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Effects of graphene addition on mechanical properties of polycrystalline diamond compact
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effects of graphene addition on mechanical properties of polycrystalline diamond compact Zhaoran Chena, Dejiang Mab, Shanmin Wangb, Wenhao Daia, Siqi Lia, Yiqing Zhuc, Baochang Liua,d,e,∗ a
College of Construction Engineering, Jilin University, Changchun, China Southern University of Science and Technology, Shenzhen, China c Shanghai Institute of Geological Engineering Exploration, Shanghai, China d Key Laboratory of Drilling and Exploitation Technology in Complex Conditions of Ministry of Natural Resources, Changchun, 130026, China e State Key Laboratory of Superhard Materials, Changchun, 130021, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Polycrystalline diamond Graphene Wear resistance Thermal conductivity Electrical conductivity
Polycrystalline diamond compact (PDC) cutters are used widely for mining and drilling in soft to medium hard rock formations. During drilling in very hard and strong rock formations, the rapid wear of the polycrystalline diamond layer results in a low service life of drilling bits. To improve the performance of PDC cutters, we adopted a high-temperature, high-pressure (HTHP) sintering method (5.5–6.0 GPa and 1350–1500 °C) in the current research by adding a certain amount of graphene to raw materials, and we successfully prepared a new type of high-performance diamond composite PDC-G (graphene was added to PDC). We investigated the microstructure, residual stress, hardness, wear resistance, thermal conductivity, and electrical conductivity of the as-synthesized PDC-G. Compared with PDC without graphene, the hardness and wear resistance of PDC-G with 0.1 wt% graphene addition were enhanced by 75% and 33%, respectively. Moreover, the electrical conductivity of PDC prepared by graphene strengthening was improved 42-fold. The strengthening mechanism of PDC-G mainly occurred as a result of the lubricating effect of graphene between diamond particles; hence, a more dense and uniform structure was formed in the polycrystalline diamond layer after HTHP sintering.
1. Introduction Polycrystalline diamond compact (PDC) generally is prepared by the sintering of diamond and a hard metal substrate under high-temperature, high-pressure (HTHP) conditions [1,2]. PDC is widely used in different geological engineering applications, such as geological core drilling and oil exploitation drilling [3,4]. PDC drill bits were one of the major technological innovations in the field of drilling during the 1980s [5,6]. Currently, the drilling footage completed by PDC bits accounts for 90% of the total footage of the world oil and gas drilling market [7]. The wear resistance of PDC drill bits in hard rocks needs to be improved, however [8]. During the synthesis of polycrystalline diamond, the added Group VIII metal (iron, cobalt, nickel) catalyst can accelerate the sintering process, and eventually cause diamond grain to grow interactively with other particles. The sintering process of this kind of polycrystalline diamond can be summarized as follows: Under HTHP conditions, the molten metal cobalt sweeps and catalyzes the diamond particles, and
∗
the surface of the diamond shows a graphitization phenomenon. When the graphitized diamond reaches saturation, and once it is in the diamond stable region, the diamond will precipitate. In the process of diamond dissolution and precipitation, cobalt exists in liquid form between diamond particles migrates, and the diamond grains combine with each other while cobalt migrates to form D-D bonding [9,10]. Diamond particles gradually break under high pressure, however, and most of these fragmented particles remain in their original positions. Thus, they cannot fill the generated gaps between coarse grains [11]. Furthermore, grain boundaries of broken diamond particles cannot be filled with the binder [12,13]. Graphene is widely used to enhance the performance of ceramic materials [14–16]. Graphene can form a transfer film at the friction interface, which results in a good self-lubricating effect. The presence of graphene causes the shear stress to be dispersed in the surface layer, which leads to plastic deformation at the wearable interface to produce a grain refinement layer [17]. When a certain amount of graphene is added to ceramic materials, graphene sheets come into contact with
Corresponding author. College of Construction Engineering, Jilin University, Changchun, China. E-mail address: [email protected] (B. Liu).
https://doi.org/10.1016/j.ceramint.2020.01.150 Received 30 October 2019; Received in revised form 24 December 2019; Accepted 15 January 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zhaoran Chen, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.01.150
Ceramics International xxx (xxxx) xxx–xxx
Z. Chen, et al.
(XRD) and Raman analyses to detect the compositional elements of the samples. A laser thermal conductivity meter with a temperature range of 30–500 K was used to measure the thermal conductivity of the samples by the flash method. Vicker's hardness tests were carried out under a load of 10 N for a fixed indentation time of 15 s. To reveal bonding states in the samples, we conducted SEM analyses. Fig. 2(a) and (b), respectively, display the structure of the HTHP cell assembly and the HTHP sintering process.
each other to form a three-dimensional conductive network [18,19]. Moreover, graphene is also added to achieve the desired strengthening and toughening effects, such as crack deflection, branching, graphene bridging, fracture, and pulling out in ceramic substrates [20–23]. In the present study, we used graphene and mixed-size diamond particles to effectively suppress the “bridge” between diamond particles, to promote the filling of broken diamond voids, and to achieve the uniform distribution of the binder phase. The addition of graphene was more conducive to the sliding and rearrangement of diamond particles and reducing the particle breakage. We have added three kinds of diamonds with different particle sizes, and the small particles are embedded in the gaps between the large particles, which is one reason for the increased density. At the same time, some carbon microcrystals dissolved in the cobalt binder fill the gaps between the large particles and grow together with them to form D-D bonding. It makes the individual particles more firmly bonded, and the wear resistance is improved.In addition, we analyzed the microstructure, residual stress, hardness, wear resistance, thermal conductivity, and electrical conductivity of the as-synthesized PDC-graphene composite.
3. Results and discussion 3.1. PDC with graphene and cobalt under HTHP sintering conditions Shul'zhenkoa [24,25] revealed the transformation of multilayer graphenes into diamond crystals at high pressure (7.7 GPa) and temperature (1700 °C) under the presence of carbon solvents (Ni–Mn alloy, iron). In the current research, we sintered PDC samples at 5.5–6 GPa and 1500°C-1600 °C. In order to verify whether graphene will be transformed into diamond or keep its original properties at 5.5–6 GPa and 1500–1600 °C (PDC synthesis temperature and pressure conditions), two sets of comparative experiments were performed (Fig. 3). The conditions of high temperature and high pressure treatment process were the same as those of the synthetic PDC. One system contains graphene and cobalt without any other additives, and the other system contains graphite and cobalt without any other additives. The results of comparative experiments were shown in Fig. 3. The raw materials did not contain diamond fine powder. XRD results in Fig. 3(a) reveal that graphene kept its properties in the graphene-Co system after HTHP treatment. Raman spectra in Fig. 3(b) confirm that graphite in the graphite-Co system was transformed into diamond crystals after HTHP treatment (pink peak at 1332 cm−1 indicates the diamond peak). The blue peak of graphene-Co signifies that graphene maintained its properties after HTHP treatment. The comparative experiments of graphite-Co system and grapheneCo system without any other additive (Fig. 3), which proves that graphene can maintain its original properties at 6 GPa and 1600 °C. When we added graphene to diamond crystals to sinter PDC, graphene kept its properties. Hence, graphene yielded good electrical and thermal conductivity in the sintered PDC sample.
2. Materials and methods 2.1. Experimental materials and conditions This experiment used diamond powders with different particle sizes of 30–50 μm (main particle size), 4–8 μm (crude-micron particle size), and 1–2 μm (fine-micron particle size). The weight ratio of different diamond particles (W30-50, W4-8, and W1-2, respectively) was 70:15:15. We used cobalt (Co) powder as the binder and graphene with a thickness of 6–8 nm and width of 5 μm to prepare PDC-G composites. We used cemented tungsten carbide (WC) with 16 wt% Co as the substrate. We added polyvinyl pyrrolidone (PVP) to graphene by ultrasonic dispersion in ethanol for 4 h. We carried out mixing of 85–95 wt% diamond powder, 4–10 wt% cobalt powder, and 0.05–0.3 wt% graphene under ball milling for 8 h. After the complete evaporation of alcohol, we treated the mixed powder in a vacuum furnace at 3.0 × 10−3 Pa and 1000 °C for 6 h to remove impurities. After vacuum treatment, we contained the mixture in a graphite capsule and then subjected it to HTHP treatments at 5–6 GPa and 1350°C-1600 °C in a 6 × 1000-ton large volume cubic press. Fig. 1 shows scanning electron microscopy (SEM) images of graphene and diamond particles. The size distribution of diamond particles and graphene is presented in Fig. 1(c). During sintering, graphene was coated on the surface of diamond particles to form a transfer film at the friction interface. Graphene filled the generated gaps between diamond particles and reduced the frictional resistance between them, thus enhancing the compactness of PDC. A stable and compact polycrystalline sinter was formed to improve the strength and wear resistance of polycrystalline diamond. We sintered the recovered synthetic products into bulk cylinders with a height of 10 mm and diameter of 27 mm and then polished them to achieve a smooth mirror surface. We performed X-ray diffraction
3.2. XRD analysis It is evident in Fig. 4 that the sintered PDC-G sample was mainly composed of diamond, cobalt, and tungsten carbide. The peak value of diamond crystals in the PDC-G sample was sharper than that in the ordinary PDC sample. This result indicated the better diamond crystallization in PDC-G. Moreover, no graphite phase existed in the wellsintered PDC-G sample within the current resolution, which indicated that PDC-G was fabricated in the of the diamond crystals.
Fig. 1. SEM micrographs of (a) graphene and (b) diamond; (c) Size distribution of diamond particles and graphene: (1) 30–50 μm, (2) 4–8 μm, (3) 1–2 μm, and (4) graphene. 2
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Fig. 2. (a) Different parts of the HTHP sintering cell:1-pyrophyllite, 2-HTHP cell assembly, 3-conductive steel cap, 4-conductive molybdenum sheet, 5-pyrophyllite, 6-carbon column, 7-pyrophylinite tube, 8-graphite tube, 9-graphite sheet, 10-sample, 11-MgO tube, 12-NaCl plate and (b) HPHT sintering process.
3.3. Raman analysis
3.5. Electrical conductivity
To perform Raman analysis, we selected five points (inside points A, B, C and boundary points D, E) on the diamond layer of sintered PDC-G sample (Fig. 5(a)). We observed the appearance of the diamond’ characteristic peak (1332 cm−1) at the boundary as well as inside the PDC-G sample; however, the graphite characteristic peak (G peak) at 1580 cm−1 was not detected (Fig. 5(b)); hence, this signified that graphitization did not occur in the PDC-G sample (the sample was pure polycrystalline diamond). The relationship between Raman spectra and residual stresses in Equation (1) [26] reveals that the addition of a certain amount of graphene significantly reduced the residual stress intensity in PDC-G (Table 1 and Fig. 6). The intensity of the compressive stress, which was generated in the middle of the PDC surface, gradually decreased from the center to the edges. Moreover, the generated compressive stress changed to tensile stress at the peripheral edges.
It is well known that graphene manifests excellent electrical conductivity. The electron mobilities in graphene and silicon are 150000 cm2/(V•s) and 14000 cm2/(V•s), respectively; hence, the transmission speed of electrons in graphene is 100 times that in silicon [16]. In the current experiment, we used the four-probe method to measure the electrical conductivity of graphene in the PDC samples [28]. The diamond layer surface of PDC and PDC-G was contacted by four equidistant metal probes. A direct current ‘I’ was passed through the outer two probes, and the voltage drop ‘V’ between these two probes was measured by a potentiometer. According to Equation (2), the measured current ‘I’ and voltage ‘V’ were directly converted into resistance values.
σ = [v0-v(cm
−1
)]/2.88,
C=
(1)
20π 1 S1
+
1 S2
−
1 S1 + S 2
−
1 S 2 + S3
ρ0 =
V C I
(2)
The electrical resistance values of PDC and PDC-G (0.1 wt% graphene) were measured as 452.16 ± 15 Ω m and 10.048 ± 0.6 Ω m, respectively (Fig. 9). We found the electrical resistance to be inversely proportional to electrical conductivity; hence, in comparison to PDC without graphene, the electrical conductivity of PDC-G was improved by about 42 times. Therefore, we inferred that there were more D-D bonds in the PDC sample synthesized with the optimal graphene content.
−1
where σ is residual stress (MPa), v0 = 1332 cm , and v is the experimental Raman shift in the PDC sample (cm-1). 3.4. SEM analysis Fig. 7 shows that the interface of the polycrystalline diamond sintered with graphene was denser in comparison to that of PDC without graphene and that the particle size distribution (15–20 μm) was more uniform [27]. It is also clear from Fig. 8(a) that a small amount of WC and Co from cemented carbide substrate penetrated the diamond layer, thus allowing cemented carbide layers to firmly bond to the diamond layer. The energy-dispersive X-ray spectroscopy (EDS) analysis at the PDC-G interface is depicted in Fig. 8(b).
3.6. Thermal conductivity We used a laser thermal conductivity meter (LFA467 Germany) to measure thermal diffusivity. The thermal conductivity of PDC and PDCG was calculated by Equation (3) [10,29,30].
Fig. 3. XRD and Raman spectra of graphite and graphene before and after HPHT treatment. 3
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Fig. 4. XRD patterns of PDC and PDC-G.
Fig. 5. (a) Inside points (A, B, C) and boundary points (D, E); (b) Raman spectra at different points. Table 1 Residual stresses in PDC and PDC-G samples. Residual stress (MPa) POINT A C E
a=
λ cρ
PDC −809 −597.2 232
PDC-G −256.9 −152.7 121.5
(3)
where a is thermal diffusivity, λ is the coefficient of thermal conductivity, c is the specific heat capacity, and ρ is density. With the increasing temperature, the increase in the thermal conductivity of PDC-G became more prominent (Fig. 10). When the temperature reached 500 K, the coefficient of thermal conductivity of PDCG (with the optimal graphene content) increased by about 60%. 3.7. Mechanical properties Fig. 11 shows that the optimal graphene content was 0.1 wt%. When the graphene content in the mixed powder exceeded 0.1 wt%, the agglomeration of graphene became serious. During sintering, diamond particles were completely coated. This phenomenon hindered the direct bonding between them, decreased the grain boundary strength, increased the porosity of the composite material, and facilitated the easy shedding of the lubricating film. When the graphene content was lower than 0.1 wt%, the addition of graphene was not sufficient to fill the
Fig. 6. Raman shifts in PDC and PDC-G samples at A, C, E points.
generated pores under high pressure. Thereby, the friction reduction was not effective. Under high temperature and pressure conditions, diamond particles are in direct contact with each other. Diamonds will break when they 4
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Fig. 7. SEM images of PDC and PDC-G samples.
contact each other under high pressure. Most of the diamond particles produced by the fragmentation are still concentrated in the original location, and large voids are formed between coarse-grained diamonds. Graphene has a large specific surface area and a good lubrication effect. The addition of a certain proportion of graphene can play a good lubricator between diamond particles, so as to reduce friction resistance between particles under high pressure conditions. Uniformly dispersed graphene can promote the slip arrangement between diamond particles of different particle sizes and reduce the gaps between diamond particles. Finally, the compactness of the synthesized PDC is improved, and then the wear resistance of the PDC is increased. When the graphene content was 0.1 wt%, graphene was uniformly distributed on the surface of diamond particles. Compared with PDC without graphene, the hardness and wear resistance of PDC-G with 0.1 wt% graphene addition were improved by 75% and 33%, respectively.
Fig. 9. Electrical conductivity of PDC and PDC-G.
4. Conclusion Based on this research, we adopted an HTHP sintering method to improve the performance of PDC cutters. Our conclusions are as follows: 1. PDC-G composites were successfully synthesized under HTHP sintering. 2. Compared with PDC without graphene, the electrical conductivity of PDC-G with 0.1 wt% graphene addition was improved by about 42 times. 3. When the temperature reached 500 K, the coefficient of thermal conductivity of PDC-G was enhanced by about 60%. 4. Compared with PDC without graphene, the hardness and wear resistance of PDC-G with 0.1 wt% graphene addition were improved by 75% and 33%, respectively. 5. The excellent electrical and thermal conductivity and mechanical properties of graphene effectively improved the comprehensive performance of PDC.
Fig. 10. Thermal conductances of PDC and PDC-G.
Fig. 8. SEM and EDS analyses at the PDC-G interface. 5
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Fig. 11. Mechanical properties of PDC-G with different graphene contents.
Funding
[13] J. Gu, K. Huang, Role of cobalt of polycrystalline diamond compact (PDC) in drilling process, Diam. Relat. Mater. 66 (2016) 98–101. [14] A. Nieto, A. Bisht, D. Lahiri, C. Zhang, A. Agarwal, Graphene reinforced metal and ceramic matrix composites: a review, Int. Mater. Rev. 62 (5) (2016) 241–302. [15] Z. Pan, L. He, L. Qiu, A.H. Korayem, G. Li, J.W. Zhu, F. Collins, D. Li, W.H. Duan, M.C. Wang, Mechanical properties and microstructure of a graphene oxide–cement composite, Cement Concr. Compos. 58 (2015) 140–147. [16] O.V. Yazyev, Y.P. Chen, Polycrystalline graphene and other two-dimensional materials, Nat. Nanotechnol. 9 (10) (2014) 755–767. [17] M. Belmonte, C. Ramírez, J. González-Julián, J. Schneider, P. Miranzo, M.I. Osendi, The beneficial effect of graphene nanofillers on the tribological performance of ceramics, Carbon 61 (2013) 431–435. [18] H. Seiner, P. Sedlák, M. Koller, M. Landa, C. Ramírez, M.a.I. Osendi, M. Belmonte, Anisotropic elastic moduli and internal friction of graphene nanoplatelets/silicon nitride composites, Compos. Sci. Technol. 75 (2013) 93–97. [19] C. Ramirez, P. Miranzo, M. Belmonte, M.I. Osendi, P. Poza, S.M. Vega-Diaz, M. Terrones, Extraordinary toughening enhancement and flexural strength in Si3N4 composites using graphene sheets, J. Eur. Ceram. Soc. 34 (2) (2014) 161–169. [20] J.-H. Shin, S.-H. Hong, Fabrication and properties of reduced graphene oxide reinforced yttria-stabilized zirconia composite ceramics, J. Eur. Ceram. Soc. 34 (5) (2014) 1297–1302. [21] J. Llorente, M. Belmonte, Friction and wear behaviour of silicon carbide/graphene composites under isooctane lubrication, J. Eur. Ceram. Soc. 38 (10) (2018) 3441–3446. [22] P. Miranzo, M. Belmonte, M.I. Osendi, From bulk to cellular structures: a review on ceramic/graphene filler composites, J. Eur. Ceram. Soc. 37 (12) (2017) 3649–3672. [23] J. Llorente, B. Román-Manso, P. Miranzo, M. Belmonte, Tribological performance under dry sliding conditions of graphene/silicon carbide composites, J. Eur. Ceram. Soc. 36 (3) (2016) 429–435. [24] A.A. Shul’zhenko, L. Jaworska, V.G. Gargin, A.N. Sokolov, A.S. Nikolenko, V.V. Strelchuk, Dry mixing of diamond and n-layered graphene powders substantially different in density and particle size, High Press. Res. 38 (1) (2017) 53–61. [25] A.A. Shul’zhenko, L. Jaworska, A.N. Sokolov, V.G. Gargin, N.N. Belyavina, Phase transformations of n-layer graphenes into diamond at high pressures and temperatures, J. Superhard Mater. 39 (2) (2017) 75–82. [26] H.A. Crostack, U. Selvadurai-Lassl, W. Tillmann, M. Gathen, C. Kronholz, T. Wroblewski, A. Rothkirch, Residual stresses in sintered diamond-cobalt composites, Mater. Sci. Forum 524–525 (2006) 787–792. [27] Y. Liu, C. Hu, J. Men, W. Feng, L. Cheng, L. Zhang, Effect of diamond content on microstructure and properties of diamond/SiC composites prepared by tape-casting and CVI process, J. Eur. Ceram. Soc. 35 (8) (2015) 2233–2242. [28] M. Yamashita, T. Nishii, H. Mizutani, Resistivity measurement by dual-configuration four-probe method, Jpn. J. Appl. Phys. 42 (Part 1, No. 2A) (2003) 695–699. [29] L. Jaworska, M. Szutkowska, P. Klimczyk, M. Sitarz, M. Bucko, P. Rutkowski, P. Figiel, J. Lojewska, Oxidation, graphitization and thermal resistance of PCD materials with the various bonding phases of up to 800°C, Int. J. Refract. Metals Hard Mater. 45 (2014) 109–116. [30] A.F. Lisovskii, N.A. Bondarenko, Thermodynamic study of the doping of the diamond-WC-Co composition with silicides of transition metals, J. Superhard Mater. 34 (4) (2012) 239–242.
This work was financially supported by the National Natural Science Foundation of China (41572357) and the Science and Technology project of Jilin Province Education Department (JJKH20180087KJ). Declaration of competing interest The authors declare no conflicts of interest. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2020.01.150. References [1] T. Irifune, A. Kurio, S. Sakamoto, T. Inoue, H. Sumiya, K.-i. Funakoshi, Formation of pure polycrystalline diamond by direct conversion of graphite at high pressure and high temperature, Phys. Earth Planet. Inter. 143–144 (2004) 593–600. [2] C. Liu, Z. Kou, D. He, Y. Chen, K. Wang, B. Hui, R. Zhang, Y. Wang, Effect of removing internal residual metallic phases on wear resistance of polycrystalline diamond compacts, Int. J. Refract. Metals Hard Mater. 31 (2012) 187–191. [3] D. Belnap, A. Griffo, Homogeneous and structured PCD/WC-Co materials for drilling, Diam. Relat. Mater. 13 (10) (2004) 1914–1922. [4] X. Wang, Z. Wang, D. Wang, L. Chai, A novel method for measuring and analyzing the interaction between drill bit and rock, Measurement 121 (2018) 344–354. [5] D.q. Guo, C. Hou, Development of PDC drill bits for MWD directional drilling in underground coal mine, Procedia Earth and Planet. Sci. 3 (2011) 440–445. [6] V. Kanyanta, S. Ozbayraktar, K. Maweja, Effect of manufacturing parameters on polycrystalline diamond compact cutting tool stress-state, Int. J. Refract. Metals Hard Mater. 45 (2014) 147–152. [7] Y. Ma, Z. Huang, Q. Li, Y. Zhou, S. Peng, Cutter layout optimization for reduction of lateral force on PDC bit using Kriging and particle swarm optimization methods, J. Pet. Sci. Eng. 163 (2018) 359–370. [8] M. Yahiaoui, L. Gerbaud, J.Y. Paris, J. Denape, A. Dourfaye, A study on PDC drill bits quality, Wear 298–299 (2013) 32–41. [9] J.N. Boland, X.S. Li, Microstructural characterisation and wear behaviour of diamond composite materials, Materials 3 (2) (2010) 1390–1419. [10] S. Liu, L. Han, Y. Zou, P. Zhu, B. Liu, Polycrystalline diamond compact with enhanced thermal stability, J. Mater. Sci. Technol. 33 (11) (2017) 1386–1391. [11] Y. Liu, C. Hu, W. Feng, J. Men, L. Cheng, L. Zhang, Microstructure and properties of diamond/SiC composites prepared by tape-casting and chemical vapor infiltration process, J. Eur. Ceram. Soc. 34 (15) (2014) 3489–3498. [12] J. Li, W. Yue, W. Qin, C. Wang, Approach to controllable tribological properties of sintered polycrystalline diamond compact through annealing treatment, Carbon 116 (2017) 103–112.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effects of graphene addition on mechanical properties of polycrystalline diamond compact Zhaoran Chena, Dejiang Mab, Shanmin Wangb, Wenhao Daia, Siqi Lia, Yiqing Zhuc, Baochang Liua,d,e,∗ a
College of Construction Engineering, Jilin University, Changchun, China Southern University of Science and Technology, Shenzhen, China c Shanghai Institute of Geological Engineering Exploration, Shanghai, China d Key Laboratory of Drilling and Exploitation Technology in Complex Conditions of Ministry of Natural Resources, Changchun, 130026, China e State Key Laboratory of Superhard Materials, Changchun, 130021, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Polycrystalline diamond Graphene Wear resistance Thermal conductivity Electrical conductivity
Polycrystalline diamond compact (PDC) cutters are used widely for mining and drilling in soft to medium hard rock formations. During drilling in very hard and strong rock formations, the rapid wear of the polycrystalline diamond layer results in a low service life of drilling bits. To improve the performance of PDC cutters, we adopted a high-temperature, high-pressure (HTHP) sintering method (5.5–6.0 GPa and 1350–1500 °C) in the current research by adding a certain amount of graphene to raw materials, and we successfully prepared a new type of high-performance diamond composite PDC-G (graphene was added to PDC). We investigated the microstructure, residual stress, hardness, wear resistance, thermal conductivity, and electrical conductivity of the as-synthesized PDC-G. Compared with PDC without graphene, the hardness and wear resistance of PDC-G with 0.1 wt% graphene addition were enhanced by 75% and 33%, respectively. Moreover, the electrical conductivity of PDC prepared by graphene strengthening was improved 42-fold. The strengthening mechanism of PDC-G mainly occurred as a result of the lubricating effect of graphene between diamond particles; hence, a more dense and uniform structure was formed in the polycrystalline diamond layer after HTHP sintering.
1. Introduction Polycrystalline diamond compact (PDC) generally is prepared by the sintering of diamond and a hard metal substrate under high-temperature, high-pressure (HTHP) conditions [1,2]. PDC is widely used in different geological engineering applications, such as geological core drilling and oil exploitation drilling [3,4]. PDC drill bits were one of the major technological innovations in the field of drilling during the 1980s [5,6]. Currently, the drilling footage completed by PDC bits accounts for 90% of the total footage of the world oil and gas drilling market [7]. The wear resistance of PDC drill bits in hard rocks needs to be improved, however [8]. During the synthesis of polycrystalline diamond, the added Group VIII metal (iron, cobalt, nickel) catalyst can accelerate the sintering process, and eventually cause diamond grain to grow interactively with other particles. The sintering process of this kind of polycrystalline diamond can be summarized as follows: Under HTHP conditions, the molten metal cobalt sweeps and catalyzes the diamond particles, and
∗
the surface of the diamond shows a graphitization phenomenon. When the graphitized diamond reaches saturation, and once it is in the diamond stable region, the diamond will precipitate. In the process of diamond dissolution and precipitation, cobalt exists in liquid form between diamond particles migrates, and the diamond grains combine with each other while cobalt migrates to form D-D bonding [9,10]. Diamond particles gradually break under high pressure, however, and most of these fragmented particles remain in their original positions. Thus, they cannot fill the generated gaps between coarse grains [11]. Furthermore, grain boundaries of broken diamond particles cannot be filled with the binder [12,13]. Graphene is widely used to enhance the performance of ceramic materials [14–16]. Graphene can form a transfer film at the friction interface, which results in a good self-lubricating effect. The presence of graphene causes the shear stress to be dispersed in the surface layer, which leads to plastic deformation at the wearable interface to produce a grain refinement layer [17]. When a certain amount of graphene is added to ceramic materials, graphene sheets come into contact with
Corresponding author. College of Construction Engineering, Jilin University, Changchun, China. E-mail address: [email protected] (B. Liu).
https://doi.org/10.1016/j.ceramint.2020.01.150 Received 30 October 2019; Received in revised form 24 December 2019; Accepted 15 January 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zhaoran Chen, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.01.150
Ceramics International xxx (xxxx) xxx–xxx
Z. Chen, et al.
(XRD) and Raman analyses to detect the compositional elements of the samples. A laser thermal conductivity meter with a temperature range of 30–500 K was used to measure the thermal conductivity of the samples by the flash method. Vicker's hardness tests were carried out under a load of 10 N for a fixed indentation time of 15 s. To reveal bonding states in the samples, we conducted SEM analyses. Fig. 2(a) and (b), respectively, display the structure of the HTHP cell assembly and the HTHP sintering process.
each other to form a three-dimensional conductive network [18,19]. Moreover, graphene is also added to achieve the desired strengthening and toughening effects, such as crack deflection, branching, graphene bridging, fracture, and pulling out in ceramic substrates [20–23]. In the present study, we used graphene and mixed-size diamond particles to effectively suppress the “bridge” between diamond particles, to promote the filling of broken diamond voids, and to achieve the uniform distribution of the binder phase. The addition of graphene was more conducive to the sliding and rearrangement of diamond particles and reducing the particle breakage. We have added three kinds of diamonds with different particle sizes, and the small particles are embedded in the gaps between the large particles, which is one reason for the increased density. At the same time, some carbon microcrystals dissolved in the cobalt binder fill the gaps between the large particles and grow together with them to form D-D bonding. It makes the individual particles more firmly bonded, and the wear resistance is improved.In addition, we analyzed the microstructure, residual stress, hardness, wear resistance, thermal conductivity, and electrical conductivity of the as-synthesized PDC-graphene composite.
3. Results and discussion 3.1. PDC with graphene and cobalt under HTHP sintering conditions Shul'zhenkoa [24,25] revealed the transformation of multilayer graphenes into diamond crystals at high pressure (7.7 GPa) and temperature (1700 °C) under the presence of carbon solvents (Ni–Mn alloy, iron). In the current research, we sintered PDC samples at 5.5–6 GPa and 1500°C-1600 °C. In order to verify whether graphene will be transformed into diamond or keep its original properties at 5.5–6 GPa and 1500–1600 °C (PDC synthesis temperature and pressure conditions), two sets of comparative experiments were performed (Fig. 3). The conditions of high temperature and high pressure treatment process were the same as those of the synthetic PDC. One system contains graphene and cobalt without any other additives, and the other system contains graphite and cobalt without any other additives. The results of comparative experiments were shown in Fig. 3. The raw materials did not contain diamond fine powder. XRD results in Fig. 3(a) reveal that graphene kept its properties in the graphene-Co system after HTHP treatment. Raman spectra in Fig. 3(b) confirm that graphite in the graphite-Co system was transformed into diamond crystals after HTHP treatment (pink peak at 1332 cm−1 indicates the diamond peak). The blue peak of graphene-Co signifies that graphene maintained its properties after HTHP treatment. The comparative experiments of graphite-Co system and grapheneCo system without any other additive (Fig. 3), which proves that graphene can maintain its original properties at 6 GPa and 1600 °C. When we added graphene to diamond crystals to sinter PDC, graphene kept its properties. Hence, graphene yielded good electrical and thermal conductivity in the sintered PDC sample.
2. Materials and methods 2.1. Experimental materials and conditions This experiment used diamond powders with different particle sizes of 30–50 μm (main particle size), 4–8 μm (crude-micron particle size), and 1–2 μm (fine-micron particle size). The weight ratio of different diamond particles (W30-50, W4-8, and W1-2, respectively) was 70:15:15. We used cobalt (Co) powder as the binder and graphene with a thickness of 6–8 nm and width of 5 μm to prepare PDC-G composites. We used cemented tungsten carbide (WC) with 16 wt% Co as the substrate. We added polyvinyl pyrrolidone (PVP) to graphene by ultrasonic dispersion in ethanol for 4 h. We carried out mixing of 85–95 wt% diamond powder, 4–10 wt% cobalt powder, and 0.05–0.3 wt% graphene under ball milling for 8 h. After the complete evaporation of alcohol, we treated the mixed powder in a vacuum furnace at 3.0 × 10−3 Pa and 1000 °C for 6 h to remove impurities. After vacuum treatment, we contained the mixture in a graphite capsule and then subjected it to HTHP treatments at 5–6 GPa and 1350°C-1600 °C in a 6 × 1000-ton large volume cubic press. Fig. 1 shows scanning electron microscopy (SEM) images of graphene and diamond particles. The size distribution of diamond particles and graphene is presented in Fig. 1(c). During sintering, graphene was coated on the surface of diamond particles to form a transfer film at the friction interface. Graphene filled the generated gaps between diamond particles and reduced the frictional resistance between them, thus enhancing the compactness of PDC. A stable and compact polycrystalline sinter was formed to improve the strength and wear resistance of polycrystalline diamond. We sintered the recovered synthetic products into bulk cylinders with a height of 10 mm and diameter of 27 mm and then polished them to achieve a smooth mirror surface. We performed X-ray diffraction
3.2. XRD analysis It is evident in Fig. 4 that the sintered PDC-G sample was mainly composed of diamond, cobalt, and tungsten carbide. The peak value of diamond crystals in the PDC-G sample was sharper than that in the ordinary PDC sample. This result indicated the better diamond crystallization in PDC-G. Moreover, no graphite phase existed in the wellsintered PDC-G sample within the current resolution, which indicated that PDC-G was fabricated in the of the diamond crystals.
Fig. 1. SEM micrographs of (a) graphene and (b) diamond; (c) Size distribution of diamond particles and graphene: (1) 30–50 μm, (2) 4–8 μm, (3) 1–2 μm, and (4) graphene. 2
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Fig. 2. (a) Different parts of the HTHP sintering cell:1-pyrophyllite, 2-HTHP cell assembly, 3-conductive steel cap, 4-conductive molybdenum sheet, 5-pyrophyllite, 6-carbon column, 7-pyrophylinite tube, 8-graphite tube, 9-graphite sheet, 10-sample, 11-MgO tube, 12-NaCl plate and (b) HPHT sintering process.
3.3. Raman analysis
3.5. Electrical conductivity
To perform Raman analysis, we selected five points (inside points A, B, C and boundary points D, E) on the diamond layer of sintered PDC-G sample (Fig. 5(a)). We observed the appearance of the diamond’ characteristic peak (1332 cm−1) at the boundary as well as inside the PDC-G sample; however, the graphite characteristic peak (G peak) at 1580 cm−1 was not detected (Fig. 5(b)); hence, this signified that graphitization did not occur in the PDC-G sample (the sample was pure polycrystalline diamond). The relationship between Raman spectra and residual stresses in Equation (1) [26] reveals that the addition of a certain amount of graphene significantly reduced the residual stress intensity in PDC-G (Table 1 and Fig. 6). The intensity of the compressive stress, which was generated in the middle of the PDC surface, gradually decreased from the center to the edges. Moreover, the generated compressive stress changed to tensile stress at the peripheral edges.
It is well known that graphene manifests excellent electrical conductivity. The electron mobilities in graphene and silicon are 150000 cm2/(V•s) and 14000 cm2/(V•s), respectively; hence, the transmission speed of electrons in graphene is 100 times that in silicon [16]. In the current experiment, we used the four-probe method to measure the electrical conductivity of graphene in the PDC samples [28]. The diamond layer surface of PDC and PDC-G was contacted by four equidistant metal probes. A direct current ‘I’ was passed through the outer two probes, and the voltage drop ‘V’ between these two probes was measured by a potentiometer. According to Equation (2), the measured current ‘I’ and voltage ‘V’ were directly converted into resistance values.
σ = [v0-v(cm
−1
)]/2.88,
C=
(1)
20π 1 S1
+
1 S2
−
1 S1 + S 2
−
1 S 2 + S3
ρ0 =
V C I
(2)
The electrical resistance values of PDC and PDC-G (0.1 wt% graphene) were measured as 452.16 ± 15 Ω m and 10.048 ± 0.6 Ω m, respectively (Fig. 9). We found the electrical resistance to be inversely proportional to electrical conductivity; hence, in comparison to PDC without graphene, the electrical conductivity of PDC-G was improved by about 42 times. Therefore, we inferred that there were more D-D bonds in the PDC sample synthesized with the optimal graphene content.
−1
where σ is residual stress (MPa), v0 = 1332 cm , and v is the experimental Raman shift in the PDC sample (cm-1). 3.4. SEM analysis Fig. 7 shows that the interface of the polycrystalline diamond sintered with graphene was denser in comparison to that of PDC without graphene and that the particle size distribution (15–20 μm) was more uniform [27]. It is also clear from Fig. 8(a) that a small amount of WC and Co from cemented carbide substrate penetrated the diamond layer, thus allowing cemented carbide layers to firmly bond to the diamond layer. The energy-dispersive X-ray spectroscopy (EDS) analysis at the PDC-G interface is depicted in Fig. 8(b).
3.6. Thermal conductivity We used a laser thermal conductivity meter (LFA467 Germany) to measure thermal diffusivity. The thermal conductivity of PDC and PDCG was calculated by Equation (3) [10,29,30].
Fig. 3. XRD and Raman spectra of graphite and graphene before and after HPHT treatment. 3
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Fig. 4. XRD patterns of PDC and PDC-G.
Fig. 5. (a) Inside points (A, B, C) and boundary points (D, E); (b) Raman spectra at different points. Table 1 Residual stresses in PDC and PDC-G samples. Residual stress (MPa) POINT A C E
a=
λ cρ
PDC −809 −597.2 232
PDC-G −256.9 −152.7 121.5
(3)
where a is thermal diffusivity, λ is the coefficient of thermal conductivity, c is the specific heat capacity, and ρ is density. With the increasing temperature, the increase in the thermal conductivity of PDC-G became more prominent (Fig. 10). When the temperature reached 500 K, the coefficient of thermal conductivity of PDCG (with the optimal graphene content) increased by about 60%. 3.7. Mechanical properties Fig. 11 shows that the optimal graphene content was 0.1 wt%. When the graphene content in the mixed powder exceeded 0.1 wt%, the agglomeration of graphene became serious. During sintering, diamond particles were completely coated. This phenomenon hindered the direct bonding between them, decreased the grain boundary strength, increased the porosity of the composite material, and facilitated the easy shedding of the lubricating film. When the graphene content was lower than 0.1 wt%, the addition of graphene was not sufficient to fill the
Fig. 6. Raman shifts in PDC and PDC-G samples at A, C, E points.
generated pores under high pressure. Thereby, the friction reduction was not effective. Under high temperature and pressure conditions, diamond particles are in direct contact with each other. Diamonds will break when they 4
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Fig. 7. SEM images of PDC and PDC-G samples.
contact each other under high pressure. Most of the diamond particles produced by the fragmentation are still concentrated in the original location, and large voids are formed between coarse-grained diamonds. Graphene has a large specific surface area and a good lubrication effect. The addition of a certain proportion of graphene can play a good lubricator between diamond particles, so as to reduce friction resistance between particles under high pressure conditions. Uniformly dispersed graphene can promote the slip arrangement between diamond particles of different particle sizes and reduce the gaps between diamond particles. Finally, the compactness of the synthesized PDC is improved, and then the wear resistance of the PDC is increased. When the graphene content was 0.1 wt%, graphene was uniformly distributed on the surface of diamond particles. Compared with PDC without graphene, the hardness and wear resistance of PDC-G with 0.1 wt% graphene addition were improved by 75% and 33%, respectively.
Fig. 9. Electrical conductivity of PDC and PDC-G.
4. Conclusion Based on this research, we adopted an HTHP sintering method to improve the performance of PDC cutters. Our conclusions are as follows: 1. PDC-G composites were successfully synthesized under HTHP sintering. 2. Compared with PDC without graphene, the electrical conductivity of PDC-G with 0.1 wt% graphene addition was improved by about 42 times. 3. When the temperature reached 500 K, the coefficient of thermal conductivity of PDC-G was enhanced by about 60%. 4. Compared with PDC without graphene, the hardness and wear resistance of PDC-G with 0.1 wt% graphene addition were improved by 75% and 33%, respectively. 5. The excellent electrical and thermal conductivity and mechanical properties of graphene effectively improved the comprehensive performance of PDC.
Fig. 10. Thermal conductances of PDC and PDC-G.
Fig. 8. SEM and EDS analyses at the PDC-G interface. 5
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Fig. 11. Mechanical properties of PDC-G with different graphene contents.
Funding
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