Journal of Alloys and Compounds 750 (2018) 972e979
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Design, fabrication and characterization of multi-layer graphene reinforced nanostructured functionally graded cemented carbides Jialin Sun a, b, Jun Zhao a, b, *, Feng Gong a, b, Zuoli Li a, b, Xiuying Ni a, b a Key Laboratory of High Efﬁciency and Clean Mechanical Manufacture of MOE, School of Mechanical Engineering, Shandong University, Jinan, 250061, PR China b National Demonstration Center for Experimental Mechanical Engineering Education (Shandong University), Jinan, 250061, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 February 2018 Received in revised form 6 April 2018 Accepted 8 April 2018 Available online 10 April 2018
Effect of predesigned multi-layer graphene (MLG) gradient on the microstructure as well as mechanical properties of MLG/WC-Co hardmetal has been investigated. Varied organic solvents and surfactants were used to identify the best combination of dispersing medium and dispersant for the dispersion of MLG. Six kinds of designed functionally graded MLG/WC-Co alloys were prepared employing two-step sintering (TSS) method. Results demonstrated that MLG can enhance the densiﬁcation process and inhibit the grain growth. Meanwhile, MLG with a predesigned gradient opposite the predesigned cobalt gradient can maximally enhance the stability of predesigned cobalt gradient during liquid phase sintering, the generation of residual surface compressive stress and the mechanical properties of MLG/WC-Co composite. The enhancing mechanisms of cobalt gradient stability were systematically discussed by combining theoretical consideration with experimentation. © 2018 Elsevier B.V. All rights reserved.
Keywords: Functionally graded materials Multi-layer graphene Dispersion Cobalt gradient stability Mechanical properties
1. Introduction Cemented carbides or hardmetals enjoy proud history since it was invented in 1923 by Karl Schroeter at Osram Studiengesellschaft in Germany . The ﬁrst products were wire drawing dies. Over the coming decades, cemented carbide products have covered various industry sectors; almost 67% of total production of hardmetals goes into metal cutting tools, about 13% for mining, oil drilling and tunneling industries and 11% and 9% for wood working and construction industries, respectively [2e4]. WC-Co is essentially a composite of the ceramic phase tungsten carbide (WC) and the metal binder cobalt (Co). The tungsten carbide phase endows the alloy extremely high hardness, wear resistance and strength, whilst the metallic binder phase is responsible for its excellent fracture toughness and impact resistance. Functionally graded cemented carbide (FGCC) has been identiﬁed as the most versatile approach to balance the hardness versus toughness for cemented carbides and then advance them to further wide applications. For instance, WC-Co graded composites
* Corresponding author. Key Laboratory of High Efﬁciency and Clean Mechanical Manufacture of MOE, School of Mechanical Engineering, Shandong University, Jinan, 250061, PR China. E-mail address: [email protected]
(J. Zhao). https://doi.org/10.1016/j.jallcom.2018.04.108 0925-8388/© 2018 Elsevier B.V. All rights reserved.
featuring a Co-enriched and cubic-phase free layer in surface zone was deﬁned as the ideal substrate for coated cutting tools due to its perfect combination of fracture toughness and wear resistance. To date, some solutions have been established to prepare WC-Co graded composites such as inﬁltration, carburized sintering, solidphase sintering and so on [5e8]. However, none of them is demonstrated at the industrial level for their complex, low performance-cost ratio and/or unfavorable mechanical properties. Liquid phase sintering (LPS) is identiﬁed as the most viable option for preparing WC-Co graded composites. Whilst initially appealing, caution should be exercised as that liquid cobalt migration resulting in homogenization of cobalt associated with LPS is still the main obstacle to fabricate dense WC-Co with stable cobalt gradient. Recently, Fang et al.  proposed that cobalt gradient during LPS can be maintained or developed via manipulating initial carbon gradient prior to sintering. They claimed that cobalt migrated in the identical direction of carbon diffusion during LPS. However, it was quite difﬁcult to design and control the carbon gradient to obtain FGCC with desirable cobalt gradient and no formation of deleterious h-phase. Being superior to free carbon, graphene has shared progressive application as ideal reinforcement phase for materials including ceramics [3,9,10], polymers [11,12] and metals [13,14], owing to its unprecedented combination of fascinating two-dimensional sp2
J. Sun et al. / Journal of Alloys and Compounds 750 (2018) 972e979
structure with considerable theoretical speciﬁc surface area (~2630 m2 g1) coupled with extraordinary intrinsic mechanical properties (e.g., stiffness 300e400 N m1, breaking strength 42 N m1, tensile strength 130 GPa, Young's modulus 0.5e1 TPa, spring constant 1e5 N m1 and thermal conductivity -1 -1 3000e5000 W m K ) [15e17]. Furthermore, graphene is also considered as a green nanomaterial possessing no toxic for human and the environment . It is proved that graphene has manifested remarkable effectiveness in reinforcing ZrB2-based ceramics , ZO2-based ceramics , Al2O3-based ceramics , TaCbased ceramics , WC-based ceramics  and Si3N4-based ceramics . To the best of our knowledge, few researches have been conducted on graphene as Co-gradient stability phase and reinforcement phase for WC-Co graded materials. However, previous investigations  have demonstrated that carbon nanotubes reinforced WC-Co composites enjoyed superior mechanical properties to the pure WC-Co alloys, due to the extraordinary mechanical properties of carbon nanotubes, the toughing and bonding role of carbon nanotubes occurred in the WC-Co matrix. Raﬁee et al.  suggested that sheet-like reinforcement was more effective to induce crack deﬂection than tubular-like reinforcement did, implying that graphene has more signiﬁcant toughening effect than carbon nanotubes. Besides the intrinsic mechanical property of ﬁller material, the dispersion lever of the nanoscale reinforcement phase in the host matrix is also a critical factor to qualitatively deduce the microstructure and properties of the host materials. In this paper, we investigated various methods to achieve a good dispersion level of graphene. Two-step sintering (TSS) methodology, put forward by Chen and Wang  as a promising approach to obtain high-density ﬁnal sintered compact with nano-sized grains, has been extensively applied to a large body of materials such as Al2O3-based nanoceramics , Y2O3 , SiC nanostructured ceramics , BaTiO3 nanometric composition  and so on. It is proposed that a temperature interval exists between when grain boundaries migration start and densiﬁcation occurs . According to the TSS methodology, the green body would be heated to a relatively higher temperature and then immediately cooled to a lower temperature with a long soaking time. In the present research, functionally graded MLG/WC-Co nanostructured composites were fabricated by employing twostep hot-pressing sintering, so as to avoid the possible damage to the featured structure of MLG. The inﬂuences of MLG as Cogradient stability phase and reinforcement phase on WC-Co graded composites were investigated. The sintering behavior and mechanical properties of so-designed MLG/WC-Co materials were evaluated. The enhancing mechanisms of cobalt gradient stability were systematically discussed by combining theoretical consideration with experimentation. 2. Experimental details 2.1. Material preparation The raw materials utilized in the present study were all commercially available, including tungsten carbide (WC, 400 nm, 99.9% purity, Shanghai Chaowei nanotechnology Co. Ltd., China), cobalt (Co, 500 nm, 99.9% purity, Shanghai Chaowei nanotechnology Co. Ltd., China), polyethyleneglycol (PEG, 99.9% purity, Sinopharm Chemical Reagent Co. Ltd., China), pyrrolidone (PVP, 99%, Tianjing Kermel Chemical Reagent Science and Technology Co. Ltd., China), N-methyl-2-pyrrolidone (NMP, 99%, Tianjing Kermel Chemical Reagent Science and Technology Co. Ltd., China), N-Dimethyl formamide (DMF, 99%, Tianjing Kermel Chemical
Reagent Science and Technology Co. Ltd., China), sodium dodecyl benzene sulfonate (SDBS 99%, Tianjing Kermel Chemical Reagent Science and Technology Co. Ltd., China) and MLG (thickness: 1e10 nm and the diameter: 1e5 mm) . The compositions of powder mixtures for WC-based ceramics are given in Table 1. In this paper, the starting materials WC and Co powders were ultrasonically dispersed by PVP and PEG in absolute alcohol for 30min maintaining a temperature of 80 C and a pH of 9.0. MLGs were ﬁrstly ultrasonically dispersed in NMP as the assistance of surface active agent PVP for 1 h maintaining a temperature of 80 C. Then, MLG suspension was dropped into the micro-nanopowder suspension with dropper pipette under strong agitation conditions followed by another 1 h ultrasonic dispersion. After dispersing, the mixed slurries were milled for 20 h in a high energy attrition mill with cemented carbide milling ball (ball-to-powder mass ratio is 25:1).
2.2. Two-step hot-pressing sintering The powder mixtures were loaded into a circular die (42 mm in diameter) in the following order: surface layer mixtures, interlayer mixtures, core layer mixtures, interlayer mixtures and then surface layer mixtures. Finally, the composite powders were sintered through two-step hot-pressing in an inductive hot-pressing vacuum furnace (Model: ZRC85-25 T, China). In this paper, the sintering temperatures were measured employing thermocouples. It has been proved that the highest temperature of heating element is not located on the central position but the position above the central position. The distance is about the 15% of the whole heating element height. Therefore, we choose to put the thermocouples at the location which is above the central position 15% of the whole heating element height. So, the measuring temperature in this paper is equal to or a little higher than the sintering temperature. The resultant composite powders were sintered in an inductive hot-pressing vacuum furnace with the following cycle: a) Heated from room temperature to 1200 C with a heating rate of 60 C/ min; b) Holding at 1200 C for 1min; c) Heating from 1200 C to 1400 C with a heating rate of 50 C/min; d) Holding at 1400 C for 5min; e) Immediately cooled down from 1400 C to 1300 C at 50 C/min; f) Holding at 1300 C for 90min; g) Furnace-cooled from 1300 C to room temperature. During the whole sintering process, the pressure in the die was kept at 40 MPa.
Table 1 Compositions of powder mixtures for designed samples (wt.%). Composites
FGCCA-SL FGCCA-IL FGCCA-CL FGCCB-SL FGCCB-IL FGCCB-CL FGCCC-SL FGCCC-IL FGCCC-CL FGCCD-SL FGCCD-IL FGCCD-CL FGCCE-SL FGCCE-IL FGCCE-CL FGCCF-SL FGCCF-IL FGCCF-CL
95 90 85 94.9 89.9 84.9 94.95 89.9 84.85 94.9 89.95 84.975 94.85 89.9 84.95 94.8 89.85 84.9
5 10 15 5 10 15 5 10 15 5 10 15 5 10 15 5 10 15
e e e 0.1 0.1 0.1 0.05 0.1 0.15 0.1 0.05 0.025 0.15 0.1 0.05 0.2 0.15 0.1
J. Sun et al. / Journal of Alloys and Compounds 750 (2018) 972e979
2.3. Microstructural characterization The microstructures of the samples were observed by a scanning electron microscope (SEM, QUANTAFEG 250, FEI Inc., USA) and high-resolution transmission electron microscopy (TEM, 20UTWIN, PHILIPS, Holland). The element distribution was investigated by energy dispersive spectroscopy (EDS, X-MAX30, Oxford Instruments Inc., UK). X-ray diffraction (XRD, D8ADVANCE, Bruker AXS Inc., Germany) and Raman spectroscopy (LabRam HR Evolution, HORIBA JOBIN YVON S.A.S.) were conducted to characterize the phase identiﬁcation of the specimens. 2.4. Mechanical measurements Flexural strength of the specimens was determined by threepoint bending tests with a 0.5 mm/min loading rate and a span of 14.5 mm. Surface residual stress was measured by X-stress (XSTRESS 3000, G2R system, Stresstech Oy Inc., Finland). The principle of X-ray measurement is based on Bragg's law, the sinesquare-psi (sin2J) method. CrKa radiation and 138.5 diffraction angle were chosen. The residual stress was measured at ﬁve different locations in two perpendicular directions. 3. Results and discussions 3.1. MLG dispersion Identical amount of MLGs were ﬁrstly ultrasonically dispersed in different kinds of organic solvents in identical volume including absolute alcohol, NMP and DMF for 1 h maintaining a temperature of 80 C. Then the MLG dispersions were left for several days. In order to insight into the dispersing capability of the three organic solvents to MLG, a UVevis Spectrometer (UV-1700, Shimadzu Group, Japan) was employed to measure the absorbance spectra of the three MLG stable solution supernates as illustrated in Fig. 1. The spectra in the condition of NMP and DMF suggested that NMP and DMF had a positive dispersing ability towards MLGs, whilst no absorption was detected in the case of ethanol, authenticating that ethanol possessed underdeveloped dispersing ability to MLGs. In conclusion, NMP enjoyed the most favorable dispersing ability to MLGs, and then DMF, the poorest ethanol. Since the dispersion quality of graphene in solvents plays a
crucial role in processing graphene reinforced composites, ionic surfactants SDBS  and non-ionic surfactants PVP  were utilized as stabilizers to further improve the dispersion of graphene. Fig. 2 depicted the absorbance pattern of MLGs suspension settled down for two weeks, with different PVP and SDBS contents (0, 25, 50, 75 and 100 wt% of MLG). It is highlighted that the absorbance of the MLGs was increased with increasing the surfactants content at low contents for PVP and SDBS. In other words, marked improvements were obtained in the MLGs dispersion in NDP in case of low content addition of PVP or SDBS. The surfactants acted as a spacer to hinder MLG reaggregation sterically by adsorbing on the MLGs surface . The absorbance reached maximum value at a PVP content of 75 wt%, whereas that for SDBS was 50 wt%. On the other hand, PVP exhibited a more pronounced dispersing ability towards MLG than SDBS, for the reason that PVP enjoyed a higher solubility in NMP than in SDBS. In conclusion, MLGs utilized in our research can be best dispersed in NMP as the assistance of 75 wt% PVP of MLGs. 3.2. Phase constitutions and microstructures The XRD patterns of sintered WC-Co graded samples were depicted in Fig. 3(a). No obvious differences were identiﬁed among them and no other new phases were detected, implying that MLG and WC or Co did not react with each other and no interface phase existed among them. Furthermore, the absence of detrimental phases including h-phase, W2C phase coupled with graphite phase in the obtained specimens suggested that MLG was capable of suppressing the formation of h-phase, W2C phase and graphite phase. Raman spectra were performed to characterize the structural integrity of MLG in the ball milled MLG/WC-Co powder and sintered samples. Three characteristic peaks at ～1350 cm1 (D band), ～1580 cm1 (G band) and ～2710 cm1 (2D band) were observed for both ball milled MLG/WC-Co powder and sintered samples (Fig.3 (b) and (c)), conﬁrming the survival of MLGs after undergoing as-designed ultrasonic dispersion, ball milling and TSS sintering process [19,35]. It is also detected that D, G and 2D peaks shifted to higher frequency for sintered specimens in comparison with ball milled MLGs, especially 2D peak illustrated a ~10 cm1 shift. The shift can be suggestive of compressive stress applied on MLGs during TSS [17,36]. ID/IG ratio is closely associated with the structural defects and disorder, while I2D/IG ratio sheds light on
NMP DMF Ethanol
Wavelength (nm) Fig. 1. UVevis absorption spectra of graphene dispersions in a variety of organic solvents left for three days.
J. Sun et al. / Journal of Alloys and Compounds 750 (2018) 972e979
Fig. 2. Effect of (a) SDBS and (b) PVP addition on dispersion of graphene NMP solvent settled for two weeks.
Fig. 3. (a) X-ray diffraction patterns of polished surfaces of sintered WC-Co graded materials, (b) Raman spectroscopy of ball milled MLG and sintered WC-Co graded materials, (c) a summary of D, G and 2D peak shift, G peak shift, intensity ratios of D bands and G bands, 2D bands and G bands and (d) surface residual stress of WC-Co graded materials.
graphene structure . It is generally accepted that I2D/IG below 1 is indicative of graphite-like structure . Compared with ball milled MLGs, the ID/IG ratio of FGCC-C, D and E showed slight increase but still very low, revealing the curling of MLGs within host composites and sintered materials being low-defect and fairly ordered composites [9,19,37]. Bulk sintered compacts had the higher I2D/IG ratio than ball milled MLGs, indicating that the graphene structure was not damaged and the numbers of few layer MLGs was increased after the sintering process. It is noted from Fig. 3(d) that the surface residual stresses of MLG-reinforced specimens were all compressive stress. As stressed by Zhao , the increased coefﬁcient from the surface layer to core
layer is effective to induce the generation of residual compressive stress in the surface of functionally graded materials during the cooling process of the thermal cycle required to fabricate the materials. Because the coefﬁcient of thermal expansion of Co (12.5 106/ C) is higher than that of WC (3.84 106/ C), an increased content of Co from surface layer to core layer and a decreased content of WC from surface layer to core layer may be beneﬁcial for the generation of residual compressive stress on the surface of the material. In case of FGCC-A, the surface residual stress is tensile stress for the disappearance of Co gradient during LPS. Furthermore, FGCC-F enjoyed the highest residual compressive stress. MLG has a negative thermal expansion coefﬁcient
J. Sun et al. / Journal of Alloys and Compounds 750 (2018) 972e979
(8.0 106/ C) , thus, a diminished content of MLG from surface layer to core layer is helpful in the generation of residual compressive stress. According to N. Scuor et al. [40,41], the residual compressive stress on material surface can signiﬁcantly contribute to the improvement of fracture toughness of the material. TEM was performed to obtain further characterization of the microstructure. TEM and HRTEM images of FGCC-E are illustrated in Fig. 4. MLGs were found to be located at the alloy grain boundaries of the host matrix and semi-wrap around the host matrix grains as shown in Fig. 4 (a) and (b), which was beneﬁcial to suppress the grain growth. Majority of the observed MLGs were below 10 nm in thickness as present in Fig. 4 (c). Besides, a slice of MLGs were distinguished easily possessing a thickness about 2 nm, which means that the MLG consists of only several layers of graphene as demonstrated in Fig. 4 (d). Furthermore, it is observed from Fig. 4 (c) and (d) that the grain boundary between MLGs and matrix grain is a clean interface without diffusion layer, further evidencing that MLG and WC or Co does not react with each other [42,43]. Fig. 5 depicted the fractured surface of sintered FGCC. It is evidenced from Fig. 5 (a) that a homogeneous dispersion of MLGs in the WC-Co matrix was obtained. Additionally, MLGs exhibited a preferential orientation in the host matrix in the plane perpendicular to the direction of applied pressing, which is consistent with the reports by Seiner et al. . This alignment has been reported to give rise to the substantial enhancements in mechanical properties as toughness , tribological properties , electrical properties  and thermal properties . Compared with FGCC-A as demonstrated in Fig. 5 (c), it kept a decreased pore and a narrower grain size distribution absenting abnormal growth in FGCC-E as shown in Fig. 5 (b), suggesting that MLG was effective to facilitate
the sintering process and suppress the grain growth. Possessing a considerable electrical conductivity (106 S m1 ) and excellent thermal conductivity (6 103 W m-1 K-1 http://www.sciencedirect. com/science/article/pii/S0264127517307931 ), MLGs may play a role as heat conducting plates facilitating the heating process by improving the distribution of the current and heat during the heating process, whereas act as heat-sink plate hindering the grain growth by boosting the cooling rate during the cooling process. 3.3. Co gradient after sintering Fig. 6 shows curves characterising the Co distribution of the model surface layer, interlayer and core layer after sintering. It is obvious that the MLG-free specimen as FGCC-A presented a completely predesigned Co gradient loss, whilst the MLG reinforced samples as FGCC-D, E and F maintained the predesigned Co gradient perfectly. Additionally, FGCC-B illustrated a partial homogenization of Co gradient and FGCC-C demonstrated nearly a completely homogeneous Co distribution. It is proffered that a proper gradient of initial MLG prior to sintering was of great signiﬁcance to enhance the Co gradient stability during LPS . To further conﬁrm the conclusion of MLG enhancing predesigned Co gradient stability, SEM micrographs of cross section and EDX line maps of Co elements from surface layer to core layer were made for FGCC-A and FGCC-E as shown in Fig. 7. It can be seen that FGCC-A possessed a complete homogenization of the structure and an eliminated Co gradient, whereas FGCC-E exhibited a noticeably laminated structure and the desired Co gradient as predesigned. It is generally preferred that the migration pressure of liquid phase is dependent on three key factors - the volume fraction of
Fig. 4. TEM and HRTEM images of FGCC-E, (a) (b) MLGs wrapping matrix grains, (c) a magniﬁed image of MLGs with thickness about 10 nm, (d) a magniﬁed image of MLGs with thickness about 2 nm.
J. Sun et al. / Journal of Alloys and Compounds 750 (2018) 972e979
Fig. 5. (a) Preferential orientation of MLGs in FGCC-E matrix after two-step hot-pressing sintering, (b) the fracture surface of FGCC-E and (c) the fracture surface of FGCC-A.
FGCC-A FGCC-B FGCC-C FGCC-D FGCC-E FGCC-F
Co content (wt.%)
10 5 Co
Depth (mm) Fig. 6. Co contents after two-step sintering.
liquid Co, WC grain size and C content . Other things being equal, Co will migrate from region with high Co to region with low Co, from region with coarse grain to region with ﬁne grain region, from region with excessive C to region with deﬁcient C. In this paper, we introduce pre-designed gradients of both Co contents and MLG contents. The Co drift in FGCC comprising a similar hard phase average size and C content but different MLG contents can be explained when taking into consideration the three following critical factors. A straightforward reason for MLG-induced Co gradient stability maybe the ab plane of MLGs possessing a preferential orientation perpendicular to the direction of predesigned Co gradient as proved in Fig. 5. MLG appears to form continuous MLG walls partly blocking the Co drift paths along the direction perpendicular to the walls. It can be concluded that if the liquid migration pressure as a function of liquid Co volume fraction was insufﬁciently through the MLG wall, the drift paths would be changed, and then the liquid phase drift paths distance was increased. So, there is no doubt that the MLG walls can conspicuously enhance the predesigned cobalt gradient stability. This means that the predesigned cobalt gradient stability can be enhanced by introducing MLG regardless of the MLG gradient, in other words, MLG addition can seriously reduce the Co gradient susceptibility. Furthermore, as commented above, acting as a heat conducting plates, MLG accelerated the densiﬁcation process, which can result in a noticeable increase of the amount of liquid cobalt phase. As a
consequence, a gradient of liquid cobalt phase was formed along the MLG gradient. Thus, the liquid cobalt phase difference as a function of predesigned cobalt gradient can be diminished by introducing a predesigned MLG gradient opposite the predesigned cobalt gradient. In addition, MLG suppressed the grain growth by boosting the cooling rate, hence, a predesigned MLG gradient may lead to an average grain size difference. The liquid migration pressure as a function of average grain size can somewhat negative the liquid migration pressure resulting from the liquid Co fraction difference by introducing a predesigned MLG gradient opposite the predesigned cobalt gradient. Therefore, compared with a predesigned homogeneous MLGs or a MLG gradient similar to the predesigned cobalt gradient, a MLG gradient opposite the predesigned cobalt gradient can be more effective to enhance the stability of predesigned cobalt gradient. Additionally, it should be taken into account that MLGs appeared to be wrapped or anchored around the WC and Co grains. Obviously, Co drift speed was lower when several Co gains were wrapped together. Furthermore, it was possible that MLGs wrapping WC or Co gains would also lower the wettability of Co to WC. As a consequence, compared with the region comprising low content of MLGs, the region with high content of MLGs possesses lower wettability of Co to WC. Other things being same, Co will drift from the region with lower wettability into the region with higher wettability . Therefore, MLG gradient opposite the predesigned cobalt gradient can enhance the stability of predesigned cobalt gradient to the most extent. On the basis of the discussion above, a desired predesigned cobalt gradient can be maintained perfectly in functionally graded cemented carbides not comprising the h-phase by introducing a predesigned MLG gradient opposite the predesigned cobalt gradient. 3.4. Mechanical properties As demonstrated in Table 2, the FGCC specimens fabricated in our research were quite dense, owing to the TSS methods and high external pressure applied for alloy consolidation. FGCC-E comprising MLG exhibited the highest relative density of 99.8%, while MLG-free sample FGCC-A possessed a relative low densiﬁcation (98.3%), implying that MLG facilitated the consolidation process. However, excessive MLG addition may result in agglomeration of MLGs, which has an unfavorable effect on the distribution of current and heat thus lead to a dreadful densiﬁcation as FGCC-F. Flexural strength of WC-Co composites containing MLG has also
J. Sun et al. / Journal of Alloys and Compounds 750 (2018) 972e979
Fig. 7. SEM micrographs of cross section and EDX line maps of Co elements on the green line (a) FGCC-A (b) FGCC-E. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the Web version of this article.)
Table 2 Physical and Mechanical properties of as-designed FGCC reinforced by MLG. Material
Relative density (%) Flexural strength (MPa)
98.3 ± 0.6 1632.3 ± 26.7
99.7 ± 0.3 1878.4 ± 30.5
99.4 ± 0.4 1791.6 ± 31.3
99.6 ± 0.1 1990.7 ± 35.6
99.8 ± 0.2 2070.2 ± 26.9
99.3 ± 0.3 1996.3 ± 40.1
been profoundly improved. FGCC-E shared a 26.8% enhancement in comparison with FGCC-A regarding ﬂexural strength. It is generally accepted that there are three factors determining the effectiveness of reinforcement: the densiﬁcation level of host composites, the interface bonding of the reinforced phase to the host matrix as well as the dispersion degree of reinforced phase in the host materials . Besides, a ﬁner grain size plays an indispensable role in the improvement of ﬂexural strength . 4. Conclusions Functionally graded MLG/WC-Co cemented carbides are successfully fabricated by employing two-step hot-pressing sintering. The effects of MLG as Co-gradient stability phase and reinforcement phase on FGCCs were comprehensively investigated. (1) The most homogeneous MLGs distribution was obtained by ultrasonication with NMP as the assistance of 75 wt% PVP of MLG. MLGs played a role as heat conducting plates facilitating the heating process by improving the distribution of the current and heat during the heating process, whilst acted
as heat-sink plate hindering the grain growth by boosting the cooling rate during the cooling process. (2) Addition of MLG to the WC-Co system can increase the freedom to tailor the cobalt gradient of FGCCs. During liquid phase sintering, the stability of predesigned cobalt gradient can be signiﬁcantly enhanced by introducing a predesigned gradient of multilayer graphene opposite the predesigned gradient of cobalt. The three most important stability enhancing mechanisms are MLG preferential orientation, MLG induced generation of the liquid cobalt phase and MLG wrapping matrix grains. Acknowledgements This work is supported by the National Natural Science Foundation of China (51775315). References  Z.Z. Fang, M.C. Koopman, H.T. Wang, Cemented tungsten carbide hardmetal an introduction, in: D. Mari, L. Llanes, V.K. Sarin (Eds.), Comprehensive Hard Materials, vol. 1, Elsevier Oxford, 2014, pp. 123e138.
J. Sun et al. / Journal of Alloys and Compounds 750 (2018) 972e979  L. Prakash, Introduction to hardmetals - fundamentals and general applications of hardmetals, in: D. Mari, L. Llanes, V.K. Sarin (Eds.), Comprehensive Hard Materials, vol. 1, Elsevier Oxford, 2014, pp. 29e90.  C.M. Fernandes, A.M.R. Senos, Cemented carbide phase diagrams: a review, Int. J. Refract. Metals Hard Mater. 29 (2011) 405e418.  G.S. Upadhyaya, Materials science of cemented carbides - an overview, Mater. Des. 22 (2001) 483e489.  J. Sun, J. Zhao, Z. Li, X. Ni, Y. Zhou, A. Li, Effects of initial particle size distribution and sintering parameters on microstructure and mechanical properties of functionally graded WC-TiC-VC-Cr3C2-Co hard alloys, Ceram. Int. 43 (2017) 2686e2696.  Z.Z. Fang, O.O. Eso, Liquid phase sintering of functionally graded WC-Co composites, Scr. Mater. 52 (2005) 785e791.  A.F. Lisovsky, N.V. Tkachenko, Composition and structure of cemented carbides produced by MMT-process, Powder Metall. Int. 23 (1991) 157e161.  G.H. Lee, S. Kang, Sintering of nano-sized WC-Co powders produced by a gas reduction-carburization process, J. Alloy. Compd. 419 (2006) 281e289. mez, F.M. Figueiredo, M. Terrones,  C. Ramírez, S.M. Vega-Diaz, A. Morelos-Go M.I. Osendi, et al., Synthesis of conducting graphene/Si3N4 composites by spark plasma sintering, Carbon 57 (2013) 425e432.  L.S. Walker, V.R. Marotto, M.A. Raﬁee, N. Koratkar, E.L. Corral, Toughening in graphene ceramic composites, ACS Nano 5 (2011) 3182e3190.  Y. Cui, S.I. Kundalwal, S. Kumar, Gas barrier performance of graphene/polymer nanocomposites, Carbon 98 (2016) 313e333.  K. Pumar, S. Yu, F. Shahzad, S.M. Hong, Y.H. Kim, C.M. Koo, Ultrahigh electrically and thermally conductive self-aligned graphene/polymer composites using large-area reduced graphene oxides, Carbon 101 (2016) 120e128.  D. Villaroman, X. Wang, W. Dai, L. Gan, R. Wu, Z. Luo, et al., Interfacial thermal resistance across graphene/Al2O3 and graphene/metal interfaces and postannealing effects, Carbon 123 (2017) 18e25.  S. Bashirvand, A. Montazeri, New aspects on the metal reinforcement by carbon nanoﬁllers: a molecular dynamics study, Mater. Des. 91 (2016) 306e313.  K.S. Novoselov, A.K. Geim, S. Morozov, D. Jiang, K. Matsnelson, I. Grigorieva, et al., Two-dimensional gas of massless Dirac fermions in graphene, Nature 438 (2005) 197.  A.K. Geim, Graphene: status and prospects, Science 324 (2009) 1530e1534.  L. Zhang, W. Liu, C. Yue, T. Zhang, P. Li, Z. Xing, et al., A tough graphene nanosheet/hydroxyapatite composite with improved in vitro biocompatibility, Carbon 61 (2013) 105e115.  L. Bao, Z.L. Zhang, Z.Q. Tian, L. Zhang, C. Liu, Y. Lin, et al., Electrochemical tuning of luminescent carbon nanodots: from preparation to luminescence mechanism, Adv. Mater. 23 (2011) 5801e5806.  Y. An, J. Han, X. Zhang, W. Han, Y. Cheng, P. Hu, G. Zhao, Bioinspired high toughness graphene/ZrB2 hybrid composites with hierarchical architectures spanning several length scales, Carbon 107 (2016) 209e216.  S. Gii, G. Dhosh, C.K. Das, Growth of vertically aligned tunable polyaniline on graphene/ZrO2 nanocomposites for supercapacitor energy-storage application, Adv. Funct. Mater. 24 (2014) 1312e1324.  Z. Gao, L. Zhao, Effect of nano-ﬁllers on the thermal conductivity of epoxy composites with micro-Al2O3 particles, Mater. Des. 66 (2015) 176e182.  A. Nieto, D. Lahiri, A. Agarwal, Graphene NanoPlatelets reinforced tantalum carbide consolidated by spark plasma sintering, Mater. Sci. Eng. A 582 (2013) 338e346.  J. Sun, J. Zhao, M. Chen, Y. Zhou, X. Ni, Z. Li, F. Gong, Multilayer graphene reinforced functionally graded tungsten carbide nano-composites, Mater. Des. 134 (2017) 171e180.  F. Zhang, J. Shen, J. Sun, Processing and properties of carbon nanotubes-nanoWC-Co composites, Mater. Sci. Eng. A 381 (2004) 86e91.  M.A. Raﬁee, J. Raﬁee, I. Srivastava, Z. Wang, H. Song, Z.Z. Yu, et al., Fracture and fatigue in graphene nanocomposites, Small 6 (2010) 179e183.  I.W. Chen, X.H. Wang, Sintering dense nanocrystalline ceramics without ﬁnalstage grain growth, Nature 404 (2000) 168e171.  J. Li, Y. Ye, Densiﬁcation and grain growth of Al2O3 nanoceramics during pressureless sintering, J. Am. Ceram. Soc. 89 (2006) 139e143.  X.H. Wang, P.L. Chen, I.W. Chen, Two-step sintering of ceramics with constant
grain-size, I. Y2O3, J. Am. Ceram. Soc. 89 (2006) 431e437.  Y.I. Lee, Y.W. Kim, M. Mitomo, D.Y. Kim, Fabrication of dense nanostructured silicon carbide ceramics through two-step sintering, J. Am. Ceram. Soc. 86 (2003) 1803e1805.  H.T. Kim, Y.H. Han, Sintering of nanocrystalline BaTiO3, Ceram. Int. 30 (2004) 1719e1723. , P. Sajgalík, rek, Two-Stage sintering of  K. Bodisova D. Galusek, P. Svan ca alumina with submicrometer grain size, J. Am. Ceram. Soc. 90 (2007) 330e332.  J. Sun, J. Zhao, Multi-layer graphene reinforced nano-laminated WC-Co composites, Mat. Sci. Eng. A 723 (2018) 1e7.  H. Chang, L. Tang, Y. Wang, J. Jiang, J. Li, Graphene ﬂuorescence resonance energy transfer aptasensor for the thrombin detection, Anal. Chem. 82 (2010) 2341e2346.  A.S. Wajid, S. Das, F. Irin, H.T. Ahmed, J.L. Shelburne, D. Parviz, et al., Polymerstabilized graphene dispersions at high concentrations in organic solvents for composite production, Carbon 50 (2012) 526e534.  O.T. Picot, V.G. Rocha, C. Ferraro, N. Ni, E. D'Elia, S. Meille, et al., Using graphene networks to build bioinspired self-monitoring ceramics, Nat. Commun. 8 (2017) 14425.  Z.H. Ni, H.M. Wang, Y. Ma, J. Kasim, Y.H. Wu, Z.X. Shen, Tunable stress and controlled thickness modiﬁcation in graphene by annealing, ACS Nano 2 (2008) 1033e1039.  C. Ramirez, F.M. Figueiredo, P. Miranzo, P. Poza, M.I. Osendi, Graphene nanoplatelet/silicon nitride composites with high electrical conductivity, Carbon 50 (2012) 3607e3615.  G. Zheng, J. Zhao, Y. Zhou, Friction and wear behaviors of Sialon-Si3N4 graded nano-composite ceramic materials in sliding wear tests and in cutting processes, Wear 41 (2012) 290e291.  S. Lee, J.Y. Hong, J. Jang, Multifunctional graphene sheets embedded in silicone encapsulant for superior performance of light-emitting diodes, ACS Nano 7 (2013) 5784e5790.  I. Konyashin, B. Ries, F. Lachmann, A.T. Fry, Gradient WC-Co hardmetals: theory and practice, Int. J. Refrac. Met. 36 (2013) 10e21.  N. Scuor, E. Lucchini, S. Maschio, S.L. Casto, V. SergoInt, Wear mechanisms and residual stresses in alumina-based laminated cutting tools, J. Refract. Metal, Wear 258 (2005) 1372e1378.  Y.F. Chen, J.Q. Bi, C.L. Yin, G.L. You, Microstructure and fracture toughness of graphene nanosheets/alumina composites, Ceram. Int. 40 (2014) 13883e13889.  Y. Zhao, K.N. Sun, W.L. Wang, Y.X. Wang, X.L. Sun, Y.J. Liang, et al., Microstructure and anisotropic mechanical properties of graphene nanoplatelet toughened biphasic calcium phosphate composite, Ceram. Int. 39 (2013) 7627e7634. k, M. Koller, M. Landa, C. Ramírez, M. Belmonte, Anisotropic  H. Seiner, P. Sedla elastic moduli and internal friction of graphene nanoplatelets/silicon nitride composites, Compos. Sci. Technol. 75 (2013) 93e97.  H. Porwal, P. Tatarko, S. Grasso, J. Khaliq, I. Dlouhý, M.J. Reece, Graphene reinforced alumina nano-composites, Carbon 64 (2013) 359e369.  P. Rutkowski, P. Klimczyk, L. Jaworska, L. Stobierski, A. Dubiel, Thermal properties of pressure sintered alumina-graphene composites, J. Therm. Anal. Calorim. 122 (2015) 105e114.  R.T. Weitz, A. Yacoby, Nanomaterials: graphene rests easy, Nat. Nanotechnol. 5 (2010) 699e700.  A. Nasser, K. Shinagawa, H. El-Hofy, A.A. Moneim, Enhancing stability of Co gradient in nano-structured WC-Co functionally graded composites using graphene additives, J. Ceram. Soc. Jpn. 124 (2016) 1191e1198.  P. Fan, J. Guo, Z.Z. Fang, P. Prichard, Design of cobalt gradient via controlling carbon content and WC grain size in liquid-phase-sintered WC-Co composite, Int. J. Refract. Met. Hard. Mater 27 (2009) 256e260.  I. Konyashin, B. Ries, F. Lachmann, A.T. Fry, Gradient WC-Co hardmetals: theory and practice, Int. J. Refract. Met. Hard. Mater 36 (2013) 10e21.  L. Yu, J. Yang, T. Qiu, J. Zhang, L. Pan, Microstructure, mechanical, and thermal properties of (ZrB2þZrC)/Zr3[Al(Si)]4C6 composite, J. Am. Ceram. Soc. 97 (2014) 2950e2956.