coatings

Article Enhancement of Oxidation Resistance via Chromium on Particles

Xuliang Zhang 1,2, Youhong Sun 1,2,3, Qingnan Meng 1,2,*, Jinhao Wu 1,2 and Linkai He 1,2

1 College of Construction Engineering, Jilin University, Changchun 130026, China; [email protected] (X.Z.); [email protected] (Y.S.); [email protected] (J.W.); [email protected] (L.H.) 2 Key Lab of Drilling and Exploitation Technology in Complex Conditions, Ministry of Natural Resources, Changchun 130061, China 3 School of Engineering and Technology, China University of Geosciences, Beijing 100083, China * Correspondence: [email protected]

Abstract: To improve the oxidation resistance of diamond, chromium boron carbide (Cr–B–C) coatings were synthesized through high temperature state synthesis and molten salt method on diamond particles in this paper. After holding the raw material at 900 ◦C for 2 h, the diamond surface was completely and uniformly covered by Cr–B–C coatings. Oxidation resistance of the diamond coated Cr–B–C was determined by the thermogravimetric analysis (TGA). The results revealed that the Cr–B–C coatings held the for 100%-mass in air atmosphere until 1151 ◦C, ◦ ◦ which was much better than the uncoated diamonds (720 C) and the B4C-coated diamonds (1090 C). When Cr–B–C-coated diamond was annealed in air, Cr2O3 and B2O3 were formed as oxygen barrier layer to protect diamond from oxidation. The formation of B2O3 with high temperature fluidity was conducive to avoiding Cr2O3 delamination due to volume expansion during oxidation in air. Furthermore, the presence of Cr O provided lasting protection by reducing the evaporation of B O .  2 3 2 3  The oxidation products (B2O3 and Cr2O3) prove a complementary functional protection on diamond particles from oxidation. Citation: Zhang, X.; Sun, Y.; Meng, Q.; Wu, J.; He, L. Enhancement of Oxidation Resistance via Chromium Keywords: diamond; chromium boron carbide coating; thermogravimetric analysis; oxidation resistance Boron Carbide on Diamond Particles. Coatings 2021, 11, 162. https:// doi.org/10.3390/coatings11020162 1. Introduction Academic Editor: Ali Dolatabadi Diamond is renowned for its outstanding physical, electrical and mechanical proper- Received: 6 January 2021 ties, including high hardness (synthetic diamond, 70–100 GPa), highest thermal conduc- Accepted: 28 January 2021 tivity at room temperature (2 × 1013 W/m·K), and extremely low coefficient of thermal Published: 30 January 2021 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 Publisher’s Note: MDPI stays neutral cutting tools, saw blade segments, grinding wheels, drill bits and heat sinks. Generally with regard to jurisdictional claims in speaking, diamond is not devoted to working directly, but acts as the reinforcing phase published maps and institutional affil- of composites. As cutting elements or tools for heat transfer, these composites are fre- iations. quently 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 Copyright: © 2021 by the authors. immediately escaping to external environment and a large amount of oxygen corrosion Licensee MDPI, Basel, Switzerland. pits [5]. In addition, the defects cause serious loss of mechanical properties, which severely This article is an open access article limits the lifetime and efficiency of diamond MMCs [3]. Therefore, the various applications distributed under the terms and of diamond tools at a high temperature requires a protective barrier without any damage conditions of the Creative Commons to diamond particles. Attribution (CC BY) license (https:// According to previous research, enhancement in the oxidation resistance of diamond creativecommons.org/licenses/by/ has been explored for decades to drive diamond for working efficiently or manufacturing 4.0/).

Coatings 2021, 11, 162. https://doi.org/10.3390/coatings11020162 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 162 2 of 9

at a higher temperature, such as mineral exploration and heat transfer applications [6]. Due to the development of the diamond oxygen barrier, especially the protective coating on the 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, good fluidity at high temperature and excellent wettability of materials [7–9]. However, the hydrolysis of B2O3 at ambient humidity can cause the coating to peel off during heating owing to evaporation or expansion [10]. In our previous research, the boron carbide (B4C) coating was covered on the surface of diamond particles by forming a B2O3 oxygen barrier layer which supplied a secondary protection of diamond to the enhancement of oxidation resistance during annealing. When the B2O3 oxygen barrier layer completely evaporated at 1000 ◦C for sustained heating time over 2 h, 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 to improve the oxidation resistance of diamond, the carbides coating materials prior to oxidation of diamond by forming an oxide layer are generally 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 (60, 180, 360 min). After 360 min of heat treatment, all uncoated diamonds were completely transformed into graphite, whereas the diamonds with TiC coatings exhibited 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). In addition, the TiC coatings can also protect diamond particles from direct contact with cobalt, thereby reducing the occurrence of cobalt-catalytic graphitization. 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 improved the oxidation resistance of refractories. However, the carbide coating was very easy to convert into oxide during heat treatment in air, and the oxide often caused the coating to fall off due to the internal stress caused by volume expansion, which showed weak adhesion to diamond. 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 and steel, but it is rarely used in diamond, except for the Ti–B–C coating on diamond which can be found in our previous research [16]. Moreover, the 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 1 h 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 Cr–B–C coatings on oxidation resistance has been investigated.

2. Materials and Methods 2.1. Materials and Preparation Synthetic high pressure high temperature (HPHT) diamond particles (SCDF90, 90– 116 µm, Zhengzhou Sino-Crystal Diamond Co., Ltd., Zhengzhou, China) were supplied. Coatings 2021, 11, 162 3 of 9

The following regents were used: chromium powder (Cr, 75 µm, purity 99.95%), boron powder (B, 10–20 µm, purity 99.0%), boric acid (H3BO3, purity 99.5%), NaCl (purity 99.5%), KCl (purity 99.5%) which was provided by Sinopharm Chemical Reagent Co., Ltd. (Shang- hai, China). For forming the chromium boron carbide coating on diamond particles, two steps were adopted. Firstly, in order to the form boron carbide (B4C) coating on diamond particles, 30.00 g diamond particles were immersed in a mixture of 39.60 g boron (B) and 27.60 g boric acid (H3BO3) powders. The diamond-powder mixture was strongly me- chanically stirred at room temperature for 2 h. This mixture was placed in an alumina container and placed into a tube furnace under argon. B4C coating was synthesized on diamond particles by heating diamond-powder mixture at 1200 ◦C for 6 h. Argon stopped flowing when the tube furnace reached room temperature. The samples were cooled in a tube furnace treated with dilute nitric acid to remove the soluble phases in the product. The coated diamonds were separated from the excess B powders by a sieve. Then, the mixture consisting of 8.70 g of Cr, 12.25 g of NaCl, and 17.60 g of KCl powders were grinded along a same circle direction for 2 h at room temperature before immersing 20.00 g of B4C-coated diamond particles. This mixture was placed in an alumina container and placed into a tube furnace under argon. Cr–B–C coating was synthesized on diamond particles by heating raw material to 900 ◦C for 2 h. When the tube furnace was cooled, the samples were immediately immersed in water to remove inorganic salt. Finally, the coated diamonds were then separated from excess Cr powders by a sieve.

2.2. Experiments and Characterization The microstructure of the samples was characterized by a Bruker D8 (Bruker, Karl- sruhe, Germany) with a Cu Kα source X-ray diffraction (XRD). The surface morphologies and topographies of the coatings were characterized by Hitachi S-4800 scanning electron microscope (SEM, Tokyo, Japan). Thermalgravimetric analysis (TGA) was performed using a Netzsch STA449F3 (Selbu, Germany). The temperature of the TGA was increased from room temperature (20 ◦C) to 1200 ◦C at a rate of 5 ◦C/min in air atmosphere.

3. Results and Discussion 3.1. Phase Analysis

Figure1 demonstrates the XRD patterns for the uncoated, B 4C-coated and Cr-B-C- coated diamond particles. As shown in Figure1, the peak located at 43.92 ◦ was observed in all the patterns, which corresponded to diamond (JCPDF#06-0675). In order to clearly observe other peaks of coating on diamond particles, the high-intensity peak of single crystal diamond substrate was truncated partially. Three peaks located at 23.49◦, 34.95◦ ◦ and 37.82 can be assigned to the B4C structure in comparison to the standard PDF cards (JCPDF#35-0798). To identify the two-step synthetic composite coatings, five peaks located at 44.93◦, 46.16◦, 56.32◦, 63.38◦, and 75.69◦ were observed which corresponded to the CrB structure (JCPDF#32-0277), and four peaks located at 39.19◦, 42.54◦, 44.16◦ and 50.19◦ were observed which corresponded to the Cr7C3 structure (JCPDF#36-1482). Therefore, ◦ the coatings of the Cr–B–C synthesized at 900 C consisted of CrB and Cr7C3. Figure2 shows the SEM images for the diamond particles with different coatings. After a high temperature process, a complete coating coverage is obtained at the B4C- coated diamond and Cr–B–C-coated diamond, and no cracking or spalling is found on the diamond surface. As shown in Figure2b, a large number of small particles are deposited on the surface of diamond due to the formation of B4C particles. As a contrast, the Cr– B–C-coated diamond (Figure2c) shows a relative smoother surface without any obvious particles. In addition, B4C-coated diamond can still maintain polyhedron shape, but the shape of the Cr–B–C-coated diamond has changed from sharp corners to nearly spherical. This also indicates that the two-step synthesis method of Cr–B–C coatings continue to grow on the basis of the B4C coating, and the thickness of the coating is greater than that of B4C. Coatings 2021, 11, x FOR PEER REVIEW 4 of 9

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shape of the Cr–B–C-coated diamond has changed from sharp corners to nearly spherical. shapeThis also of the indicates Cr–B–C-coated that the diamond two-step has synthe changsised method from sharp of Cr–B–C corners coatings to nearly continue spherical. to Coatings 2021, 11, 162 4 of 9 Thisgrow also on theindicates basis of that the the B4C two-step coating, andsynthe thesis thickness method ofof the Cr–B–C coating coatings is greater continue than that to growof B4C. on the basis of the B4C coating, and the thickness of the coating is greater than that of B4C.

FigureFigure 1.1. XRDXRD patternspatterns forfor uncoateduncoated diamond diamond particles, particles, diamond diamond particles particles with with B4 BC4C coating, coating, Cr–B–C Cr– Figurecoating,B–C coating, 1. respectively. XRD respectively. patterns for uncoated diamond particles, diamond particles with B4C coating, Cr– B–C coating, respectively.

Figure 2. SEM images for (a) uncoated diamond, (b) B4C-coated diamond, (c) Cr–B–C-coated diamond. Figure 2. SEM images for (a) uncoated diamond, (b) B4C-coated diamond, (c) Cr–B–C-coated diamond. Figure 2. SEM images for (a) uncoated diamond, (b)B4C-coated diamond, (c) Cr–B–C-coated diamond. Possible reaction equations between diamond, Cr and B4C are listed as follows: Possible reaction equations between diamond, Cr and B4CC are are listed as follows: C + 4B = B4C, (1) C + 4B = B4C, (1) C + 4B = B4C, (1) B4C + 4Cr = 4CrB + C, (2) (2) B4BC4C + + 4Cr 4Cr = = 4CrB 4CrB + + C, C, (2) 3B4C + 7Cr = Cr7C3 + 12B, (3) 3B4C + 7Cr = Cr7C3 + 12B, (3) 3B4C + 7Cr = Cr7C3 + 12B, (3) (4) Cr7CrC37C+3 7B+ 7B = = 7CrB 7CrB + + 3C, 3C, (4) Cr7C3 + 7B = 7CrB + 3C, (4) EffectsEffects ofof temperaturetemperature on on the the Gibbs Gibbs free free energy energy (∆ (GΔ)G of) of reactions reactions (1)–(4) (1)–(4) are are shown shown in Figurein FigureEffects3. It 3. is ofIt well istemperature well known known that on that athe reaction a Gibbsreaction can free can only energy only occur occur (Δ whenG) whenof reactions its its∆G ΔisG below(1)–(4)is below zero,are zero, shown and and a insmallera smallerFigure∆ 3. GΔ GItgives isgives well rise rise known to to an aneasy that easy reaction.a reaction.reaction The canThe Equationonly Equation occur (1) (1) when showed showed its Δ that Gthat is B belowB4C4C waswas zero, formedformed and ◦ abyby smaller diamonddiamond ΔG gives andand CrCr rise sincesince to an thethe easy ∆ΔGG reaction. << 00 atat 12001200 The Equation°C.C. AtAt thethe (1) heatingheating showed processprocess that B 4 ofCof wasthethe Cr–B–CformedCr–B–C bysystem,system, diamond thethe inorganic inorganicand Cr since salt salt (NaCl the(NaCl Δ andG and < KCl)0 KCl)at 1200 added added °C. in At thein the raw rawheating mixture mixture process acted acted as of a catalystasthe a Crcatalyst–B that–C system,cannotthat cannot participatethe inorganicparticipate in thesalt in reaction (NaClthe reaction and actually, KCl) actually, andadded chromium and in chromiumthe raw was mixture immediately was immediately acted reactedas a catalyst reacted with ◦ thatBwith4C cannot to B form4C to participate CrBform (as CrB shown (asin theshown in reaction Equation in Equation actually, (2), the (2), ∆andG the< chromium− Δ50G kcal< −50from was kcal 0–900immediately from 0–900C) and °C)reacted Cr 7andC3 ◦ with(asCr7 shownC 3B (as4C toshown in form Equation in CrB Equation (as (3), shown the (3),∆G the in< 0EquationΔG when < 0 when temperature (2), temperaturethe ΔG >< 500−50 > kcal500C). In°C).from the In meantime,0–900 the meantime, °C) and the ◦ Crformedthe7C formed3 (as Cr shown7C Cr3 7transformedC in3 transformed Equation to (3), CrB to the CrB continuously Δ Gcontinuously < 0 when as temperature the as the temperature temperature > 500 rose°C). rose In further the further meantime, to 900 to 900C ◦ the(as shownformed in Cr Equation7C3 transformed (4), the ∆ toG

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°C (as shown in Equation (4), the ΔG < −60 kcal). Finally, when the mixture maintained °C (as shown in Equation (4), the ΔG < −60 kcal). Finally, when the mixture maintained 900 °C for 2 h, the formed CrB was the end product, with a small amount of Cr7C3 that 900 °C for 2 h, the formed CrB was the end product, with a small amount of Cr7C3 that Coatings 2021, 11, 162 had not yet reacted completely, which agreed well with the results of the XRD patterns5 of 9 had not yet reacted completely, which agreed well with the results of the XRD patterns (Figure 1). (Figure 1).

Figure 3. Relationships of temperature on Gibbs free energy (ΔG) of the reactions (1)–(4). Figure 3. Relationships of temperaturetemperature on Gibbs free energy ((∆ΔG)) ofof thethe reactionsreactions (1)–(4).(1)–(4). 3.2.3.2. OxidationOxidation ResistanceResistance 3.2. Oxidation Resistance OxidationOxidation resistanceresistance ofof thethe uncoated, B4C-coatedC-coated and and Cr–B–C-coated Cr–B–C-coated diamond diamond parti- par- Oxidation resistance of the uncoated, B4C-coated and Cr–B–C-coated diamond parti- ticlescles was was determined determined by by the the TGA. TGA. As Asshown shown in Figure in Figure 4, for4, forthe theuncoated uncoated diamond diamond parti- cles was determined by the TGA. As shown in Figure 4, for the uncoated diamond parti- particles,cles, the TGA the TGA curve curve displays displays an obvious an obvious prot protectionection failure failure while while the heating the heating temperature temper- cles, the TGA curve displays an obvious protection failure while the heating temperature aturereaches reaches 720 °C, 720 indicating◦C, indicating the oxidation the oxidation resistance resistance of the uncoated of the uncoated diamond diamond can reach can 720 reaches 720◦ °C, indicating the oxidation resistance of the uncoated diamond can reach 720 reach°C. The 720 TGAC. curve The TGA for the curve B4C-coated for the Bdiamond4C-coated displays diamond a weight displays gain a from weight 700 gain to 855 from °C, °C. The TGA curve for the B4C-coated diamond displays a weight gain from 700 to 855 °C, 700and to slowly 855 ◦C, decreases and slowly to 100% decreases in weight to 100% until in weight1090 °C. until The 1090weight◦C. gain The was weight attributed gain was to and slowly decreases to 100% in weight until 1090 °C. The weight gain was attributed to attributedthe formation to the of formation B2O3, and ofthe B 2fastO3, weight and the decrease fast weight due decrease to the evaporation due to the evaporationof B2O3 accord- of the formation of B2O3, and the fast weight decrease due to the evaporation of B2O3 accord- Bing2O 3toaccording our previous to our research previous [11,16]. research For [11 th,16e Cr–B–C-coated]. For the Cr–B–C-coated diamond diamondparticles, particles,the TGA ing to our previous research [11,16]. For the Cr–B–C-coated diamond◦ particles, the TGA thecurve TGA shows curve that shows the weight that the increases weight increasesfrom 599 fromto 1118 599 °C, to and 1118 thenC, decreases and then decreasesdirectly to curve shows that the weight increases◦ from 599 to 1118 °C, and then decreases directly to directly100% mass to 100% at 1151 mass °C. at 1151 C. 100% mass at 1151 °C.

Figure 4. TGA results for the uncoated diamond particles, diamond particles with B4C coating, Cr–B–C coating, respectively.

The inflection points of the curve in Figure4 are made into the data histogram as

shown in Figure5. From the initial temperature of weight gain, the Cr–B–C coating (599 ◦C) ◦ and B4C coating (700 C) are both lower than the oxidation temperature of diamond in air Coatings 2021, 11, x FOR PEER REVIEW 6 of 9

Figure 4. TGA results for the uncoated diamond particles, diamond particles with B4C coating, Cr– B–C coating, respectively.

Coatings 2021, 11, 162 6 of 9 The inflection points of the curve in Figure 4 are made into the data histogram as shown in Figure 5. From the initial temperature of weight gain, the Cr–B–C coating (599 °C) and B4C coating (700 °C) are both lower than the oxidation temperature of diamond (720in air◦ C),(720 indicating °C), indicating that the that coatings the coatings can play can a play protective a protective role before role before diamond diamond oxidation. oxi- ◦ Fromdation. the From initial the temperature initial temperature of weight of weight loss, the loss, Cr–B–C the Cr–B–C coating coating (1118 (1118C) and °C) the and B 4theC ◦ ◦ coatingB4C coating (855 (855C) °C) are are much much higher higher than than that that of of the the uncoated uncoated diamond diamond (720 (720 °C)C) asas well,well, indicatingindicating thatthat bothboth Cr–B–CCr–B–C coatingcoating andand BB44CC coatingcoating hadhad goodgood protectiveprotective effect,effect, andand thethe Cr–B–CCr–B–C coatingcoating has has better better effect effect on on delaying delaying the the oxidation oxidation of diamondof diamond than than the the B4C B coating.4C coat- Froming. From the initial the initial temperature temperature of protection of protecti failure,on failure, when the when diamond the diamond mass loss mass is reduced loss is toreduced 100%, Cr–B–Cto 100%, coating Cr–B–C (1151 coating◦C) is(1151 better °C) for is maintainingbetter for maintaining the quality the of diamondquality of than dia- ◦ Bmond4C coating than B (10904C coatingC). In(1090 summary, °C). In summary, the Cr–B–C the coating Cr–B–C exhibits coating the exhibits best protection the best pro- of diamondtection ofparticles diamond from particles oxidation from inoxidation this research. in this research.

FigureFigure 5.5. InitialInitial temperaturetemperature ofof thethe threethree typestypes ofof diamonddiamond particlesparticles inin TGATGA results.results.

◦ TheThe Cr–B–C-coatedCr–B–C-coated diamonddiamond particlesparticles afterafter annealingannealing atat 11511151 °CC withwith aa heatingheating raterate ◦ ◦ ◦ ◦ ◦ ofof 55 °C/minC/min in air, werewere observedobserved ofof fourfour peakspeaks locatedlocated atat 33.68 33.68°,, 36.3236.32°,, 41.1841.18°,, 54.8854.88° which corresponded to Cr O (JCPDF#38-1479) and other four peaks located at 39.19◦, which corresponded to Cr22O33 (JCPDF#38-1479) and other four peaks located at 39.19°, 42.54◦, 44.16◦ and 50.19◦ can be corresponded to Cr C (as shown in Figure6). Therefore, 42.54°, 44.16° and 50.19° can be corresponded to Cr77C33 (as shown in Figure 6). Therefore, the Cr–B–C coatings after annealing at 1151 ◦C were composed of Cr O and part of Cr C the Cr–B–C coatings after annealing at 1151 °C were composed of Cr2 2O3 3 and part of Cr7 7C33 thatthat hashas notnot beenbeen oxidized.oxidized. As shown in Figure7, after annealing at 1151 ◦C in air, the edges and corners of the As shown in Figure 7, after annealing at 1151 °C in air, the edges and corners of the uncoated diamond were confusion, which displayed a severe surface corrosion (Figure7a). uncoated diamond were confusion, which displayed a severe surface corrosion (Figure A similar phenomenon occurred on the B4C-coated diamond (Figure7b), but displayed a 7a). A similar phenomenon occurred on the B4C-coated diamond (Figure 7b), but dis- pitting corrosion. In contrast, the Cr–B–C-coated diamond possessed a complete coverage played a pitting corrosion. In contrast, the Cr–B–C-coated diamond possessed a complete of edges and corners, and stayed a large area of squama structure stayed on the diamond coverage of edges and corners, and stayed a large area of squama structure stayed on the surface (Figure7c). diamond surface (Figure 7c). In order to visually observe the Cr–B–C coating after oxidation in Figure7c, the sample In order to visually observe the Cr–B–C coating after oxidation in Figure 7c, the sam- was enlarged to obtain the SEM images shown in Figure8a–c. It can be seen from Figure8 ple was enlarged to obtain the SEM images shown in Figure 8a–c. It can be seen from that the crystal form and angular region of the diamond maintained very well. On the surface Figure 8 that the crystal form and angular region of the diamond maintained very well. of the Cr–B–C coating, large squama structures appeared, which was related to the oxidation ofOn coating the surface components of the Cr–B–C at high temperature.coating, large When squama the structures oxidation appeared, product on which the diamond was re- lated to the oxidation of coating components at high temperature. When the oxidation surface evaporated and exfoliated, the original fluid B2O3 showed holes and voids, bringing newproduct oxygen on the flow diamond channels surface to the diamondevaporated which and damaged exfoliated, the the protection original offluid the B coating.2O3 showed

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Coatings 2021, 11, x FOR PEER REVIEW 7 of 9 Coatings 2021, 11, 162 7 of 9 holes and voids, bringing new oxygen flow channels to the diamond which damaged the holesprotection and voids, of the bringing coating. new oxygen flow channels to the diamond which damaged the holesprotection and voids, of the bringingcoating. new oxygen flow channels to the diamond which damaged the protection of the coating.

FigureFigure 6.6. XRDXRD patternspatterns forfor thethe Cr–B–C-coatedCr–B–C-coated diamonddiamond particlesparticles afterafter annealingannealing atat 11511151◦ °C.C. Figure 6. XRD patterns for the Cr–B–C-coated diamond particles after annealing at 1151 °C. Figure 6. XRD patterns for the Cr–B–C-coated diamond particles after annealing at 1151 °C.

(c-1) (c-1) (c-1)

Figure 7. SEM images for (a) uncoated diamond, (b) B4C-coated diamond and (c) Cr–B–C-coated diamond, after annealing Figureat 1151 7. °C: SEM (c-1 images) the squama for (a) uncoatedon diamond diamond, surface. (b ) B4C-coated diamond and (c) Cr–B–C-coated diamond, after annealing Figure 7. SEM images for (a) uncoated diamond, (b)B4C-coated diamond and (c) Cr–B–C-coated diamond, after annealing at 1151 ◦°C: (c-1) the squama on diamond surface. Figureat 1151 7.C: SEM (c-1 images) the squama for (a) onuncoated diamond diamond, surface. (b) B4C-coated diamond and (c) Cr–B–C-coated diamond, after annealing at 1151 °C: (c-1) the squama on diamond surface.

Figure 8. SEM images for Cr–B–C coated diamond after annealing at 1151 °C: (a) the squama on diamond surface; (b) the Figureexfoliation 8. SEM and images evaporation for Cr–B–C of coating; coated (c di) theamond new afterchannel annealing for oxygen. at 1151 °C: (a) the squama on diamond surface; (b) the exfoliation and evaporation of coating; (c) the new channel for oxygen. Figure 8. 8. SEMSEM images images for for Cr–B–C Cr–B–C coated coated diamond diamond after after annealing annealing at 1151 at 1151°C: (a◦)C: the (a squama) the squama on diamond on diamond surface; surface; (b) the Combined with above results, the Cr–B–C coating fully covers on the diamond sur- (exfoliationb) the exfoliation and evaporation and evaporation of coating; of coating; (c) the new (c) the channel new channel for oxygen. for oxygen. face Combined(Figure 2c). with The aboveCr–B–C results, coating the is Cr–B–C oxidized coating to a layer fully ofcovers B2O3 onand the Cr diamond2O3 (Figure sur- 6) face (Figure 2c). The Cr–B–C coating is oxidized to a layer of B2O3 and Cr2O3 (Figure 6) duringCombinedCombined annealing. with The above above weight results, results, is increased the the Cr–B–C Cr–B–C due coating coatingto the Cr–B–C fully fully covers covers coating on on theabsorbing the diamond diamond oxygen sur- sur- duringatoms aboveannealing. 599 °C The (Figure weight 5, theis increased initial temperature due to the of Cr–B–C weight coatinggain with absorbing Cr-B-C coating). oxygen faceface (Figure 2 2c).c). TheThe Cr–B–CCr–B–C coatingcoating isis oxidizedoxidizedto to a a layer layer of of B B22O33 and Cr 2O33 (Figure(Figure 66)) atoms above 599 °C (Figure 5, the initial temperature of weight gain with Cr-B-C coating). duringduringFigure annealing.9 illustrates The Thethe oxidationweight weight is is mechanismincreased increased due due of Cr–B–Cto to the the Cr–B–C Cr–B–C coating coating coatingon diamond absorbing absorbing particles. oxygen The atomsFigureatomsCr–B–C above above9 illustratescoating 599 599 consisted°C◦C the (Figure (Figure oxidation of 55,, CrBthe mechanism initialand Cr temperature7C3 willof Cr–B–C be oxidized of coatingweight prior gain on diamond to with diamond. Cr-B-C particles. Whencoating). The the FigureCr–B–CFiguretemperature9 9 illustrates coatingillustrates of consistedanneal the the oxidation oxidation increases of CrB mechanism mechanism andgradually, Cr7C of3 will of Cr–B–Cthe Cr–B–C befluid oxidized coating of coating B2O on prior3 (fusion diamondon diamondto diamond. point, particles. particles. 450 When The°C) The Cr–effi-the temperature of anneal increases gradually, the fluid of B2O3 (fusion point, 450 °C) effi- Cr–B–CB–Cciently coating releasescoating consisted consistedthe stress of CrB ofcaused andCrB Cr and by7C 3theCrwill7 Cexpansion3 be will oxidized be oxidized of priorCr2O3 to ,prior which diamond. to preventsdiamond. When the the When desqua- temper- the ciently releases the stress caused by the expansion of Cr2O3, which ◦prevents the desqua- temperatureaturemate ofof anneal Cr2 Oof3 increases fromanneal volume increases gradually, expansion. gradually, the fluid In of addition,the B2O fluid3 (fusion ofB2 OB point,32 Oheals3 (fusion 450 the C) exfoliationpoint, efficiently 450 °C)of releases Cr effi-2O3 mate of Cr2O3 from volume expansion. In addition, B2O3 heals the exfoliation of Cr2O3 cientlythewhich stress releasesmight caused lead the by tostress the the expansion causeddefects byon of Crthethe2 O expansiondiam3, whichond prevents surface.of Cr2O 3 theMoreover,, which desquamate prevents the evaporation of the Cr2 Odesqua-3 from of which might lead to the defects on the diamond surface. Moreover, the evaporation of matevolume of expansion.Cr2O3 from In volume addition, expansion. B2O3 heals In the addition, exfoliation B2O of3 heals Cr2O 3thewhich exfoliation might lead of Cr to2 theO3 whichdefects might on the lead diamond to the surface. defects Moreover,on the diam theond evaporation surface. Moreover, of B2O3 is the suppressed evaporation due of to

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B2O3 is suppressed due to the coverage of Cr2O3. The oxidation products (B2O3 and Cr2O3) provethe coverage as complementary of Cr2O3. The functional oxidation protection products (B on2O the3 and diamond Cr2O3) particles prove as from complementary oxidation. Asfunctional the temperature protection reached on the diamond 1151 °C, particlesthe squama from (Figures oxidation. 8a and As the 9) on temperature the diamond reached sur- ◦ 1151face isC, caused the squama by the (Figures evaporation8a and of9) B on2O the3 completely diamond surface (Figure is 8b, caused agree by with the evaporationthe XRD result of thatB2O 3thecompletely diamond (Figure surface8b, is agree covered with theby Cr XRD2O3 result after thatannealing), the diamond revealing surface a isnew covered channel by (FigureCr2O3 after 8c) annealing),for oxygen revealingthrough athe new barrier channel and (Figure corroding8c) for the oxygen diamond through particles, the barrier leading and tocorroding a directly the protection diamond particles, failure (agree leading well toa with directly the protectionFigure 4). failureThus, (agreethe Cr–B–C-coated well with the diamondFigure4). Thus,remains the intact Cr–B–C-coated after annealing diamond in air remains and reveals intact after the best annealing protection in air andagainst reveals oxi- dationthe best of protection diamond against particles. oxidation of diamond particles.

Figure 9. Schematic illustration of the oxidation mechanismmechanism related to the Cr–B–C coatedcoated diamond.diamond.

4. Conclusions 4. Conclusions After holding the raw material at 900 °C for 2 h, the diamond surface was completely ◦ and uniformlyAfter holding covered the raw by materialCr–B–C atcoatings 900 C forconsisting 2 h, the of diamond CrB and surface Cr7C3 was. The completely results of TGAand uniformly revealed coveredthat the byCr–B–C Cr–B–C coatings coatings can consisting effectively of CrBprevent and Crthe7C diamond3. The results from of being TGA oxidatedrevealed thatduring the heating Cr–B–C in coatings air. The can formed effectively B2O3 preventand Cr2 theO3 acted diamond as oxygen from being barrier oxidated layers toduring protect heating the diamond in air. The from formed oxidatio B2On.3 andFurthermore, Cr2O3 acted the as formation oxygen barrier of B2O layers3 with high to protect tem- peraturethe diamond fluidity from was oxidation. conducive Furthermore, to avoiding theCr2O formation3 delamination of B2O due3 with to volume high temperature expansion fluidity was conducive to avoiding Cr O delamination due to volume expansion during during oxidation in air. In addition, the2 presence3 of Cr2O3 provided lasting protection by oxidation in air. In addition, the presence of Cr O provided lasting protection by reducing reducing the evaporation of B2O3. The enhancement2 3 of oxidation resistance via chromium boronthe evaporation carbide on of diamond B2O3. The particles enhancement is of great of oxidationsignificance resistance for various via applications chromium boron with elevatedcarbide on temperature, diamond particles such as is mineral of great exploration significance and for variousheat transfer applications applications. with elevated temperature, such as mineral exploration and heat transfer applications. Author Contributions: Conceptualization, X.Z. and Y.S.; methodology, X.Z. and Q.M.; software, AuthorL.H.; validation, Contributions: Y.S.; formalConceptualization, analysis, X.Z. X.Z.and andQ.M.; Y.S.; investigation, methodology, J.W. X.Z.; writing—original and Q.M.; software, draft preparation,L.H.; validation, X.Z. Y.S.;and Q.M.; formal writing—review analysis, X.Z. and and Q.M.;editing, investigation, Q.M. All authors J.W.; have writing—original read and agreed draft to preparation,the published X.Z. version and Q.M.;of the writing—reviewmanuscript. and editing, Q.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Natural Science Foundation of China (Nos. 41872181 and 41502344).Funding: This research was funded by the Natural Science Foundation of China (Nos. 41872181 and 41502344). Institutional Review Board Statement: Not applicable. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available in article. Data Availability Statement: The data presented in this study are available in article. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Bovenkerk, H.P.; Bundy, F.P.; Hall, H.T.; Strong, H.M.; Wentorf, R.H. Preparation of diamond. Nature 1959, 184, 1094–1098. Nature 1959 184 1.2. Bovenkerk,Zhao, Z.; Xu, H.P.; B.; Tian, Bundy, Y. Recent F.P.; Hall, advances H.T.; Strong,in superhard H.M.; materials. Wentorf, R.H.Annu. Preparation Rev. Mater. ofRes. diamond. 2016, 46, 383–406. , , 1094–1098. 3. [Boland,CrossRef J.N.;] Li, X.S. Microstructural characterisation and wear behaviour of diamond composite materials. Materials 2010, 3, Annu. Rev. Mater. Res. 2016 46 2. Zhao,1390–1419. Z.; Xu, B.; Tian, Y. Recent advances in superhard materials. , , 383–406. [CrossRef]

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