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titanium carbide composites of a fine-grained structure and improved mechanical properties
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Graphene nanosheet/titanium carbide composites of a fine-grained structure and improved mechanical properties Xia Liua, Jianlin Lib,n, Xiaowei Yua, Hongwei Fana, Qing Wanga, Shan Yana, Lianjun Wanga,nn, Wan Jianga a
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, PR China b School of Materials and Chemical Engineering, Hainan University, Haikou 570228, PR China Received 18 July 2015; accepted 14 August 2015
Abstract Dense graphene nanosheets (GNSs)/titanium carbide (TiC) composites have been produced from graphene oxide (GO)/TiC composite powders by spark plasma sintering. It is unexpected to observe that an introduction of 1.0 vol% GNSs from GO completely stops TiC grain growth by pinning their grain boundaries and densification is completed under the confinement of the flexible GNSs. Such a mechanism assumedly comes from the ultra-thin structure of GNSs, which indicates a crucial role GNSs may play in ceramic processing and has not been reported previously. Compared with monolithic TiC, the flexural strength of GNSs/TiC composites is significantly improved as a result of the refinement of matrix grains and excellent strength of GNSs, while the fracture toughness is enhanced due mainly to crack deflection, GNSs bridging and pull-out. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Grain boundaries; Mechanical properties; Graphene; Titanium carbide
1. Introduction Recently, the mono-layered 2 dimensional graphene has attracted enormous interest for its fascinating physical properties, given the Young0 s modulus of E¼ ¼ 1.0 TPa, third-order elastic stiffness of D¼ ¼ 2.0 TPa and intrinsic strength of σint ¼ ¼ 130 GPa [1]. Compared with other materials, graphene or graphene nanosheets (GNSs) have a great specific surface area and they do not form agglomerates in a matrix when handled appropriately, suggesting they are a potential reinforcement for ceramic composites. Among those earliest reports on graphene/ceramic composites is our research of in situ produced C/TiC composites, which had a flexural strength and fracture toughness of 480 MPa and 6.5 MPa m1/2, respectively [2]. Later we prepared fully dense GNSs/Al2O3 composites and observed that this composite had n
Corresponding author. Tel.: þ86 65 67904342; fax: þ 86 65 67909081. Corresponding author. Tel.: þ 86 21 67792835; fax: þ86 21 67792855. E-mail addresses: [email protected] (J. Li), [email protected] (L. Wang). nn
very high electrical conductivity with a low GNS content. More importantly, the charge carrier type could be manipulated by controlling the content of GNSs, enabling the graphene contained ceramic composites to be used as energy conversion material in high temperature environment [3,4]. We found that by adding only 1.2 vol% of GNSs, the average grain size of Al2O3 matrix is much finer than that of monolithic Al2O3 [5]. Besides, Centeno et al. [6] applied a simple, fast and scalable method to produce GNSs/Al2O3 composites by SPS. More recently, we adopted an in situ strategy for fabrication of reduced graphene oxide/fused silica (rGO/FS) composites. Results showed that the addition of 1 wt% GO sheets to FS resulted in 72% increase in Vickers hardness, and 74% in the fracture toughness [7]. Dusza et al. [8] prepared GNSs/Si3N4 composites containing 1 wt% GNSs and reported an increase of about 43% in fracture toughness over the pure Si3N4. Liu et al. [9] fabricated GNS/ZTA composites with 0.81 vol% GNSs and found an increase of nearly 40% in fracture toughness. In view of these achievements, graphene/ceramic composite has been a hot topic worthy further and detailed study. But to
http://dx.doi.org/10.1016/j.ceramint.2015.08.071 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: X. Liu, et al., Graphene nanosheet/titanium carbide composites of a fine-grained structure and improved mechanical properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.071
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date, to our knowledge no research has been carried out on TiC composites reinforced by graphene. Titanium carbide (TiC), as one of the most important hightemperature structural ceramics, has been used in various applications due to its high melting point, good strength and high hardness, good thermal stability, wear and erosion resistance [10–15]. However, like most other ceramics, TiC possesses a low toughness and poor fracture toughness, which hinder its applications as an advanced structural material. An effective way to overcome this problem is to fabricate TiC based composites. Metal bound TiC was expected to excel in these composites, however, due to the low melting point and being easy to soften at higher temperatures, such TiC materials suffered huge drawbacks at high temperature use. To obtain improved high temperature properties, intermetallic aluminides Ni3Al and FeAl were used as the binder phase in TiC composites, but success is rather limited [16–18]. Another way is to fabricate ceramic particles (SiC, Al2O3, TiB2, TiN) reinforced TiC composites [19,20]. Our previous work on TiC composites demonstrated Al2O3–TiC bulk composites by spark plasma sintering (SPS) had much improved mechanical and electrical conduction properties simultaneously [21]. Compared to particles, carbon nanotubes (CNTs) and carbon fibers (Cf) [22] are better to bear the load and prevent crack propagation in matrix for their large aspect ratios. The work of Song et al. [23] on TiC composites reinforced with 20 vol% short Cf showed that fibers could remarkably increased both the room-temperature and elevatedtemperature strength and fracture toughness. Katsuyoshi et al. [24] reported that the mechanical properties of TiC matrix composites were remarkably improved with an additive of 0.35 wt% CNTs. In this work, we used expanded graphite as the starting material to fabricate GO colloid. Dense bulk composites were prepared from mixtures of TiC powder and GO colloid by SPS. During sintering, GO layers were reduced to GNSs at high temperatures. The aim of our research is to investigate the influence of GNSs on the structure and mechanical properties of the as-prepared GNSs/TiC bulk composites. 2. Experimental 2.1. Preparation of GO/TiC composite powders In this work, the GO was prepared by the modified hummers method reported elsewhere [4,25]. Briefly, commercial expandable graphite (160–50 N, Grafguard, USA) (1 g) was added to a flask and filled with concentrated sulfuric acid (25 ml) at room temperature, followed by addition of potassium permanganate (3.5 g) slowly at 0 1C (ice bath). The asprepared GO precipitated quickly because of the strong acid environment and the clear supernatant was decanted after a few hours. The precipitate mixture was washed with de-ionized water and centrifuged at 4000–11000 rpm for 30 min for several times to remove impurities. Finally, a GO aqueous solution was prepared from the gelatinous mixture by ultrasonic processing.
The commercially available TiC powder (Japan New Metals Co., Ltd.), with an average particle size of 1.3 μm was used in this study. As-received TiC (10 g) was directly poured into a beaker without treatment. Then water (500 ml) was added followed by ultrasonic stirring for one hour to ensure that the powder could be fully dispersed in water. The GO aqueous solution was added dropwise to the suspensions under ultrasonic stirring. The products were separated by the rotary evaporator followed by drying at 70 1C. The as-prepared mixture was blended on a planetary ball miller (Nanjing NanDa Instrument Plant Co, Ltd., QM-3SP2) with a rate of 200 r/min for 8 h before being dried at 80 1C for 12 h. 2.2. Sintering of GNSs/TiC bulk composites Bulk composite samples were prepared using SPS apparatus (Dr. Sinter 725; Sumitomo Coal Mining Co, Tokyo, Japan). GO/TiC powders were loaded into a 15 mm inner diameter graphite die and sintered in a vacuum of 6 Pa. The heating rate was 100 1C/min and soaking time was 3 min. A uniaxial pressure of 60 MPa was applied from 1000 1C upwards and maintained during the dwelling at 1550 1C. 2.3. Characterization The sintered bulk samples were grinded and polished by a polishing machine (UNIPOL-802, Shenyang Kejing Autoinstrument Co., Ltd.). Density measurements were conducted using the Archimedes' method. The morphology and microstructure of as-prepared samples were characterized by a field emission scanning electron microscopy (FESEM, Hitachi S-4800) and transmission electron microscopy (TEM, 2100F, Japan). Grain sizes were measured by the linear intercept method [26]. D ¼ 1:56
C MN
ð1Þ
where D is the average grain size, C is the total length of test line used, N is the number of intercepts, and M is the magnification of the photomicrograph. About 400 intercepts were counted for each measurement. The flexural strength was examined by three-point bending test. The testing was performed using a DS-II multifunctional desktop tester with a cross-head speed of 0.05 mm/min. Four samples were used for each run. It was calculated using the following equation Eq. (2) [27]: σ¼
3PL 2bh2
ð2Þ
where P is the load at the fracture point, L is the span length, b is the sample breadth and h is the sample thickness. The Vickers hardness and fracture toughness of the samples were determined by Vickers indentation technique (FV-700, Future-Tech Corporation) at a load of 3.0 Kg f (29.4 N) with a dwell of 5 s on carefully polished surfaces. Six measurements were conducted for each sample to calculate the average value.
Please cite this article as: X. Liu, et al., Graphene nanosheet/titanium carbide composites of a fine-grained structure and improved mechanical properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.071
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Fig. 1. FESEM micrographs of thermally etched surfaces of sintered samples (a) monolithic TiC; (b) 1.0 vol% GNSs composite; (c) 3.0 vol% GNSs composite; and (d) average grain size (blue) and relative density (red) of the sintered samples with various GNSs contents. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. TEM images of the raw TiC particles (a) and (b).
Please cite this article as: X. Liu, et al., Graphene nanosheet/titanium carbide composites of a fine-grained structure and improved mechanical properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.071
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Hardness was calculated using Eq. (3): H v ¼ 1:854
W d2
where 2c is the length of crack and P is the applied force and β¼ 681. ð3Þ 3. Results and discussion
where W is the applied load and d is the mean value of the diagonal length. KIC was obtained by indentation method using Eq. (4) [28]: K IC ¼ PðπcÞ 3=2 cot β
ð4Þ
Fig. 3. Diagrams showing the grain boundary migration driven by surface energy.
3.1. Microstructure of the as-prepared samples As is known, thermal treatment is one of the most effective methods to reduce GO which is thermally unstable. Thermal reduction of GO involves the removal of oxygenated functional by formation of carboneous species (CO2, CO) to restore the sp2 bonding network by directly heating GO to 1050 1C in a furnace [29,30]. Since SPS is a high temperature process under a low atmosphere pressure (4 1000 1C and P o 6 Pa in this work), there is no need of additional reduction procedure. After sintering, GNSs/TiC composite pellets were obtained with GO layers being reduced to GNSs. In order to accurately determine grain size of these samples, we heated the polished samples in evacuated tube at 1500 1C for 40 min to obtain the thermally etched surface, as shown in Fig. 1a–c. The average grain size of 1.0 vol% GNSs composite is calculated as 1.3 μm (Fig. 1d, blue line), which is of the same size of raw TiC grain and a small part of that of monolithic TiC ( 6.0 μm). The relative densities of as-sintered samples are only slightly affected by the GNSs (Fig. 1d, red line).
Fig. 4. FESEM micrographs of fractured surfaces of (a) Pure TiC ceramic and (b–d) GNSs/TiC composite containing 3.0 vol% GNSs. Please cite this article as: X. Liu, et al., Graphene nanosheet/titanium carbide composites of a fine-grained structure and improved mechanical properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.071
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Generally, grain growth takes place during powder sintering, which is usually affected by second phase particles distributed in the matrix. When foreign particles pin on the grain boundary, the final grain size of the matrix are to be significantly reduced. In our work here, the introduction of GNSs greatly prevented the TiC grains developing. It should be noted that with a 1.0 vol% GNSs addition, the TiC grains seems to have completely stopped growing, say, keeping their
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original size, 1.3 μm (Fig. 2). This is really not expected and has not been reported previously, but it is understandable that more GNSs cannot further reduce the grain size (Fig. 1d). As shown in Fig. 3, if grain boundaries can migrate over GNSs pinning there, that is, GNSs are later wrapped in matrix grains, the grain growth is sure to happen. The driving force behind grain growth is the interfacial energy of areas a and b.
Fig. 5. TEM and HRTEM images of a GNSs/TiC composite containing 3.0 vol% GNSs, (a) GNSs surrounding TiC grains; (b) GNSs with different thickness around TiC grains and (c) Ti and C element line scannings across over two TiC grains. Please cite this article as: X. Liu, et al., Graphene nanosheet/titanium carbide composites of a fine-grained structure and improved mechanical properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.071
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Fig. 6. Variation of mechanical properties with GNSs contents for the GNSs/ TiC composites. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Therefore, the ratio of GNS size to the grain lateral surface is vital. A too small ratio will not effectively trigger the migration of the grain boundary and thus the growth of the grain. Obviously, a thin layer is more effective than a cubic particle of the same volume in dragging the grain boundary migration (Fig. 3, from left to right). The key point here is that what percentage of GNSs is needed to completely prohibit matrix grain growth. Under usual conditions, grain size is roughly depicted by following relationship, D ¼ d=V d
ð5Þ
where D is the average grain size, d and Vd are, respectively, size and volume content of the second phase particles. In this work, if 1.0 vol% particulate second phase works to completely suppress the TiC grain growth, that is, Vd ¼ 1%, D ¼ 1:3 μm (original grain size), d should be 13 nm by Eq. (5). However, a few, not many, pores as big as 100 nm are wrapped inside TiC grains, as shown in Fig. 4a. Therefore, it is reasonable to assume 100 nm may be a critical size, that is, pores with a size of around 100 nm are just able to stop TiC grain boundary migration and retard grain growth. Since solid particles work much better than pores in dragging grain boundary migration, it is reasonable to assume that particles of a size of tens of nm can effectively pin grain boundaries. As a result, tiny particles of a size of 13 nm will not effectively suppress matrix grain growth. In view of that GNSs are very thin sheets with a huge specific surface area, they work much better than particles of the same volume to retard matrix grain growth. Again by Eq. (5), let Vd ¼ 1%, D ¼ 1:3 μm, we have d¼ 13 nm. When this 13 nm particle of a volume of 2200 (133) nm3 is sliced into a 0.3– 1 nm thick GNS (1–3 layers), it has a cross section area of 2200–6600 nm2, namely, a size of 50–80 nm. Our previous tests showed most GNSs produced in our work have 1 to 3 layers, indicating most GNSs act as cubic of a size of 80 nm, rather than 13 nm, to retard TiC grain growth (Fig. 3), in agreement with the above argument. This estimation here is rather rough but convinces us that the structure of ultra-thin GNSs plays the key role in refining TiC matrix grains. A similar phenomenon was observed when we studied GNSs/Al2O3 composites. It was
found that Al2O3 matrix grain size leveled off when GNSs contents increased to 2.0 vol% [31], however, the Al2O3 matrix grains were 80–100% bigger than that of original grain size. Considering that Al2O3 grains formed hard agglomerations and GNSs could not penetrate into these agglomerations, we concluded that these Al2O3 grains grew inside their agglomerations that were confined by GNSs. The suspension of TiC matrix grain growth does not stop the densification of the composites. At this moment, to coordinate the deformation of neighboring grains, TiC grains change their original morphology due to applied pressure as well as the tendency to reduce the total surface area. Here the flexibility of GNSs is primarily important in coordinating the deformation of neighboring grains. Careful examinations have been carried out on the fracture surface of the as-prepared samples to investigate the distribution of GNSs in the matrix. The FESEM micrographs of monolithic TiC show the coarsening grains after sintering (Fig. 4a). Some small pores are present inside TiC grains, indicating the driving force behind the matrix grain growth is huge, saying, pores cannot drag the movement of TiC grain boundaries. For GNSs/TiC samples, by contrast, no pores inside grains are found. GNSs are distributed homogeneously in matrix along TiC grain boundaries, and no GNSs are found to be located inside TiC grains (Fig. 4b–d). Compared with monolithic TiC ceramic (Fig. 4a), the introduction of GNS greatly restrains the growth of TiC grains, reducing grain size from 6 mm to 1.3 mm (Fig. 1d). The introduction of GNSs does not change the fracture mode of TiC materials, all samples exhibiting a mix of both inter-granular and trans-granular facture. Further characterization of the microstructure of sintered composites is performed by TEM. Fig. 5 shows representative TEM and HRTEM images of the composite containing 3.0 vol% GNSs. These graphene sheets are tucked and wrap around TiC grains, following the TiC grain boundaries and connecting with each other to form a network structure (Fig. 5a), which is similar to the observation in Fig. 4d. It is observed that GNSs with thickness 2–3.5 nm are well dispersed in the matrix along grain boundaries (Fig. 5b). However, due to their ultra-thin structure, one or two layers GNSs are difficult to be imaged by HRTEM. Some GNSs overlaps exist in the grain boundaries (as seen in Fig. 5c, left image), and no GNSs are found inside TiC grains. Shown in Fig. 5c is Ti and C element line scannings across over two TiC grains separated by a GNS, showing the GNSs are dispersed along TiC grain boundaries. All the above observations clearly demonstrate that GNSs located along TiC grain boundaries attenuate the driving force of TiC grain boundary migration and thus restrain the matrix grain growth. This is a never reported phenomena caused by GNSs. To our understanding, only GNSs can achieve such a success, because only GNS has the most specific surface area among all ceramic reinforcements used in a matrix. Previously, researchers knew grain refinement might be caused by tiny particles distributed in the matrix, but did not expect a tiny addition of a second phase could suppress the grain growth completely, which is of significant importance in materials preparation.
Please cite this article as: X. Liu, et al., Graphene nanosheet/titanium carbide composites of a fine-grained structure and improved mechanical properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.071
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Fig. 7. FESEM micrographs of indentation induced cracks (a); crack bridging at platelets (b), and crack deflection at platelets (c) (arrow indicates the crack extension direction).
3.2. Mechanical properties of the as-prepared samples The common way to improve mechanical properties of ceramic materials is to introduce reinforcement into the matrix. The addition of GNSs in this work has a significant effect on the flexural strength of the composites (Fig. 6, red line). Compared with monolithic TiC ceramics, a 60% improvement in flexural strength is observed for 3.0 vol% GNSs/TiC composite, which, according to the Hall–Petch relationship, can be partly attributed to the grain refinement of TiC matrix. Another factor is the excellent strength of GNSs. As shown in Fig. 4, the GNSs run along TiC grain boundaries and form a strong mechanical grip or friction force between matrix and GNSs [8], ensuring the load transferring from the matrix to GNSs. Similar phenomenon was reported in Al2O3 ceramic system [31,32]. As another essential and important parameter for mechanical properties of bulk samples, fracture toughness (KIC) was obtained using nano-indentation technique. As depicted in Fig. 6 (black line), the composites with 3.0 vol% GNS exhibit the highest fracture toughness of 4.0 MPa m1/2, an increase of 26% over the monolithic TiC ceramic. Similar to conventional fiber-reinforced TiC ceramics [23,33], pull-out and crack deflection and bridging are observed long the crack extension. As shown in Fig. 4c and d, we can observe pulled-out GNSs
along the grain boundary firmly embedded in the TiC matrix and forming a large area of interface between the sheet and TiC matrix. During the densification process, flexible GNSs are bent and embedded between the TiC grains because of the force applied by neighbor grains. The close contact enables the platelets to anchor at and bind with matrix grains. It is expected that the energy required to pull out a sheet is much greater than that of a fiber due to “sheet wrapping” around the matrix grain boundaries and the increased binding area with the matrix [8]. Fracture toughness of the composites is therefore believed to be greatly improved due to the strong interfacial friction in the interface between GNSs and matrix. On the other hand, the two-dimensional graphene is soft when bearing bending force while rigid when bearing tensile force. As shown in Fig. 7, when a crack generated by external force propagates and meets with a GNS, it is arrested and deflected in-plane, resulting in the formation of crack deflection and bridging, which retard the crack extending [8,9,34]. Besides, the addition of GNSs is expected to contribute to the elevated temperature strength and thermal shock resistance of the composites [11,35]. For example, to inhibit grain boundary sliding and creep, slow the expansion of the crack spread, and retain a good toughening and strengthening effect. Further research is underway.
Please cite this article as: X. Liu, et al., Graphene nanosheet/titanium carbide composites of a fine-grained structure and improved mechanical properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.071
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With the increasing of GNSs fraction, the Vickers hardness of GNSs/TiC composites is also slightly increased by about 20% (Fig. 6, blue line), which is almost certainly due to the fine grain size. 4. Conclusions Dense GNSs/TiC bulk composites have been fabricated by SPS from GO colloid and TiC powders. During the sintering process, the GO layers are well reduced to GNSs. The introduction of GNSs results in the grain refinement of TiC matrix and completely stops the TiC grain growth, which is an important finding in our work. Compared with monolithic TiC, the flexural strength and Vickers hardness of GNSs/TiC composites have been improved due mainly to the refinement of matrix grains. Crack deflection, GNSs bridging and pull-out contribute to the increase in the fracture toughness. Acknowledgments This work was supported by Program for Shanghai Committee of Science and Technology (No. 13NM1400101), NSFC (No. 51262006), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, No. 20110075110007), PCSIRT (No. IRT1221), the Fundamental Research Funds for the Central Universities (No. 2232012C3-01) and DHU Distinguished Young Professor Program. References [1] C. Lee, X.D. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (2008) 385–388. [2] J.L. Li, L.J Wang, G.Z. Bai, W. Jiang, Microstructure and mechanical properties of in-situ produced TiC/C nanocomposite, Scr. Mater. 52 (2005) 867–871. [3] Y.C Fan, L.J. Wang, J.L. Li, J.Q. Li, S.K. Sun, F. Chen, L.D. Chen, W. Jiang, Preparation and electrical properties of graphene nanosheet/ Al2O3 composites, Carbon 48 (2010) 1743–1749. [4] Y.C. Fan, W. Jiang, A. Kawasaki, Highly conductive few-layer graphene/ Al2O3 nano composites with tunable charge carrier type, Adv. Funct. Mater. 22 (2012) 3882–3889. [5] Y.C. Fan, M. Estili, G. Igarashi, W. Jiang, A. Kawasaki, The effect of homogeneously dispersed few-layer graphene on microstructure and mechanical properties of Al2O3 nanocomposites, J. Eur. Ceram. Soc. 34 (2014) 443–451. [6] A. Centeno, V.G. Rocha, B. Alonso, A. Fernández, C.F. GutierrezGonzalez, R. Torrecillas, Graphene for tough and electroconductive alumina ceramics, J. Eur. Ceram. Soc. 33 (2013) 3201–3210. [7] B.B. Chen, X. Liu, X.Q. Zhao, Z. Wang, L.J. Wang, W. Jiang, J.L. Li, Preparation and properties of reduced graphene oxide/fused silica composites, Carbon 77 (2014) 66–75. [8] J. Dusza, J. Morgiel, A. Duszová, L. Kvetková, M. Nosko, P. Kun, C. Balázsi, Microstructure and fracture toughness of Si3N4 þ graphene platelet composites, J. Eur. Ceram. Soc. 32 (2012) 3389–3397. [9] J. Liu, H. Yan, K. Jiang, Mechanical properties of graphene plateletreinforced alumina ceramic composites, Ceram. Int. 39 (2013) 6215–6221. [10] N. Durlu, Titanium carbide based composites for high temperature applications, J. Eur. Ceram. Soc. 19 (1999) 2415–2419. [11] G.M. Song, Y. Zhou, S.J.L. Kang, D.Y. Yoon, Effect of carbon fibers on the thermophysical properties of TiC composites, J. Mater. Sci. Lett. 21 (2002) 1733–1736.
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CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Graphene nanosheet/titanium carbide composites of a fine-grained structure and improved mechanical properties Xia Liua, Jianlin Lib,n, Xiaowei Yua, Hongwei Fana, Qing Wanga, Shan Yana, Lianjun Wanga,nn, Wan Jianga a
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, PR China b School of Materials and Chemical Engineering, Hainan University, Haikou 570228, PR China Received 18 July 2015; accepted 14 August 2015
Abstract Dense graphene nanosheets (GNSs)/titanium carbide (TiC) composites have been produced from graphene oxide (GO)/TiC composite powders by spark plasma sintering. It is unexpected to observe that an introduction of 1.0 vol% GNSs from GO completely stops TiC grain growth by pinning their grain boundaries and densification is completed under the confinement of the flexible GNSs. Such a mechanism assumedly comes from the ultra-thin structure of GNSs, which indicates a crucial role GNSs may play in ceramic processing and has not been reported previously. Compared with monolithic TiC, the flexural strength of GNSs/TiC composites is significantly improved as a result of the refinement of matrix grains and excellent strength of GNSs, while the fracture toughness is enhanced due mainly to crack deflection, GNSs bridging and pull-out. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Grain boundaries; Mechanical properties; Graphene; Titanium carbide
1. Introduction Recently, the mono-layered 2 dimensional graphene has attracted enormous interest for its fascinating physical properties, given the Young0 s modulus of E¼ ¼ 1.0 TPa, third-order elastic stiffness of D¼ ¼ 2.0 TPa and intrinsic strength of σint ¼ ¼ 130 GPa [1]. Compared with other materials, graphene or graphene nanosheets (GNSs) have a great specific surface area and they do not form agglomerates in a matrix when handled appropriately, suggesting they are a potential reinforcement for ceramic composites. Among those earliest reports on graphene/ceramic composites is our research of in situ produced C/TiC composites, which had a flexural strength and fracture toughness of 480 MPa and 6.5 MPa m1/2, respectively [2]. Later we prepared fully dense GNSs/Al2O3 composites and observed that this composite had n
Corresponding author. Tel.: þ86 65 67904342; fax: þ 86 65 67909081. Corresponding author. Tel.: þ 86 21 67792835; fax: þ86 21 67792855. E-mail addresses: [email protected] (J. Li), [email protected] (L. Wang). nn
very high electrical conductivity with a low GNS content. More importantly, the charge carrier type could be manipulated by controlling the content of GNSs, enabling the graphene contained ceramic composites to be used as energy conversion material in high temperature environment [3,4]. We found that by adding only 1.2 vol% of GNSs, the average grain size of Al2O3 matrix is much finer than that of monolithic Al2O3 [5]. Besides, Centeno et al. [6] applied a simple, fast and scalable method to produce GNSs/Al2O3 composites by SPS. More recently, we adopted an in situ strategy for fabrication of reduced graphene oxide/fused silica (rGO/FS) composites. Results showed that the addition of 1 wt% GO sheets to FS resulted in 72% increase in Vickers hardness, and 74% in the fracture toughness [7]. Dusza et al. [8] prepared GNSs/Si3N4 composites containing 1 wt% GNSs and reported an increase of about 43% in fracture toughness over the pure Si3N4. Liu et al. [9] fabricated GNS/ZTA composites with 0.81 vol% GNSs and found an increase of nearly 40% in fracture toughness. In view of these achievements, graphene/ceramic composite has been a hot topic worthy further and detailed study. But to
http://dx.doi.org/10.1016/j.ceramint.2015.08.071 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: X. Liu, et al., Graphene nanosheet/titanium carbide composites of a fine-grained structure and improved mechanical properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.071
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date, to our knowledge no research has been carried out on TiC composites reinforced by graphene. Titanium carbide (TiC), as one of the most important hightemperature structural ceramics, has been used in various applications due to its high melting point, good strength and high hardness, good thermal stability, wear and erosion resistance [10–15]. However, like most other ceramics, TiC possesses a low toughness and poor fracture toughness, which hinder its applications as an advanced structural material. An effective way to overcome this problem is to fabricate TiC based composites. Metal bound TiC was expected to excel in these composites, however, due to the low melting point and being easy to soften at higher temperatures, such TiC materials suffered huge drawbacks at high temperature use. To obtain improved high temperature properties, intermetallic aluminides Ni3Al and FeAl were used as the binder phase in TiC composites, but success is rather limited [16–18]. Another way is to fabricate ceramic particles (SiC, Al2O3, TiB2, TiN) reinforced TiC composites [19,20]. Our previous work on TiC composites demonstrated Al2O3–TiC bulk composites by spark plasma sintering (SPS) had much improved mechanical and electrical conduction properties simultaneously [21]. Compared to particles, carbon nanotubes (CNTs) and carbon fibers (Cf) [22] are better to bear the load and prevent crack propagation in matrix for their large aspect ratios. The work of Song et al. [23] on TiC composites reinforced with 20 vol% short Cf showed that fibers could remarkably increased both the room-temperature and elevatedtemperature strength and fracture toughness. Katsuyoshi et al. [24] reported that the mechanical properties of TiC matrix composites were remarkably improved with an additive of 0.35 wt% CNTs. In this work, we used expanded graphite as the starting material to fabricate GO colloid. Dense bulk composites were prepared from mixtures of TiC powder and GO colloid by SPS. During sintering, GO layers were reduced to GNSs at high temperatures. The aim of our research is to investigate the influence of GNSs on the structure and mechanical properties of the as-prepared GNSs/TiC bulk composites. 2. Experimental 2.1. Preparation of GO/TiC composite powders In this work, the GO was prepared by the modified hummers method reported elsewhere [4,25]. Briefly, commercial expandable graphite (160–50 N, Grafguard, USA) (1 g) was added to a flask and filled with concentrated sulfuric acid (25 ml) at room temperature, followed by addition of potassium permanganate (3.5 g) slowly at 0 1C (ice bath). The asprepared GO precipitated quickly because of the strong acid environment and the clear supernatant was decanted after a few hours. The precipitate mixture was washed with de-ionized water and centrifuged at 4000–11000 rpm for 30 min for several times to remove impurities. Finally, a GO aqueous solution was prepared from the gelatinous mixture by ultrasonic processing.
The commercially available TiC powder (Japan New Metals Co., Ltd.), with an average particle size of 1.3 μm was used in this study. As-received TiC (10 g) was directly poured into a beaker without treatment. Then water (500 ml) was added followed by ultrasonic stirring for one hour to ensure that the powder could be fully dispersed in water. The GO aqueous solution was added dropwise to the suspensions under ultrasonic stirring. The products were separated by the rotary evaporator followed by drying at 70 1C. The as-prepared mixture was blended on a planetary ball miller (Nanjing NanDa Instrument Plant Co, Ltd., QM-3SP2) with a rate of 200 r/min for 8 h before being dried at 80 1C for 12 h. 2.2. Sintering of GNSs/TiC bulk composites Bulk composite samples were prepared using SPS apparatus (Dr. Sinter 725; Sumitomo Coal Mining Co, Tokyo, Japan). GO/TiC powders were loaded into a 15 mm inner diameter graphite die and sintered in a vacuum of 6 Pa. The heating rate was 100 1C/min and soaking time was 3 min. A uniaxial pressure of 60 MPa was applied from 1000 1C upwards and maintained during the dwelling at 1550 1C. 2.3. Characterization The sintered bulk samples were grinded and polished by a polishing machine (UNIPOL-802, Shenyang Kejing Autoinstrument Co., Ltd.). Density measurements were conducted using the Archimedes' method. The morphology and microstructure of as-prepared samples were characterized by a field emission scanning electron microscopy (FESEM, Hitachi S-4800) and transmission electron microscopy (TEM, 2100F, Japan). Grain sizes were measured by the linear intercept method [26]. D ¼ 1:56
C MN
ð1Þ
where D is the average grain size, C is the total length of test line used, N is the number of intercepts, and M is the magnification of the photomicrograph. About 400 intercepts were counted for each measurement. The flexural strength was examined by three-point bending test. The testing was performed using a DS-II multifunctional desktop tester with a cross-head speed of 0.05 mm/min. Four samples were used for each run. It was calculated using the following equation Eq. (2) [27]: σ¼
3PL 2bh2
ð2Þ
where P is the load at the fracture point, L is the span length, b is the sample breadth and h is the sample thickness. The Vickers hardness and fracture toughness of the samples were determined by Vickers indentation technique (FV-700, Future-Tech Corporation) at a load of 3.0 Kg f (29.4 N) with a dwell of 5 s on carefully polished surfaces. Six measurements were conducted for each sample to calculate the average value.
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Fig. 1. FESEM micrographs of thermally etched surfaces of sintered samples (a) monolithic TiC; (b) 1.0 vol% GNSs composite; (c) 3.0 vol% GNSs composite; and (d) average grain size (blue) and relative density (red) of the sintered samples with various GNSs contents. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. TEM images of the raw TiC particles (a) and (b).
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Hardness was calculated using Eq. (3): H v ¼ 1:854
W d2
where 2c is the length of crack and P is the applied force and β¼ 681. ð3Þ 3. Results and discussion
where W is the applied load and d is the mean value of the diagonal length. KIC was obtained by indentation method using Eq. (4) [28]: K IC ¼ PðπcÞ 3=2 cot β
ð4Þ
Fig. 3. Diagrams showing the grain boundary migration driven by surface energy.
3.1. Microstructure of the as-prepared samples As is known, thermal treatment is one of the most effective methods to reduce GO which is thermally unstable. Thermal reduction of GO involves the removal of oxygenated functional by formation of carboneous species (CO2, CO) to restore the sp2 bonding network by directly heating GO to 1050 1C in a furnace [29,30]. Since SPS is a high temperature process under a low atmosphere pressure (4 1000 1C and P o 6 Pa in this work), there is no need of additional reduction procedure. After sintering, GNSs/TiC composite pellets were obtained with GO layers being reduced to GNSs. In order to accurately determine grain size of these samples, we heated the polished samples in evacuated tube at 1500 1C for 40 min to obtain the thermally etched surface, as shown in Fig. 1a–c. The average grain size of 1.0 vol% GNSs composite is calculated as 1.3 μm (Fig. 1d, blue line), which is of the same size of raw TiC grain and a small part of that of monolithic TiC ( 6.0 μm). The relative densities of as-sintered samples are only slightly affected by the GNSs (Fig. 1d, red line).
Fig. 4. FESEM micrographs of fractured surfaces of (a) Pure TiC ceramic and (b–d) GNSs/TiC composite containing 3.0 vol% GNSs. Please cite this article as: X. Liu, et al., Graphene nanosheet/titanium carbide composites of a fine-grained structure and improved mechanical properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.071
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Generally, grain growth takes place during powder sintering, which is usually affected by second phase particles distributed in the matrix. When foreign particles pin on the grain boundary, the final grain size of the matrix are to be significantly reduced. In our work here, the introduction of GNSs greatly prevented the TiC grains developing. It should be noted that with a 1.0 vol% GNSs addition, the TiC grains seems to have completely stopped growing, say, keeping their
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original size, 1.3 μm (Fig. 2). This is really not expected and has not been reported previously, but it is understandable that more GNSs cannot further reduce the grain size (Fig. 1d). As shown in Fig. 3, if grain boundaries can migrate over GNSs pinning there, that is, GNSs are later wrapped in matrix grains, the grain growth is sure to happen. The driving force behind grain growth is the interfacial energy of areas a and b.
Fig. 5. TEM and HRTEM images of a GNSs/TiC composite containing 3.0 vol% GNSs, (a) GNSs surrounding TiC grains; (b) GNSs with different thickness around TiC grains and (c) Ti and C element line scannings across over two TiC grains. Please cite this article as: X. Liu, et al., Graphene nanosheet/titanium carbide composites of a fine-grained structure and improved mechanical properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.071
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Fig. 6. Variation of mechanical properties with GNSs contents for the GNSs/ TiC composites. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Therefore, the ratio of GNS size to the grain lateral surface is vital. A too small ratio will not effectively trigger the migration of the grain boundary and thus the growth of the grain. Obviously, a thin layer is more effective than a cubic particle of the same volume in dragging the grain boundary migration (Fig. 3, from left to right). The key point here is that what percentage of GNSs is needed to completely prohibit matrix grain growth. Under usual conditions, grain size is roughly depicted by following relationship, D ¼ d=V d
ð5Þ
where D is the average grain size, d and Vd are, respectively, size and volume content of the second phase particles. In this work, if 1.0 vol% particulate second phase works to completely suppress the TiC grain growth, that is, Vd ¼ 1%, D ¼ 1:3 μm (original grain size), d should be 13 nm by Eq. (5). However, a few, not many, pores as big as 100 nm are wrapped inside TiC grains, as shown in Fig. 4a. Therefore, it is reasonable to assume 100 nm may be a critical size, that is, pores with a size of around 100 nm are just able to stop TiC grain boundary migration and retard grain growth. Since solid particles work much better than pores in dragging grain boundary migration, it is reasonable to assume that particles of a size of tens of nm can effectively pin grain boundaries. As a result, tiny particles of a size of 13 nm will not effectively suppress matrix grain growth. In view of that GNSs are very thin sheets with a huge specific surface area, they work much better than particles of the same volume to retard matrix grain growth. Again by Eq. (5), let Vd ¼ 1%, D ¼ 1:3 μm, we have d¼ 13 nm. When this 13 nm particle of a volume of 2200 (133) nm3 is sliced into a 0.3– 1 nm thick GNS (1–3 layers), it has a cross section area of 2200–6600 nm2, namely, a size of 50–80 nm. Our previous tests showed most GNSs produced in our work have 1 to 3 layers, indicating most GNSs act as cubic of a size of 80 nm, rather than 13 nm, to retard TiC grain growth (Fig. 3), in agreement with the above argument. This estimation here is rather rough but convinces us that the structure of ultra-thin GNSs plays the key role in refining TiC matrix grains. A similar phenomenon was observed when we studied GNSs/Al2O3 composites. It was
found that Al2O3 matrix grain size leveled off when GNSs contents increased to 2.0 vol% [31], however, the Al2O3 matrix grains were 80–100% bigger than that of original grain size. Considering that Al2O3 grains formed hard agglomerations and GNSs could not penetrate into these agglomerations, we concluded that these Al2O3 grains grew inside their agglomerations that were confined by GNSs. The suspension of TiC matrix grain growth does not stop the densification of the composites. At this moment, to coordinate the deformation of neighboring grains, TiC grains change their original morphology due to applied pressure as well as the tendency to reduce the total surface area. Here the flexibility of GNSs is primarily important in coordinating the deformation of neighboring grains. Careful examinations have been carried out on the fracture surface of the as-prepared samples to investigate the distribution of GNSs in the matrix. The FESEM micrographs of monolithic TiC show the coarsening grains after sintering (Fig. 4a). Some small pores are present inside TiC grains, indicating the driving force behind the matrix grain growth is huge, saying, pores cannot drag the movement of TiC grain boundaries. For GNSs/TiC samples, by contrast, no pores inside grains are found. GNSs are distributed homogeneously in matrix along TiC grain boundaries, and no GNSs are found to be located inside TiC grains (Fig. 4b–d). Compared with monolithic TiC ceramic (Fig. 4a), the introduction of GNS greatly restrains the growth of TiC grains, reducing grain size from 6 mm to 1.3 mm (Fig. 1d). The introduction of GNSs does not change the fracture mode of TiC materials, all samples exhibiting a mix of both inter-granular and trans-granular facture. Further characterization of the microstructure of sintered composites is performed by TEM. Fig. 5 shows representative TEM and HRTEM images of the composite containing 3.0 vol% GNSs. These graphene sheets are tucked and wrap around TiC grains, following the TiC grain boundaries and connecting with each other to form a network structure (Fig. 5a), which is similar to the observation in Fig. 4d. It is observed that GNSs with thickness 2–3.5 nm are well dispersed in the matrix along grain boundaries (Fig. 5b). However, due to their ultra-thin structure, one or two layers GNSs are difficult to be imaged by HRTEM. Some GNSs overlaps exist in the grain boundaries (as seen in Fig. 5c, left image), and no GNSs are found inside TiC grains. Shown in Fig. 5c is Ti and C element line scannings across over two TiC grains separated by a GNS, showing the GNSs are dispersed along TiC grain boundaries. All the above observations clearly demonstrate that GNSs located along TiC grain boundaries attenuate the driving force of TiC grain boundary migration and thus restrain the matrix grain growth. This is a never reported phenomena caused by GNSs. To our understanding, only GNSs can achieve such a success, because only GNS has the most specific surface area among all ceramic reinforcements used in a matrix. Previously, researchers knew grain refinement might be caused by tiny particles distributed in the matrix, but did not expect a tiny addition of a second phase could suppress the grain growth completely, which is of significant importance in materials preparation.
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Fig. 7. FESEM micrographs of indentation induced cracks (a); crack bridging at platelets (b), and crack deflection at platelets (c) (arrow indicates the crack extension direction).
3.2. Mechanical properties of the as-prepared samples The common way to improve mechanical properties of ceramic materials is to introduce reinforcement into the matrix. The addition of GNSs in this work has a significant effect on the flexural strength of the composites (Fig. 6, red line). Compared with monolithic TiC ceramics, a 60% improvement in flexural strength is observed for 3.0 vol% GNSs/TiC composite, which, according to the Hall–Petch relationship, can be partly attributed to the grain refinement of TiC matrix. Another factor is the excellent strength of GNSs. As shown in Fig. 4, the GNSs run along TiC grain boundaries and form a strong mechanical grip or friction force between matrix and GNSs [8], ensuring the load transferring from the matrix to GNSs. Similar phenomenon was reported in Al2O3 ceramic system [31,32]. As another essential and important parameter for mechanical properties of bulk samples, fracture toughness (KIC) was obtained using nano-indentation technique. As depicted in Fig. 6 (black line), the composites with 3.0 vol% GNS exhibit the highest fracture toughness of 4.0 MPa m1/2, an increase of 26% over the monolithic TiC ceramic. Similar to conventional fiber-reinforced TiC ceramics [23,33], pull-out and crack deflection and bridging are observed long the crack extension. As shown in Fig. 4c and d, we can observe pulled-out GNSs
along the grain boundary firmly embedded in the TiC matrix and forming a large area of interface between the sheet and TiC matrix. During the densification process, flexible GNSs are bent and embedded between the TiC grains because of the force applied by neighbor grains. The close contact enables the platelets to anchor at and bind with matrix grains. It is expected that the energy required to pull out a sheet is much greater than that of a fiber due to “sheet wrapping” around the matrix grain boundaries and the increased binding area with the matrix [8]. Fracture toughness of the composites is therefore believed to be greatly improved due to the strong interfacial friction in the interface between GNSs and matrix. On the other hand, the two-dimensional graphene is soft when bearing bending force while rigid when bearing tensile force. As shown in Fig. 7, when a crack generated by external force propagates and meets with a GNS, it is arrested and deflected in-plane, resulting in the formation of crack deflection and bridging, which retard the crack extending [8,9,34]. Besides, the addition of GNSs is expected to contribute to the elevated temperature strength and thermal shock resistance of the composites [11,35]. For example, to inhibit grain boundary sliding and creep, slow the expansion of the crack spread, and retain a good toughening and strengthening effect. Further research is underway.
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With the increasing of GNSs fraction, the Vickers hardness of GNSs/TiC composites is also slightly increased by about 20% (Fig. 6, blue line), which is almost certainly due to the fine grain size. 4. Conclusions Dense GNSs/TiC bulk composites have been fabricated by SPS from GO colloid and TiC powders. During the sintering process, the GO layers are well reduced to GNSs. The introduction of GNSs results in the grain refinement of TiC matrix and completely stops the TiC grain growth, which is an important finding in our work. Compared with monolithic TiC, the flexural strength and Vickers hardness of GNSs/TiC composites have been improved due mainly to the refinement of matrix grains. Crack deflection, GNSs bridging and pull-out contribute to the increase in the fracture toughness. Acknowledgments This work was supported by Program for Shanghai Committee of Science and Technology (No. 13NM1400101), NSFC (No. 51262006), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, No. 20110075110007), PCSIRT (No. IRT1221), the Fundamental Research Funds for the Central Universities (No. 2232012C3-01) and DHU Distinguished Young Professor Program. References [1] C. Lee, X.D. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (2008) 385–388. [2] J.L. Li, L.J Wang, G.Z. Bai, W. Jiang, Microstructure and mechanical properties of in-situ produced TiC/C nanocomposite, Scr. Mater. 52 (2005) 867–871. [3] Y.C Fan, L.J. Wang, J.L. Li, J.Q. Li, S.K. Sun, F. Chen, L.D. Chen, W. Jiang, Preparation and electrical properties of graphene nanosheet/ Al2O3 composites, Carbon 48 (2010) 1743–1749. [4] Y.C. Fan, W. Jiang, A. Kawasaki, Highly conductive few-layer graphene/ Al2O3 nano composites with tunable charge carrier type, Adv. Funct. Mater. 22 (2012) 3882–3889. [5] Y.C. Fan, M. Estili, G. Igarashi, W. Jiang, A. Kawasaki, The effect of homogeneously dispersed few-layer graphene on microstructure and mechanical properties of Al2O3 nanocomposites, J. Eur. Ceram. Soc. 34 (2014) 443–451. [6] A. Centeno, V.G. Rocha, B. Alonso, A. Fernández, C.F. GutierrezGonzalez, R. Torrecillas, Graphene for tough and electroconductive alumina ceramics, J. Eur. Ceram. Soc. 33 (2013) 3201–3210. [7] B.B. Chen, X. Liu, X.Q. Zhao, Z. Wang, L.J. Wang, W. Jiang, J.L. Li, Preparation and properties of reduced graphene oxide/fused silica composites, Carbon 77 (2014) 66–75. [8] J. Dusza, J. Morgiel, A. Duszová, L. Kvetková, M. Nosko, P. Kun, C. Balázsi, Microstructure and fracture toughness of Si3N4 þ graphene platelet composites, J. Eur. Ceram. Soc. 32 (2012) 3389–3397. [9] J. Liu, H. Yan, K. Jiang, Mechanical properties of graphene plateletreinforced alumina ceramic composites, Ceram. Int. 39 (2013) 6215–6221. [10] N. Durlu, Titanium carbide based composites for high temperature applications, J. Eur. Ceram. Soc. 19 (1999) 2415–2419. [11] G.M. Song, Y. Zhou, S.J.L. Kang, D.Y. Yoon, Effect of carbon fibers on the thermophysical properties of TiC composites, J. Mater. Sci. Lett. 21 (2002) 1733–1736.
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Please cite this article as: X. Liu, et al., Graphene nanosheet/titanium carbide composites of a fine-grained structure and improved mechanical properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.071