Study of Phase Transformations in ZrO₂ Ceramics Stabilized by Y₂O₃ and Their Role in Changing Strength Characteristics and Heat Resistance

Review Report Form 1

The abstract is informative but could be more concise. Consider summarizing key findings more effectively.

The authors thank the reviewer for this comment; corrections have been made to the text of the article and the abstract has been corrected.

The results obtained reflect the determination of the possibility of the stability enhancement of the strength and thermal parameters of zirconium-containing ceramic materials via addition of a stabilizing additive in the form of Y₂O₃. According to the data obtained, a connection between phase transformations caused by the addition of the stabilizing Y₂O₃ additive and an increase in strength characteristics, the growth of which is also due to the formation of the dispersion hardening effect at the grain size reduction was established. It was found that the formation of the stabilized Zr(Y)O₂ phase results in thermal conductivity reduction, while the formation of the perovskite phase YZrO₃ has a positive effect on the thermophysical parameters of ceramics. It was determined that the change in phase composition associated with the formation of the tetragonal Zr(Y)O₂ phase at low dopant concentrations leads to an increase in strength characteristics, but a decrease in thermal conductivity, while the formation of the YZrO₃ phase in the structure contributes to a growth not only in strength, but also thermophysical parameters.

Some sentences are lengthy and complex. Breaking them down could enhance readability.

The authors thank the reviewer for this comment; corrections have been made to the text of the article.

Consider adding more specific keywords related to the experimental techniques used in the study.

The authors thank the reviewer for this comment; the keywords have been corrected.

ZrO₂ ceramics; stability enhancement of strength properties; polymorphic phase transformations; high-strength ceramics; changes in thermophysical properties

The introduction is well-written but could benefit from a clearer research gap statement. Highlighting the novelty more explicitly would be helpful.

The authors thank the reviewer for this comment. The text of the article has been corrected and the introduction has been expanded.

The novelty of the research consists in the determination of the optimal compositions of ZrO₂ ceramics with high strength characteristics, as well as thermophysical parameters that play a very important role in determination of the applicability of these ceramics as structural materials for nuclear reactors, in particular, materials for inert matrices of dispersed nuclear fuel. Enhancement of stability to external influences (mechanical loads, pressure, aggressive environments, thermal expansion), as well as maintenance of the stability of the crystalline structure by prevention of the effects of polymorphic transformations during the operation of ceramics, is one of the key tasks, the solution of which will make it possible to obtain high-strength ceramics used as inert matrices for dispersed nuclear fuel, the operation of which is accompanied by extreme conditions (high temperature and high doses of radiation).

Selection of ZrO₂ and Y₂O₃– The authors selected ZrO₂ and Y₂O₃ for stabilization and phase transformation studies. However, Al, Si, or Zn are commonly used to enhance thermal conductivity. Please clarify the specific advantages of ZrO₂-Y₂O₃ over these alternatives.

The authors thank the reviewer for this comment, below is the response, which supplemented the article regarding the possibilities of using other types of stabilizing additives.

The choice of Y₂O₃ as a stabilizing additive is primarily due to its high compatibility with zirconium dioxide, the combination of which allows for a significant reduction in the temperature of polymorphic transformations during the manufacturing process of ceramics, as well as an increase in the stability of the formed phase composition to subsequent thermal effects during operation. At the same time, consideration of the possibility of using such elements as Al, Si, or Zn or their oxides as stabilizing additives to improve the thermophysical parameters was excluded due to the fact that the combination of zirconium dioxide with aluminum or silicon oxides leads to the formation of a structure of the "massive matrix with inclusions in the form of ZrO₂ grains" type, which in turn does not allow growth of the strength properties due to the effect of dispersion hardening, and also does not allow restraining the processes of polymorphic transformations of ZrO₂ at high temperatures.

The SEM images are useful, but their discussion in the text could be expanded to highlight significant morphological differences.

The authors thank the reviewer for this comment; the following information describing the observed structural changes associated with morphological features has been added to the text of the article.

Figure 5 reveals the results of the change in the grain morphology of the studied ZrO₂ ceramics depending on the variation in the concentration of Y₂O₃ in the composition after thermal sintering at a temperature of 1200 °C. The SEM image data reflect the change in the shape of the grains and their agglomeration associated with phase transformations that occur during thermal sintering with different variations in the stabilizing additive concentration.

As can be seen from the data presented, in the case of thermal annealing of ZrO₂ ceramics without addition of a stabilizing dopant, the grain morphology is represented by agglomerates of large spherical particles. The addition of the stabilizing dopant Y₂O₃ leads to the formation of two types of particles: large spherical and a finely dispersed fraction surrounding large agglomerates. Such changes in grain shape, in particular, the formation of a finely dispersed fraction, are associated with phase changes caused by the addition of the stabilizing Y₂O₃ dopant to the composition of ceramics. It should be noted that an elevation in the concentration of the Y₂O₃ dopant leads to an increase in the proportion of the finely dispersed fraction in the composition of ceramics. Comparing the results of X-ray phase analysis (data on the weight contributions of the established phases) and the ratio of large and fine fractions in the composition of ceramics, it can be concluded that the formation of the fine fraction is associated with phase polymorphic transformations of the m-ZrO₂ → t-Zr(Y)O₂ type, arising due to a change in the concentration of the stabilizing dopant in the composition of ceramics. The observed growth in the fine fraction in the composition of ceramics with an increase in the concentration of the stabilizing Y₂O₃ dopant leads to the fact that larger agglomerates are completely covered with small grains, thereby creating the effect of “large core – fine shell” filling the intergranular space, and also creating a large number of grain boundaries that have a positive effect on hardening.

SEM Scale Bar– The scale bars in SEM images are not clearly mentioned. Ensure that the magnification and scale bars are properly labeled for each image.

The authors thank the reviewer for this comment; corrections have been made to the SEM images.

The methodology is described well, but specifying the error margins in measurements would strengthen the reproducibility of results.

The authors thank the reviewer for this comment; information on the methodology for measuring measurement errors has been added to the text of the article.

All experimental work related to the determination of strength and thermal parameters was carried out in several parallels in order to eliminate errors and inaccuracies in measurements, as well as to determine the magnitude of measurement errors, the magnitude of which amounted to no more than 1-2 % deviation from the measured values.

While some references are included, a more detailed comparison with similar studies would reinforce the significance of the findings, please make a table.

The authors thank the reviewer for this comment. The following table has been added to the text of the article.

Table 2 demonstrates the results of a comparative analysis of the strength characteristics of the studied ceramics with a number of works obtained by other authors, reflecting the possibilities of modification of ZrO₂ ceramics and the role of modification in hardening and enhancement of stability to external influences.

Table 2. Results of comparative analysis

The discussion of phase transformations could be improved with a deeper explanation of the thermodynamic aspects involved.

The authors thank the reviewer for this comment. The following information has been added to the text of the article, concerning the determination of the influence of thermodynamic aspects on the processes of phase transformations.

In this case, the formation of the t-Zr(Y)O₂ and YZrO₃ phases is caused not only by the effects of partial substitution, but also by the thermodynamic effects of polymorphic transformations m-ZrO₂ → t-Zr(Y)O₂. In this case, the addition of yttrium oxide to the composition of ZrO₂ ceramics leads to an increase in the number of oxygen vacancies due to dissociative substitution reactions of the type Y₂O₃→2Y_Zr′+V_O ^●●+3O_O^×, which in turn affects the entropy due to the exothermic contribution of vacancies, which leads to a decrease in the temperature at which polymorphic transformation reactions occur (in the case of unstabilized m-ZrO₂, the polymorphic transformation into t-ZrO₂ occurs at a temperature of 1450 K). Also, the reduction in grain size caused by mechanical grinding of samples, observed at addition of Y₂O₃ to the composition, also contributes to a shift in thermodynamic equilibrium, which leads to an acceleration of the processes of polymorphic transformations.

Some figures lack detailed explanations of axes and units. Ensure clarity in all graphs for better interpretation.

The authors thank the reviewer for this comment; corrections have been made to the text of the article.

The transition between different subsections could be smoother. Consider using subheadings for better organization.

The authors thank the reviewer for this comment, however the introduction of subheadings into the article is left to the discretion of the editors.

More quantitative discussion on hardness and flexural strength variations could enhance the results' impact.

The authors thank the reviewer for this comment; corrections have been made to the text of the article, and the description of the observed hardening effects has been supplemented.

It should be noted that the change in the phase composition of ceramics due to the formation of inclusions in the form of t-Zr(Y)O₂ and YZrO₃, which is also accompanied by a change in grain size, leads to a growth in resistance to external influences. In this case, the observed hardening effects (increase in hardness and bending strength), as well as an increase in resistance to softening processes caused by thermal shock exposure, indicate a positive effect of phase changes. The observed hardening effects in this case can be explained by the effects of dispersion hardening associated with an increase in the proportion of finely dispersed fraction in the samples, creating a large number of grain boundaries that prevent the propagation of cracks under external influences, and also enhance resistance to oxidation processes under thermal influences.

The manuscript presents optical characterization, but the optical band gap values for all samples are not provided. Please calculate and discuss the band gap variation using Tauc’s plot or another suitable method.

The authors thank the reviewer for this comment, however, in this paper the optical properties are presented in order to reflect the effects associated with the formation of oxygen vacancies in the structure, as well as the overall transmittance.

In the future, we will definitely take this remark into account and conduct a detailed study of the effects associated with changes in the width of the band gap in the samples and the role of phase transformations in these changes.

The method used to determine thermal conductivity should be explained in more detail. Please specify the exact parameters measured, the experimental conditions, and any assumptions made during calculations.

The authors thank the reviewer for this comment; a description of the thermal conductivity measurements has been added to the text of the article.

The measurements were carried out on samples in the form of tablets with a diameter of 10 mm and a thickness of about 1 mm, placed on a heating element with a heating temperature recorded using a thermocouple. A thermocouple was also placed on the back side of the sample, allowing the temperature of the sample to be recorded from the back side. Based on the temperature difference measurements, the thermal conductivity coefficient was determined taking into account the size of the sample, its density and the type of material.

The explanation of thermal conductivity trends could be strengthened with additional theoretical background.

The authors thank the reviewer for this comment, the correction has been made to the text of the article, and the description of the observed effects of increased thermal conductivity has been expanded.

The presence of the perovskite phase YZrO₃, which has higher thermal conductivity, also contributes to the change in thermophysical parameters, which leads to a positive effect not only in hardening of ceramics, but also in increasing thermal conductivity, which eliminates the effect of local overheating. In this case, the growth of thermophysical parameters with an increase in the contribution of the perovskite YZrO₃ phase in the composition of ceramics is due to the higher thermal conductivity of this phase compared to the monoclinic phase ZrO₂ and t-Zr(Y)O₂, the displacement of which leads to an increase in the heat transfer rate. Thus, by variation of the composition of ceramics, it is possible to control not only the strength characteristics, but also the thermophysical parameters, which play an important role in determination of the applicability potential of ceramics.

The conclusion effectively summarizes findings, but consider briefly mentioning possible applications and future research directions.

The study provides valuable insights, but a brief discussion on potential real-world applications and the scalability of this material for industrial use would improve the impact of the work.

The authors thank the reviewer for this comment; the conclusion has been expanded.

The obtained results can be further used to develop the technology of industrial production and to scale up the technology of manufacture of ceramics and ceramic powder used as structural materials for creation of inert matrices of dispersed nuclear fuel. In the future, research will be continued in the field of study of the resistance of synthesized ceramics to radiation damage, as well as the possibility of containing it due to phase changes associated with the addition of stabilizing dopants to the composition of ceramics.

Ensure all references are formatted consistently according to the journal's guidelines.

The authors thank the reviewer for this comment; corrections have been made to the text of the article.

A few minor typographical errors are present. A final proofreading pass is recommended.

The authors thank the reviewer for this comment; corrections have been made to the text of the article.

Review Report Form 2

1) In introduction, the motivation and important (difference) of this work should be further presented.

The authors thank the reviewer for this comment; the introduction has been expanded.

The novelty of the research consists in the determination of the optimal compositions of ZrO₂ ceramics with high strength characteristics, as well as thermophysical parameters that play a very important role in determination of the applicability of these ceramics as structural materials for nuclear reactors, in particular, materials for inert matrices of dispersed nuclear fuel. Enhancement of stability to external influences (mechanical loads, pressure, aggressive environments, thermal expansion), as well as maintenance of the stability of the crystalline structure by prevention of the effects of polymorphic transformations during the operation of ceramics, is one of the key tasks, the solution of which will make it possible to obtain high-strength ceramics used as inert matrices for dispersed nuclear fuel, the operation of which is accompanied by extreme conditions (high temperature and high doses of radiation).

  2) Line 71 what does “resistance” refer to?

The authors thank the reviewer for this comment; corrections have been made to the text of the article.

After a series of similar tests (10 consecutive tests), the values ​​of hardness and bending strength were measured, which made it possible to determine the influence of such temperature effects on the change in the strength properties of ceramics.

  3) Figs. 1 and 2 should be moved into section 3.

The authors thank the reviewer for this comment; these figures have been moved.

  4) The format of tables should be revised as three-line table.

The authors thank the reviewer for this comment; corrections have been made to the table.

  5) How is the testing accuracy of the lattice parameters? If possible, add the deviation value of the calculation.

The authors thank the reviewer for this comment; the error is given in the table.

  6) Horizontal lines on words should be deleted in figures.

The authors thank the reviewer; the figures have been corrected.

  7) Raman spectroscopy is different with XRD, so the bond type instead of phase should be marked in Fig. 3.

The authors thank the reviewer for this comment; corrections have been made to Figure 3.

  8) How to determine the distribution of composition and phase shown in Fig. 6?

The authors thank the reviewer for this comment; corrections have been made to the text of the article.

The calculation of inclusions of the phases formed in the composition of ceramics in the case of addition of Y₂O₃ to them was performed using the standard method of estimation of weight contributions. This method includes determination of the areas of all reflections characteristic of the established main and impurity phases with subsequent calculation of their contribution to the diffraction pattern or Raman spectrum. This method allows quite accurate determination of the change in phase composition in samples in the case of the formation of impurity inclusions, the content of which exceeds 0.1 wt.%.

  9) Why did authors cite the image of reference in Fig. 7?

In this case, the observed effect was established in the work [30], in which the authors showed the relationship between grain sizes and phase composition of ceramic samples.

10) Please explain the mechanism of action of oxygen vacancies.

In this case, the addition of yttrium oxide to the composition of ZrO₂ ceramics leads to an increase in the number of oxygen vacancies due to dissociative substitution reactions of the type Y₂O₃→2Y_Zr′+V_O ^●●+3O_O^×, which in turn affects the entropy due to the exothermic contribution of vacancies, which leads to a decrease in the temperature at which polymorphic transformation reactions occur (in the case of unstabilized m-ZrO₂, the polymorphic transformation into t-ZrO₂ occurs at a temperature of 1450 K). Also, the reduction in grain size caused by mechanical grinding of samples, observed at addition of Y₂O₃ to the composition, also contributes to a shift in thermodynamic equilibrium, which leads to an acceleration of the processes of polymorphic transformations.

11) How to measure and calculate the parameter of resistance to cracking in Figs. 9 and 11?

The crack resistance parameters were calculated by comparing the bending strength values for the samples in the initial state and after thermal shock tests, which made it possible to determine the effect of changes in the phase composition of ceramics on the crack resistance during ceramic degradation.

Review Report Form 3

1. Abstract – This section does not match the article's overall quality. It should clearly present the context, objectives, methods, and results concisely.

2. Abstract – The current text lacks quantitative values from the findings. A revision including key data is recommended for greater impact.

The authors thank the reviewer for this comment, the correction has been made to the text of the article, and the abstract of the work has been revised taking into account the comments of the two reviewers.

3. Introduction – The section is overly lengthy relative to the number of references (only 16). A broader literature review, particularly recent studies, is needed to better support the claims.

The authors thank the reviewer for this comment, corrections have been made to the text of the article, the number of references has been expanded, and the introduction has been expanded.

4. Materials and Methods – The description is confusing. Adding a flowchart summarizing the synthesis process would significantly improve clarity.

The authors thank the reviewer for this comment; the correction has been made to the description of the methods, and some of the results have been moved to the appropriate section.

5. Methods and Results – From line 116 onward, some content pertains to results rather than methodology. This should be revised to retain only synthesis and experimental procedures.

The authors thank the reviewer for this comment, corrections have been made to the text of the article, and the results have been transferred to the appropriate section.

6. Figure 2 – To confirm the identified phases, the authors should provide Rietveld refinement plots derived from experimental diffractograms.

The phase composition was refined using weight contributions; the Rietveld method was not used in this case. The method for determination of weight contributions is described in the article.

7. Figure Standardization – Figures 9 and 10 are not formatted consistently with the others. Adjustments are needed to ensure uniformity.

The authors thank the reviewer for this comment; corrections have been made.

8. References – Such a well-developed article deserves a stronger bibliographic foundation. Only 20 references are insufficient; incorporating more recent studies would reinforce the theoretical framework.

The authors thank the reviewer for this comment; corrections have been made to the text of the article and the number of references has been expanded.

Review Report Form 4

1 The introduction should describe in detail the phase transformations of zirconium oxide and the features of polymorphs (crystal structure, properties, thermal stability).

The authors thank the reviewer for this comment; corrections have been made to the text of the article.

As is known, one of the key features of ZrO₂ is the polymorphic transformations that occur under thermal influence, which consist of the transformation of the monoclinic phase into a tetragonal one at temperatures above 1450 K, from tetragonal to cubic at temperatures of about 2600 – 2700 K. Moreover, these polymorphic transformations are usually accompanied by a change in the volume of ceramics, which leads to an alteration in their strength properties due to the metastability of the tetragonal phase and the instability of the high-temperature cubic phase, which can result in accelerated destruction processes in the case of prolonged exposure to high temperatures. One of the ways to maintain the stability of the phase composition of ZrO₂ ceramics when used in extreme conditions is to stabilize their crystal structure during the manufacturing process by addition of stabilizing additives to the composition in the form of Y₂O₃, MgO, Al₂O₃, etc. The use of stabilizing additives allows acceleration of the processes of polymorphic transformations during the synthesis process, which results in sintering temperature reduction, and allows elevation of resistance to external influences due to substitution effects.

2 The scale of the images is not discernible in Figure 1.

The authors have made corrections to the presentation of the figures, the scale has been corrected.

3 The authors use an unconventional way of expressing the amount of dopants - concentration in M units. It is necessary to explain what this is in the understanding of the authors.

The authors thank the reviewer for this comment; corrections have been made to the text of the article. In this case, the ratio of the main component (ZrO₂) and the dopant (Y₂O₃) was taken in a molar ratio normalized to 1, i.e. in the case of using 0.05 M Y₂O₃, the concentration of ZrO₂ was 0.95 M. The choice of mole fraction representation is determined by the standard method for synthesis of such ceramics.

4 In Figure 3, the type and symmetry of oscillations should be given.

The authors thank the reviewer for this comment; corrections have been made to the text of the article.

5 It is unclear how the authors of the article calculate the content of impurities. This calculation should be demonstrated using one composition as an example.

The authors thank the reviewer for this comment; corrections have been made to the text of the article.

The calculation of inclusions of the phases formed in the composition of ceramics in the case of addition of Y₂O₃ to them was performed using the standard method of estimation of weight contributions. This method includes determination of the areas of all reflections characteristic of the established main and impurity phases with subsequent calculation of their contribution to the diffraction pattern or Raman spectrum. This method allows quite accurate determination of the change in phase composition in samples in the case of the formation of impurity inclusions, the content of which exceeds 0.1 wt.%.

6 Why should yttrium replace zirconium?

The authors thank the reviewer for this comment; corrections have been made to the text of the article.

The choice of Y₂O₃ as a stabilizing additive is primarily due to its high compatibility with zirconium dioxide, the combination of which allows for a significant reduction in the temperature of polymorphic transformations during the manufacturing process of ceramics, as well as an increase in the stability of the formed phase composition to subsequent thermal effects during operation. Also, due to its structural features, the substitution of zirconium by yttrium results in minimal distortions of the crystal lattice of the tetragonal phase due to the small difference in ionic radii (Y³⁺  ~ 0.9 Å, Zr⁴⁺ ~ 0.79 Å). At the same time, at maintenance of electroneutrality, the substitution of zirconium by yttrium leads to the formation of oxygen vacancies, the formation of which leads to a decrease in the deformation elastic distortions of the tetragonal phase. Also, the tendency to a variable valence state of yttrium ions allows minimization of the contribution of electronic defects in the structure.