Study on Damage of 4H-SiC Single Crystal through Indentation and Scratch Testing in Micro–Nano Scales

Round 1

Reviewer 1 Report

The ratio c/a is used, but not introduced in the abstract. It is first introduced in chapter 2.3.

Pay attention to correct cross references. Figure 2 is present twice, whereas figure 3 does not exist. In Chapter 5.3, Figure 6 is referenced, which shows a schematic comparison of the scratch test and does not show peeling and crack bifurcation, as can be read in the text.

The overall image quality is too poor. In figure 12, the labeled features are often not tracable because of the blurred photos. Make sure the labels are clearly legible.

Statistical data should be added in Table 3 (number indentations, standard deviations ...)

Line 193: "There are pop-in events in all of loading curve, however, the unloading curve is relatively smooth except the last indentation which occurs the peeling under a greater load." Is there any interpretation possible?

Fig. 10, fig. 12: The direction of the arrows is unusual. They should point to the cracks and not vice versa.

Your sentences are often too long and suffer from word confusion.

Author Response

Response to Reviewer 1 Comments

 

1. The ratio c/a is used, but not introduced in the abstract. It is first introduced in chapter 2.3.

 

Thanks for the suggestions. We explained the radio c/a in detail in line 13-14.

2. Pay attention to correct cross references. Figure 2 is present twice, whereas figure 3 does not exist. In Chapter 5.3, Figure 6 is referenced, which shows a schematic comparison of the scratch test and does not show peeling and crack bifurcation, as can be read in the text.

We are sorry about the confusion caused by our negligence. We have revised the correct figure number in line 113, 203 and 226.

 

3. The overall image quality is too poor. In figure 12, the labeled features are often not tracable because of the blurred photos. Make sure the labels are clearly legible.

Thanks for the suggestions. In order to show the details more clearly, we increased the resolution of Figure 12 from 400 to 600 dpi.

 

4. Statistical data should be added in Table 3 (number indentations, standard deviations ...)

Thanks for the suggestions. We added the standard deviation of c in Table 3. With the increase of c, the standard deviation of c also increases.

 

5. Line 193: "There are pop-in events in all of loading curve, however, the unloading curve is relatively smooth except the last indentation which occurs the peeling under a greater load." Is there any interpretation possible?

We added the explanation of ‘pop-in events’ in line 197: sudden depth burst occurred in the load-displacement curves. We do not have a very good theoretical explanation for this phenomenon, and we will pay attention to this aspect of research in the future .

 

6. Fig. 10, fig. 12: The direction of the arrows is unusual. They should point to the cracks and not vice versa.

Thanks for the suggestions. We revised the direction of the arrows in Figure 10 and 12.

 

7. Your sentences are often too long and suffer from word confusion.

Thanks for the suggestions. In order to eliminate the confusion caused by writing, we used the MDPI English editing services.

 

Author Response File: Author Response.docx

Reviewer 2 Report

The authors in this manuscript have performed indentation and scratch tests on 4H-SiC single crystals samples and reported data including force-depth curves, dimensions of cracks and characterization using SEM. However it is unclear what original contribution the authors are making. All the tests seems very standard. Moreover, the determination of KIC from the test data seems to be quite unconvincing as the trend of KIc vs c/a in Figure 9b is varying over a wide range. The only other possibly original contribution seems to be the determination of a ‘modified’ normal force-scratch depth but hardly any details are given, especially why the depth of the elastic recovery is chosen to be what it is. Only once reference is given. Other comments and questions are listed below:

1. In the abstract, what do the authors mean by elastic recovery? Abstract also contains others terms such as c/a ratio and micro-nano machining which are not explained. The reviewer suggests to remove these from the abstract or explain.
2. Page 1, line 32: “properties of SiC determine the difficulty to manufacture them that it is easy to occur microcracks and micro damages in the process such as grinding and wiresaw machining.” – correct grammar and English.
3. In section 2.2, the authors mention that (c) is the subsurface cracking stage – however there also exist radial cracks which are not exactly subsurface correct?
4. In figure 2, the damage size ‘s’ which is marked in red is not visible clearly.
5. Page 4, line 103, “Numerous studies about indentation…” Split up this sentence into 2 or more and check grammar and English usage. This sentence is too long right now.
6. Page 4, line 114, “An experiential method..” It is not clear what the authors mean by an ‘experential method’.
7. Page 4, line 117, “when the rate, c/a..” Is this a typo, do the authors mean ‘ratio’?
8. Page 4, line 126, “The fracture toughness of Palmqvist crack..” changing the writing here. Fracture toughness measure is not ‘of the Palmqvist crack”.
9. If the parameter x_v is material dependent, please remove the text in the brackets on page 4, line 128 because it makes it seem like x_v is a simple constant for the Berkovich indenter.
10. Page 7, line 195: “last indentation which occurs the peeling under a greater load” check English usage and grammar.
11. Page 7, line 194, please explain in more detail ‘pop-in’ events.
12. In section 4.2, what are lateral force and normal depth and how are they measured?
13. Page 8, line 204, “curve begin to appear jagged fluctuations” “as the continuation of scratch” correct grammar.
14. Page 9, line 212, “As expected from Eq. (5), the approximately linear relationship (R2=0.99) between normal force and c3/2 can be observed for 4H-SiC single crystal shown in Figure 8(a)”, the figure reference appears to be wrong. The equation reference appears to be wrong as well. Equation 5 which the authors reference is the stress intensity factor from indentation with an assumption of a half penny crack.
15. Figure 9b appears to have a typo in equation numbers as well, please correct.
16. Page 9, line 218, please explain what ‘nano-grooves’ here mean?
17. Page 9, line 223, “the peeling and crack bifurcation are observed as shown in Figure 6”. The authors appear to have either referenced the wrong figure number or a figure is missing. Either way, they are suggested to add this figure in figure 10 for comparison.
18. Page 9, line 224, “This leads to the termination of the crack effective extension” The authors need to explain in detail. Without a figure or explanation, this does not mean anything.
19. Page 9, line 225, “From the above, an appropriate value of c/a ratio is significant for fracture toughness calculation of 4H-SiC single crystal.” Rewrite this sentence.
20. Page 9, line 225, “It is seen from Figure 9 that a value of c/a ratio, 2.2, used to determine the crack systems for 4H-SiC single crystal seems to be more reasonable.” Why is a c/a ratio of 2.2 chosen? Why not 2.0 or 2.5? Please provide clear reasoning.
21. Page 9, line 227, “The fracture toughness of 4H-SiC single crystal on (0001) face is 3.37 MPa•m1/2 via Eq. (5) and 3.17 MPa•m1/2 via Eq. (6).” These numbers don’t seem correct looking at Figure 9b. One seems to be around 2.25 MPa.m0.5 and the other seems to be around 3.75 MPa.m0.5. Moreover, the values of KIC seem to change drastically between c/a ratios of 1.75 and 3.0
22. Page 10, line 253, “Meanwhile, it is found that there are numerous micro cracks perpendicular to the scratching direction within the groove, because the residual stress where the indenter impacts on the material subsurface makes the median crack extension to the groove surface, thus forming the micro crack.” Please re-write this, it is difficult to understand.
23. Page 10, line 262, “with the sustaining aggrandize of normal force” please rewrite, this is not clear.
24. It is not clear what Figure 13 shows. What ‘edge’ is seen here? Are these from the same scratch experiment? What force values are given here?

Author Response

Response to Reviewer 2 Comments

 

 

1. In the abstract, what do the authors mean by elastic recovery? Abstract also contains others terms such as c/a ratio and micro-nano machining which are not explained. The reviewer suggests to remove these from the abstract or explain.

Thanks for the suggestions. We give a further explanation of elastic recovery in page 1, line 23. We explained the radio c/a in detail in page 1, line 13-14. We used the ultra precision instead of micro-nano.

 

2. Page 1, line 32: “properties of SiC determine the difficulty to manufacture them that it is easy to occur microcracks and micro damages in the process such as grinding and wiresaw machining.” – correct grammar and English.

We are sorry about the confusion caused by our negligence. We rewrite it: Machining precision of these parts by SiC is often required to achieve a micrometer or nanometer level, but the hard and brittle properties of SiC make it difficult to manufacture them due to easily occurring microcracks and microdamages during processes such as grinding and wiresaw machining.

In order to eliminate the confusion caused by writing, we used the MDPI English editing services.

 

3. In section 2.2, the authors mention that (c) is the subsurface cracking stage – however there also exist radial cracks which are not exactly subsurface correct?

We are sorry about the confusion caused by our negligence. We modified the Figure 2, as shown below.

4. In figure 2, the damage size ‘s’ which is marked in red is not visible clearly.

Thanks for the suggestions. We marked the damage size ‘s’ in a conspicuous place, shown in the figure above.

 

5. Page 4, line 103, “Numerous studies about indentation…” Split up this sentence into 2 or more and check grammar and English usage. This sentence is too long right now.

We are sorry about the confusion caused by our negligence. We rewrite it: Numerous studies regarding the indentation method were carried out over the past half-century, starting with Palmqvist initially putting forward the potential values of indentation-induced cracking to characterize the toughness of brittle materials [44] due to its advantages such as simple operation, prepared sample facilities, and no sample shape or size requirements.

 

6. Page 4, line 114, “An experiential method..” It is not clear what the authors mean by an ‘experential method’.

Thanks for the suggestions. We deleted ‘experiential’.

 

7. Page 4, line 117, “when the rate, c/a..” Is this a typo, do the authors mean ‘ratio’?

We are sorry about the confusion caused by our negligence. It is ‘ratio’ and we revised it.

 

8. Page 4, line 126, “The fracture toughness of Palmqvist crack..” changing the writing here. Fracture toughness measure is not ‘of the Palmqvist crack”.

Thanks for the suggestions. We add ‘calculation formula’ in page 5, line 130.

 

9. If the parameter x_v is material dependent, please remove the text in the brackets on page 4, line 128 because it makes it seem like x_v is a simple constant for the Berkovich indenter.

Thanks for the suggestions. We remove the sentence ‘The parameters of α in equation (5) and xv in equation (6) are determined through experiment, therefore, the values of α and xv corresponding to different materials may be different’.

10. Page 7, line 195: “last indentation which occurs the peeling under a greater load” check English usage and grammar.

We are sorry about the confusion caused by our negligence. We rewrite it: except for the last indentation where peeling occurred under a greater load.

 

11. Page 7, line 194, please explain in more detail ‘pop-in’ events.

Thanks for the suggestions. We added the explanation of ‘pop-in events’ in page 7 line 205-206: sudden depth bursts in the load-displacement curves.

 

12. In section 4.2, what are lateral force and normal depth and how are they measured?

Thanks for the suggestions. The lateral force is the force collected in scratch direction, and the normal depth is the depth of penetration of indenter into sample. They all are collected by the sensor of TI 950.

 

13. Page 8, line 204, “curve begin to appear jagged fluctuations” “as the continuation of scratch” correct grammar.

We are sorry about the confusion caused by our negligence. We rewrite it: When the peelings and debris were observed in the sample surface, both the lateral force versus lateral displacement curve and the normal depth versus lateral displacement curve began to show jagged fluctuations, which were more severe as the scratch continued

 

14. Page 9, line 212, “As expected from Eq. (5), the approximately linear relationship (R2=0.99) between normal force and c3/2 can be observed for 4H-SiC single crystal shown in Figure 8(a)”, the figure reference appears to be wrong. The equation reference appears to be wrong as well. Equation 5 which the authors reference is the stress intensity factor from indentation with an assumption of a half penny crack.

Thanks for the suggestions. We deleted the sentence of relationship between normal force and c3/2. Accordingly, we deleted Figure 8(a). This has no effect on the structure of the article. We revised the equation number in Figure 9.

 

15. Figure 9b appears to have a typo in equation numbers as well, please correct.

We are sorry about the confusion caused by our negligence. We revised the equation number in Figure 9

 

16. Page 9, line 218, please explain what ‘nano-grooves’ here mean?

Thanks for the suggestions. The nano-grooves are left on the sample surface after the CMP (chemical mechanical polishing) treatment. That is to say, the nano-grooves existed before the scratch test.

 

17. Page 9, line 223, “the peeling and crack bifurcation are observed as shown in Figure 6”. The authors appear to have either referenced the wrong figure number or a figure is missing. Either way, they are suggested to add this figure in figure 10 for comparison.

We are sorry about the confusion caused by our negligence. We revised the Figure number (Figure 7).

 

18. Page 9, line 224, “This leads to the termination of the crack effective extension” The authors need to explain in detail. Without a figure or explanation, this does not mean anything.

Thanks for the suggestions. We combined the two sentences into one: When the value of c/a ratio is large (such as more than 2.9), the peeling and crack bifurcation are observed as shown in Figure 7. which lead to the termination of the crack effective extension.

 

19. Page 9, line 225, “From the above, an appropriate value of c/a ratio is significant for fracture toughness calculation of 4H-SiC single crystal.” Rewrite this sentence.

We are sorry about the confusion caused by our negligence. We rewrite it: From the above, an appropriate value of c/a ratio is significant to calculate the fracture toughness of 4H-SiC single crystals.

 

20. Page 9, line 225, “It is seen from Figure 9 that a value of c/a ratio, 2.2, used to determine the crack systems for 4H-SiC single crystal seems to be more reasonable.” Why is a c/a ratio of 2.2 chosen? Why not 2.0 or 2.5? Please provide clear reasoning.

Thanks for the suggestions. We added the comparison results of different c/a ratio, shown in Table 4. Table 4 shows that a c/a ratio of 2, where the results of two calculations are the closest, used to determine the crack system of a 4H-SiC single crystal was reasonable.

Table 4. The fracture toughness of SiC according to different c/a ratios.

c/a ratio	KIC-Eq(5) (MPa•m1/2)	KIC –Eq(6) (MPa•m1/2)
2.0	3.42	3.47
2.1	3.28	3.43
2.2	3.17	3.37
2.3	3.04	3.35
2.4	2.94	3.3
2.5	2.83	3.27

 

21. Page 9, line 227, “The fracture toughness of 4H-SiC single crystal on (0001) face is 3.37 MPa•m1/2 via Eq. (5) and 3.17 MPa•m1/2 via Eq. (6).” These numbers don’t seem correct looking at Figure 9b. One seems to be around 2.25 MPa.m0.5 and the other seems to be around 3.75 MPa.m0.5. Moreover, the values of KIC seem to change drastically between c/a ratios of 1.75 and 3.0

We are sorry about the confusion caused by our negligence. An explanation is added after Eq.(5), ‘where the c/a ratio is less than 2’, and another explanation is added after Eq.(6), ‘where the c/a ratio is more than 2’. We rewrite it: The fracture toughness of a 4H-SiC single crystal on the (0001) face was shown to be 3.42 MPa•m1/2 via employment of Eq. (5) (where the c/a ratio is less than 2) and 3.47 MPa•m1/2 via employment of Eq. (6) (where the c/a ratio is more than 2).

 

22. Page 10, line 253, “Meanwhile, it is found that there are numerous micro cracks perpendicular to the scratching direction within the groove, because the residual stress where the indenter impacts on the material subsurface makes the median crack extension to the groove surface, thus forming the micro crack.” Please re-write this, it is difficult to understand.

We are sorry about the confusion caused by our negligence. We rewrite it: Meanwhile, numerous microcracks perpendicular to the scratching direction were found within the groove, because the residual stress where the indenter impacted on the material subsurface caused median crack extension to the groove surface, thus forming the microcrack.

 

23. Page 10, line 262, “with the sustaining aggrandize of normal force” please rewrite, this is not clear.

We are sorry about the confusion caused by our negligence. We rewrite it: As shown in Figure 12(e) and 12(f), with sustained increase in normal force, the brittle fracture began to dominate the material removal and a large amount of variably sized debris of appeared due to the crack system no longer being able to stably extend; instead, this was replaced by a dynamic fracture when the applied load exceeded the critical load [35].

 

24. It is not clear what Figure 13 shows. What ‘edge’ is seen here? Are these from the same scratch experiment? What force values are given here?

Thanks for the suggestions. The edge damage sizes of five characteristic positions generated relatively large peeling of the scratched groove are measured through SEM. These are from the same scratch experiment. We added the explanation of Figure 13 in page 12, line 296.

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript presents results on indentation and scratch tests, which were conducted on the (0001) surface of 4H-SiC single crystals. It seems to be well-done and well-described experimental work, which can be published in the Journal. Nevertheless, I have a few comments and suggestions:

1. Incorrect ref order, 22 detected after 19. You jumped the numbers [20, 21] in between. Please revise and renumber the references so they appear in sequential numerical order throughout the text.
2. Line 199 “Figure 4 shows …”. Could it be Figure 8?
3. Figure 12 c,d. Microcracks are almost indistinguishable. I recommend fixing the photos and making them more contrasting.
4. I would recommend that the authors add more critical views on possible future developments in the field of research.

Author Response

Response to Reviewer 3 Comments

 

1. Incorrect ref order, 22 detected after 19. You jumped the numbers [20, 21] in between. Please revise and renumber the references so they appear in sequential numerical order throughout the text.

We are sorry about the confusion caused by our negligence. We revised the references in Table 1.

 

2. Line 199 “Figure 4 shows …”. Could it be Figure 8?

We are sorry about the confusion caused by our negligence. We revised the Figure number in line 212, .

 

3. Figure 12 c,d. Microcracks are almost indistinguishable. I recommend fixing the photos and making them more contrasting.

Thanks for the suggestions. In order to show the details more clearly, we increased the resolution of Figure 12 from 400 to 600 dpi.

 

4. I would recommend that the authors add more critical views on possible future developments in the field of research.

Thanks for the suggestions. With the deepening of this research topic, we will add more critical views on possible future developments in the field of damage and crack for brittle materials in future research.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

No further comments.