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Preparation and characterization of vitrified CeO2 coated cBN composites
Ceramics International 45 (2019) 19704–19709
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Preparation and characterization of vitrified CeO2 coated cBN composites Jiang Shi a b c
a,b
, Feng He
a,b,∗
a,b
, Junlin Xie
c
, Xiaoqing Liu , Hu Yang
T
b
State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, 430070, China School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, 430070, China Center for Materials Research and Analysis, Wuhan University of Technology, Wuhan, 430070, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ceria cBN Vitrified bonds Chemical bonding
The performances of vitrified cBN composites are deeply affected by the wettability of vitrified bonds on cBN particles. CeO2 coated cBN particles were successfully prepared for the further improvement of the covering and wetting of cBN by vitrified bonds. The microstructure and properties of vitrified cBN composites were characterized by scanning electron microscope (SEM), hot stage microscope (HSM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and flexural strength. Results showed that the prepared CeO2 coating on the surface of cBN was uniform and dense. Besides, the improved wettability of vitrified bonds on CeO2 coated cBN particles accompanied with the formation of Ce–O–Al and N–Si confirmed by XPS were supposed to conduce to enhancing the holding power of the vitrified bonds to cBN particles, which resulted in increasing the flexural strength of vitrified cBN composites by 9.16%. Thus, coating cBN with CeO2 was a potential and effective method to obtain vitrified cBN composites with higher flexural strength.
1. Introduction High-performance superhard grinding tools have found numerous applications in modern advanced manufacturing technology for exhibiting high processing precision, high efficiency, and long service life. Owing to their excellent chemical stability and hardness, superhard abrasives, mainly including diamond and cubic boron nitride (cBN), are of foremost importance in the performance of superhard grinding tools [1]. During the process of preparing superhard grinding tools at high sintering temperature, vitrified bonds, which are considered to contain partial or whole amorphous phase, play critical roles in tight bonding the abrasives together. However, the smooth surfaces with a low surface free energy of diamond and cBN offer a significant challenge to the complete spreading and wetting by vitrified bonds on their surfaces at sintering temperature [2], which is responsible for the weak holding power of vitrified bonds on abrasives. Thus, the diamond or cBN particles are unable to be tightly consolidated and the shedding of abrasives may easily occur during the processing of workpieces with the superhard grinding tools, resulting in a low processing efficiency, short service life, and high processing cost. In order to improve the infiltration of vitrified bonds on superhard abrasives, three typical methods had been discussed by researchers: (1) Pre-oxidation of abrasives. Researchers found that the pre-oxidation of cBN particles under 900 °C for
∗
60 min is effective for improving the wetting on cBN by vitrified bonds [3], while the degree of pre-oxidation for abrasives is known to be uncontrollable which might induce the property deterioration of abrasives. (2) Surface metallization of abrasives. Metal plating on abrasives including Ti [4,5], Ni [6], and Cr [7] can enhance the wetting of the abrasives with vitrified bonds by forming chemical bonding between the metal plating and the abrasives. However, complex and expensive equipment are necessary for achieving the surface metallization. (3) Oxide coating on abrasives. Experiments had proved that oxide coating on abrasives, such as SiO2 [8], TiO2 [9], Al2O3 [10], ZnO [11], and TiO2–Al2O3 [12], contributed to improving the initial oxidation temperature of the abrasives and the ability to be wetted by vitrified bonds during high sintering temperature. Given the fact that a controllable coating morphology can be achieved with simple preparation process, the oxide coating on abrasives is supposed to be the most potential and simplest solution to the difficult spreading and wetting by vitrified bonds on abrasive particles compared with the former two methods. Recently, researchers had found that the CeO2 coated SiO2 showed improved polishing performance in comparison to original SiO2 [13], demonstrating that CeO2 was feasible to be considered as the shell of abrasives. Nevertheless, no previous studies have investigated the CeO2 coated superhard abrasives in detail. In view of the above, the utilization of CeO2 as a coating layer on superhard abrasives is supposed to be
Corresponding author. School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, 430070, China. E-mail address: [email protected] (F. He).
https://doi.org/10.1016/j.ceramint.2019.06.222 Received 17 May 2019; Received in revised form 20 June 2019; Accepted 21 June 2019 Available online 22 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 19704–19709
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Fig. 1. Schematic of the preparation process of CeO2 coated cBN particles.
an effective method to improve the high-temperature wettability of the vitrified bond to the abrasives and upgrade the performance of the vitrified diamond/cBN composites. Meanwhile, it will, in turn, enrich the type of oxide coating on superhard abrasives. In this study, CeO2 coated cBN was designed and fabricated with the assistance of polyvinyl pyrrolidone (PVP), the mechanism of CeO2 coating on cBN was also analyzed. Besides, the microstructure and flexural strength of vitrified cBN composites were investigated. Finally, the chemical bonding between cBN, CeO2, and vitrified bonds was studied. 2. Experimental section
particles. In addition, flexural strength of vitrified cBN composites were conducted using a three-point test method in a universal material tester (AG-IC50KN, Shimadzu, Japan) at a crosshead speed of 0.5 mm/min, a fulcrum span of 25 mm, a loading speed of 9.8 ± 0.1 N/S. X-ray photoelectron spectroscopy (XPS) measurements were performed on ESCALAB 250Xi (Thermo Fisher Scientific, USA). Besides, a hot stage microscope (HSM, HM867, TA Instruments, USA) was employed to measure the contact angle between the vitrified bonds and original cBN films or CeO2 coated cBN films at 860 °C, where the cBN films were polycrystalline cBN tools (SNGN090304, HLCBN superhard, China) with smooth surface, and the preparation of CeO2 coated cBN films were the same as synthesis of CeO2 coated particles shown in Fig. 1. For measuring the contact angle, the vitrified bond powers were pressing into a cylinder with a diameter of 2 mm and a height of 4 mm. Then the cylindrical specimen was placed on the above mentioned cBN films with a dimension of 9.52 × 9.52 × 3.18 mm. And the contact angle test was carried out after the cBN films with cylindrical vitrified bond specimen on it was placed in HSM. Besides, thin VCeBC were prepared by using a Helios NanoLab G3 (Thermo Fisher Scientific, USA), then high angle annular dark field (HAADF) scanning transmission electron microscope (STEM) image, EDS mapping were collected by a Talos F200S (Thermo Fisher Scientific, USA) with EDS (X-Max 50, Oxford instruments, UK). 3. Results and discussions
2.1. Synthesis of CeO2 coated cBN and vitrified cBN composites Fig. 1 showed the preparation process of CeO2 coated cBN particles. Firstly, 70 ml solution A (ethanol solution of 0.1 g PVP and 1.0 g cBN), 20 ml solution B (1.25 mol/L hexamethylene tetramine (HMT) aqueous solution), and 20 ml solution C (0.25 mol/L Ce(NO3)3·6H2O·aqueous solution) were prepared. Then, solution A was stirred in a magnetic stirrer at room temperature for 0.5 h in a 250 ml beaker, and a 10 min continuous stirring was carried out to obtain a completely dispersed mixed solution after solution C was added into solution A. Finally, the solution B was added, drop by drop, into the above mentioned mixture, and vigorously stirring at 75 °C was performed for 2 h to obtain a milky white suspension. The suspension was then allowed to stand for 30 min, suction filtered, and washed with deionized water and ethanol, respectively. Finally, the CeO2 coated cBN particles were successfully obtained after the products were dried at 100 °C for 12 h. Vitrified bonds were prepared by a traditional melt-quenching method which was shown in our previous study [14]. In addition, vitrified cBN composites were prepared as follows: original cBN particles or CeO2 coated cBN particles, vitrified bonds, and dextrin were well mixed, and 2.0 g mixed powders were pressed into 40 × 6 × 5 mm under 50 MPa for 1 min. Here, vitrified original cBN composites containing original cBN particles were marked as “VOBC” and vitrified CeO2 coated cBN composites including CeO2 coated cBN particles were named as “VCeBC” for the convenience of description. The above regular shaped composites were sintered in a muffle furnace. The sintering schedule was firstly heated to 500 °C at 2.5 °C/ min, then raised to 860 °C at 5 °C/min and held at this temperature for 2 h. After that, the furnace was cooled down to 575 °C at 5 °C/min and the temperature was held for 1 h. Finally, the vitrified cBN composites were furnace-cooled. 2.2. Measurement methods A field emission scanning electron microscope (FESEM, ULTRA PLUS, Zeiss, Germany) equipped with an energy dispersive spectrometer (EDS, X-Max 50, Oxford instruments, UK) was used to observe the microstructure of original cBN and CeO2 coated cBN particles. X-ray diffraction (XRD, D8 Advance, Bruker, Germany) with Cu Kα radiation source under a 2θ range of 10 °–70° at a rate of 10°/min was utilized to identify the phase composition of original cBN and CeO2 coated cBN
3.1. Characterization of CeO2 coated cBN particles Fig. 2 shows the SEM and EDS spectrum of original cBN and CeO2 coated cBN. Comparing Fig. 2 (a) with Fig. 2 (b), it was clearly observed that the surface of original cBN was smooth with only some impurities adhering to it, while a thin and uniform coating composed of nanoscale particles was found to cover the surface of CeO2 coated cBN. Besides, Fig. 2 (c) revealed that the existence of B and N in the original cBN and no other elements except for Al, Si, and Pt which was a result of the conductive carbon adhesive tapes and platinum coating on samples for finishing the SEM tests were detected. However, Fig. 2 (d) confirmed the presence of Ce in the surface of CeO2 coated cBN, since the thickness of the coating was thin, only 0.28 at. % Ce was detected indicating the main elemental composition of the coating on cBN was Ce. XRD patterns of original cBN particles and CeO2 coated cBN particles are shown in Fig. 3. MgO, which was evidenced by the diffraction peaks at 2θ = 42.82° and 74.51°, might be introduced during the synthesis of original cBN [15]. It could be found that the most prominent difference between the two kinds of cBN particles in XRD was that weak diffraction peaks of CeO2 crystal phase had been detected in CeO2 coated cBN particles. Combined with the analysis of Fig. 2, the uniform, dense, and flat coatings precipitated on the surface of cBN in Fig. 2 (b) were confirmed to be CeO2 coatings. Moreover, comparing the intensity of diffraction peak of cBN for original cBN particles and CeO2 coated cBN particles, the cBN peak intensity of CeO2 coated cBN was found to be weaker than original cBN, this was ascribed to the fact that the dense CeO2 coating on the surface of cBN prevented the diffraction of inner cBN particles to some degree. This phenomenon was also reported in CeO2 coated SiO2 particles [16]. XPS was used to characterize the chemical bonding state between the coatings and cBN. The N 1s peak shown in Fig. 4 (a) had been fitted by two binding energy peak at 397.4 eV and 398.4 eV corresponding to N–Ce bonding [17] and N–B bonding [18], respectively. The presence of N–Ce indicates that the bonding between CeO2 and cBN is stable and compact. Besides, Ce 3d spectra in Fig. 4 (b) had been deconvoluted into four pairs of the spin-orbital doublet peaks: 883.2 eV (v)/901.8 eV (u), 887.0 eV (vI)/904.5 eV (uI), 891.0 eV (vII)/907.5 eV (uII), and 899.1 eV (vIII)/917.0 eV (uIII), which were consistent with the results in the literature [19]. The peaks denoted as v, vII, vIII, u, uII, and uIII are characteristic of Ce4+ and the peaks named as vI and uI correspond to
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Fig. 2. SEM images of (a) original cBN, (b) CeO2 coated cBN; and EDS spectra of (c) area M labeled in (a) and (d) area N labeled in (b).
Ce3+ [20]. In addition, the content of Ce3+ can be calculated by the following equation [21]:
Ce3 +% =
Fig. 3. XRD pattern of original cBN particles and CeO2 coated cBN particles.
(A v Ι + A uΙ ) (Av + A v Ι + A v ΙI + A v ΙII + Au + A uΙ + A uΙI + A v ΙII )
(1)
where A represents the area corresponding to each peak in Fig. 4 (b), and the calculation result for the Ce3+ content of the CeO2 coated on the cBN surface was about 20.34%. The formation mechanism of CeO2 coated cBN is proposed illustrated in Fig. 5. It was shown that PVP and cBN were connected by hydrogen bonding after solution A was stirred in a magnetic stirrer at room temperature for 0.5 h. Then HMT which acted as slow-release precipitant would slowly hydrolyze to generate ammonia and formaldehyde under a continuous heating and stirring (Eq. (2)), and ammonia would further hydrolyze to produce OH− (Eq. (3)) which would result in the connection between a large amount of OH− with PVP [22]. According to the literature, PVP is a non-ionic surfactant and tends to be significantly adsorbed onto Lewis acid sites in oxides by hydrogen bonding [23], while Ce3+ cation acted as Lewis acid sites [24] in the mixed solution so that PVP could link with Ce3+. Meanwhile, the exchange reaction would occur between Ce(NO)3 and OH− (Eq. (4)).
Fig. 4. XPS N 1s (a) and Ce 3d (b) spectra of the CeO2 coated cBN particles. 19706
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Fig. 7. The contact angle of vitrified bond on cBN films at 860 °C; (a) Original cBN, (b) CeO2 coated cBN.
Fig. 5. Synthetic mechanism of CeO2 coating on cBN particles.
Thus, the incorporation of PVP was favorable for the adhesion of Ce3+ onto the surface of cBN particles. Thereafter, these Ce3+ adsorbed on cBN would combine with OH− in the mixed solution to form Ce (OH)3·nH2O (Eq. (5)), thereby a core-shell structure was synthesized. Besides, Ce3+ was promoted to be oxidized to Ce4+ in alkaline aqueous solution [25]. Consequently, a Ce(OH)4 coating was generated on the surface of cBN (Eq. (6)). When the composites were dried, Ce(OH)4 was supposed to dehydrate to form CeO2 (Eq. (7)) [26]. Finally, CeO2coated cBN particles were obtained. Moreover, during the process of synthesis, PVP showed good ability to promote the nucleated CeO2 to aggregate with each other and grow in an anisotropic direction, inhibiting the agglomeration of CeO2 coating [27,28]. Therefore, the uniform dispersion of CeO2 coating on the cBN surface was achieved.
C6 H12 N4 + 6H2 O→ 6H2 CO + 4NH3
(2)
NH+4
(3)
NH3 + H2 O→
+
OH−
Ce − O− NO2 + OH− → Ce − OH + NO−3
(4)
Ce3 +
+ nH2 O→ Ce(OH)3 ·nH2 O
(5)
4Ce(OH)3·nH2 O+ O2 → 4CeO2 ·nH2 O+ 4H3 O+ + 2O22 −
(6)
CeO2 ·nH2 O→ CeO2 + nH2 O
(7)
+
3OH−
3.2. Characterization of vitrified cBN composites Fig. 6 shows SEM images of the cross-section for VOBC and VCeBC, due to the same preparation conditions for the vitrified cBN composites and the SEM tests of these two samples, the probability of the fracture surface occurring at the interface of cBN and vitrified bonds was considered to be equal for VOBC and VCeBC. Besides, during the sintering of vitrified cBN composites, the melting and flowing vitrified bonds would wet and spread at the surfaces of cBN particles, the worse the wetting state between cBN and vitrified was, the more the bare cBN particles would stay in the vitrified cBN composites, which indicated the weaker bonding between the cBN and vitrified bonds. Thus, in Fig. 6, it was clearly evident that the number of bare cBN particles which were unable to be covered and wetted by vitrified bonds in VOBC
Fig. 6. SEM images of the cross-section for (a) VOBC and (b) VCeBC. The bare cBN particles which were not wetted and covered by vitrified bonds were colored in turquoise.
was less than which was shown in VCeBC, indicating that CeO2 coating on cBN promoted the wettability of vitrified bonds onto the cBN surface when sintering temperature was 860 °C. Fig. 7 illustrates the contact angle of vitrified bond on cBN films at 860 °C, it could be seen that the contact angle between vitrified bonds and original cBN at 860 °C was 49°, while it was 42° when cBN was coated with CeO2. As it is well known that the closer the contact angle was to 0°, the more improved wetting and stronger adhesion of the vitrified bond on the cBN surface will be achieved. Hence, the rough CeO2 layer on the surface enhances the wettability of the cBN by vitrified bonds under sintering temperature compared with the smooth original cBN surface. Correspondingly, the flexural strength of VOBC was 114.09 MPa, while it was 124.54 MPa for VCeBC. The above results reveal that the wettability of vitrified bonds onto cBN has been improved by CeO2 coating on the surface of cBN which is beneficial for enhancing the flexural strength of vitrified cBN composites. Fig. 8 shows the HAADF-STEM image and EDS mapping of VCeBC. As observed in Fig. 8 (a), the left darker part in this figure is determined as cBN, and the right part corresponds to the vitrified bonds wetting on the surface of cBN, and the thickness of this part is about 184 nm. Moreover, the cBN is proved to be tightly bonded with vitrified bond for no obvious cracks are observed at the interface of cBN and vitrified bonds, indicating that good wetting and strong adhesion of the vitrified bond on the cBN surface. Furthermore, the existence and distribution of Si and Al on the surface of cBN in Fig. 8 (b) and (c) further confirms the completely spreading and wetting of vitrified bonds on cBN. Besides, in comparison of adding CeO2 into vitrified bonds during the preparation of bonds, the CeO2 coating on the cBN surface has a double effect on improving the high-temperature wetting of the vitrified bonds on cBN, on the one hand the rough CeO2 coating on cBN is beneficial for the wetting of the cBN by vitrified bonds at 860 °C (Fig. 7), on the other hand the CeO2 will diffuse from cBN surface into the vitrified bond at 860 °C (Fig. 8 (d)) which contributes in improving the fluidity of the vitrified bonds [29]. Thus, an improved wetting and stronger adhesion of the vitrified bond on the cBN bonds had been achieved.
Fig. 8. (a) HAADF-STEM image of VCeBC; (b)–(d) EDS elemental mapping results: (b) Si, (c) Al, (d) Ce.
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Fig. 9. XPS N 1s (a) and Ce 3d (b) spectra of vitrified CeO2 coated cBN composites.
Fig. 9 presents XPS spectra of the N 1s and Ce 3d spectra of VCeBC. The N 1s photoelectron peak split into two peaks, see Fig. 9 (a). The XPS photoelectron peak at 398.4 eV indicates the N–B bonding originating from cBN particles. However, the N 1s peak at 397.7 eV assigned to N–Si bonding [30] is due to the photoelectron peak that originates from the chemical bonding between cBN and vitrified bonds for the source of Si was vitrified bonds. Besides, the binding energy of the deconvoluted peak of Ce 3d in Fig. 9 (b) was same as the ones shown in Fig. 4 (b) except for uIII. The peak of uIII was found to be 912.5 eV for VCeBC (Fig. 9 (b)), while it was 917.0 eV for CeO2 coated cBN particles (Fig. 4 (b)). Actually, the shift in binding energy of uIII was also found in the mixed oxides of CeO2–Al2O3 studied by other researchers [31,32], providing evidence of an interaction between CeO2 and Al2O3 with the formation of CeAlO3 (Eq. (8)).
Al2O3 + Ce2 O3 → 2CeAlO3
(8)
The thermodynamic calculation of Eq. (8) showed the ΔGr (Gibbs free energy) of CeAlO3 formed by the reaction of Al2O3 with Ce2O3 at 860 °C was −55.7 kJ/mol, demonstrating that the above motioned reaction could spontaneously occur at 860 °C. Therefore, chemical bonding between CeO2 and vitrified bonds was theoretically feasible and reasonable to be formed. The following explanation is presented to help clarify the formation of chemical bonding between CeO2 coated cBN and vitrified bonds after a comprehensive analysis of Figs. 8 and 9: the CeO2 coatings on cBN surface would enter into vitrified bond through the self-diffusion reaction when the composites were sintered at 860 °C. Then CeAlO3 precursor generated by the reaction of CeO2 and Al2O3 would exist on the surface of Al2O3 [33], with the nucleation and growth of CeAlO3 precursor, the stable chemical bonding between CeO2 and vitrified bond would generate. Furthermore, compared with the content of Ce3+ was 20.34% for CeO2 coated cBN particles, it was 35.82% in Fig. 9 (b) for vitrified CeO2 coated cBN composites according to Eq. (1). An increasing in concentration of Ce3+ in vitrified CeO2 coated cBN composites was attributed to the diffusion of CeO2 into glass structure, which tended to release free oxygen to destroy the glass network structure of vitrified bonds when the composites were sintered at 860 °C [34]. Besides, the fact that Ce4+ is easily reduced to Ce3+ under high temperature is another reason for the increasing concentration of Ce3+ in vitrified CeO2 coated cBN composites [35]. As for Eq. (8), the more content of Ce3+ in CeO2 is, the more intense the reaction of CeAlO3 production would be, which promotes the formation of stable chemical bonds between the CeO2 and vitrified bonds. Hence, the combination of Ce–O–Al bonding formed between the CeO2 coating and vitrified bonds as well as the N–Si bonding between the vitrified bonds and the cBN particles provides stable and reliable chemical bonding for improving the holding power of the vitrified bonds to cBN particles. Thus, the presence of the CeO2 coating on cBN abrasives is theoretically beneficial to
the enhanced flexural strength of the vitrified cBN composites. 4. Conclusion In this work, CeO2 coated cBN particles and vitrified CeO2 coated cBN composites were successfully prepared. The results demonstrated that the uniform and dense CeO2 coating on cBN promoted the wettability of the cBN by vitrified bonds and the Ce would diffuse into vitrified bonds when the vitrified CeO2 coated cBN composites were sintered at 860 °C. Besides, the prepared vitrified CeO2 coated cBN composites showed 9.16% higher strength in bending than vitrified cBN composites, which was ascribed to the stable and reliable chemical bonding originated from the combination of Ce–O–Al and N–Si bonding in vitrified CeO2 coated cBN composites. Hence, the holding power of the vitrified bonds to cBN particles was enhanced after the cBN particles were coated with CeO2, which was contributed to strengthening the bonding between cBN and vitrified bonds. Acknowledgments This work was supported by research fund of Center for Materials Research and Analysis, Wuhan University of Technology (2018KFJJ11). Besides, the authors were grateful to Dr. De Fang (Center for Materials Research and Analysis, Wuhan University of Technology, Wuhan 430070, China) for his assistance with the XPS analyses. References [1] J.F.G. Oliveira, E.J. Silva, C. Guo, et al., Industrial challenges in grinding, CIRP Ann. - Manuf. Technol. 58 (2) (2009) 663–680. [2] N. Yan, W. Miao, Y. Zhao, et al., Effects of titania films on the oxidation resistance and dispersibility of ultrafine diamond, Mater. Lett. 141 (2015) 92–95. [3] X. Yang, J. Bai, W. Jing, et al., Strengthening of low-temperature sintered vitrified bond cBN grinding wheels by pre-oxidation of cBN abrasives, Ceram. Int. 42 (7) (2016) 9283–9286. [4] R. Chang, J. Zang, Y. Wang, et al., Study of Ti-coated diamond grits prepared by spark plasma coating, Diam. Relat. Mater. 77 (2017) 72–78. [5] R. Chang, Y. Wang, J. Zang, et al., Investigation of Ti coatings on cubic boron nitride (cBN) grits by discharge treatment in spark plasma sintering system, Adv. Powder Technol. 28 (9) (2017) 2281–2287. [6] Y. Zhao, B. Zhang, N. Yao, et al., Improved field emission properties from metalcoated diamond films, Diam. Relat. Mater. 16 (3) (2007) 650–653. [7] Y. Zhu, L. Wang, W. Yao, et al., The interface diffusion and reaction between Cr layer and diamond particle during metallization, Appl. Surf. Sci. 171 (1–2) (2001) 143–150. [8] D. Zhao, Z. Wang, Y. Xi, et al., Preparation of silica-coated ultrafine diamond and dispersion in ceramic matrix, Mater. Lett. 113 (24) (2013) 134–137. [9] W. Miao, N. Yan, Y. Zhao, et al., Synthesis and application of titania-coated ultrafine diamond abrasive particles, Ceram. Int. 42 (7) (2016) 8884–8890. [10] X. Yan, X. Li, X. Wang, et al., Synthesis of nano-diamond/alumina composite by detonation method, Diam. Relat. Mater. 77 (2017) 79–83. [11] Y. Wang, Y. Yuan, X. Cheng, et al., Inhibiting the oxidation of diamond during preparing the vitrified dental grinding tools by depositing a ZnO coating using direct urea precipitation method, Mater. Sci. Eng. C 53 (2015) 23–28. [12] W. Hu, L. Wan, X. Liu, et al., Effect of TiO2/Al2O3 film coated diamond abrasive particles by sol-gel technique, Appl. Surf. Sci. 257 (13) (2011) 5777–5783.
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Preparation and characterization of vitrified CeO2 coated cBN composites Jiang Shi a b c
a,b
, Feng He
a,b,∗
a,b
, Junlin Xie
c
, Xiaoqing Liu , Hu Yang
T
b
State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, 430070, China School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, 430070, China Center for Materials Research and Analysis, Wuhan University of Technology, Wuhan, 430070, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ceria cBN Vitrified bonds Chemical bonding
The performances of vitrified cBN composites are deeply affected by the wettability of vitrified bonds on cBN particles. CeO2 coated cBN particles were successfully prepared for the further improvement of the covering and wetting of cBN by vitrified bonds. The microstructure and properties of vitrified cBN composites were characterized by scanning electron microscope (SEM), hot stage microscope (HSM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and flexural strength. Results showed that the prepared CeO2 coating on the surface of cBN was uniform and dense. Besides, the improved wettability of vitrified bonds on CeO2 coated cBN particles accompanied with the formation of Ce–O–Al and N–Si confirmed by XPS were supposed to conduce to enhancing the holding power of the vitrified bonds to cBN particles, which resulted in increasing the flexural strength of vitrified cBN composites by 9.16%. Thus, coating cBN with CeO2 was a potential and effective method to obtain vitrified cBN composites with higher flexural strength.
1. Introduction High-performance superhard grinding tools have found numerous applications in modern advanced manufacturing technology for exhibiting high processing precision, high efficiency, and long service life. Owing to their excellent chemical stability and hardness, superhard abrasives, mainly including diamond and cubic boron nitride (cBN), are of foremost importance in the performance of superhard grinding tools [1]. During the process of preparing superhard grinding tools at high sintering temperature, vitrified bonds, which are considered to contain partial or whole amorphous phase, play critical roles in tight bonding the abrasives together. However, the smooth surfaces with a low surface free energy of diamond and cBN offer a significant challenge to the complete spreading and wetting by vitrified bonds on their surfaces at sintering temperature [2], which is responsible for the weak holding power of vitrified bonds on abrasives. Thus, the diamond or cBN particles are unable to be tightly consolidated and the shedding of abrasives may easily occur during the processing of workpieces with the superhard grinding tools, resulting in a low processing efficiency, short service life, and high processing cost. In order to improve the infiltration of vitrified bonds on superhard abrasives, three typical methods had been discussed by researchers: (1) Pre-oxidation of abrasives. Researchers found that the pre-oxidation of cBN particles under 900 °C for
∗
60 min is effective for improving the wetting on cBN by vitrified bonds [3], while the degree of pre-oxidation for abrasives is known to be uncontrollable which might induce the property deterioration of abrasives. (2) Surface metallization of abrasives. Metal plating on abrasives including Ti [4,5], Ni [6], and Cr [7] can enhance the wetting of the abrasives with vitrified bonds by forming chemical bonding between the metal plating and the abrasives. However, complex and expensive equipment are necessary for achieving the surface metallization. (3) Oxide coating on abrasives. Experiments had proved that oxide coating on abrasives, such as SiO2 [8], TiO2 [9], Al2O3 [10], ZnO [11], and TiO2–Al2O3 [12], contributed to improving the initial oxidation temperature of the abrasives and the ability to be wetted by vitrified bonds during high sintering temperature. Given the fact that a controllable coating morphology can be achieved with simple preparation process, the oxide coating on abrasives is supposed to be the most potential and simplest solution to the difficult spreading and wetting by vitrified bonds on abrasive particles compared with the former two methods. Recently, researchers had found that the CeO2 coated SiO2 showed improved polishing performance in comparison to original SiO2 [13], demonstrating that CeO2 was feasible to be considered as the shell of abrasives. Nevertheless, no previous studies have investigated the CeO2 coated superhard abrasives in detail. In view of the above, the utilization of CeO2 as a coating layer on superhard abrasives is supposed to be
Corresponding author. School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, 430070, China. E-mail address: [email protected] (F. He).
https://doi.org/10.1016/j.ceramint.2019.06.222 Received 17 May 2019; Received in revised form 20 June 2019; Accepted 21 June 2019 Available online 22 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. Schematic of the preparation process of CeO2 coated cBN particles.
an effective method to improve the high-temperature wettability of the vitrified bond to the abrasives and upgrade the performance of the vitrified diamond/cBN composites. Meanwhile, it will, in turn, enrich the type of oxide coating on superhard abrasives. In this study, CeO2 coated cBN was designed and fabricated with the assistance of polyvinyl pyrrolidone (PVP), the mechanism of CeO2 coating on cBN was also analyzed. Besides, the microstructure and flexural strength of vitrified cBN composites were investigated. Finally, the chemical bonding between cBN, CeO2, and vitrified bonds was studied. 2. Experimental section
particles. In addition, flexural strength of vitrified cBN composites were conducted using a three-point test method in a universal material tester (AG-IC50KN, Shimadzu, Japan) at a crosshead speed of 0.5 mm/min, a fulcrum span of 25 mm, a loading speed of 9.8 ± 0.1 N/S. X-ray photoelectron spectroscopy (XPS) measurements were performed on ESCALAB 250Xi (Thermo Fisher Scientific, USA). Besides, a hot stage microscope (HSM, HM867, TA Instruments, USA) was employed to measure the contact angle between the vitrified bonds and original cBN films or CeO2 coated cBN films at 860 °C, where the cBN films were polycrystalline cBN tools (SNGN090304, HLCBN superhard, China) with smooth surface, and the preparation of CeO2 coated cBN films were the same as synthesis of CeO2 coated particles shown in Fig. 1. For measuring the contact angle, the vitrified bond powers were pressing into a cylinder with a diameter of 2 mm and a height of 4 mm. Then the cylindrical specimen was placed on the above mentioned cBN films with a dimension of 9.52 × 9.52 × 3.18 mm. And the contact angle test was carried out after the cBN films with cylindrical vitrified bond specimen on it was placed in HSM. Besides, thin VCeBC were prepared by using a Helios NanoLab G3 (Thermo Fisher Scientific, USA), then high angle annular dark field (HAADF) scanning transmission electron microscope (STEM) image, EDS mapping were collected by a Talos F200S (Thermo Fisher Scientific, USA) with EDS (X-Max 50, Oxford instruments, UK). 3. Results and discussions
2.1. Synthesis of CeO2 coated cBN and vitrified cBN composites Fig. 1 showed the preparation process of CeO2 coated cBN particles. Firstly, 70 ml solution A (ethanol solution of 0.1 g PVP and 1.0 g cBN), 20 ml solution B (1.25 mol/L hexamethylene tetramine (HMT) aqueous solution), and 20 ml solution C (0.25 mol/L Ce(NO3)3·6H2O·aqueous solution) were prepared. Then, solution A was stirred in a magnetic stirrer at room temperature for 0.5 h in a 250 ml beaker, and a 10 min continuous stirring was carried out to obtain a completely dispersed mixed solution after solution C was added into solution A. Finally, the solution B was added, drop by drop, into the above mentioned mixture, and vigorously stirring at 75 °C was performed for 2 h to obtain a milky white suspension. The suspension was then allowed to stand for 30 min, suction filtered, and washed with deionized water and ethanol, respectively. Finally, the CeO2 coated cBN particles were successfully obtained after the products were dried at 100 °C for 12 h. Vitrified bonds were prepared by a traditional melt-quenching method which was shown in our previous study [14]. In addition, vitrified cBN composites were prepared as follows: original cBN particles or CeO2 coated cBN particles, vitrified bonds, and dextrin were well mixed, and 2.0 g mixed powders were pressed into 40 × 6 × 5 mm under 50 MPa for 1 min. Here, vitrified original cBN composites containing original cBN particles were marked as “VOBC” and vitrified CeO2 coated cBN composites including CeO2 coated cBN particles were named as “VCeBC” for the convenience of description. The above regular shaped composites were sintered in a muffle furnace. The sintering schedule was firstly heated to 500 °C at 2.5 °C/ min, then raised to 860 °C at 5 °C/min and held at this temperature for 2 h. After that, the furnace was cooled down to 575 °C at 5 °C/min and the temperature was held for 1 h. Finally, the vitrified cBN composites were furnace-cooled. 2.2. Measurement methods A field emission scanning electron microscope (FESEM, ULTRA PLUS, Zeiss, Germany) equipped with an energy dispersive spectrometer (EDS, X-Max 50, Oxford instruments, UK) was used to observe the microstructure of original cBN and CeO2 coated cBN particles. X-ray diffraction (XRD, D8 Advance, Bruker, Germany) with Cu Kα radiation source under a 2θ range of 10 °–70° at a rate of 10°/min was utilized to identify the phase composition of original cBN and CeO2 coated cBN
3.1. Characterization of CeO2 coated cBN particles Fig. 2 shows the SEM and EDS spectrum of original cBN and CeO2 coated cBN. Comparing Fig. 2 (a) with Fig. 2 (b), it was clearly observed that the surface of original cBN was smooth with only some impurities adhering to it, while a thin and uniform coating composed of nanoscale particles was found to cover the surface of CeO2 coated cBN. Besides, Fig. 2 (c) revealed that the existence of B and N in the original cBN and no other elements except for Al, Si, and Pt which was a result of the conductive carbon adhesive tapes and platinum coating on samples for finishing the SEM tests were detected. However, Fig. 2 (d) confirmed the presence of Ce in the surface of CeO2 coated cBN, since the thickness of the coating was thin, only 0.28 at. % Ce was detected indicating the main elemental composition of the coating on cBN was Ce. XRD patterns of original cBN particles and CeO2 coated cBN particles are shown in Fig. 3. MgO, which was evidenced by the diffraction peaks at 2θ = 42.82° and 74.51°, might be introduced during the synthesis of original cBN [15]. It could be found that the most prominent difference between the two kinds of cBN particles in XRD was that weak diffraction peaks of CeO2 crystal phase had been detected in CeO2 coated cBN particles. Combined with the analysis of Fig. 2, the uniform, dense, and flat coatings precipitated on the surface of cBN in Fig. 2 (b) were confirmed to be CeO2 coatings. Moreover, comparing the intensity of diffraction peak of cBN for original cBN particles and CeO2 coated cBN particles, the cBN peak intensity of CeO2 coated cBN was found to be weaker than original cBN, this was ascribed to the fact that the dense CeO2 coating on the surface of cBN prevented the diffraction of inner cBN particles to some degree. This phenomenon was also reported in CeO2 coated SiO2 particles [16]. XPS was used to characterize the chemical bonding state between the coatings and cBN. The N 1s peak shown in Fig. 4 (a) had been fitted by two binding energy peak at 397.4 eV and 398.4 eV corresponding to N–Ce bonding [17] and N–B bonding [18], respectively. The presence of N–Ce indicates that the bonding between CeO2 and cBN is stable and compact. Besides, Ce 3d spectra in Fig. 4 (b) had been deconvoluted into four pairs of the spin-orbital doublet peaks: 883.2 eV (v)/901.8 eV (u), 887.0 eV (vI)/904.5 eV (uI), 891.0 eV (vII)/907.5 eV (uII), and 899.1 eV (vIII)/917.0 eV (uIII), which were consistent with the results in the literature [19]. The peaks denoted as v, vII, vIII, u, uII, and uIII are characteristic of Ce4+ and the peaks named as vI and uI correspond to
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Fig. 2. SEM images of (a) original cBN, (b) CeO2 coated cBN; and EDS spectra of (c) area M labeled in (a) and (d) area N labeled in (b).
Ce3+ [20]. In addition, the content of Ce3+ can be calculated by the following equation [21]:
Ce3 +% =
Fig. 3. XRD pattern of original cBN particles and CeO2 coated cBN particles.
(A v Ι + A uΙ ) (Av + A v Ι + A v ΙI + A v ΙII + Au + A uΙ + A uΙI + A v ΙII )
(1)
where A represents the area corresponding to each peak in Fig. 4 (b), and the calculation result for the Ce3+ content of the CeO2 coated on the cBN surface was about 20.34%. The formation mechanism of CeO2 coated cBN is proposed illustrated in Fig. 5. It was shown that PVP and cBN were connected by hydrogen bonding after solution A was stirred in a magnetic stirrer at room temperature for 0.5 h. Then HMT which acted as slow-release precipitant would slowly hydrolyze to generate ammonia and formaldehyde under a continuous heating and stirring (Eq. (2)), and ammonia would further hydrolyze to produce OH− (Eq. (3)) which would result in the connection between a large amount of OH− with PVP [22]. According to the literature, PVP is a non-ionic surfactant and tends to be significantly adsorbed onto Lewis acid sites in oxides by hydrogen bonding [23], while Ce3+ cation acted as Lewis acid sites [24] in the mixed solution so that PVP could link with Ce3+. Meanwhile, the exchange reaction would occur between Ce(NO)3 and OH− (Eq. (4)).
Fig. 4. XPS N 1s (a) and Ce 3d (b) spectra of the CeO2 coated cBN particles. 19706
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Fig. 7. The contact angle of vitrified bond on cBN films at 860 °C; (a) Original cBN, (b) CeO2 coated cBN.
Fig. 5. Synthetic mechanism of CeO2 coating on cBN particles.
Thus, the incorporation of PVP was favorable for the adhesion of Ce3+ onto the surface of cBN particles. Thereafter, these Ce3+ adsorbed on cBN would combine with OH− in the mixed solution to form Ce (OH)3·nH2O (Eq. (5)), thereby a core-shell structure was synthesized. Besides, Ce3+ was promoted to be oxidized to Ce4+ in alkaline aqueous solution [25]. Consequently, a Ce(OH)4 coating was generated on the surface of cBN (Eq. (6)). When the composites were dried, Ce(OH)4 was supposed to dehydrate to form CeO2 (Eq. (7)) [26]. Finally, CeO2coated cBN particles were obtained. Moreover, during the process of synthesis, PVP showed good ability to promote the nucleated CeO2 to aggregate with each other and grow in an anisotropic direction, inhibiting the agglomeration of CeO2 coating [27,28]. Therefore, the uniform dispersion of CeO2 coating on the cBN surface was achieved.
C6 H12 N4 + 6H2 O→ 6H2 CO + 4NH3
(2)
NH+4
(3)
NH3 + H2 O→
+
OH−
Ce − O− NO2 + OH− → Ce − OH + NO−3
(4)
Ce3 +
+ nH2 O→ Ce(OH)3 ·nH2 O
(5)
4Ce(OH)3·nH2 O+ O2 → 4CeO2 ·nH2 O+ 4H3 O+ + 2O22 −
(6)
CeO2 ·nH2 O→ CeO2 + nH2 O
(7)
+
3OH−
3.2. Characterization of vitrified cBN composites Fig. 6 shows SEM images of the cross-section for VOBC and VCeBC, due to the same preparation conditions for the vitrified cBN composites and the SEM tests of these two samples, the probability of the fracture surface occurring at the interface of cBN and vitrified bonds was considered to be equal for VOBC and VCeBC. Besides, during the sintering of vitrified cBN composites, the melting and flowing vitrified bonds would wet and spread at the surfaces of cBN particles, the worse the wetting state between cBN and vitrified was, the more the bare cBN particles would stay in the vitrified cBN composites, which indicated the weaker bonding between the cBN and vitrified bonds. Thus, in Fig. 6, it was clearly evident that the number of bare cBN particles which were unable to be covered and wetted by vitrified bonds in VOBC
Fig. 6. SEM images of the cross-section for (a) VOBC and (b) VCeBC. The bare cBN particles which were not wetted and covered by vitrified bonds were colored in turquoise.
was less than which was shown in VCeBC, indicating that CeO2 coating on cBN promoted the wettability of vitrified bonds onto the cBN surface when sintering temperature was 860 °C. Fig. 7 illustrates the contact angle of vitrified bond on cBN films at 860 °C, it could be seen that the contact angle between vitrified bonds and original cBN at 860 °C was 49°, while it was 42° when cBN was coated with CeO2. As it is well known that the closer the contact angle was to 0°, the more improved wetting and stronger adhesion of the vitrified bond on the cBN surface will be achieved. Hence, the rough CeO2 layer on the surface enhances the wettability of the cBN by vitrified bonds under sintering temperature compared with the smooth original cBN surface. Correspondingly, the flexural strength of VOBC was 114.09 MPa, while it was 124.54 MPa for VCeBC. The above results reveal that the wettability of vitrified bonds onto cBN has been improved by CeO2 coating on the surface of cBN which is beneficial for enhancing the flexural strength of vitrified cBN composites. Fig. 8 shows the HAADF-STEM image and EDS mapping of VCeBC. As observed in Fig. 8 (a), the left darker part in this figure is determined as cBN, and the right part corresponds to the vitrified bonds wetting on the surface of cBN, and the thickness of this part is about 184 nm. Moreover, the cBN is proved to be tightly bonded with vitrified bond for no obvious cracks are observed at the interface of cBN and vitrified bonds, indicating that good wetting and strong adhesion of the vitrified bond on the cBN surface. Furthermore, the existence and distribution of Si and Al on the surface of cBN in Fig. 8 (b) and (c) further confirms the completely spreading and wetting of vitrified bonds on cBN. Besides, in comparison of adding CeO2 into vitrified bonds during the preparation of bonds, the CeO2 coating on the cBN surface has a double effect on improving the high-temperature wetting of the vitrified bonds on cBN, on the one hand the rough CeO2 coating on cBN is beneficial for the wetting of the cBN by vitrified bonds at 860 °C (Fig. 7), on the other hand the CeO2 will diffuse from cBN surface into the vitrified bond at 860 °C (Fig. 8 (d)) which contributes in improving the fluidity of the vitrified bonds [29]. Thus, an improved wetting and stronger adhesion of the vitrified bond on the cBN bonds had been achieved.
Fig. 8. (a) HAADF-STEM image of VCeBC; (b)–(d) EDS elemental mapping results: (b) Si, (c) Al, (d) Ce.
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Fig. 9. XPS N 1s (a) and Ce 3d (b) spectra of vitrified CeO2 coated cBN composites.
Fig. 9 presents XPS spectra of the N 1s and Ce 3d spectra of VCeBC. The N 1s photoelectron peak split into two peaks, see Fig. 9 (a). The XPS photoelectron peak at 398.4 eV indicates the N–B bonding originating from cBN particles. However, the N 1s peak at 397.7 eV assigned to N–Si bonding [30] is due to the photoelectron peak that originates from the chemical bonding between cBN and vitrified bonds for the source of Si was vitrified bonds. Besides, the binding energy of the deconvoluted peak of Ce 3d in Fig. 9 (b) was same as the ones shown in Fig. 4 (b) except for uIII. The peak of uIII was found to be 912.5 eV for VCeBC (Fig. 9 (b)), while it was 917.0 eV for CeO2 coated cBN particles (Fig. 4 (b)). Actually, the shift in binding energy of uIII was also found in the mixed oxides of CeO2–Al2O3 studied by other researchers [31,32], providing evidence of an interaction between CeO2 and Al2O3 with the formation of CeAlO3 (Eq. (8)).
Al2O3 + Ce2 O3 → 2CeAlO3
(8)
The thermodynamic calculation of Eq. (8) showed the ΔGr (Gibbs free energy) of CeAlO3 formed by the reaction of Al2O3 with Ce2O3 at 860 °C was −55.7 kJ/mol, demonstrating that the above motioned reaction could spontaneously occur at 860 °C. Therefore, chemical bonding between CeO2 and vitrified bonds was theoretically feasible and reasonable to be formed. The following explanation is presented to help clarify the formation of chemical bonding between CeO2 coated cBN and vitrified bonds after a comprehensive analysis of Figs. 8 and 9: the CeO2 coatings on cBN surface would enter into vitrified bond through the self-diffusion reaction when the composites were sintered at 860 °C. Then CeAlO3 precursor generated by the reaction of CeO2 and Al2O3 would exist on the surface of Al2O3 [33], with the nucleation and growth of CeAlO3 precursor, the stable chemical bonding between CeO2 and vitrified bond would generate. Furthermore, compared with the content of Ce3+ was 20.34% for CeO2 coated cBN particles, it was 35.82% in Fig. 9 (b) for vitrified CeO2 coated cBN composites according to Eq. (1). An increasing in concentration of Ce3+ in vitrified CeO2 coated cBN composites was attributed to the diffusion of CeO2 into glass structure, which tended to release free oxygen to destroy the glass network structure of vitrified bonds when the composites were sintered at 860 °C [34]. Besides, the fact that Ce4+ is easily reduced to Ce3+ under high temperature is another reason for the increasing concentration of Ce3+ in vitrified CeO2 coated cBN composites [35]. As for Eq. (8), the more content of Ce3+ in CeO2 is, the more intense the reaction of CeAlO3 production would be, which promotes the formation of stable chemical bonds between the CeO2 and vitrified bonds. Hence, the combination of Ce–O–Al bonding formed between the CeO2 coating and vitrified bonds as well as the N–Si bonding between the vitrified bonds and the cBN particles provides stable and reliable chemical bonding for improving the holding power of the vitrified bonds to cBN particles. Thus, the presence of the CeO2 coating on cBN abrasives is theoretically beneficial to
the enhanced flexural strength of the vitrified cBN composites. 4. Conclusion In this work, CeO2 coated cBN particles and vitrified CeO2 coated cBN composites were successfully prepared. The results demonstrated that the uniform and dense CeO2 coating on cBN promoted the wettability of the cBN by vitrified bonds and the Ce would diffuse into vitrified bonds when the vitrified CeO2 coated cBN composites were sintered at 860 °C. Besides, the prepared vitrified CeO2 coated cBN composites showed 9.16% higher strength in bending than vitrified cBN composites, which was ascribed to the stable and reliable chemical bonding originated from the combination of Ce–O–Al and N–Si bonding in vitrified CeO2 coated cBN composites. Hence, the holding power of the vitrified bonds to cBN particles was enhanced after the cBN particles were coated with CeO2, which was contributed to strengthening the bonding between cBN and vitrified bonds. Acknowledgments This work was supported by research fund of Center for Materials Research and Analysis, Wuhan University of Technology (2018KFJJ11). Besides, the authors were grateful to Dr. De Fang (Center for Materials Research and Analysis, Wuhan University of Technology, Wuhan 430070, China) for his assistance with the XPS analyses. References [1] J.F.G. Oliveira, E.J. Silva, C. Guo, et al., Industrial challenges in grinding, CIRP Ann. - Manuf. Technol. 58 (2) (2009) 663–680. [2] N. Yan, W. Miao, Y. Zhao, et al., Effects of titania films on the oxidation resistance and dispersibility of ultrafine diamond, Mater. Lett. 141 (2015) 92–95. [3] X. Yang, J. Bai, W. Jing, et al., Strengthening of low-temperature sintered vitrified bond cBN grinding wheels by pre-oxidation of cBN abrasives, Ceram. Int. 42 (7) (2016) 9283–9286. [4] R. Chang, J. Zang, Y. Wang, et al., Study of Ti-coated diamond grits prepared by spark plasma coating, Diam. Relat. Mater. 77 (2017) 72–78. [5] R. Chang, Y. Wang, J. Zang, et al., Investigation of Ti coatings on cubic boron nitride (cBN) grits by discharge treatment in spark plasma sintering system, Adv. Powder Technol. 28 (9) (2017) 2281–2287. [6] Y. Zhao, B. Zhang, N. Yao, et al., Improved field emission properties from metalcoated diamond films, Diam. Relat. Mater. 16 (3) (2007) 650–653. [7] Y. Zhu, L. Wang, W. Yao, et al., The interface diffusion and reaction between Cr layer and diamond particle during metallization, Appl. Surf. Sci. 171 (1–2) (2001) 143–150. [8] D. Zhao, Z. Wang, Y. Xi, et al., Preparation of silica-coated ultrafine diamond and dispersion in ceramic matrix, Mater. Lett. 113 (24) (2013) 134–137. [9] W. Miao, N. Yan, Y. Zhao, et al., Synthesis and application of titania-coated ultrafine diamond abrasive particles, Ceram. Int. 42 (7) (2016) 8884–8890. [10] X. Yan, X. Li, X. Wang, et al., Synthesis of nano-diamond/alumina composite by detonation method, Diam. Relat. Mater. 77 (2017) 79–83. [11] Y. Wang, Y. Yuan, X. Cheng, et al., Inhibiting the oxidation of diamond during preparing the vitrified dental grinding tools by depositing a ZnO coating using direct urea precipitation method, Mater. Sci. Eng. C 53 (2015) 23–28. [12] W. Hu, L. Wan, X. Liu, et al., Effect of TiO2/Al2O3 film coated diamond abrasive particles by sol-gel technique, Appl. Surf. Sci. 257 (13) (2011) 5777–5783.
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