by plastic deformation; while the surface fracture and slight abrasive wear were the wear mechanisms of composite coating”. However, there was not further discussions on the phenomena,
5. In addition, the sentence: " SiO₂ was present in the coating, due to the oxidation of SiAlON and probably Si₃N₄ as well" is a plausible hypothesis and not a conclusion. This should be removed or offered only as a conjecture or hypothesis.

Author Response

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Reviewer 2 Report

Few minor changes required. 1) Initially you claim that SiAlON are promising material but you don't specify which are the application areas. 2)In your case, using TiO₂, did you try pure anatase/rutile or brookite? If so, which are the outcomes? Are those comparable with the mixture rutile/anatase of your final coating? 3) Why did you determine the porosity through an image analyzer (ImageJ) rather than obtaining clear and unmistakable BET N2-adsorption pore distribution curves? The peaks associated to rutile are of low intensity and very limited (I count only 2). Did you try to spray pirolysis only TiO₂? What is the XRD of the resulting TiO₂ (pure anatase initially)? Monocrystalline or polycrystalline? If so, is just anatase/rutile or also brookite?

Thanks

I attach the corrected file for your attention

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

In the paper „Microstructure and Wear Behavior of Plasma-Sprayed TiO2-SiAlON Composite Coating” the author's report (i) the production of feedstock powder prior to spraying, (ii) the obtaining of plasma-sprayed TiO2-SiAlON composite coating, and (iii) the characterization of them (comparative studies of substrate and coating) from the microstructural and tribological point of view. The production of feedstock powders consisted of homogeneous mixing of Si3N4, Al2O3, and AlN raw powders according to the stoichiometry ratio with ethyl alcohol, then vacuum drying, sintering in the argon atmosphere, and crushing in accordance with the literature data; the samples thus obtained were mixed with commercially TiO2 anatase in a planetary ball mill, manually granulated, using an organic binder, and subsequently dried in vacuum, to remove binder and moisture. 

Preparation of the TiO2- 20%SiAlON composite coating was made with a high –energy atmospheric plasma spraying system to deposit TiO2-SiAlON coating onto a surface of stainless steel. The surface of stainless steel has been previously prepared (grit blasted and decreased in ultrasonic acetone bath and then preheated) to improve the adhesion of the coating prior to spraying.

After the investigations of granulated powder morphology, the authors highlighted that the granulated particles were polygonal with a very narrow size, which means that they had good flowability and has the advantage to be delivered into the plasma in a controlled fusion. Another advantage of the dense agglomerates granulated particles was that ensures that the SiAlON particles inside could be less heated due to the protection of TiO2 in the spray process.  Regarding the microstructural characterization of the coating, the study has also shown that the SiO2  formed during the spraying process facilitates the formation of the tribo-film and the pores in the coating is considered beneficial since it allows the oil retention for the boundary or starved lubrication conditions. These results are also confirmed by investigations on wear and tribological behavior. The lubricated coating exhibits the best wear resistance compared to the other investigated samples. The authors concluded that the wear rate of the TiO2-SiAlON coating was just 1/3 of the substrate under both dry and starved lubrication friction conditions and the composite coating also had a lower coefficient of friction than 316 stainless steel substrates. Also, the wear mechanisms of the substrate were mainly severe abrasive wear and adhesive wear, accompanied by plastic deformation; while the surface fracture and slight abrasive wear were the wear mechanisms of the composite coating.

This paper, as a whole,  is of interest to the readers, due to the scientific data provided in a clear manner. As a result, I agree with the publication of this paper.

Author Response

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Round 2

Reviewer 1 Report

The results are adequately interpreted and that is why I recommend this manuscript for publication.

Author Response

Thanks for your comments.

Reviewer 2 Report

Authors have satisfied my comments. Good job.

Author Response

Thanks for your comments.

Reviewer 3 Report

I accept the second revised version of the manuscript for publication.

Author Response

Thanks for your comments.