A Study on the Effects of Liquid Phase Formation Temperature and the Content of Sintering Aids on the Sintering of Silicon Nitride Ceramics
Round 1
Reviewer 1 Report
The authors have studied the effects of liquid phase formation temperature and content of sintering aids on the sintering of silicon nitride ceramics. The research is well designed and presented clearly. A good comparative analysis of existing publications concerning the tasks set in the work is performed. The methodological section of the manuscript is presented in sufficient detail. Three sintering aids, namely Y2O3-Al2O3 (YA), Y2Si2O7 (Y2S), and Y2Si2O7-Al6Si2O13 (Y2SM), with homologous elements and different liquid phase formation temperatures were developed, which allowed formation of a liquid phase with SiO2 on the surface of Si3N4 at varying temperatures. The authors showed that excessive liquid phase can improve density and refine grain size but increases weight loss rate during sintering due to volatilization.
However, some shortcomings should be corrected to make the manuscript acceptable for publication in Crystals.
(1) In the title of the manuscript, it is suggested “… the Effects of …” in plural.
(2) In the heading of Table 1, it is advisable to replace “Mass fraction (wt%)” with “Fraction (wt%)”, as the unit is given in parentheses.
(3) In the caption of Figure 7, it is advisable to mention relative density, namely “Figure 7. The sintering volume shrinkage and relative density of Si3N4 ceramics at different temperatures.”.
(4) In Lines 349–354, to substantiate the corresponding statement, the authors should add the following sentence (before the Conclusions section). “This also allows implementing a barrier effect of elongated grains, which improves fracture toughness of the ceramics, as found in Si3N4–Y2O3 ceramics with the average grain size in the range of 0.3 to 0.5 μm [Reference].” The suggested reference is: https://doi.org/10.1007/BF02355530
Minor editing of English language is required.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript "Study on the effect of liquid phase formation temperature and content of sintering aids on the sintering of silicon nitride ceramics" studies the natural sintering of Si3N4 based materials with three different sintering aids. The influence of the nature of the additive on the sintering temperature, the densification level and the grain size is assessed. The work is interesting, however some information is lacking. Major revisions are required and are listed below.
Introduction
A lot of studies were performed in Europe and in the USA about the sintering of Si3N4 with the assessment of different sintering aids. However, the authors cite a few of them. Thus, the authors should improve the introduction and complete the list of reference.
Section 2 : Materials and methods
p2, l77: the authors write "trinary". Do they mean "ternary"?
Section 3: Results and discussion
p4, l146-151 : the authors describe the synthesis of Y2S sintering aids and explain the formation of Y2SiO5 at 1600°C. The peak identification is not complete in figure 1 (1600°C). Some peaks are not identified and do not correspond to Y2Si2O7. The authors should identify all the peaks on the XRD pattern of the synthesis at 1600°C. Moreover, the authors indicate that the amount of Y2Si2O5 is small. Can the authors quantify this amount using XRD refinement?
p6, fig 2 : reference is lacking
p7: the authors mention a weight loss of the samples depending on the nature of the additive. They explain this phenomenon as the volatilization of the sintering aids. However, to confirm this point, the authors should quantify the amount of the sinterings aids after sintering using XRD and compare with the initial amount. Moreover, XRD patterns of sintered samples should be added. In addition, Si3N4 can decompose at high temperature if no N2 pressure is applied (~10bar). Thus, XRD patterns and the sintering aids quantification are mandatory to confirm that the weight loss is only due to the volatilization of the additives.
p11, l310 : the authors write "Y2O3-Al2O3" instead of Y2Si2O7
p11: the authors should talk about the evolution of the Y2Si2O7 phase. Indeed, in p4 and 5 the authors described the formation of Y2SiO5 at a temperature of 1600°C. As sinterings are performed at 1500, 1600, 1700 and 1800°C, thus the formation of Y2SiO5 can occur. The authors should discuss about this point or show that this phase is not detected in the material. Thus, XRD patterns of sintered Si3N4 materials are necessary.
p11-12: How is measured the grain size? The grains are not equiaxe thus the diameter is not the best parameter to consider. Can the authors consider the aspect ratio of the Si3N4 grains?
Minor editing of English language required
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Reviewer 3 Report
Manuscript ID: crystals - 2486601
Dear Editor,
The introduction section is well represented and describes the background of the current ongoing research.
1-line 40: tzhe???it is a mistake. may be the.....
2-checmical acronyms in the reference should be corrected, just for example, Al2O3 into Al2O3 and ..etc.
3-line 80: the temperature range order either should be in descendant order (not common) or ascendant order ( common). Accordingly, rearrange references 23-25.
4-Why Focused Ion Beam (FIB) was not used in the current study?
5- Have the authors used FESEM or SEM, or FESEM attached with EDX (EDS)?
6-line 135: Xα
7-Figure 8 (d,h) Y2S20 !
8-wt.% instead of wt%
9-Why the generation temperatures and holding time for SN-YA, SN-Y2S and SN-Y2SM are different with those of described in lines 146-148?
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The authors have taken into account all the comments of the reviewer. Th paper can be published.
Some added sentences need corrections.
