Yttria Ceria Co-Stabilized Zirconia Reinforced with Alumina and Strontium Hexaaluminate
Round 1
Reviewer 1 Report
This draft presents an interesting, well-designed and systematic study of feasibility on zircona based ceramic composites. The experimental part is very consistent, and comprises manufacturing as well as mechanical properties testing (bending strength, fracture toughness, hardness) and a low thermal degradation analysis by accelerated aging experiments. It is very clearly described, and the results are adequately shown and discussed.
However, minor changes are necessary to correct some errors and especially for the sake of clarity. Some of them are indicated below, and others have been hand-written on the draft, and have been scanned and enclosed. There are some underlined words with no comments, so do not worry about that.
Title: I would prefer the term “co-stabilized” instead of costabilized, but this is just a suggestion.
Abstract: I would indicate that the sintering has been carried out at different temperatures.
Page 2. Line 54. “…provided the microstructures are perfect” Please explain what do you mean by “perfect”, maybe fully dense??? Without pores? Homogeneous?
Page 2. Line 78. “…the lack of stabilizer in the Sr hexa-aluminate mixture was compensated by addition of yttria (see equation one)”. However, no yttria appears in equation one, so this reference to eq. 1 is misleading to me.
Page 3 line 121. “…Occurrence of intergranular alumina grains were also detected”. Do you mean that some alumina grains have grown around a zirconia grain which then remains embeded??? Please try to explain what you mean by intergranular in the text.
Same question in page 3 line 126.
Page 4, line 130. Figure 1 caption. Please indicate a b c and d on the pictures for better identification.
Figure 1. Please label the pictures a b c and d. (Additionally to existing labels)
Figure 2. Please label the micrographs as a b and c (additionally to existing labels)
Line 139. “… crack propagation alumina grains at junctions can lead cracks into the matrix (left).” I do not understand this sentence. Could you please explain it??
Page 4-5. Labels and arrows indicating the TZP facts mentioned in the text would be helpful, especially for fig. 2.
Page 7 line 196. “… assuming that the threshold toughness is in the same range as previously published for.... “
It seems to me that that the sentence is incomplete.
Page 7, line 208. “The time relevant for biomedical applications is t=10 h, which corresponds to 30-35 years in vivo….” This is not precisely correct. Recent papers (Gremillard et al., Acta Mater 2018) show how the correspondence between degradation at 134ºC and degradation in vivo, at human body temp. depends on the aging kinetic and a general correspondence cannot be established for all the materials.
Page 7 line 222. Reference to the figure 7 c and d should be made. “…Some types of materials” should be replaced by “some sintering temperatures” or similar.
Figure 7. Please remove the [-] symbols from the axis labels.
Page 8, line 234. “…Are able to provide more damage tolerance.... “ More than the matrix???? Please specify.
Page 8, line 235. “As Swain has shown in detail……”
The mentioned transition from defect size-controlled strength to transformation-controlled strength has been observed in the present study or this whole sentence makes reference to Swain’s findings?? It is not very clear.
Page 9 line 264. “…samples containing high fractions of SA6 show different ageing kinetics…” Different from the rest of samples in the study?
Page 9, line 273. “The toughening by crack deflection and bridging is less pronounced than the effect….” Can you specify for which composites???
Page 9, line 276. “In general... as the size of the matrix and its transformability gets coarser”. Maybe better “as the grain size of the matrix gets coarser and its transformability increases”.
Comments for author File: 
Comments.pdf
Author Response
The authors would like to express their gratitude for reviewer1 to provide such an in depth review and a manuscipt with annotations. In fact many revisions regarding ccontent and language were necessary.
The reply to reviewer comments in detail:
Title: was changed to co-stabilized (germans have a strange preference for composite words)
Abstract: comment to sintering temperatures was added
Page 2, line 54: was changed to: "provided that the microstructure is dense, defect-free and homogeneous"...
Page 2 line 78: the reference was removed, a comment on the amount of yttria per 100g of SrZrO3 was added.
Page 3 line 121: it seems that we used the wrong terminology, the sentence was re-written:
"Some intragranular alumina grains trapped inside the TZP matrix grains were detected"
page 3 line 126: was changed to "Zirconia grains inside SA6 precipitates are more frequently observed.."
page 4 line 130: was changed...
Figure 1 and 2 labels were appended as requested
Line 139: the whole paragraph was re-written after adding labels and arrows to figure 2a-c:
page 4-5...
"Fracture of alumina grains is not observed (Fig. 2a). Depending on the angle of incidence, the orientation of adjacent zirconia grains and grain boundaries either de-bonding (D) of the alumina/TZP boundary occurs (the most frequent effect), the crack is stopped (S) or enters adjacent zirconia grains causing transgranular fracture (T) (Fig. 2a). Moreover, cracks can be stopped in the tough matrix and be re-initiated in a distance of a few µm (RI). In the 15SA6 composites crack deflection at TZP/SA6 boundaries (D) is observed (Fig. 2b). SA6 grains as obstacles in the crack paths may moreover lead to more complex interaction (Fig. 2c). Here the crack first breaks through a TZP grain (T), is then deflected (D) for a short distance along the TZP/SA6 boundary. Then the crack breaks through the SA6 platelet (T) and exits along the SA6/SA6 lamella boundary (D) and is stopped in the TZP matrix. Approximately 2 µm away from this location the crack is re-initiated (RI) along a TZP/TZP grain boundary until it is finally terminated within a transgranularly fractured TZP grain (T)."
Page 7 line 196 was changed to:
"In a previously published paper the threshold toughness of a compositionally similar 1Y6Ce-10A-10SA6 composite was measured (KIO = 4.6 MPa√m)."
Page 7 line 208: " the proposed reference was added" the paragraph was re-written to make clearer that the statement is just a rule of the thumb:
"According to a recent publication by Wei and Gremillard the correlations between microstructure, sintering conditions composition and ageing kinetics are complex [33]. Data on Y-Ce-TZP are yet not available. Emanating from a simplistic a priori assumption we may guess that the time relevant for biomedical applications is ta = 10 h which corresponds to 30-35 years in vivo, a realistic upper lifetime limit for a biomedical implant."
page 7 line 222: was done...
Figure 7 [-] removed..
Page 8 line 234: was changed. "The zirconia matrix shows high transformability and thereby provides high fracture resistance.
Page 8 line 235: "As Swain has shown in detail for different zirconia materials a transition from defect size controlled strength to transformation controlled strength is observed above a certain level of R-curve derived fracture resistance.."
"In the present case the strength of the materials is definitely transformation limited as the R-curve dependent toughness of 6-7 MPa√m is above this critical level defined by Swain.."
Page 9 line 264: wasa changed to: "Samples containing 15 vol% SA6 tend to show different ageing characteristics than alumina-richer materials in terms of ageing velocity and ageing mechanism.
Page 9 line 273 and 276: the whole paragraph was re-written:
"Total toughness actually declined slightly by further increase of SA6 fraction. Transformation toughening was identified as the dominant toughing mechanism (ΔKICT ~4-5.5 MPa√m). The contribution of other non-TT effects such as crack deflection and bridging was calculated from the difference (ΔKICNTT ~1.5-2.5 MPa√m). Deflection and bridging effects were, however not exclusively observed for SA6 containing materials, they also occur in TZP/alumina materials. A possible explanation for the inefficiency of the pure in-situ platelet reinforcement may be found in a paper by Evans et al.. It is stated that in composites with randomly oriented reinforcement elements which show incomplete de-bonding it is the toughness and not the strength of the reinforcement elements which governs the toughness of the composites [37]. Occurrence of de-bonding depends on the interfacial strength, the fracture energy of the reinforcement and the elastic mismatch α of reinforcement and matrix. Alumina has a higher Young’s modulus than SA6 (400 GPa vs. ~290 GPa). This should favor de-bonding of alumina/TZP interfaces compared to SA6/TZP interfaces due to higher α. Chen stated that hexaaluminates have a low cohesive strength, so that it is very likely that the ratio between the fracture energy of the SA6/TZP interface Γi and the fracture energy of the SA6 Γf is higher than in case of an alumina/TZP combination [38].
page 9 line 276: was changed to "In general, SA6 addition slightly promoted LTD, probably indirectly as the size of the matrix and gets coarser and its transformability increases"
Reviewer 2 Report
Abstract
Line 19: 2 דshow” – i.e. “ …exhibit high …”?
Line 20: please indicate the hardness in the vickers units as in the main body of the publication – to my knowledge, indicating hardness in GPa is rather uncommon.
Materials and Methods
Material synthesis: Since the size of the SA6 phases and the distribution of ions are subject of discussion in the publication – were there any attempts to determine the particle size of the SrZrO3 after milling? Or differently put: how was the particle size distribution of the slurry / the components after milling?
Ageing investigations: how many samples were used for one data point (e.g. composition and ageing time identical)? Were there multiple measurements per sample, for example at different places?
Results
Line 130: In the legend of fig. 1, the second sample name seems to contain an “A” too much (15ASA6 --> 15SA6 ?)
Line 256: … amount of monoclinic phase in the as fired …
Low temperature ageing:
Since this is a very interesting point, I suggest to supplement additional data. In order not to overload fig. 6, possibly a second figure (6b) could be added, showing Vm vs. composition at a given ageing time (i.e. 10 h). If multiple measurements were evaluated per data point: is it possible / sensible to indicate error bars? Or would the error be smaller than 3 %, e.g. smaller than the symbols?
Fig. 7: Unfortunately, the many, partially crossing lines in the figures do not help the interpretability of the graphs. I suggest to only show the regression lines which were used for the “n1” evaluation while skipping the “n2” regression lines; the actual values for n1 (and n2 if desired) could be displayed in a separate table. Also, error bars would be useful – or at least an indication of the fitting quality (i.e. R2 value) of the regression lines; the latter information could be integrated in the aforementioned table as well.
Discussion
Line 250 ff: As “advocatus diaboli” it could be objected why the present synthesis pathway (all in one) was chosen instead of a two-step method or a wet-chemistry approach. Therefore, please state also some arguments in favour of your approach, i.e. possible avoidance of stabilizer depleted zones at the grain boundaries because of the newly formed ZrO2, simplicity, etc.
Line 261: Comment to sentence “Alumina seems to …”: Other authors have noticed this as well, if I recall correctly, it was explained through a reduction of water / hydroxide diffusivity along grain boundaries. A quick browse turned up the following publication, others surely exist: DOI 10.1016/j.actbio.2012.11.027
Author Response
The author would like to thank reviewer 2 for the thorough review, The review presented some interesting questions and recommendations which were introduced into the manuscript:
The reply in detail:
Material synthesis: particle size of the SrZrO3: we measured particle sizes of all dispersions during milling by laser granulometry, however we did not check the SrZrO3 separately, a comment is added in the text.
"Mean particle sizes after 2h milling were d50 = 0.46-0.48 µm in all cases (d10 ~ 0.2 µm, d90 ~ 1.1 µm)."
how many samples were used for one data point ?
see page 3 line 111: "(Autoclave test, 134°C, 3 bar saturated water vapor, exposition times 1-100 h, a fresh sample was used for each ageing time and composition) "
Figure 6: we really discussed about that before making the first submission, I then decided to show some selcted plots to visualize the spread, interesting idea the plot would be still be extremely confusing, so we decided to keep the figure as it was.
Figure7: the reviewer is right these multiple regressions are extemely cosusing, we left out the slope n2 for the high ageing times and just plot the slope n at the onset of ageing.
Concerning the quality of the regressions: I checked the regressions with 3 or 4 data points again an found that they all have R²-values of > 0.97 (of course for the slopes containing only 2 data points this makes no sense)
page 7 line 229f: "Therefore, only the first stage ageing behavior was linearly fitted to calculate the nucleation factor, the coefficients of determination for the linear fits containing 3-4 data points in fig. 7 are significant (R² > 0.97)."
Discussion:
synthesis route:
line 250:
Of course we already tried all the varieties and finally came back to this - at first sight - strange combination. Milling both stabilizers does not work as the attritor is unable to homogenize the yttria good enough, maybe with a milling process providing higher energy (this is something we will try in the future). Coating of both stabilizers is tricky as the ammonium cerium (IV) nitrate is so extremely oxidizing that the whole process must be done in water notin alcohol which leads to hard agglomeration. All these tests are not published so we decided to give just a short comment.
page 2 line 75: "The coating process was chosen to avoid inhomogeneous yttria distribution."
line 261: the proposed reference was added in the introduction:
page 2 line 52: "Alumina reduces the water/hydroxide diffusivity along grain boundaries [16]"
