Xiang Gea,
Mingqi Chena,
Jitong Wanga,
Donghui Longa,
Licheng Linga,
Wenming Qiao*ab,
Isao Mochidac and
Seong-Ho Yoon*c
aState Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China. E-mail: qiaowm@ecust.edu.cn; Fax: +86-021-64252914; Tel: +86-021-64253730
bKey Laboratory of Specially Functional Polymeric Materials and Related Technology (East China University of Science and Technology), Ministry of Education, China
cInstitute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga Koen, Kasuga, Fukuoka, Japan. E-mail: yoon@cm.kyushu-u.ac.jp; Fax: +81-92-583-7879; Tel: +81-92-583-7959
First published on 11th January 2016
Monolithic carbon nanofiber/carbon (M-CNF/C) composites were fabricated through facile liquid impregnation and hot pressing technologies, using monolithic carbon nanofibers and phenolic resin as reinforcement and carbon matrix precursor, respectively. The M-CNFs are uniformly dispersed in the M-CNF/C composites and display strong interfacial bonding with carbon matrix. Compared with powdered carbon nanofiber reinforced (P-CNF/C) composites, M-CNF/C composites exhibit significantly higher improvement in electrical conductivity, thermal conductivity and mechanical properties. The M-CNF/C composites also exhibit much lower friction coefficients (0.09–0.12) and wear losses (0.12–0.43 mg) than those of P-CNF/C composites. The superior enhancement is attributed to the unique 3D interconnected structure and high integrity of M-CNFs, which could dramatically increase conductive channels, significantly improve mechanical properties and remarkably decrease friction coefficient and wear loss of M-CNF/C composites. Furthermore, the electrical, thermal, mechanical and tribological properties of M-CNF/C composites could be adjusted by using M-CNFs with different bulk densities. The present work suggests the M-CNF/C composites a widespread potential as high-performance tribological materials.
Recently, CNFs were used as fillers in composites in order to obtain good tribological properties.21–23 Being similar to carbon nanotubes and graphene, CNFs are expected to possess unique advantages in tribological applications due to their layered structures.24 The attractive characteristics of the layered structures could be summarized as follows:21–24 (i) remarkably improved mechanical properties of the composites with increased strength, yield points and fracture toughness, which could reduce the fatigue and failures resulting from brittleness; (ii) significantly superior thermal conductivity which can help dissipate the heat from friction, reducing plastic deformation and plasticity-induced damage of the composites; (iii) good solid lubrication capability due to graphitic structures (van der Waals forces between the layers and covalent bond within the graphitic planes) which could efficiently reduce friction coefficient and improve wear resistance of the composites. Therefore, the tribological materials reinforced by CNFs display remarkable improvement in mechanical, thermal and frictional performances.21–24
However, CNFs were generally synthesized in the form of nano-sized and fluffy powders, making their handling and large scale application difficult.1,2,25,26 As for application in composites, powdered CNFs (P-CNFs) reinforced composites only display moderate enhancement in many respects, including electrical, thermal and mechanical properties. Besides, P-CNFs tend to agglomerate when dispersed in matrix, resulting in nonuniformity and unstability of the composites.17 One promising way to solve the above problems is to introduce macroscopic CNFs, such as monoliths, foams, sponges and areogels, which could offer a great opportunity to develop high-performance composites.1,2,25,26 So far, very little attempt, to our knowledge, has been made to study the effect of macroscopic CNFs structures on the fabrication of macroscopic CNF/matrix composites. Also, there are no systematic studies on the properties, including electrical, thermal, mechanical and tribological behaviors of macroscopic CNF/matrix composites. Therefore, a study on the fabrication of macroscopic CNF/matrix composites as well as their structure and properties will provide a significantly important guideline to design distinguished high-performance composites for various applications.
In the previous work,1,2 our group reported a new kind of monolithic CNFs (M-CNFs) synthesized by one-step facile chemical vapour deposition (CVD). Such M-CNFs exhibit high porosity, controllable bulk density, high specific surface area and good mechanical properties. In the present work, inspired by the processing methods of carbon/carbon composites, the M-CNFs were used to fabricate monolithic carbon nanofiber/carbon (M-CNF/C) composites through liquid impregnation and hot pressing technologies. Such fabrication can avoid dispersion difficulty by impregnating carbon matrix precursor into the M-CNFs, thus process a unique advantage in producing M-CNF/C composite structures, in which CNFs are uniformly distributed in the matrix. The aim of this investigation is to explore the influence of the M-CNFs additions on the electrical, thermal, mechanical and tribological properties of M-CNF/C composites. Meanwhile, advantages of M-CNF/C composites could be evaluated by taking powdered carbon nanofiber/carbon (P-CNF/C) composites and carbon matrix with similar fabrication as a contrast.
Morphologies of the samples were observed under scanning electron microscope (SEM; JOEL JSM3360LV) and transmission electron microscope (TEM; JOEL JSM2010).
X-ray diffraction (XRD) was performed on a Rigaku D/Max 2550VB/PC (Rigaku, Cu Kα radiation). Raman spectra were performed on a Raman microscope (InVia Reflex) with an excitation wavelength of 514.5 nm.
Electrical resistivity was measured using a four-point method at room temperature, and the electrical conductivity was calculated based on the electrical resistivity. Thermal conductivity was measured by flash method using the LFA 447 Nanoflash® (NETZSCH, Capitola, CA) at room temperature. Specimens with a dimension of Φ 12.7 × 3 mm were prepared from the center of the hot-pressed samples for the two measurements.
Compression test was performed on a universal testing machine (3367, Instron) with sample size of Φ 15 × 15 mm from the center of the hot-pressed samples, conducted at a loading speed of 1 mm min−1. Five samples were tested for each category to get an average value.
The friction test was performed on a ball-on-disk friction apparatus (GSM-Tribometer) at room temperature and ambient atmosphere. Friction occurred due to contact of the static ball and disk rotation by the motor (Fig. S2†). The test was performance under conditions of load: 5 N, sliding radius: 4.1 mm, sliding speed: 0.02, 0.05, 0.08, and 0.12 m s−1, relative humidity: 40% and test duration: 1 h. Before the test, samples were cleaned with acetone in a supersonic bath for 10 min. The wear loss was obtained by measuring the weight loss with a microbalance to detect weight changes as small as 10−5 g. Each experiment was repeated three times and the average values were expressed in the results. After the wear tests, the worn and un-worn surfaces of each sample were observed under SEM.
Fig. 2 Morphologies of P-CNFs and M-CNFs: macro morphologies of P-CNFs and M-CNFs (a), SEM images of P-CNFs (b) and M-CNFs (c) (d), TEM images of P-CNFs (e) and M-CNFs (f). |
The P-CNFs and M-CNFs were then used to fabricate P-CNF/C and M-CNF/C composites, respectively. There is no obvious difference between the two kinds of composites from the macro morphologies (insets in Fig. 3a and b). However, significant differences could be observed under SEM from the fracture surfaces of the P-CNF/C and M-CNF/C composites. P-CNF/C composites show a rough fracture surface with clear sliding steps (Fig. 3a) whose formation strongly depends on CNF pull-out and CNF/matrix interface separation. In contrast, M-CNF/C composites give a relatively smooth fracture surface without obvious fracture steps, where M-CNFs are broken directly with carbon matrix rather than just pulled out of the matrix (Fig. 3b). The change of fracture behavior means the increase of the cohesion in M-CNF/C composites. TEM image shows obvious CNF pull-out and interface separation in P-CNF/C composites (Fig. 3c), which is in accordance with SEM observation. Nevertheless, M-CNFs are coated with carbon matrix and there is no sign of interface separation, indicating stronger interfacial bonding between CNFs and carbon matrix (Fig. 3d). The superior interfacial bonding is attributed to the 3D interconnected networks of M-CNFs with high integrity and uniformity in the composites (Fig. 1).
Fig. 3 Micro and macro morphologies of P-CNF/C composites (a), micro and macro morphologies of M-CNF/C composites (b), TEM image of P-CNF/C composites (c), TEM image of M-CNF/C composites (d). |
The density of M-CNFs is controllable through changing growth conditions such as growth time and catalyst amount.1,2 Therefore, a series of M-CNF/C composites with different CNF contents could be fabricated by using M-CNFs with different densities. As demonstrated in Table 1, the density of M-CNF/C composites gradually increases to the maximum at 33% of M-CNF content, and then decrease as M-CNF content increases. Such decrease of density could result from lower porosity of M-CNFs with higher density, which may increase diffusion barriers of impregnation.
Samples | CNFs content (wt%) | M-CNFs density (mg cm−3) | M-CNFs porosity a (%) | Composite density (g cm−3) | Electrical conductivity (S cm−1) | Thermal conductivity (W m−1 K−1) |
---|---|---|---|---|---|---|
a Porosity was estimated based on calculations by using a density of 1.75.1,29 | ||||||
Carbon matrix | — | — | — | 1.32 | 16.31 | 0.16 |
P-CNF/C-22 | 22 | — | — | 1.61 | 29.24 | 0.64 |
M-CNF/C-18 | 18 | 21.3 | 98.8 | 1.55 | 53.52 | 1.03 |
M-CNF/C-22 | 22 | 50.9 | 97.1 | 1.63 | 79.39 | 1.21 |
M-CNF/C-33 | 33 | 69.5 | 96.0 | 1.64 | 123.78 | 1.88 |
M-CNF/C-40 | 40 | 110.3 | 93.7 | 1.58 | 154.24 | 2.04 |
Both P-CNF/C and M-CNF/C composites show a prominent reflection at ca. 26° and a weak peak at ca. 43° assigned to graphitic carbon (planes 002 and 100, respectively30) in XRD pattern (Fig. S3†). They both exhibit two bands at around 1600 cm−1 and 1350 cm−1 in Raman spectra (Fig. S4†), corresponding to the high crystalline graphite vibration in the tangential stretching mode and the disorder induced phonon mode.31 Although P-CNF/C and M-CNF/C composites exhibit higher graphitization than carbon matrix, resulting from the existence of CNFs, there is very little difference in XRD pattern or Raman spectra for the two composites (Fig. S3 and S4†).
Fig. 4 Compressive stress–strain curves of carbon matrix (a), P-CNF/C (a) and M-CNF/C composites (b), compressive modulus of carbon matrix, P-CNF/C and M-CNF/C composites (c). |
Fig. 5 Variation of friction coefficient of carbon matrix, P-CNF/C and M-CNF/C composites with respect to sliding time performed at 0.12 m s−1. |
Fig. 6 shows the mean friction coefficient for carbon matrix, P-CNF/C and M-CNF/C composites during the friction test. The mean friction coefficient of carbon matrix exceeds 0.2, and that of P-CNF/C-22 is approximately 0.17. However, M-CNF/C composites show much lower values of approximately 0.09–0.12, and they display a concave type dependency as M-CNF content increases and give the lowest value at M-CNF/C-33. Interestingly, the mean friction coefficients of M-CNF/C composites are even superior to that of most C/C composites reported in the literature. As demonstrated in Table 2, they are much lower than that of chopped carbon fiber reinforced C/C composites,36,37 2D carbon cloth and felt reinforced C/C composites,38,39 3D needled carbon fiber reinforced C/C composites40–42 and special additive coated C/C composites.43,44 They are comparable to that of CNT-doped C/C composites,45 but slightly higher than that of CNT added C/C composites.46 Nevertheless, the fabrication process of M-CNF/C composites does not require modified reinforcement or a second reinforcement, which is much more facile than that of both CNT-doped and CNT added composites.
Fig. 6 Mean friction coefficients of carbon matrix, P-CNF/C and M-CNF/C composites performed at 0.12 m s−1. |
Samples | Reinforcement | Friction coefficient | Ref. |
---|---|---|---|
C/C composite | Semi-random chopped PAN-based carbon fibers | 0.25–0.4 | 36 |
PAN/phenolic-based C/C | Chopped PAN-based carbon fiber | 0.32–0.45 | 37 |
2D C/C composite | PAN-based carbon cloth | 0.27–0.55 | 38 |
C/C composite | 2D PAN-based carbon fiber felt | 0.22–0.36 | 39 |
C/C composite | 3D needled carbon fiber | 0.19–0.32 | 40 |
C/C PAN-CVI | 3D needled carbon fiber | 0.26–0.38 | 41 |
C/C composite | 3D ex-PAN fiber mat | 0.15–0.35 | 42 |
MoSi impregnated C/C | MoSi, PAN-based carbon fiber | 0.25–0.62 | 43 |
2D C/C composite | Si nanoadditives, commercial 2D C/C | 0.22–0.62 | 44 |
CNT-doped C/C | CNT, stacking carbon felt | 0.09–0.14 | 45 |
CNT added C/C | CNT, carbon fiber | 0.08–0.11 | 46 |
M-CNF/C composite | M-CNFs | 0.09–0.12 | This work |
Fig. 7 shows the wear loss of carbon matrix, P-CNF/C and M-CNF/C composites during the friction test. Generally, the wear losses of M-CNF/C composites (0.12–0.43 mg) are much lower than that of carbon matrix and P-CNF/C composites. Additionally, M-CNF/C-33 shows the lowest wear loss of about 0.12 mg among the M-CNF/C composites as M-CNFs content increases. The mean friction coefficient and wear loss of M-CNF/C composites exhibit the lowest value at the same best M-CNFs content. Such low and controllable friction coefficients and wear loss indicate that the M-CNF/C composites are expected to be useful as new tribological materials.47
A high performance friction material should possess stable friction under varying operating speeds.47–49 The influence of sliding velocity on friction coefficient was studied at carbon matrix, P-CNF/C-22 and M-CNF/C-22 (Fig. 8). The friction coefficients of carbon matrix and P-CNF/C-22 show high dependence on sliding velocity, and give higher friction coefficient at higher sliding velocity. Nonetheless, the mean friction coefficient of M-CNF/C-22 shows almost no change when tested at different sliding velocities. It is interesting to note that the M-CNF/C composites display highly stable and low friction coefficient against sliding velocity conditions, which may further indicate the great potential of M-CNF/C composites as excellent tribological materials.
Fig. 8 The variation of mean friction coefficient with sliding velocity for carbon matrix, P-CNF/C and M-CNF/C composites. |
Friction coefficient and wear loss is closely related to the friction film which acts as a kind of lubricant.50–52 Fig. 9 shows SEM images of the border of the worn and unworn areas of carbon matrix, P-CNF/C and M-CNF/C composites. For carbon matrix, there are irregular and random pores on both of the original and worn surface, which could be the reason for the unstable friction behavior, high friction coefficient and high wear loss (Fig. 5). The worn surface of P-CNF/C composites is much smoother than M-CNF/C composites, indicating that P-CNF/C composites tend to be abraded more easily than M-CNF/C composites during the friction test. On the worn surface of M-CNF/C composites, there are some convex parts, which could result from the mixture of M-CNFs and carbon matrix. The convex parts indicate that the 3D interconnected networks of M-CNFs could make the M-CNF/C composites much more difficult to be abraded. The convex parts may further act as a solid lubricant when worn away from the surface.47 It is believed that the convex parts and worn particles produce film-like lubricity on the surface, which finally results in the low friction coefficient and wear loss of M-CNF/C composites.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26049c |
This journal is © The Royal Society of Chemistry 2016 |