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C composites
Diamond & Related Materials 95 (2019) 99–108
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Influence of diamond graphitization on the microstructure and performance of micro-diamond modified C/C composites ⁎
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Lei Chena, Xin Yanga,b, , Qizhong Huanga, , Cunqian Fanga, Anhong Shia, Ruirui Liua a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China Science and Technology of Advanced Functional Composite Laboratory, Aerospace Research Institute of Materials & Processing Technology, Beijing 100076, China
A R T I C LE I N FO
A B S T R A C T
Keywords: C/C composites Micro-diamond particles Graphitization Microstructure Performance
Micro-diamond modified C/C composites (C/C-D composites) were fabricated using pressureless infiltration (PI) and chemical vapor infiltration (CVI) methods. And the influence of diamond graphitization on the microstructure and performance of C/C-D composites were investigated. During further graphitization process of diamond particles (1600 °C), thicker graphite layer formed on diamond surface accompanied by volume expansion, which induced stress graphitization to surrounding PyC. Meanwhile, the graphite/PyC interface bonding strength improved significantly after diamond graphitization. Compared with the low strength retention rate of C/C composites (46.8%) after heat treated at 1600 °C, a high strength retention rate was achieved for C/ C-D composites (87.1%) benefiting from the enhanced graphite/PyC interface bonding strength after diamond graphitization. And the thermal conductivity of the as-prepared C/C-D composites increased by 32.5% with the addition of diamond particles (9.97 wt%). It could be expected that C/C composites with optimal microstructure and excellent performances can be obtained by adjusting the content and graphitization degree of diamond particles in future work.
1. Introduction Carbon fiber reinforced carbon matrix (C/C) composites have aroused extensive attention due to their excellent properties, such as high specific strength, high specific modulus, low thermal expansion coefficient and outstanding thermal shock resistance, which make them the most promising candidate for thermal structural materials used in aerospace and aircraft fields [1–4]. Besides the commonly used PANbased carbon fibers, other kinds of carbon materials with outstanding physical and thermal properties, such as carbon nanotubes [5–9], carbon nanofibers [10,11] and graphene [12,13], have been also applied as reinforcement phase to improve the mechanical and thermal properties of C/C composites. Stress graphitization of matrix carbon has occurred at the reinforcement phase/matrix interfaces due to the mismatch of thermal expansion coefficient and has significant influence on the performance of C/C composites [14,15]. Therefore, investigation on the microstructure of reinforcement phase/matrix interface is vitally important for the development of advanced C/C composites with adequate performances. Diamond particles have been widely used as reinforcement phase in polymer and ceramic composites that require better mechanical and
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thermal properties [16–18], owing to their outstanding properties, including high strength, high thermal conductivity, superior wear and erosion resistance. SiC ceramics modified by diamond particles were reported to possess excellent fracture strength and enhanced thermal conductivity [19–21]. In recent years, diamond particles have also been introduced into C/SiC and C/C composites to improve the thermal conductivity and performance of the composites. The thermal conductivity of C/SiC-Diamond composites prepared by CVI and RMI methods in a recent study got a near twofold enhancement and an improvement in ablation resistance of micro-diamond modified C/SiC composites fabricated by PIP method was also realized [22,23]. In a recent study, micro-diamond particles were firstly introduced into C/C composites and the thermal conductivity of the composites was improved by 19.6% with a diamond particle content of 2 vol% [24]. However, additional properties as well as the relationship between microstructure and performance of C/C-diamond composites need to be further investigated. It is well known that diamond is the thermodynamically unstable form of carbon and the graphitization of diamond particles is an unavoidable process under the composite fabrication or application condition which involves high temperatures [25–27]. And it is worth noting that the graphitization process of diamond is
Correspondence to: X. Yang, State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China. Corresponding author. E-mail addresses: [email protected] (X. Yang), [email protected] (Q. Huang).
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https://doi.org/10.1016/j.diamond.2019.04.003 Received 21 January 2019; Received in revised form 19 March 2019; Accepted 2 April 2019 Available online 03 April 2019 0925-9635/ © 2019 Elsevier B.V. All rights reserved.
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C/C and C/C-D composites were analyzed by Raman spectra (LabRAM HR800) using an incident laser with a wavelength of 488 nm. The texture of PyC in both C/C and C/C-D composites was determined by polarized light microscopy (PLM, Leica DM4000M). The hardness and elastic modulus of PyC in C/C and C/C-D composites were characterized using an ultra nanoindentation tester (UNHT+MCT, CSM Instruments, Swiss) with a loading rate of 10 mN/min. The samples used in the PLM and nanoindentation study were firstly mounted in epoxy resin and allowed to harden, followed by subsequent grind and polish process. At least 5 positions were examined on each sample in nanoindentation test and the average hardness and elastic modulus were adopted. Scanning electron microscope (SEM, FEI Nova Nano230) was applied to characterize the morphology of the composites as well as the fracture surfaces. The microstructure of diamond particles under various graphitization stages and the diamond/PyC interface structure were characterized by TEM and HRTEM (FEI Titan G2 60-300). For TEM and HRTEM observation, the diamond particles or powders scraped from C/C and C/C-D composites were firstly dispersed in ethanol by ultra-sonication and then dropped on a copper grid. Mechanical properties of C/C and C/C-D composites were characterized via three-point bending test on a universal material tester (Instron3369) using samples with a dimension of 55 × 10 × 4 mm3. The span and loading rate were 40 mm and 1 mm/min respectively. And four samples for each group of composites were tested to obtain the average bending strength. Thermal conductivity of the composites was obtained using a laser thermal conducting instrument (LFA 457). The sample size and heating rate were about Φ 12.5 × 2.5 mm3 and 5 °C/min, respectively.
accompanied by volume expansion which can cause stress accumulation at the diamond/matrix interface and influence the microstructure of matrix carbon. Moreover, graphite particles with different microstructures, such as onion like carbon (OLC) [28–31] and closed curved graphite structures (CCGS) [32,33], could be obtained by annealing diamond particles with different sizes. Therefore, the microstructure of C/C-diamond composites, especially the matrix structure, will experience significant transformation after diamond graphitization which will influence the performance of C/C-diamond composites at a large extent. Nevertheless, to the best of our knowledge, the influence of diamond graphitization on the microstructure and performance of C/C composites have not been reported yet. In this study, micro-diamond modified C/C composites (C/C-D composites) were fabricated and a 1600 °C heat treatment process was also conducted to realize diamond particle graphitization. The graphitization process of micro-diamond particles was investigated systematically, based on which the influence of diamond graphitization on the diamond/pyrolytic carbon (PyC) interface structure was discussed. And the relationship between microstructure and mechanical, thermal properties of C/C-D composites was also analyzed. 2. Experimental 2.1. Materials fabrication Micro-diamond modified C/C composites (C/C-D composites) were fabricated by pressureless infiltration (PI) and chemical vapor infiltration (CVI) methods using 2.5D needle punched carbon fiber felt (with a density of 0.24 g/cm3) as preform. A pyrolytic carbon layer was firstly deposited on carbon fibers before the subsequent procedure. Micro-diamond particles (Changsha Naiqiang Superabrasives Co. Ltd) with an average particle size of 1.27 μm were introduced into the preform by PI method. Firstly, diamond particles were dispersed in ethyl alcohol (AR, Tianjin HengXing Chemical Reagent Co. Ltd) by ultrasonic agitation for 10 min with a power of 50 W and frequency of 40 KHz, forming a suspension liquid. Then, the preform was immerged into the suspension liquid and the apparatus was vacuumized. After being dried at 80 °C for 5 h, the ethyl alcohol was evaporated and diamond particles were introduced into the carbon fiber preform successfully. The above process was repeated until the content of diamond reached the calculated value (calculated with a final density of 1.7 g/ cm3 and a diamond content of 10 wt%). After being heat treated at 1400 °C for 1 h, the above diamond modified carbon fiber preform was densified by CVI process using propylene as carbon source and nitrogen as carrier gas. The final density of the C/C-D composites is about 1.7 g/ cm3 and the diamond content is calculated to be 9.97 wt%. C/C composites without diamond were also fabricated using the same procedure for comparison. To fully understand the graphitization and microstructure evolution of micro-diamond, the diamond particles were annealed in argon atmosphere from 1200 to 1600 °C with a heating rate of 10 °C/min. After 1 h holding at preset temperature, the samples were furnace cooled to room temperature. And finally, a group of C/C and C/C-D composites were heat treated at 1600 °C for 1 h (the exact condition for fully diamond graphitization) to explore the influence of diamond graphitization on the microstructure and mechanical performance of the composites.
3. Results and discussion 3.1. Microstructure transformation of diamond particles The graphitization process of diamond can be affected by many factors, including diamond particle size, crystallinity as well as pressure. Compared with the intensive research on the nano-diamond graphitization process, the investigation on the micro-diamond graphitization is still limited. In order to clarify the graphitization temperature of the diamond particles used in this study, the graphitization process was analyzed and distinctive microstructure was formed on the middle stage of graphitization, which gives a new sight into the graphitization mechanism of diamond particles. The original diamond particles used in this study present an irregular polygonal shape with a median size (Dv (50)) of 1.27 μm (Fig. S1). It can be seen in Fig. 1 that the graphitization of diamond particles begins at 1300 °C, indicated by the appearance of diffraction peak at 26.4° (Fig. 1a) and G bands at 1580 cm−1 (Fig. 1c) which is reported to originate from an ideal graphitic lattice vibration mode with E2g symmetry [34]. With the temperature increasing, the intensity of diamond peaks in both XRD and Raman spectra weaken while that of graphite peaks increase, which confirms the further graphitization of diamond. When the temperature reaches 1600 °C, as shown in Fig. 1b and d, the graphitization process accomplishes and the yielded graphite possesses high degree of crystallinity, manifested by the intensive and narrow diffraction peak of graphite (002) plane as well as the low ID1/IG value. Existence of edge carbon atoms (with imperfect hexagonal structure) and relatively small particle size (determined by original diamond particles) after graphitization are believed to account for the appearance of D bands in Fig. 1d [35]. The microstructures of micro-diamond particles heat treated at different temperatures are shown in Fig. 2. It can be concluded from the microstructure transformation process and the morphology of graphite phase (Fig. 2c and d) that the graphitization process begins from the diamond surface and proceeds inwards to the core gradually with the increase of temperature. The transformed region at different positions of diamond surface possesses nearly the same thickness, and the
2.2. Characterization The bulk density and open porosity of C/C and C/C-D composites were measured by Archimedes method. X-ray diffraction (XRD, Rigaku Dmax/2550VB + 18 kW) was used to characterize the phase constitution of diamond particles before and after heat treatment with CuKα radiation (0.154 nm) at 40 kV and 200 mA. The graphitization degree of diamond particles at different graphitization stages as well as the PyC in 100
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Fig. 1. XRD and Raman spectra of diamond particles annealed at different temperatures: (a) 1200–1500 °C, XRD; (b) 1600 °C, XRD; (c) 1200–1400 °C, Raman; (d) 1600 °C, Raman.
3.2. Influence of diamond graphitization on the microstructure of PyC
particles tend to maintain their original shape. Meanwhile, the direct growth of diamond {111} planes into graphite (002) planes could be observed in Fig. 2a and b, as marked by the yellow squares. And the corresponding fast Fourier transformation (FFT) patterns illustrate a parallel relationship between graphite (002) and diamond {111} planes. Moreover, it is worth noting that two sets of graphene layers with different crystal orientation, parallel to diamond (111) and (111) planes respectively, are presented in Fig. 2b, which demonstrates a uniform transformation tendency of {111} diamond planes. Different from previously reported CCGS [32], the diamond particles annealed at 1400 °C in this research present three distinctive regions (Fig. 2b), the outer graphite region with flat and ordered graphene layers, the transition region with newly formed graphene layers, as well as the inner untransformed diamond core, which gives a direct clue to the phase transformation process. Combining the graphitization mechanism proposed by many researchers earlier [26] with the above analysis, the graphitization process can be summarized as follows. In-situ transformation of diamond {111} planes to graphite (002) planes is the main process, which is accompanied by bond breaking and shrinkage of interatomic distance. The equal transformation possibility of all the {111} planes results in the formation of complicated but locally ordered transition region. Plenty of point defects and dislocations exist in this transition region. Decrease of surface and distortion energy is believed to be the driving force for the formation of ordered graphene layers. After the fulfillment of graphitization, graphite particles with high crystallinity are obtained (Figs. 1b and 2f).
The microstructure of C/C and C/C-D composites before and after 1600 °C heat treatment was characterized by SEM and PLM. In C/C composites, as shown in Fig. 3a and b, PyC deposits around the carbon fibers and forms lamellar structure. After heat treated at 1600 °C, the texture of PyC presents little change (Fig. 3a and b) due to the relatively low temperature compared with commonly used graphitization temperature (> 2000 °C) for C/C composites. In C/C-D composites, diamond particles distribute between carbon fibers and PyC, combining well with matrix carbon. Different from the morphology of C/C composites, in C/C-D composites, diamond particles act as nucleation sites for the deposition of PyC; and the preferential orientation of PyC parallel to carbon fibers is disturbed due to the irregular surface of diamond particles. Moreover, the influence of diamond addition on the structure of PyC is mainly reflected in the diamond stacking and adjacent area (Fig. 4c and d). After 1600 °C heat treatment, the texture of PyC adjacent to diamond particles presents better texture, and the diamond particles become smooth (Fig. 3d), resulting from the further graphitization. Raman spectrum was used to characterize the microstructure transformation of PyC after heat treatment and the results are shown in Fig. 5. All the spectra of C/C and C/C-D composites were fitted with four Lorentzian-shaped bands (G, D1, D2, D4) and one Gaussian-shaped band (D3), and the fitted peak parameters as well as the ID1/IG values are summarized in Table S1. It could be found that, before heat 101
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Fig. 2. TEM, HRTEM and corresponding FFT patterns of diamond particles heat treated at different temperatures: (a, c) 1300 °C; (b, d) 1400 °C; (e, f) 1600 °C.
ID1/IG value of C/C-D composites decreases obviously after heat treated at 1600 °C, especially for the diamond stacking area. Compared with the commonly used graphitization temperature (2000–2800 °C) for C/C composites, 1600 °C is a relatively low temperature at which the type of PyC would not experience significant change. Nevertheless, due to the stress accumulation caused by volume expansion of diamond during graphitization process, the texture of PyC around the diamond particles improves significantly. The obvious decrease of ID1/IG value in diamond stacking area mainly results from the highly ordered graphene layers formed by graphitization of diamond particles. Nanoindentation experiment was conducted to investigate the plasticity of PyC in both C/C and C/C-D composites. And the mean hardness as well as elastic modulus of PyC were presented in insert tables of corresponding load-displacement curves, as shown in Fig. 6. The highest hardness (8.07 GPa) and elastic modulus (39.13 GPa) were measured for PyC in as-prepared C/C-D composites, while the lowest
treatment, the ID1/IG values of C/C-D composites in both diamond stacking area and adjacent area are higher than those of C/C composites, indicating a lower graphitization degree. During the deposition process, due to the small particle size, irregular shape and stacking of diamond, PyC depositing around the stacking particles presents random orientation with diamond as nucleation core, thus breaking the longrange preferential orientation of PyC and resulting in more defects and lower graphitization degree. Therefore, the ID1/IG value of C/C-D composites is higher than that of C/C composites, especially in the diamond stacking area where the deposition space of PyC is further restricted to pores among diamond particles. After heat treated at 1600 °C, the FWHM of both D and G bands for C/C and C/C-D composites decrease obviously, indicating an increase in graphitization degree. For C/C composites, the ID1/IG value presents no obvious change after heat treatment, indicating little enhancement in PyC texture, in consistent with the SEM and PLM results. However, the 102
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Fig. 3. Microstructure of C/C (a, b) and C/C-D (c, d) composites before (a, c) and after (b, d) 1600 °C heat treatment. The interface between diamond stacking area and PyC are marked in red dash lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. PLM of C/C (a, b) and C/C-D (c, d) composites before (a, c) and after (b, d) 1600 °C heat treatment. 103
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Fig. 5. Raman spectra and curve fitting results of PyC in C/C and C/C-D composites before (a, b, c) and after (d, e, f) 1600 °C heat treatment: (a, d) C/C composite; (b, e) adjacent area in C/C-D composite; (c, f) diamond stacking area in C/C-D composites.
the transformed graphite layer on diamond particles after heat treatment (Fig. 7c and d). Limited space caused by the stacking of diamond particles together with the existence of surrounding PyC restricts the volume expansion during diamond graphitization, which causes stress accumulation in the material, resulting in stress graphitization of adjacent PyC. And the PyC around diamond particles in C/C-D composites presents better texture compared with that in C/C composites after heat treatment (Fig. 7b and d). Different from the condition when diamond particles are annealed alone, the restricted space in C/C-D composites hinders the graphitization of diamond and diamond particles at intermediate graphitization stages are factually observed in C/C-D composites (Fig. S2).
hardness (2.85 GPa) and elastic modulus (21.09 GPa) were measured for PyC in C/C-D composites after heat treatment. Compared with the slight decrease of hardness (5.14 GPa to 4.14 GPa) and elastic modulus (33.24 GPa to 27.02 GPa) for C/C composites after heat treatment, this significant reduce for C/C-D composites is proposed to be induced by diamond graphitization. In consistent with the Raman results, the stress graphitization changes the microstructure of PyC towards better ordered carbon in the matrix, resulting in lower hardness and elastic modulus [36]. Fig. 7 displays the microstructure of C/C and C/C-D composites before and after 1600 °C heat treatment. It is apparent that PyC in both C/C (Fig. 7b) and C/C-D (Fig. 7d) composites show better texture after heat treated at 1600 °C, which is consistent with the Raman results. The graphene layers transformed from diamond present high graphitization degree and there is a more compact interface bonding between PyC and
Fig. 6. Load-displacement curves of PyC in C/C and C/C-D composites before (a) and after (b) 1600 °C heat treatment. The inset tables in a and b are corresponding mean hardness (H) and elastic modulus (E). 104
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Fig. 7. HRTEM images of PyC in C/C composites (a, b) and diamond-PyC interface in C/C-D composites (c, d) before (a, c) and after (b, d) 1600 °C heat treatment. (e, f) are the corresponding TEM images of diamond particles in C/C-D composites before (e) and (f) after heat treatment.
for the lower flexural strength of C/C-D composites. After being heat treated at 1600 °C, the flexural strength of C/C composites shows a 53.2% decrease while that of C/C-D composites only decreases by 12.9%, as presented in Fig. 8. The damage caused by high temperature to carbon fibers is believed to be the main cause for the degradation of flexural strength of C/C and C/C-D composites. However, it is worth noting that the C/C-D composites possess a higher flexural strength than C/C composites after heat treatment, which is opposite to the case before heat treatment.
3.3. Mechanical and thermal properties of micro-diamond modified C/C composites The load-displacement curves of C/C and C/C-D composites are shown in Fig. 8. The flexural strength of as-prepared C/C and C/C-D composites before 1600 °C heat treatment is 144.1 MPa and 119.7 MPa respectively, as illustrated in Table 1. The high open porosity (13.4%) of C/C-D composites which is nearly two times that of C/C composites, resulting from the stacking of diamond particles, is proposed to account 105
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Fig. 8. Load-displacement curves of C/C (a) and C/C-D (b) composites before and after 1600 °C heat treatment.
required to transfer the heat and load from PyC to diamond effectively. During the further graphitization process at 1600 °C, thicker graphite layer formed on diamond surface and the stress graphitization caused by diamond volume expansion resulted in better texture of surrounding PyC. In the meantime, a compact interface bonding between graphite phase and PyC formed after heat treatment, as shown in Fig. 7d. Combining the fracture surface morphologies (Fig. 9), the fracture mechanism of C/C-D composites before and after heat treatment could be proposed as Fig. 10. In the as-prepared C/C-D composites, due to the high open porosity and relatively weak interface bonding between diamond and PyC, the cracks propagate along diamond edges and stacking pores easily, resulting in a lower flexural strength. After heat treated at 1600 °C, benefiting from the enhanced interface bonding, the propagation of cracks becomes difficult and more fracture energy
Table 1 Physical, mechanical and thermal properties of C/C and C/C-D composites. Temperature
Sample
Without heat treatment 1600 °C
C/C C/C-D C/C C/C-D
Density (g/cm3)
1.73 1.69 1.65 1.59
Open porosity (%) 7.7 13.4 9.9 15.1
Bending strength (MPa) 144.1 119.7 67.4 104.2
Thermal conductivity (W/(m ∗ K)) 4.296 5.692 7.108 7.871
In C/C-D composites, the interface bonding condition between diamond and PyC is an important factor that determines the final performance of the composites. A suitable interface bonding strength is
Fig. 9. Fracture surfaces of C/C-D composites before (a, b) and after (c, d) 1600 °C heat treatment. 106
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Fig. 10. Schematic diagram of crack propagation in C/C-D composites before (a) and after (b) 1600 °C heat treatment.
Acknowledgement
would be consumed, which can in part compensate the strength reduction caused by carbon fiber damage. And a distinct rugged fracture surface could be observed in Fig. 9d. Thus, a high strength retention rate was achieved and the flexural strength of C/C-D composites was higher than that of C/C composites after heat treatment. Owing to the complicated structure of C/C-D composites, the thermal conductivity can be affected by various factors, such as the property of diamond particles, the microstructure of PyC and the interface bonding feature in the composites. Han et al. [24] recently reported a 19.6% increase in thermal conductivity of C/C composites with an addition of 2 vol% diamond particles (particle size: 20–30 μm). Generally, with the diamond particle size increasing, the thermal conductivity of composites increases due to the reduced defects and interfaces [37,38]. Despite of the relatively small diamond particles size and high porosity of C/C-D composites in this study, the thermal conductivity gets a 32.5% increase, from 4.296 W·m−1·K−1 to 5.692 W·m−1·K−1, as illustrated in Table 1, which mainly benefits from the high content and high thermal conductivity of diamond particles. After heat treated at 1600 °C, the thermal conductivity of both C/C and C/C-D composites increase prominently, benefiting from the enhanced graphitization degree of PyC and carbon fibers. However, the thermal conductivity of C/C-D composites only shows a 10.7% increase compared with that of C/C composites after heat treatment, which is believed to result from the graphitization of diamond particles [39,40].
This work was supported by “National Natural Science Foundation of China (51304249)” and “Hunan Provincial Science Foundation of China (14JJ3023)”. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.diamond.2019.04.003. References [1] K.Z. Li, X.T. Shen, H.J. Li, S.Y. Zhang, T. Feng, L.L. Zhang, Ablation of the carbon/ carbon composite nozzle-throats in a small solid rocket motor, Carbon 49 (2011) 1208–1215. [2] T. Windhorst, G. Blount, Carbon-carbon composites: a summary of recent developments and applications, Mater. Des. 18 (1997) 11–15. [3] C. Li, A. Crosky, The effect of carbon fabric treatment on delamination of 2D-C/C composites, Compos. Sci. Technol. 66 (2006) 2633–2638. [4] D. Huang, M. Zhang, Q. Huang, L. Wang, K. Tong, Mechanical property, oxidation and ablation resistance of C/C-ZrB2-ZrC-SiC composite fabricated by polymer infiltration and pyrolysis with preform of Cf/ZrB2, J. Mater. Sci. Technol. 33 (2016) 481–486. [5] C. Jie, X. Xiang, X. Peng, The effect of carbon nanotube growing on carbon fibers on the microstructure of the pyrolytic carbon and the thermal conductivity of carbon/ carbon composites, Mater. Chem. Phys. 116 (2009) 57–61. [6] L. Feng, K.Z. Li, J.H. Lu, L.H. Qi, Effect of growth temperature on carbon nanotube grafting morphology and mechanical behavior of carbon fibers and carbon/carbon composites, J. Mater. Sci. Technol. 33 (2016) 65–70. [7] P. Xiao, X.F. Lu, Y. Liu, L. He, Effect of in situ grown carbon nanotubes on the structure and mechanical properties of unidirectional carbon/carbon composites, Mater. Sci. Eng. A 528 (2011) 3056–3061. [8] Song Qiang, Kezhi, Hejun, Qiangang, Increasing the tensile property of unidirectional carbon/carbon composites by grafting carbon nanotubes onto carbon fibers by electrophoretic deposition, J. Mater. Sci. Technol. 29 (2013) 711–714. [9] L. Feng, K. Li, Z. Zhao, H. Li, L. Zhang, J. Lu, Q. Song, Three-dimensional carbon/ carbon composites with vertically aligned carbon nanotubes: providing direct and indirect reinforcements to the pyrocarbon matrix, Mater. Des. 92 (2016) 120–128. [10] X. Li, K. Li, H. Li, J. Wei, C. Wang, Microstructures and mechanical properties of carbon/carbon composites reinforced with carbon nanofibers/nanotubes produced in situ, Carbon 45 (2007) 1662–1668. [11] Q.M. Gong, Z. Li, X.W. Zhou, J.J. Wu, Y. Wang, J. Liang, Synthesis and characterization of in situ grown carbon nanofiber/nanotube reinforced carbon/carbon composites, Carbon 43 (2005) 2426–2429. [12] G. Bo, R. Zhang, M. He, L. Sun, C. Wang, L. Lei, L. Zhao, H. Cui, A. Cao, Effect of a multiscale reinforcement by carbon fiber surface treatment with graphene oxide/ carbon nanotubes on the mechanical properties of reinforced carbon/carbon composites, Compos. A: Appl. Sci. Manuf. 90 (2016) 433–440. [13] Y.J. Kwon, Y. Kim, H. Jeon, S. Cho, W. Lee, J.U. Lee, Graphene/carbon nanotube hybrid as a multi-functional interfacial reinforcement for carbon fiber-reinforced composites, Compos. Part B 122 (2017) 23–30. [14] L.J. Lanticse-Diaz, Y. Tanabe, T. Enami, K. Nakamura, M. Endo, E. Yasuda, The effect of nanotube alignment on stress graphitization of carbon/carbon nanotube composites, Carbon 47 (2009) 974–980. [15] K. Matsui, L.J. Lanticse, Y. Tanabe, E. Yasuda, M. Endo, Stress graphitization of C/C composite reinforced by carbon nanofiber, Carbon 43 (2005) 1577–1579. [16] M. Sun, B. Dai, K. Liu, K. Yao, J. Zhao, Z. Lyu, P. Wang, Y. Ding, L. Yang, J. Han,
4. Conclusions Micro-diamond modified C/C composites were fabricated by pressureless infiltration and chemical vapor infiltration method. The microstructure of C/C-D composites experienced significant transformation after diamond graphitization, especially the diamond stacking area and graphite/PyC interface, which brought prominent influence to the mechanical and thermal properties of C/C-D composites. The main conclusions obtained are as follows: 1. Diamond graphitization proceeds via the in-situ transformation of diamond {111} planes to graphite (002) planes. And the obtained graphite particles possess high degree of graphitization. 2. During the further graphitization process of diamond particles, thicker graphite layer forms on diamond surface, accompanied by volume expansion which causes stress graphitization to surrounding PyC. And the graphite/PyC interface bonding strength gets obvious improvement at the same time. 3. C/C-D composites exhibit higher strength retention rate than C/C composites after 1600 °C heat treatment due to the enhanced graphite/PyC interface bonding strength. 4. The thermal conductivity of the as-prepared C/C-D composites increases by 32.5% compared with that of C/C composites. 107
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Influence of diamond graphitization on the microstructure and performance of micro-diamond modified C/C composites ⁎
T
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Lei Chena, Xin Yanga,b, , Qizhong Huanga, , Cunqian Fanga, Anhong Shia, Ruirui Liua a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China Science and Technology of Advanced Functional Composite Laboratory, Aerospace Research Institute of Materials & Processing Technology, Beijing 100076, China
A R T I C LE I N FO
A B S T R A C T
Keywords: C/C composites Micro-diamond particles Graphitization Microstructure Performance
Micro-diamond modified C/C composites (C/C-D composites) were fabricated using pressureless infiltration (PI) and chemical vapor infiltration (CVI) methods. And the influence of diamond graphitization on the microstructure and performance of C/C-D composites were investigated. During further graphitization process of diamond particles (1600 °C), thicker graphite layer formed on diamond surface accompanied by volume expansion, which induced stress graphitization to surrounding PyC. Meanwhile, the graphite/PyC interface bonding strength improved significantly after diamond graphitization. Compared with the low strength retention rate of C/C composites (46.8%) after heat treated at 1600 °C, a high strength retention rate was achieved for C/ C-D composites (87.1%) benefiting from the enhanced graphite/PyC interface bonding strength after diamond graphitization. And the thermal conductivity of the as-prepared C/C-D composites increased by 32.5% with the addition of diamond particles (9.97 wt%). It could be expected that C/C composites with optimal microstructure and excellent performances can be obtained by adjusting the content and graphitization degree of diamond particles in future work.
1. Introduction Carbon fiber reinforced carbon matrix (C/C) composites have aroused extensive attention due to their excellent properties, such as high specific strength, high specific modulus, low thermal expansion coefficient and outstanding thermal shock resistance, which make them the most promising candidate for thermal structural materials used in aerospace and aircraft fields [1–4]. Besides the commonly used PANbased carbon fibers, other kinds of carbon materials with outstanding physical and thermal properties, such as carbon nanotubes [5–9], carbon nanofibers [10,11] and graphene [12,13], have been also applied as reinforcement phase to improve the mechanical and thermal properties of C/C composites. Stress graphitization of matrix carbon has occurred at the reinforcement phase/matrix interfaces due to the mismatch of thermal expansion coefficient and has significant influence on the performance of C/C composites [14,15]. Therefore, investigation on the microstructure of reinforcement phase/matrix interface is vitally important for the development of advanced C/C composites with adequate performances. Diamond particles have been widely used as reinforcement phase in polymer and ceramic composites that require better mechanical and
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thermal properties [16–18], owing to their outstanding properties, including high strength, high thermal conductivity, superior wear and erosion resistance. SiC ceramics modified by diamond particles were reported to possess excellent fracture strength and enhanced thermal conductivity [19–21]. In recent years, diamond particles have also been introduced into C/SiC and C/C composites to improve the thermal conductivity and performance of the composites. The thermal conductivity of C/SiC-Diamond composites prepared by CVI and RMI methods in a recent study got a near twofold enhancement and an improvement in ablation resistance of micro-diamond modified C/SiC composites fabricated by PIP method was also realized [22,23]. In a recent study, micro-diamond particles were firstly introduced into C/C composites and the thermal conductivity of the composites was improved by 19.6% with a diamond particle content of 2 vol% [24]. However, additional properties as well as the relationship between microstructure and performance of C/C-diamond composites need to be further investigated. It is well known that diamond is the thermodynamically unstable form of carbon and the graphitization of diamond particles is an unavoidable process under the composite fabrication or application condition which involves high temperatures [25–27]. And it is worth noting that the graphitization process of diamond is
Correspondence to: X. Yang, State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China. Corresponding author. E-mail addresses: [email protected] (X. Yang), [email protected] (Q. Huang).
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https://doi.org/10.1016/j.diamond.2019.04.003 Received 21 January 2019; Received in revised form 19 March 2019; Accepted 2 April 2019 Available online 03 April 2019 0925-9635/ © 2019 Elsevier B.V. All rights reserved.
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C/C and C/C-D composites were analyzed by Raman spectra (LabRAM HR800) using an incident laser with a wavelength of 488 nm. The texture of PyC in both C/C and C/C-D composites was determined by polarized light microscopy (PLM, Leica DM4000M). The hardness and elastic modulus of PyC in C/C and C/C-D composites were characterized using an ultra nanoindentation tester (UNHT+MCT, CSM Instruments, Swiss) with a loading rate of 10 mN/min. The samples used in the PLM and nanoindentation study were firstly mounted in epoxy resin and allowed to harden, followed by subsequent grind and polish process. At least 5 positions were examined on each sample in nanoindentation test and the average hardness and elastic modulus were adopted. Scanning electron microscope (SEM, FEI Nova Nano230) was applied to characterize the morphology of the composites as well as the fracture surfaces. The microstructure of diamond particles under various graphitization stages and the diamond/PyC interface structure were characterized by TEM and HRTEM (FEI Titan G2 60-300). For TEM and HRTEM observation, the diamond particles or powders scraped from C/C and C/C-D composites were firstly dispersed in ethanol by ultra-sonication and then dropped on a copper grid. Mechanical properties of C/C and C/C-D composites were characterized via three-point bending test on a universal material tester (Instron3369) using samples with a dimension of 55 × 10 × 4 mm3. The span and loading rate were 40 mm and 1 mm/min respectively. And four samples for each group of composites were tested to obtain the average bending strength. Thermal conductivity of the composites was obtained using a laser thermal conducting instrument (LFA 457). The sample size and heating rate were about Φ 12.5 × 2.5 mm3 and 5 °C/min, respectively.
accompanied by volume expansion which can cause stress accumulation at the diamond/matrix interface and influence the microstructure of matrix carbon. Moreover, graphite particles with different microstructures, such as onion like carbon (OLC) [28–31] and closed curved graphite structures (CCGS) [32,33], could be obtained by annealing diamond particles with different sizes. Therefore, the microstructure of C/C-diamond composites, especially the matrix structure, will experience significant transformation after diamond graphitization which will influence the performance of C/C-diamond composites at a large extent. Nevertheless, to the best of our knowledge, the influence of diamond graphitization on the microstructure and performance of C/C composites have not been reported yet. In this study, micro-diamond modified C/C composites (C/C-D composites) were fabricated and a 1600 °C heat treatment process was also conducted to realize diamond particle graphitization. The graphitization process of micro-diamond particles was investigated systematically, based on which the influence of diamond graphitization on the diamond/pyrolytic carbon (PyC) interface structure was discussed. And the relationship between microstructure and mechanical, thermal properties of C/C-D composites was also analyzed. 2. Experimental 2.1. Materials fabrication Micro-diamond modified C/C composites (C/C-D composites) were fabricated by pressureless infiltration (PI) and chemical vapor infiltration (CVI) methods using 2.5D needle punched carbon fiber felt (with a density of 0.24 g/cm3) as preform. A pyrolytic carbon layer was firstly deposited on carbon fibers before the subsequent procedure. Micro-diamond particles (Changsha Naiqiang Superabrasives Co. Ltd) with an average particle size of 1.27 μm were introduced into the preform by PI method. Firstly, diamond particles were dispersed in ethyl alcohol (AR, Tianjin HengXing Chemical Reagent Co. Ltd) by ultrasonic agitation for 10 min with a power of 50 W and frequency of 40 KHz, forming a suspension liquid. Then, the preform was immerged into the suspension liquid and the apparatus was vacuumized. After being dried at 80 °C for 5 h, the ethyl alcohol was evaporated and diamond particles were introduced into the carbon fiber preform successfully. The above process was repeated until the content of diamond reached the calculated value (calculated with a final density of 1.7 g/ cm3 and a diamond content of 10 wt%). After being heat treated at 1400 °C for 1 h, the above diamond modified carbon fiber preform was densified by CVI process using propylene as carbon source and nitrogen as carrier gas. The final density of the C/C-D composites is about 1.7 g/ cm3 and the diamond content is calculated to be 9.97 wt%. C/C composites without diamond were also fabricated using the same procedure for comparison. To fully understand the graphitization and microstructure evolution of micro-diamond, the diamond particles were annealed in argon atmosphere from 1200 to 1600 °C with a heating rate of 10 °C/min. After 1 h holding at preset temperature, the samples were furnace cooled to room temperature. And finally, a group of C/C and C/C-D composites were heat treated at 1600 °C for 1 h (the exact condition for fully diamond graphitization) to explore the influence of diamond graphitization on the microstructure and mechanical performance of the composites.
3. Results and discussion 3.1. Microstructure transformation of diamond particles The graphitization process of diamond can be affected by many factors, including diamond particle size, crystallinity as well as pressure. Compared with the intensive research on the nano-diamond graphitization process, the investigation on the micro-diamond graphitization is still limited. In order to clarify the graphitization temperature of the diamond particles used in this study, the graphitization process was analyzed and distinctive microstructure was formed on the middle stage of graphitization, which gives a new sight into the graphitization mechanism of diamond particles. The original diamond particles used in this study present an irregular polygonal shape with a median size (Dv (50)) of 1.27 μm (Fig. S1). It can be seen in Fig. 1 that the graphitization of diamond particles begins at 1300 °C, indicated by the appearance of diffraction peak at 26.4° (Fig. 1a) and G bands at 1580 cm−1 (Fig. 1c) which is reported to originate from an ideal graphitic lattice vibration mode with E2g symmetry [34]. With the temperature increasing, the intensity of diamond peaks in both XRD and Raman spectra weaken while that of graphite peaks increase, which confirms the further graphitization of diamond. When the temperature reaches 1600 °C, as shown in Fig. 1b and d, the graphitization process accomplishes and the yielded graphite possesses high degree of crystallinity, manifested by the intensive and narrow diffraction peak of graphite (002) plane as well as the low ID1/IG value. Existence of edge carbon atoms (with imperfect hexagonal structure) and relatively small particle size (determined by original diamond particles) after graphitization are believed to account for the appearance of D bands in Fig. 1d [35]. The microstructures of micro-diamond particles heat treated at different temperatures are shown in Fig. 2. It can be concluded from the microstructure transformation process and the morphology of graphite phase (Fig. 2c and d) that the graphitization process begins from the diamond surface and proceeds inwards to the core gradually with the increase of temperature. The transformed region at different positions of diamond surface possesses nearly the same thickness, and the
2.2. Characterization The bulk density and open porosity of C/C and C/C-D composites were measured by Archimedes method. X-ray diffraction (XRD, Rigaku Dmax/2550VB + 18 kW) was used to characterize the phase constitution of diamond particles before and after heat treatment with CuKα radiation (0.154 nm) at 40 kV and 200 mA. The graphitization degree of diamond particles at different graphitization stages as well as the PyC in 100
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Fig. 1. XRD and Raman spectra of diamond particles annealed at different temperatures: (a) 1200–1500 °C, XRD; (b) 1600 °C, XRD; (c) 1200–1400 °C, Raman; (d) 1600 °C, Raman.
3.2. Influence of diamond graphitization on the microstructure of PyC
particles tend to maintain their original shape. Meanwhile, the direct growth of diamond {111} planes into graphite (002) planes could be observed in Fig. 2a and b, as marked by the yellow squares. And the corresponding fast Fourier transformation (FFT) patterns illustrate a parallel relationship between graphite (002) and diamond {111} planes. Moreover, it is worth noting that two sets of graphene layers with different crystal orientation, parallel to diamond (111) and (111) planes respectively, are presented in Fig. 2b, which demonstrates a uniform transformation tendency of {111} diamond planes. Different from previously reported CCGS [32], the diamond particles annealed at 1400 °C in this research present three distinctive regions (Fig. 2b), the outer graphite region with flat and ordered graphene layers, the transition region with newly formed graphene layers, as well as the inner untransformed diamond core, which gives a direct clue to the phase transformation process. Combining the graphitization mechanism proposed by many researchers earlier [26] with the above analysis, the graphitization process can be summarized as follows. In-situ transformation of diamond {111} planes to graphite (002) planes is the main process, which is accompanied by bond breaking and shrinkage of interatomic distance. The equal transformation possibility of all the {111} planes results in the formation of complicated but locally ordered transition region. Plenty of point defects and dislocations exist in this transition region. Decrease of surface and distortion energy is believed to be the driving force for the formation of ordered graphene layers. After the fulfillment of graphitization, graphite particles with high crystallinity are obtained (Figs. 1b and 2f).
The microstructure of C/C and C/C-D composites before and after 1600 °C heat treatment was characterized by SEM and PLM. In C/C composites, as shown in Fig. 3a and b, PyC deposits around the carbon fibers and forms lamellar structure. After heat treated at 1600 °C, the texture of PyC presents little change (Fig. 3a and b) due to the relatively low temperature compared with commonly used graphitization temperature (> 2000 °C) for C/C composites. In C/C-D composites, diamond particles distribute between carbon fibers and PyC, combining well with matrix carbon. Different from the morphology of C/C composites, in C/C-D composites, diamond particles act as nucleation sites for the deposition of PyC; and the preferential orientation of PyC parallel to carbon fibers is disturbed due to the irregular surface of diamond particles. Moreover, the influence of diamond addition on the structure of PyC is mainly reflected in the diamond stacking and adjacent area (Fig. 4c and d). After 1600 °C heat treatment, the texture of PyC adjacent to diamond particles presents better texture, and the diamond particles become smooth (Fig. 3d), resulting from the further graphitization. Raman spectrum was used to characterize the microstructure transformation of PyC after heat treatment and the results are shown in Fig. 5. All the spectra of C/C and C/C-D composites were fitted with four Lorentzian-shaped bands (G, D1, D2, D4) and one Gaussian-shaped band (D3), and the fitted peak parameters as well as the ID1/IG values are summarized in Table S1. It could be found that, before heat 101
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Fig. 2. TEM, HRTEM and corresponding FFT patterns of diamond particles heat treated at different temperatures: (a, c) 1300 °C; (b, d) 1400 °C; (e, f) 1600 °C.
ID1/IG value of C/C-D composites decreases obviously after heat treated at 1600 °C, especially for the diamond stacking area. Compared with the commonly used graphitization temperature (2000–2800 °C) for C/C composites, 1600 °C is a relatively low temperature at which the type of PyC would not experience significant change. Nevertheless, due to the stress accumulation caused by volume expansion of diamond during graphitization process, the texture of PyC around the diamond particles improves significantly. The obvious decrease of ID1/IG value in diamond stacking area mainly results from the highly ordered graphene layers formed by graphitization of diamond particles. Nanoindentation experiment was conducted to investigate the plasticity of PyC in both C/C and C/C-D composites. And the mean hardness as well as elastic modulus of PyC were presented in insert tables of corresponding load-displacement curves, as shown in Fig. 6. The highest hardness (8.07 GPa) and elastic modulus (39.13 GPa) were measured for PyC in as-prepared C/C-D composites, while the lowest
treatment, the ID1/IG values of C/C-D composites in both diamond stacking area and adjacent area are higher than those of C/C composites, indicating a lower graphitization degree. During the deposition process, due to the small particle size, irregular shape and stacking of diamond, PyC depositing around the stacking particles presents random orientation with diamond as nucleation core, thus breaking the longrange preferential orientation of PyC and resulting in more defects and lower graphitization degree. Therefore, the ID1/IG value of C/C-D composites is higher than that of C/C composites, especially in the diamond stacking area where the deposition space of PyC is further restricted to pores among diamond particles. After heat treated at 1600 °C, the FWHM of both D and G bands for C/C and C/C-D composites decrease obviously, indicating an increase in graphitization degree. For C/C composites, the ID1/IG value presents no obvious change after heat treatment, indicating little enhancement in PyC texture, in consistent with the SEM and PLM results. However, the 102
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Fig. 3. Microstructure of C/C (a, b) and C/C-D (c, d) composites before (a, c) and after (b, d) 1600 °C heat treatment. The interface between diamond stacking area and PyC are marked in red dash lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. PLM of C/C (a, b) and C/C-D (c, d) composites before (a, c) and after (b, d) 1600 °C heat treatment. 103
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Fig. 5. Raman spectra and curve fitting results of PyC in C/C and C/C-D composites before (a, b, c) and after (d, e, f) 1600 °C heat treatment: (a, d) C/C composite; (b, e) adjacent area in C/C-D composite; (c, f) diamond stacking area in C/C-D composites.
the transformed graphite layer on diamond particles after heat treatment (Fig. 7c and d). Limited space caused by the stacking of diamond particles together with the existence of surrounding PyC restricts the volume expansion during diamond graphitization, which causes stress accumulation in the material, resulting in stress graphitization of adjacent PyC. And the PyC around diamond particles in C/C-D composites presents better texture compared with that in C/C composites after heat treatment (Fig. 7b and d). Different from the condition when diamond particles are annealed alone, the restricted space in C/C-D composites hinders the graphitization of diamond and diamond particles at intermediate graphitization stages are factually observed in C/C-D composites (Fig. S2).
hardness (2.85 GPa) and elastic modulus (21.09 GPa) were measured for PyC in C/C-D composites after heat treatment. Compared with the slight decrease of hardness (5.14 GPa to 4.14 GPa) and elastic modulus (33.24 GPa to 27.02 GPa) for C/C composites after heat treatment, this significant reduce for C/C-D composites is proposed to be induced by diamond graphitization. In consistent with the Raman results, the stress graphitization changes the microstructure of PyC towards better ordered carbon in the matrix, resulting in lower hardness and elastic modulus [36]. Fig. 7 displays the microstructure of C/C and C/C-D composites before and after 1600 °C heat treatment. It is apparent that PyC in both C/C (Fig. 7b) and C/C-D (Fig. 7d) composites show better texture after heat treated at 1600 °C, which is consistent with the Raman results. The graphene layers transformed from diamond present high graphitization degree and there is a more compact interface bonding between PyC and
Fig. 6. Load-displacement curves of PyC in C/C and C/C-D composites before (a) and after (b) 1600 °C heat treatment. The inset tables in a and b are corresponding mean hardness (H) and elastic modulus (E). 104
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Fig. 7. HRTEM images of PyC in C/C composites (a, b) and diamond-PyC interface in C/C-D composites (c, d) before (a, c) and after (b, d) 1600 °C heat treatment. (e, f) are the corresponding TEM images of diamond particles in C/C-D composites before (e) and (f) after heat treatment.
for the lower flexural strength of C/C-D composites. After being heat treated at 1600 °C, the flexural strength of C/C composites shows a 53.2% decrease while that of C/C-D composites only decreases by 12.9%, as presented in Fig. 8. The damage caused by high temperature to carbon fibers is believed to be the main cause for the degradation of flexural strength of C/C and C/C-D composites. However, it is worth noting that the C/C-D composites possess a higher flexural strength than C/C composites after heat treatment, which is opposite to the case before heat treatment.
3.3. Mechanical and thermal properties of micro-diamond modified C/C composites The load-displacement curves of C/C and C/C-D composites are shown in Fig. 8. The flexural strength of as-prepared C/C and C/C-D composites before 1600 °C heat treatment is 144.1 MPa and 119.7 MPa respectively, as illustrated in Table 1. The high open porosity (13.4%) of C/C-D composites which is nearly two times that of C/C composites, resulting from the stacking of diamond particles, is proposed to account 105
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Fig. 8. Load-displacement curves of C/C (a) and C/C-D (b) composites before and after 1600 °C heat treatment.
required to transfer the heat and load from PyC to diamond effectively. During the further graphitization process at 1600 °C, thicker graphite layer formed on diamond surface and the stress graphitization caused by diamond volume expansion resulted in better texture of surrounding PyC. In the meantime, a compact interface bonding between graphite phase and PyC formed after heat treatment, as shown in Fig. 7d. Combining the fracture surface morphologies (Fig. 9), the fracture mechanism of C/C-D composites before and after heat treatment could be proposed as Fig. 10. In the as-prepared C/C-D composites, due to the high open porosity and relatively weak interface bonding between diamond and PyC, the cracks propagate along diamond edges and stacking pores easily, resulting in a lower flexural strength. After heat treated at 1600 °C, benefiting from the enhanced interface bonding, the propagation of cracks becomes difficult and more fracture energy
Table 1 Physical, mechanical and thermal properties of C/C and C/C-D composites. Temperature
Sample
Without heat treatment 1600 °C
C/C C/C-D C/C C/C-D
Density (g/cm3)
1.73 1.69 1.65 1.59
Open porosity (%) 7.7 13.4 9.9 15.1
Bending strength (MPa) 144.1 119.7 67.4 104.2
Thermal conductivity (W/(m ∗ K)) 4.296 5.692 7.108 7.871
In C/C-D composites, the interface bonding condition between diamond and PyC is an important factor that determines the final performance of the composites. A suitable interface bonding strength is
Fig. 9. Fracture surfaces of C/C-D composites before (a, b) and after (c, d) 1600 °C heat treatment. 106
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Fig. 10. Schematic diagram of crack propagation in C/C-D composites before (a) and after (b) 1600 °C heat treatment.
Acknowledgement
would be consumed, which can in part compensate the strength reduction caused by carbon fiber damage. And a distinct rugged fracture surface could be observed in Fig. 9d. Thus, a high strength retention rate was achieved and the flexural strength of C/C-D composites was higher than that of C/C composites after heat treatment. Owing to the complicated structure of C/C-D composites, the thermal conductivity can be affected by various factors, such as the property of diamond particles, the microstructure of PyC and the interface bonding feature in the composites. Han et al. [24] recently reported a 19.6% increase in thermal conductivity of C/C composites with an addition of 2 vol% diamond particles (particle size: 20–30 μm). Generally, with the diamond particle size increasing, the thermal conductivity of composites increases due to the reduced defects and interfaces [37,38]. Despite of the relatively small diamond particles size and high porosity of C/C-D composites in this study, the thermal conductivity gets a 32.5% increase, from 4.296 W·m−1·K−1 to 5.692 W·m−1·K−1, as illustrated in Table 1, which mainly benefits from the high content and high thermal conductivity of diamond particles. After heat treated at 1600 °C, the thermal conductivity of both C/C and C/C-D composites increase prominently, benefiting from the enhanced graphitization degree of PyC and carbon fibers. However, the thermal conductivity of C/C-D composites only shows a 10.7% increase compared with that of C/C composites after heat treatment, which is believed to result from the graphitization of diamond particles [39,40].
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4. Conclusions Micro-diamond modified C/C composites were fabricated by pressureless infiltration and chemical vapor infiltration method. The microstructure of C/C-D composites experienced significant transformation after diamond graphitization, especially the diamond stacking area and graphite/PyC interface, which brought prominent influence to the mechanical and thermal properties of C/C-D composites. The main conclusions obtained are as follows: 1. Diamond graphitization proceeds via the in-situ transformation of diamond {111} planes to graphite (002) planes. And the obtained graphite particles possess high degree of graphitization. 2. During the further graphitization process of diamond particles, thicker graphite layer forms on diamond surface, accompanied by volume expansion which causes stress graphitization to surrounding PyC. And the graphite/PyC interface bonding strength gets obvious improvement at the same time. 3. C/C-D composites exhibit higher strength retention rate than C/C composites after 1600 °C heat treatment due to the enhanced graphite/PyC interface bonding strength. 4. The thermal conductivity of the as-prepared C/C-D composites increases by 32.5% compared with that of C/C composites. 107
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