Enhancement of Oxidation Resistance via Chromium Boron Carbide on Diamond Particles
Round 1
Reviewer 1 Report
- The terms 'spectra' or 'spectrum' should not be used in relation to X-ray diffraction. 'XRD patterns' is a correct use.
- On page 4, the authors present a system of six dependent equations and a figure with Gibbs Energy of reactions, which complicates the perception of the text. It was enough to bring the Gibs Energy of the formation of three phases (CrB, B4C, Cr7C3) and show its dependence on temperature.
At the same time, the authors do not consider the possibility of forming a ternary system, for example, chromium bromate.
Author Response
Point 1: The terms 'spectra' or 'spectrum' should not be used in relation to X-ray diffraction. 'XRD patterns' is a correct use.
Response 1: We have revised the 'spectra' to 'patterns' in all texts and figure captions: lines 134, 145, 188 and 225.
Point 2: On page 4, the authors present a system of six dependent equations and a figure with Gibbs Energy of reactions, which complicates the perception of the text. It was enough to bring the Gibs Energy of the formation of three phases (CrB, B4C, Cr7C3) and show its dependence on temperature.
At the same time, the authors do not consider the possibility of forming a ternary system, for example, chromium bromate.
Response 2: We have modified the reactions in Cr-B-C system from six equations to four equations. Possible reaction equations were demonstrated in Figure 3. The reactions to the decisive factors of B4C, CrB, Cr7C3 were only listed. Equation (4) represented the possibility of conversion between the two products (Cr7C3 and CrB). Besides, the corresponding text description has also been modified.
（Please see the attachment）
Figure 3. Relationships of temperature on Gibbs free energy (ΔG) of the reactions (1)-(4).
In our XRD patterns, only CrB and Cr7C3 were found on the coatings of diamond surface. We did not see the birth of ternary system products. Therefore, ternary system reactions were not taken into account when calculating Gibbs free energy of the reactions (1)-(4).
Author Response File: 
Author Response.docx
Reviewer 2 Report
Dear Authors,
On lines 29 and 30 you write:
"...hardness (synthetic diamond, 70-100 Gpa), highest thermal conductivity at room temperature (2×1013 W/m•K) and extremely low coefficient of thermal expansion (1×10-6 K)"..."
Correct it please.
Author Response
Point 1: On lines 29 and 30 you write:
"...hardness (synthetic diamond, 70-100 Gpa), highest thermal conductivity at room temperature (2×1013 W/m•K) and extremely low coefficient of thermal expansion (1×10-6 K)"..."
Correct it please.

Response 1: The referee is right and we have made several changes on lines 28 to 30: such as ‘Gpa’ to ‘GPa’ and the ‘×’ to ‘ × ’. Thanks for the reviewer's reminder.
Please see the attachment.
Author Response File: Author Response.docx
Reviewer 3 Report
Evaluation of the paper “ Enhancement of oxidation resistance via chromium boron carbide on diamond particles
- The first phrase in introduction is very confusion I suggest rephrasing it
- There the novelty of this study should be better highlighted
- Please provide some details of the results in introduction
- Overall the introduction is brief
- Please define all the acronym before their first appearance in text i.e. HPHT and other
- Please discuss the Figures individually (Figure 4 and Figure 5; Figure 7 and Figure 8,)
- You define section 3 as “Results and Discussion” but there are only discussion please further elaborate with a discussions section in which you demonstrate the validity of your evidence by data from literature
- “various applications to elevated temperature.” Please provide details which applications
Author Response
Point 1: The first phrase in introduction is very confusion I suggest rephrasing it
Response 1: We appreciate that the referee has a point in the first phrase of introduction. We separated the advantages and applications of diamond so as to reduce the reader's confusion. Diamond is renowned for its outstanding physical, electrical and mechanical properties. Then, we introduced many industrial applications of diamond, such as mineral exploration and heat transfer applications. It is also shown that diamond is usually not used directly but as the reinforcing phase of composite materials to improve the performance. These composites are often subjected to a lot of high temperatures not only in working but also during the manufacturing process. Unfortunately, diamond is easily oxidized at approximately 700 °C, leading to catastrophic loss of its mechanical properties. Therefore, for the applications of diamond composites under high temperatures require a protective barrier from oxidation. That is the logic of the first paragraph in introduction.
Point 2: There the novelty of this study should be better highlighted
Response 2: Thanks for the reviewer's proposal. In the second paragraph of introduction, we have reviewed many of applications of diamond surface coatings, including carbides and borides, and listed some advantages and disadvantages about these coatings. In order to realize the protection of diamond in a binary system product coating, we have fully explained and expounded the highlights. The mutual protection mechanism of oxidation products is widely used in the field of ceramics and steel, but it is rarely used in diamond, except for Ti-B-C coating on diamond which can be found in our previous research. Through literature review, it is found that chromium carbide has better oxidation resistance than titanium carbide. Therefore, we want to realize the protective coating of Cr-B-C system on diamond surface.
Point 3: Please provide some details of the results in introduction
Response 3: We understand what the referee is saying. We have given some data when listing the references, but the results of the references are not so sufficient. At present, we have modified and enriched the data and conclusions of the references.
Point 4: Overall the introduction is brief
Response 4: The referee is right and we have made several changes to ensure the introduction is comprehensive. Through the above three points, we reduce the confusion in the introduction, increase the applications of the research, highlight our novelty, and enrich the citation data and conclusions, hoping to give readers no misunderstanding.
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The recommendations of the reviewer from point 1 to point 4 are about the introduction. Now, we list the revised introduction below for reviewer 3. The main revisions in the revised introduction are marked on highlight.
Diamond is renowned for its outstanding physical, electrical and mechanical properties, including high hardness (synthetic diamond, 70-100 GPa), highest thermal conductivity at room temperature (2 × 1013 W/m•K) and extremely low coefficient of thermal expansion (1 × 10-6 K) [1-3]. On the industrial front, it is remarkable to note that diamond is being exploited for both its mineral exploration and heat transfer applications, such as cutting tools, saw blade segments, grinding wheels, drill bits and heat sinks. Generally speaking, diamond is not devoted to working directly, but acts as the reinforcing phase of composites. As cutting elements or tools for heat transfer, these composites are frequently subjected the generation of high temperatures. Besides, it is worth mentioning that diamond metal matrix composites (MMCs) are generally manufactured under high temperature process [4]. However, diamond is easily oxidized at approximately 700 °C when annealed in oxidative atmosphere, leading to the oxidation products of diamond immediately escapes to external environment and a large amount of oxygen corrosion pits [5]. In addition, the defects cause serious loss of mechanical properties, which severely limits the lifetime and efficiency of diamond MMCs [3]. Therefore, the various applications of diamond tools at a high temperature requires a protective barrier without any damage to diamond particles.
According to previous research, enhancement in the oxidation resistance of diamond has been explored for decades to drive diamond for working efficiently or manufacturing at a higher temperature, such as mineral exploration and heat transfer applications [6]. Due to the development of diamond oxygen barrier, especially the protective coating on diamond surface, the oxygen corrosion pits can be inhibited directly [3]. B2O3 is widely used to prevent oxidation of diamond, because of low oxygen permeability, high fluidity and good wettability on carbon materials [7-9]. However, the hydrolysis of B2O3 exposed to ambient moisture can cause the spallation of coating owing to swelling or evaporation during heating [10]. In our previous research, boron carbide (B4C) coating on diamond particles contributed to the enhancement of oxidation resistance due to formation of a B2O3 oxygen barrier layer which gave rise to secondary coating growth on diamond surface. When B2O3 oxygen barrier layer completely evaporated at 1000 °C for sustained heating time over 2 hours, the coatings had no protective effect on diamond particles. Besides, tests for oxidation resistance at a higher temperature over 1000 °C were lacking [11]. Considering the ability of carbides to improve the oxidation resistance of diamond, the carbides coating materials prior to oxidation of diamond by forming an oxide layer are treated as available options generally [5, 12]. Titanium carbide (TiC) coating was planted on diamond surfaces to protect the diamond particles from oxidation through delaying erosion temperature of diamond. Chagas et al. [13] studied the thermal damage of TiC-coated diamond under different heat treatment times (60min, 180 min, 360 min). For 360 min heat treatment all uncoated diamonds were transformed into graphite, while TiC-coated diamonds presented sharpness and crystalline integrity. The TiC-coated diamonds had stronger protective effect than the uncoated diamonds during all the heat treatment time. It was worth mentioning that the diamond will lose its protection immediately due to the spalling of oxidation products. Sha et al. [14] reported that TiC-coated diamonds had a much higher onset temperature of mass loss (868 °C) compared to the uncoated diamonds (725 °C). Moreover, the TiC barrier could also protect diamond grains from direct contact with cobalt, thus a lower cobalt-catalytic graphitization occurs. Yang et al. [15] researched that chromium carbide (Cr7C3) can take precedence over carbon to react with oxygen before the temperature reached 1048 °C. The use of Cr7C3 resulted in an improvement in oxidation resistance, and there was no deterioration in important refractory properties, such as slag resistance and thermal shock resistance, which indicated that Cr7C3 would be suitable to be used as an antioxidant in the low carbon ceramics refractories. However, the carbide coatings might not remain a strong adherence to the diamond surface after annealing in air at a high temperature owing to carbides are converted into oxides, and the internal stress caused by the volume swelling results in the exfoliation of coatings. Therefore, to protect diamond better, we need to combine the dual advantages of B2O3 and carbides. The mutual protection mechanism of oxidation products is widely used in the field of ceramics and steel, but it is rarely used in diamond, except for Ti-B-C coating on diamond which can be found in our previous research [16]. Moreover, boride coating on diamond also has a small quantity of applications. Titanium boron carbide (Ti-B-C) coatings on diamond particles were benefited to hold the complete diamond morphology on 1000 °C for 1h by forming TiO2 and B2O3 [16]. The results revealed that oxidation resistance of borides on diamond was better than that of carbides, and the oxidation products (TiO2 and B2O3) proved a complementary functional protection on diamond particles from oxidation. Therefore, metal borides coatings on diamond particles can immediately improve oxidation resistance at high temperatures. Considering that chromium carbide has a higher protective temperature (1048 °C) for diamond than titanium carbide (868 °C), therefore, Cr-B-C is a potential coating for improving oxidation resistance of diamonds.
The present study focuses on forming Cr-B-C coatings on diamond particles by high temperature process. The ability of the Cr-B-C coatings on oxidation resistance has been investigated.
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Point 5: Please define all the acronym before their first appearance in text i.e. HPHT and other
Response 5: Thanks for the reviewer's reminder. We have defined all the acronym before their first appearance in this paper, including high pressure high temperature (HPHT) and other.
Point 6: Please discuss the Figures individually (Figure 4 and Figure 5; Figure 7 and Figure 8,)
Response 6: Having read the comment about the Figure 4 and Figure 5, Figure 7 and Figure 8, we rechecked the literature and discussed the Figures individually. The inflection points of the curve in Figure 4 are made into the data histogram as shown in Figure 5. Then, the data in Figure 5 were presented and discussed separately. Similarly, for Figure 7 and Figure 8 we discussed separately. Figure 8 was actually an enlarged graph of Figure 7c, we added texts description in lines 194 to 205 and 225 to 232 to reduce the trouble to readers.
Point 7: You define section 3 as “Results and Discussion” but there are only discussion please further elaborate with a discussions section in which you demonstrate the validity of your evidence by data from literature
Response 7: Thanks for the reviewer's proposal. In this paper, the results and discussion are written together. When describing some pictures, there are not only descriptions of the results but also discussion about the data. The comments given by the reviewer are very helpful. We have paid too much attention to the discussion, and there are too few descriptions of the results accurately and clearly. Now, many descriptions of the results have been added in this paper to help readers obtain the test results more clearly. The descriptions added are highlighted in the text: lines 151 to 155, 168 to 171, 194 to 205 and 225 to 232.
Point 8: “various applications to elevated temperature.” Please provide details which applications
Response 8: We understand what the referee is saying. In the conclusion part of this sentence, we did not provide too many details owing to the confusing ' Point 1' which was reminded by the reviewer. At present, we have introduced the possible applications of the Cr-B-C coating in detail in introduction. Although many composites may not work at too much extreme temperature, the manufacturing temperature is very high so that improving the oxidation resistance is also beneficial. In the introduction, we modified the original text, explained the possible applications of this study. We have manufactured drill bits [1] using the method displayed in Figure S1. In comparison to the drill bit using un-coated diamonds, the work life for the drill bit using B4C coated diamonds (Figure S2) increases to 3 times. The results indicate that the B4C coating is beneficial for protection of diamond. Thus, next step we are planning to make drill bits with Cr-B-C coatings to test its effect. Thanks again for the reviewer's suggestion.
Please see the attachment.
Figure S1. Schematic illustration of the manufacturing process for diamond/metal composites.
Please see the attachment.
Figure S2. Optical photograph (a) and the enlargement (b) of drill bits using B4C coated diamonds after wear test.
References
1. Meng, L.; Youhong, S.; Qingnan, M.; Haidong, W.; Ke, G.; Baochang, L. Fabrication of Fe-Based Diamond Composites by Pressureless Infiltration. Materials 2016, 9 (12), 1006.
Please see the attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 3 Report
Thank you
