Effects of Laser-Remelting on the Microstructure, Hardness and Oscillating Wear Resistance of Atmospheric Plasma Sprayed Alumina-Rich Coatings

Round 1

Reviewer 1 Report

1. Abstract, line 13: ‘property profile’, be more specific.
2. Abstract, line 16: ‘effect of different parameters’, on what?
3. Materials and Methods, line 82: ‘differing in microstructure’, please mention what are these differences.    
4. Materials and Methods, line 94: Did you measure the roughness?
5. Materials and Methods, line 117: You measured the hardness on the surface, however as you mention in the text structural differences can be observed between the outer layers and those close to the interface. What is the reason for this choice?
6. Results, lines 136-138: Presence of cracks is not clear in Figure1.
7. Results, lines 145-148: Can you please give a brief summary instead of referring to REF [29].
8. Figure 4: EDS is not accurate for quantitative analysis of light elements such as O. Also, EDS is very localized, so please mention in which region of the coating where these measurements taken.
9. In Figure 7, please indicate that the results on the right side are the laser treated ones.
10. Discussion, lines 279-282: Unfortunately, the wear results are not adequately explained. Why do you observe these differences? Is it due to localized delamination and debris formation? Do you get the same wear mechanisms in all cases? Adding some higher magnification images of the wear tracks would possibly help in understanding these differences. Also have in mind that the hardness of your counter-material is similar to your coating, so by further hardening the coating you could be inducing more wear on the counter-material.      

Author Response

Thank you for your helpful comments and remarks, which I would like to address below:

1. Additional information is added.
2. This has already been mentioned: on the macro- and microstructure of the coatings and their influence on hardness and oscillating wear resistance.
3. Some additional information has been added. A detailed description is given in the frequently mentioned publication [29].
4. Information was added. For the roughness, the values Rz and Ra were determined: Rz in the as-sprayed state: blend so: approx. 42µm, blend bfc: approx. 58µm, as1: approx. 38µm; in the polished state Rz < 3 µm for all coatings.
5. The aim of this investigation was to determine the hardness of the laser remelted zone. Due to the relatively low thickness of this zone, a measurement in cross-section would only be possible with a reduction in the test load. The underlying area corresponds to the original microstructure of the coating (before laser-remelting), which hardness was already determined in the previous study with the given test force.
6. The figure has been adjusted. The cracks are visible especially in 1a and some of them were marked with arrows.
7. A brief explanation has been added to highlight the essential point of the multi-phase and single-phase nature of the coatings.
8. In contrast to XRF, light elements can also be detected with relatively good accuracy using EDS, although the simultaneous determination of Cr and oxygen is problematic (L lines Cr and K lines oxygen). Nevertheless, we consider the measurement to be suitable for the purpose presented. The purpose of the EDS measurements was to show the single or multi-phase nature of the coating once again and to show that the average composition (when considering a large measurement area) is identical in all coatings. The results presented here refer to the previous study [29] and the measuring points are also described in more detail there. In this study, new EDS measurements were performed in the laser remelted zone. The results show that regardless of the original coating structure, a similar composition was detected in all coatings, corresponding to the average composition of the original coatings. This indicates a reaction of the different lamellae as a result of the laser treatment.
9. Thank you for pointing this out. The figure has been adjusted.
10. We agree with you that the discussion of wear behavior is very brief. Unfortunately, we have not yet carried out any more detailed investigations that would allow us to make statements on the wear behavior / wear mechanism under oscillating load for the coatings shown. However, this work is planned. An increased wear on the counterbody was not detected. We assume that the strong improvement in wear behavior is mainly due to the increased hardness and compact structure of the laser remelted zone. Breaking out of individual splats, as occurs in thermal sprayed coatings, probably does not occur after laser remelting. A statement was added.

We hope we could answer your questions to your satisfaction.

Reviewer 2 Report

This is a good writting paper focusing on laser treatment of the Al-riched coatings. The following suggestions are provided to further improve it.

1) For Figue 2, does the pair (a,d),(b,e),(c,f) describe the microstructure in the same location?

2) For Figure 5, no subtitle in the caption.

3)  For Figure 7, how many data points was used to get the averaged data? The oscillating wear rate has a large deviation, what is the reason?

Author Response

Thank you for your review and helpful comments, which I am pleased to address below:

1. The images do not describe the exact same position, but are each a representative depiction of the same coating before and after laser remelting.
2. Thank you very much for the helpful advice. Description has been adjusted.
3. Three wear tracks were evaluated for each coating and type (before and after laser remelting). In our opinion, the standard deviation of the wear rate is within a normal range for thermal sprayed coatings, which is likely caused by the inhomogeneity and defects (porosity, microcracks) of thermal sprayed coatings.

We hope we could answer your questions to your satisfaction.

Reviewer 3 Report

The authors presented an interesting work, but it requires numerous improvements to be of satisfactory quality.
General remarks 
1. The novelty of the work in comparison with the state of the art or science should be indicated. 

Detailed comments 
Line 120 oscillating wear test - is it fretting?, if so, please enter the standard according to which the tests were carried out.
Fig. 2 - Are the photos shown in sections? If so, enter it in the description

Author Response

Thank you for your review. In the following, we would like to respond to your comments:

To the general remarks:
Although the binary systems (i.e., Al₂O₃-TiO₂) of the investigated ternary system (Al₂O₃-Cr₂O₃-TiO₂) have been studied for many years and have numerous applications, the ternary system has hardly been considered. Our own studies show that the simultaneous additions of Cr₂O₃ and TiO₂ can improve the properties of Al₂O₃, and solid solutions such as (Al,Cr,Ti)₂O₃ have particular potential. The manuscript therefore investigates not only the influence of laser remelting on the hardness and wear resistance of such coatings, but in particular the possibility of converting multiphase coatings into single-phase solid solution structures by laser remelting.
A sentence was added in the introduction to point this out more clearly. It is already referred to at the beginning of the discussion.

To the detailed comments:
Line 120: Low amplitudes were used in the oscillation wear test, so it can also be called fretting wear or reciprocating wear. The test was performed in accordance with ASTM G133. An information was added in the manuscript.
Fig. 2: Figure 2 shows cross-sections of the coatings before and after laser remelting. A note on this was added in the paper.

We hope we could answer your questions to your satisfaction.