Electron-Beam Sintering of Al2O3-Cr-Based Composites Using a Forevacuum Electron Source
, Ilya Bakeev 1, Anna Dolgova 1, Efim Oks 1,2, Van Tu Tran 1 and Aleksey Zenin 1
Victor Paperny
Round 1
Reviewer 1 Report
The paper describes a new technique for the synthesis of a ceramic coating containing a metal impurity, and describes some of the properties of such a coating. These materials are very promising for many applications, and this paper is of great interest to specialists in the relevant field. However, there are a number of comments to the papere and it cannot be published in its current form.
1) Figure 3 is not informative and needs to be redone. In particular, it is necessary to indicate fragments a, b, c; it is necessary to point the scale; it is necessary to clearly highlight the various components in the SEM image, especially on the left fragment
2) Why the Cr content for composite 25A in Fig. 4 very few? Need comments on this Figure.
3) What is the reason for the sharp violation of the stoichiometric composition of synthetic ceramics in relation to the initial composite, shown in Fig. 5? Why is there such a strong decrease in the Al content in the coating when using composite 50A instead of 75A? What is the main component of the coating in these cases and why is it so?
4) It is necessary to explain the physical reasons for the dependences observed in Figs. 7 and 8
Author Response
Response to Reviewer 1 Comments
Point 1: Figure 3 is not informative and needs to be redone. In particular, it is necessary to indicate fragments a, b, c; it is necessary to point the scale; it is necessary to clearly highlight the various components in the SEM image, especially on the left fragment.
Response 1: Figure 3 has been corrected according to your suggestions
Point 2: Why the Cr content for composite 25A in Fig. 4 very few? Need comments on this Figure.
Response 2: The chromium content for sample 25A is 32%. The increase in the Cr content corresponds to its addition as a percentage in the initial powder mixture for sintering.
Point 3: What is the reason for the sharp violation of the stoichiometric composition of synthetic ceramics in relation to the initial composite, shown in Fig. 5? Why is there such a strong decrease in the Al content in the coating when using composite 50A instead of 75A? What is the main component of the coating in these cases and why is it so?
Response 3: The main elements of the coating are the elements of the substrate – iron and oxygen. There was no goal here to get stoichiometric coverage.
The composition of the coating is determined by the temperature of the composite, the temperature of the substrate and ionization processes in the vacuum chamber and does not always correspond to the stoichiometry of the target
Point 4: It is necessary to explain the physical reasons for the dependences observed in Figs. 7 and 8
Response 4: Explanations are given on pages 7-9
Page7.
(1)
where
γ0 is an electrical conductivity of the conductor/dielectric, S/m;
γ0 is a temperature-independent coefficient determined by the properties of the conductor/dielectric, S/m;
k is Boltzmann’s constant, J/K;
Т is the temperature of the irradiated composite surface, K;
Eа is the conduction activation energy, eV;
Page 8.
As can be seen, the experimental points of the logarithm of the current from the inverse temperature fit into a linear dependence. On the one hand, this serves as an argument in favor of the chosen mechanism for increasing electrical conductivity with increasing temperature and the correctness of choosing formula 1 for theoretical estimates.
Page 9
The general pattern here is as follows: the thermal conductivity of ceramics of a crystalline structure, especially oxide, with an increase in temperature, as a rule, drops significantly [39 ]. This is based on the idea of heat transfer in solid non–metallic bodies by thermal elastic waves - phonons. The thermal conductivity of the composite is closely related to their microstructure and depends on the free path length of the phonons, the degree of disturbance of the harmonic oscillations of heat waves during their passage through a given substance. Phonons are also known to interact with lattice defects, grain boundaries, and other microstructure defects. The presence of a metallic phase in the form of chromium inclusions leads to a higher porosity value characteristic of composites and, as a result, negatively affect thermal conductivity. The resulting internal stresses in composites also lead to a decrease in thermal conductivity [40].
Author Response File: 
Author Response.docx
Reviewer 2 Report
The article entitled “ Electron-beam sintering of Al2O3-Cr based composites using a forevacuum electron source” by the authors A.S. Klimov, I.Yu. Bakeev, A.V. Dolgova, E.M. Oks, A.A. Zenin has been reviewed. The paper is focused on developing Cr-alumina composite materials by electron beam sintering.
The work contains interesting results on the effect of Cr-Al2O3 mixture composition on the prepared materials structure and properties. Despite the paper has a good Scientific Soundness, I think the "results and discussion" part should be expanded. In particular, there is a lack of comparison of the properties of the obtained materials with similar ones obtained by other methods, for example, by hot pressing [DOI: 10.2298/SOS0603231C]. Do the thermal conductivity and density values correlate with other known data?
In addition, I encourage authors to more clearly define the target parameters of materials in the context of specific applications. This will make it possible to draw clearer conclusions regarding the prospects of the chosen approach to obtaining composite materials.
Author Response
Response to Reviewer 2 Comments
Point 1: The work contains interesting results on the effect of Cr-Al2O3 mixture composition on the prepared materials structure and properties. Despite the paper has a good Scientific Soundness, I think the "results and discussion" part should be expanded. In particular, there is a lack of comparison of the properties of the obtained materials with similar ones obtained by other methods, for example, by hot pressing [DOI: 10.2298/SOS0603231C]. Do the thermal conductivity and density values correlate with other known data?
Response 1: We have added a comparison with the results of other researchers.
page 6
Compared with the sintering of similar composites using the hot pressing method [36], the porosity value in this work turned out to be higher. This difference may be due to the peculiarity of the electron beam method - sintering without applying pressure.
page 9
As shown in [37], the electrical conductivity of aluminum oxide-based composites can be adjusted when reinforced with conductive or semi-conductive phases (such as silicon carbide, for example) added in an amount at which they penetrate into an insulating aluminum oxide matrix. After sintering, such a composite can be used in many industries. The main factors affecting the electrical properties of composites with reinforced semiconductor phases are the volume fraction of SiC and the content of other impurities. The addition of SiC improves the electrical conductivity, which increases with an increase in the volume fraction of SiC [37]. Thus, in a composite with 20 vol.% SiC, the conductivity of 4.05×10-2 S·m-1 was measured, which is an increase of four orders of magnitude compared to the reference monolithic alumina (7.80×10-6 S·m-1).
Thermal conductivity is an important parameter in such applications of Al2O3 ceramics as high-temperature structural components, refractories, gas burners, wear parts and cutting tools. To reduce thermal shock, the thermal conductivity of the composite in all these applications should be as high as possible. It can be expected that Cr particles improve the thermal conductivity of Al2O3-based composites due to the inherent high thermal conductivity of Cr.
Page 10
The general pattern here is as follows: the thermal conductivity of ceramics of a crystalline structure, especially oxide, with an increase in temperature, as a rule, drops significantly [40]. This is based on the idea of heat transfer in solid non–metallic bodies by thermal elastic waves - phonons. The thermal conductivity of the composite is closely related to their microstructure and depends on the free path length of the phonons, the degree of disturbance of the harmonic oscillations of heat waves during their passage through a given substance. Phonons are also known to interact with lattice defects, grain boundaries, and other microstructure defects. The presence of a metallic phase in the form of chromium inclusions leads to a higher porosity value characteristic of composites and, as a result, negatively affect thermal conductivity. The resulting internal stresses in composites also lead to a decrease in thermal conductivity [41]. However, despite these negative factors, the thermal conductivity of the composite increases with an increase in the chromium content and is still higher than pure aluminum oxide.
The values of thermal conductivity measured at room temperature are somewhat lower than the data given in the literature [43] and measured for mono-cast Al2O3 (28-30 W/m K). A possible reason is the greater porosity of the materials obtained in this work.
Point 2: In addition, I encourage authors to more clearly define the target parameters of materials in the context of specific applications. This will make it possible to draw clearer conclusions regarding the prospects of the chosen approach to obtaining composite materials.
Response 2: The purpose of the work is to study the effect of electron beam irradiation on the parameters of composites after sintering. Further work will be aimed at investigating the effect of electron beam sintering modes on the properties of composites. The text of the work has been amended according to your suggestion
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
In the revised manuscript, the authors have made the necessary corrections and the manuscript can be published in its present form. However, in the explanations to formula (1), the authors used the same notation γ0 twice
