Study on the Effect of Carburizing on the Microstructure and High-Temperature Oxidation Properties of Hot-Dip Aluminum Coating on Titanium Alloy
Round 1
Reviewer 1 Report
This is an interesting work, written with clarity and coherence. The theoretical support through the bibliographical review is in accordance with the needs of the subject and its results will be an interesting path to continue future investigations.
Author Response
Response: Thanks for the reviewer's kind comments.
Reviewer 2 Report
The paper presents original experimental results of the influence of carburization of the TiAl/Ti3Al/TiAl2/TiAl3 composite coating on the kinetics of the oxidation of Ti65 titanium samples in air at 800°C. The formation of a Ti-Al coating on a titanium alloy by hot aluminizing is a widely used method for improving high-temperature oxidation resistance.
It has been shown that additional carburization of the composite coating on a titanium alloy makes it possible to significantly reduce the oxidation rate at elevated temperatures.
Results of researches show that the oxidation gain of uncoated Ti65 is about 50 times that of hot-dip aluminized and carburized Ti65, and the oxidation gain of hot-dip aluminized Ti65 is about 2.6 times that of hot-dip aluminized and carburized Ti65.
The results presented in this manuscript complement previously obtained data on the oxidation of titanium alloys with Ti-Al composite coating in air at high temperature.
The relevance of the research, the results of which are presented in the manuscript, is associated with the expansion of the use of titanium alloys with Ti-Al composite coating in advanced aircraft engines with high front temperature in the turbine to achieve high thrust-to-weight ratio, long service life and reduce fuel consumption.
The manuscript is well structured. The illustrations are of good quality and informative. The reference list is satisfactory. The conclusions and results of the research are based on the obtained experimental results.
The conclusions consistent with the evidence and arguments presented and correspond to the main aim of research.
The results presented in the manuscript may be of interest to specialists involved in the development of new technologies for improving the high-temperature oxidation resistance of titanium elements of advanced aircraft engines.
Remark: "Discussion" subsection is missing. In this section, it would be very useful to compare the results of investigations on high-temperature oxidation resistance of titanium alloys due to the carburization of the Al-Ti coating with the results obtained for some other coatings, for example, Ti-Al-Cr, Ti-Al-Cr-Y coatings, AlNbTaZrx high-entropy alloy, etc.
Author Response
-The introduction part should be extended by what has been done till now. There exist several studies about hot dip aluminizing and carburizing which have to be described in this part of the paper.
Response:Thanks for the reviewer's kind suggestion.The introductory section expands on the work done so far. Studies on hot dip aluminization and carburizing have also been described in this section. The above has been added to the introduction section of the manuscript.
-It is not clear to the readers why the structure and properties were analyzed only for the coating obtained by the hot dip aluminizing for 10 mins. The influence of the hot dip aluminizing time on the structure is an important question and is expected to give a better understanding of the structure formation of Ti-Al coatings by the above-mentioned technique.
Response:Thanks for the reviewer's kind suggestion.Through comprehensive analysis of SEM, EDS, XRD and other detection methods, it is found that TiAl3 phase layer can be formed by hot dipping aluminum plating at 760℃ for 5-20 minutes, but the thickness is different, and the growth kinetics of TiAl3 phase layer is described in section 3.1.3. When the hot dipping aluminum plating time is less than 10 minutes, the thickness of the TiAl3 phase layer is small, which is not conducive to the formation of TiAl2, TiAl, Ti3Al with appropriate thickness in the subsequent carburizing process. However, the hot dipping plating time is too long, which is not suitable for industrial production and is more likely to cause the formation and expansion of micro-cracks in the TiAl3 phase layer. Therefore, we chose to analyze the structure and properties of the coating obtained by hot dipping aluminum plating for 10 minutes. The above has been added to section 3.1.1. of this article.
-The same can be said about carburizing. It is not clear why the results for 2, 6, and 8 h are missing.
Response:Thanks for the reviewer's kind suggestion.By comprehensive SEM, EDS, XRD and other detection methods, it is found that three new Ti-Al alloy phase layers (Ti3Al, TiAl, TiAl2) can be formed by carburizing at 1050℃ for 2 to 4 hours, but their thicknesses are different, and the growth kinetics of Ti-Al alloy phase layers during carburizing is described in section 3.1.3. Carburizing 2-4 hours, Ti-Al carburizing layer contains Al2O3, TiAl2, C, TiC. When the carburizing time is less than 4h, the thickness of the Ti-Al carburizing layer and the three Ti-Al alloy phase layers (TiAl2, TiAl, Ti3Al) is small. When the carburizing time is too long, the surface brittleness of the coating increases, and the carburizing layer is easy to peel off. Therefore, we chose to analyze the structure and properties of the coating obtained by carburizing for 4 hours. The above has been added to section 3.1.1. of this article.
-The same can be said about the high-temperature resistance. After reading the captions of Figures 6,7, and 8 is not clear which coating (hot-dip aluminized for how many minutes and carburized for how many hours) was subjected to the high-temperature experiments.
Response:Thanks for the reviewer's kind suggestion.Because the structure of the coating obtained by hot dipping aluminum for 10 minutes and carburizing for 4 hours is the best, we selected the samples of this process parameter for heat exposure test to explore the high temperature oxidation resistance of the composite coating. Figure. 6, 7 and 8 are the composite coatings obtained by hot dipping aluminum plating for 10 minutes and carburizing for 4 hours. The temperature and time of hot dip aluminization and carburizing have been added to Figures 6, 7 and 8 in section 3.2.1. of the manuscript.
-The authors mentioned in the introduction that the main drawback of the Ti alloys are the low surface hardness and wear resistance. They mentioned that these limitations can be overcome by the formation of Ti-Al coating… However, data for the hardness, coefficient of friction, and wear resistance are missing in this paper. The authors should present some results.
Response:Thanks for the reviewer's kind suggestion.The surface hardness values of uncoated Ti65 sample, hot dipping aluminization (760℃×10min)Ti65 sample, hot dipping aluminization (760℃×10min) and carburizing (1050℃×4h)Ti65 sample have been added in the paper. In general, the greater the hardness of the material, the better the wear resistance, and the smaller the friction coefficient. The above has been added to section 3.3. of this article.
Reviewer 3 Report
The paper discusses the formation of Ti-Al coatings on the top of Ti alloy by hot-dip aluminizing and carburizing. The high-temperature oxidation resistance is analyzed. The topic seems interesting. The authors should pay attention to the following aspects:
The introduction part should be extended by what has been done till now. There exist several studies about hot dip aluminizing and carburizing which have to be described in this part of the paper. This will highlight the novelty of the manuscript.
It is not clear to the readers why the structure and properties were analyzed only for the coating obtained by the hot dip aluminizing for 10 mins. The influence of the hot dip aluminizing time on the structure is an important question and is expected to give a better understanding of the structure formation of Ti-Al coatings by the above-mentioned technique.
The same can be said about carburizing. It is not clear why the results for 2, 6, and 8 h are missing.
The same can be said about the high-temperature resistance. After reading the captions of Figures 6,7, and 8 is not clear which coating (hot-dip aluminized for how many minutes and carburized for how many hours) was subjected to the high-temperature experiments.
The authors mentioned in the introduction that the main drawback of the Ti alloys are the low surface hardness and wear resistance. They mentioned that these limitations can be overcome by the formation of Ti-Al coating… However, data for the hardness, coefficient of friction, and wear resistance are missing in this paper. The authors should present some results.
The language is understandable
Author Response
-The introduction part should be extended by what has been done till now. There exist several studies about hot dip aluminizing and carburizing which have to be described in this part of the paper.
Response:Thanks for the reviewer's kind suggestion.The introductory section expands on the work done so far. Studies on hot dip aluminization and carburizing have also been described in this section. The above has been added to the introduction section of the manuscript.
-It is not clear to the readers why the structure and properties were analyzed only for the coating obtained by the hot dip aluminizing for 10 mins. The influence of the hot dip aluminizing time on the structure is an important question and is expected to give a better understanding of the structure formation of Ti-Al coatings by the above-mentioned technique.
Response:Thanks for the reviewer's kind suggestion.Through comprehensive analysis of SEM, EDS, XRD and other detection methods, it is found that TiAl3 phase layer can be formed by hot dipping aluminum plating at 760℃ for 5-20 minutes, but the thickness is different, and the growth kinetics of TiAl3 phase layer is described in section 3.1.3. When the hot dipping aluminum plating time is less than 10 minutes, the thickness of the TiAl3 phase layer is small, which is not conducive to the formation of TiAl2, TiAl, Ti3Al with appropriate thickness in the subsequent carburizing process. However, the hot dipping plating time is too long, which is not suitable for industrial production and is more likely to cause the formation and expansion of micro-cracks in the TiAl3 phase layer. Therefore, we chose to analyze the structure and properties of the coating obtained by hot dipping aluminum plating for 10 minutes. The above has been added to section 3.1.1. of this article.
-The same can be said about carburizing. It is not clear why the results for 2, 6, and 8 h are missing.
Response:Thanks for the reviewer's kind suggestion.By comprehensive SEM, EDS, XRD and other detection methods, it is found that three new Ti-Al alloy phase layers (Ti3Al, TiAl, TiAl2) can be formed by carburizing at 1050℃ for 2 to 4 hours, but their thicknesses are different, and the growth kinetics of Ti-Al alloy phase layers during carburizing is described in section 3.1.3. Carburizing 2-4 hours, Ti-Al carburizing layer contains Al2O3, TiAl2, C, TiC. When the carburizing time is less than 4h, the thickness of the Ti-Al carburizing layer and the three Ti-Al alloy phase layers (TiAl2, TiAl, Ti3Al) is small. When the carburizing time is too long, the surface brittleness of the coating increases, and the carburizing layer is easy to peel off. Therefore, we chose to analyze the structure and properties of the coating obtained by carburizing for 4 hours. The above has been added to section 3.1.1. of this article.
-The same can be said about the high-temperature resistance. After reading the captions of Figures 6,7, and 8 is not clear which coating (hot-dip aluminized for how many minutes and carburized for how many hours) was subjected to the high-temperature experiments.
Response:Thanks for the reviewer's kind suggestion.Because the structure of the coating obtained by hot dipping aluminum for 10 minutes and carburizing for 4 hours is the best, we selected the samples of this process parameter for heat exposure test to explore the high temperature oxidation resistance of the composite coating. Figure. 6, 7 and 8 are the composite coatings obtained by hot dipping aluminum plating for 10 minutes and carburizing for 4 hours. The temperature and time of hot dip aluminization and carburizing have been added to Figures 6, 7 and 8 in section 3.2.1. of the manuscript.
-The authors mentioned in the introduction that the main drawback of the Ti alloys are the low surface hardness and wear resistance. They mentioned that these limitations can be overcome by the formation of Ti-Al coating… However, data for the hardness, coefficient of friction, and wear resistance are missing in this paper. The authors should present some results.
Response:Thanks for the reviewer's kind suggestion.The surface hardness values of uncoated Ti65 sample, hot dipping aluminization (760℃×10min)Ti65 sample, hot dipping aluminization (760℃×10min) and carburizing (1050℃×4h)Ti65 sample have been added in the paper. In general, the greater the hardness of the material, the better the wear resistance, and the smaller the friction coefficient. The above has been added to section 3.3. of this article.
Reviewer 4 Report
Please find below some general notice/ questions.
1.Numbering of capitals must be verified
2.Line 82-86 – please use the full sentences.
3.Line 88-92 – please use full description of process, using full sentences. Moreover please explain:
- What for cleaned by acetone by 5 min? What happened with the surface alloy in this time?
- What for cleaning by water after acetone use?
- What for cleaning by alcohol by 15 min?
4.Please use the chemical description not full names or if it is needed, use both (i.e. line 95-96).
5. Please explain, why 10 min for aluminizing was used? Please add result for only aluminizing. All images are after 4 h in 1050 what explain the thickness and phases. Fig. 4 shows the relationship between thickness TiAl3 phase and time in min. In molten aluminum the thickness should be very small ca. 6 um (after 25-30 s). The thickness of this layer should increase in 1050 and it takes hours.
6.Please explain the mechanism of diffusion (line 180). The phase formation is determined not by diffusional transport but by chemical kinetics.
7. Line 358 – how oxygen can penetrate? Line 338 – how oxygen can enter? Especially the CO2 effect is quite interesting. The mechanism of oxidation please confirm by references or verify it.
8.The presented chemical reaction should be proved or references. It is not an effect of carried out tests.
Research methodology, results and discussion must be clearly described and interrelated. Conclusions must result from the conducted research.
Author Response
-Numbering of capitals must be verified.
Response:Thanks for the reviewer's kind suggestion.Numbering of capitals has been verified.
-Line 82-86 – please use the full sentences.
Response:Thanks for the reviewer's kind suggestion.Lines 82-86 – complete sentences already used.
-Line 88-92 – please use full description of process, using full sentences. Moreover please explain:
- What for cleaned by acetone by 5 min? What happened with the surface alloy in this time?
- What for cleaning by water after acetone use?
- What for cleaning by alcohol by 15 min?
Response:Thanks for the reviewer's kind suggestion.Line 88-92 – complete sentences already used.Because acetone is a good organic solvent, both fat-soluble and water-soluble, and acetone has a low boiling point and is easy to volatilize, it is generally used to wash off the fat-soluble impurities and water-soluble impurities on the metal surface. After using acetone cleaning, use flowing water to clean off the residual acetone on the surface of the titanium alloy. After washing, the beaker containing titanium alloy and alcohol is placed in the ultrasonic cleaning machine for cleaning to completely remove the residual water on the surface of titanium alloy.
-Please use the chemical description not full names or if it is needed, use both (i.e. line 95-96).
Response:Thanks for the reviewer's kind suggestion.Chemical description and full names have been used.
-Please explain, why 10 min for aluminizing was used? Please add result for only aluminizing. All images are after 4 h in 1050 what explain the thickness and phases. Fig. 4 shows the relationship between thickness TiAl3 phase and time in min. In molten aluminum the thickness should be very small ca. 6 um (after 25-30 s). The thickness of this layer should increase in 1050 and it takes hours.
Response:Thanks for the reviewer's kind suggestion.In this paper, the structure and properties of Ti-Al alloy phase /Ti-Al carburizing composite coating prepared by hot dipping aluminum plating (760℃×10min) and then carburizing (1050℃×4h) were analyzed.Through comprehensive analysis of SEM, EDS, XRD and other detection methods, it is found that TiAl3 phase layer can be formed by hot dipping aluminum plating at 760℃ for 5-20 minutes, but the thickness is different, and the growth kinetics of TiAl3 phase layer is described in section 3.1.3. When the hot dipping aluminum plating time is less than 10 minutes, the thickness of the TiAl3 phase layer is small, which is not conducive to the formation of TiAl2, TiAl, Ti3Al with appropriate thickness in the subsequent carburizing process. However, the hot dipping plating time is too long, which is not suitable for industrial production and is more likely to cause the formation and expansion of micro-cracks in the TiAl3 phase layer. Therefore, we chose to analyze the structure and properties of the coating obtained by hot dipping aluminum plating for 10 minutes. By comprehensive SEM, EDS, XRD and other detection methods, it is found that three new Ti-Al alloy phase layers (Ti3Al, TiAl, TiAl2) can be formed by carburizing at 1050℃ for 2 to 4 hours, but their thicknesses are different, and the growth kinetics of Ti-Al alloy phase layers during carburizing is described in section 3.1.3. Carburizing 2-4 hours, Ti-Al carburizing layer contains Al2O3, TiAl2, C, TiC. When the carburizing time is less than 4h, the thickness of the Ti-Al carburizing layer and the three Ti-Al alloy phase layers (TiAl2, TiAl, Ti3Al) is small. When the carburizing time is too long, the surface brittleness of the coating increases, and the carburizing layer is easy to peel off. Therefore, we chose to analyze the structure and properties of the coating obtained by carburizing for 4 hours.Because the structure of the coating obtained by hot dipping aluminum for 10 minutes and carburizing for 4 hours is the best, we chose to analyze the structure and properties of the composite coating prepared by the process parameters. Microstructure photos of hot dip plating pure aluminum for 10min have been added to section 3.1.1. of this paper.
-Please explain the mechanism of diffusion (line 180). The phase formation is determined not by diffusional transport but by chemical kinetics.
Response:Thanks for the reviewer's kind suggestion.Phase formation is indeed determined by chemical kinetics, but the rate of phase formation can be divided into reaction control and diffusion control. When the kinetic index n <0.5, the growth is diffusion-controlled, and when n > 0.5, the growth is reaction-controlled.
-Line 358 – how oxygen can penetrate? Line 338 – how oxygen can enter? Especially the CO2 effect is quite interesting. The mechanism of oxidation please confirm by references or verify it.
Response:Thanks for the reviewer's kind suggestion.TiO2 generated by oxidation in the Ti-Al carburizing layer is loose in structure, and the generated CO2 can escape through TiO2 with loose structure, thus leaving a void in the layer. The generation of these defects will destroy the continuity of the Al2O3 phase layer and promote the further inward diffusion of oxygen.
-The presented chemical reaction should be proved or references. It is not an effect of carried out tests.
Response:Thanks for the reviewer's kind suggestion.The chemical reactions proposed in this paper have been verified or referenced. The above has been added to section 3.2.3.
Round 2
Reviewer 3 Report
The introduction is still written very poorly. Please, add citations to the statements "For example,Li, Z.W. et al..." (line 41) and "Wang, Z. et al prepared Ti-Al-Si gradient..." (line 45). Also, the results obtained by these authors should be described. Not only their experimental work. The advantage of the fabrication of the mentioned coatings is not clear to the readers. The same can be said about the ref. [20]. The novelty should be better highlighted. This part of the introduction should be supported by adding results of previous research based on hot-dip aluminizing and carburization.
The title of subparagraph 2.4. "Representation" seems not appropriate.
More details about the hardness measurements should be provided
The quality of Figure 11 is not appropriate and must be improved.
The load used for hardness measurements should be added to Figure 12.
The English language seems understandable but needs to be improved. Please, avoid sentences with more than two verbs. Also, some typo errors should be corrected.
The English language seems understandable but needs to be improved. Please, avoid sentences with more than two verbs. Also, some typo errors should be corrected.
Author Response
-The introduction is still written very poorly. Please, add citations to the statements "For example,Li, Z.W. et al..." (line 41) and "Wang, Z. et al prepared Ti-Al-Si gradient..." (line 45). Also, the results obtained by these authors should be described. Not only their experimental work. The advantage of the fabrication of the mentioned coatings is not clear to the readers. The same can be said about the ref. [20]. The novelty should be better highlighted. This part of the introduction should be supported by adding results of previous research based on hot-dip aluminizing and carburization.
Response: Thanks for the reviewer's kind suggestion. The advantages of the coatings prepared by these authors have been added to the introduction section. References [7][8][20] are the results of previous studies on hot dip aluminization and carburization of titanium alloys by some scholars, and these three references have been highlighted in this paper.
-The title of subparagraph 2.4. "Representation" seems not appropriate.
Response: Thanks for the reviewer's kind suggestion. The title of subparagraph 2.4. has been changed from "Representation" to "Characterization test".
-More details about the hardness measurements should be provided
Response: Thanks for the reviewer's kind suggestion. The sample was observed under the Vickers hardness tester microscope, measured for 3 times, and the macro hardness value was obtained by taking the average value. The penetrator type is diamond cone penetrator, the force is 3Kg, and the load holding time is 10 s. The above has been added to section 2.4. of the article.
-The quality of Figure 11 is not appropriate and must be improved.
Response: Thanks for the reviewer's kind suggestion. The quality of Figure 11 has been improved.
-The load used for hardness measurements should be added to Figure 12.
Response: Thanks for the reviewer's kind suggestion. The load for hardness measurement has been added to Figure 12.
-The English language seems understandable but needs to be improved. Please, avoid sentences with more than two verbs. Also, some typo errors should be corrected.
Response: Thanks for the reviewer's kind suggestion. Sentences with more than two verbs have been avoided. The typo has also been corrected.
Reviewer 4 Report
The article describes selected results concerning coatings after the hot dip aluminization and carburizing process. Analyzing the results, it can be seen that only selected results are included in the article, which are to indicate the formation of phases, and moreover, the formation of phases from gases (CO and O2). There is no pressure analysis or gas mixture analysis in the work that could give the effect of this interaction. In a previous review, the need to refer to publications on the proposed chemical reactions. They were only provided with the term "may be", which is insufficient for publications with research results.
On the other hand, the analysis used SEM-EDS as a method to reveal, among others, coal. Due to the specificity of the SEM-EDS test, coal cannot be identified by this method. Moreover, the results of the quantitative analysis were not included. What is the form of carbon in this layer? Is it possible at all for its occurrence and the formation of separations that can be analyzed using the methods used. How does carbon increase hardness - what is the mechanism of this strengthening? Moreover, if, according to the answer given, voids appear, do the voids cause an increase in hardness or a decrease in hardness?
The confirmation of presence of new phases needs some more experiments i.e. TEM observations.
In conclusion, the discussion should be a state-of-the-art discussion based on evidence, not conjecture.
Author Response
-The article describes selected results concerning coatings after the hot dip aluminization and carburizing process. Analyzing the results, it can be seen that only selected results are included in the article, which are to indicate the formation of phases, and moreover, the formation of phases from gases (CO and O2). There is no pressure analysis or gas mixture analysis in the work that could give the effect of this interaction. In a previous review, the need to refer to publications on the proposed chemical reactions. They were only provided with the term "may be", which is insufficient for publications with research results.
Response: Thanks for the reviewer's kind suggestion. After reviewing the literature on the principle of carburizing, we learned that CO must be produced during the generation of active carbon atoms in the carburizing process:
2C+O2=2CO
2CO=CO2+[C]
BaCO3=BaO+CO2
CO2+C=2CO
-On the other hand, the analysis used SEM-EDS as a method to reveal, among others, coal. Due to the specificity of the SEM-EDS test, coal cannot be identified by this method. Moreover, the results of the quantitative analysis were not included. What is the form of carbon in this layer? Is it possible at all for its occurrence and the formation of separations that can be analyzed using the methods used. How does carbon increase hardness - what is the mechanism of this strengthening? Moreover, if, according to the answer given, voids appear, do the voids cause an increase in hardness or a decrease in hardness?
Response: Thanks for the reviewer's kind suggestion. The distribution trend of C in the composite coating can be seen through the surface scanning results. The presence of free carbon and TiC in the composite coating was determined by XRD results. After 2 days of thermal exposure, Al4C3 and Ti2AlC were also detected in the composite coating. The hardness was measured at room temperature before thermal exposure test. At this time, the forms of carbon in the composite coating obtained by hot dipping aluminum plating and carburizing are free carbon and TiC. The reinforcement mechanism is the second phase reinforcement. No heat exposure test has been conducted, so Ti in the Ti-Al carburizing layer will not oxidize to form TiO2 with loose structure, nor will it generate CO2, so no void will be left in the layer.
-The confirmation of presence of new phases needs some more experiments i.e. TEM observations. -In conclusion, the discussion should be a state-of-the-art discussion based on evidence, not conjecture.
Response: Thanks for the reviewer's kind suggestion. The Ti-Al binary alloy phase of hot-dip aluminization of titanium alloys has been characterized by various literatures and can be confirmed only by SEM-EDS. Solid carburizing reaction products and high-temperature oxidation reactants are also book theoretical knowledge, and these phases only need to be characterized by XRD and SEM-EDS. All the phases described in this paper are analyzed by existing characterization methods. TEM is mainly used for the characterization of nanoscale solid precipitates. In the scope of this study, no further mechanism discussion has been involved. Therefore, we believe that the current characterization methods are sufficient to support the conclusions of this paper.
