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Carbon Nanofiber Arrays Grown on Three-Dimensional Carbon Fiber Architecture Substrate and Enhanced Interface Performance of Carbon Fiber and Zirconium Carbide Coating Liwen Yan, Xinghong Zhang, Ping Hu, Guangdong Zhao, Shun Dong, Dazhao Liu, Boqian Sun, Dongyang Zhang, and Jiecai Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 12, 2017

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ACS Applied Materials & Interfaces

Carbon

Nanofiber

Arrays

Grown

on

Three-

Dimensional Carbon Fiber Architecture Substrate and Enhanced Interface Performance of Carbon Fiber and Zirconium Carbide Coating Liwen Yan*, Xinghong Zhang*, Ping Hu*, Guangdong Zhao, Shun Dong, Dazhao Liu, Boqian Sun, Dongyang Zhang, Jiecai Han National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin, 150080, P. R. China KEYWORDS: carbon nanofibers, interface performance, hierarchical architectures, thermal stability, zirconium carbide, carbon fiber

ABSTRACT: Carbon nanofibers (CNFs) were grown around the carbon fiber architecture through a plasma enhanced chemical vapor deposition method to enhance the interface performance between CF architecture substrate and ZrC preceramic matrix. The synthesized 3D CF hierarchical architectures (CNFs-CF) are coated with zirconium carbide (ZrC) ceramic to enhance their antioxidant property and high temperature resistance. The composition and the crystalline phase structure of the composite were detected with the X-ray photoelectron

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spectroscopy and X-ray diffraction. The results of scanning electron microscopy show that, the as-prepared CNFs and consistent ZrC ceramic coating are uniformly covered on the surface of carbon fiber architecture substrate. The ZrC ceramic products with excellent crystallinity were got from the pyrolysis of preceramic polymer at 1600 °C in inert atmosphere. Comparing with the untreated CF, the loading of ZrC ceramics around the CNFs-CF architecture surface are increased significantly. The thermal stability and mechanical property of CNFs-CF/ZrC nanocomposites have been promoted obviously compared with the CF/ZrC ceramic nanocomposite. The prepared CNFs-CF/ZrC ceramic nanocomposite is one of the potential candidate materials for the thermal protection application.

1. INTRODUCTION Composites based carbon fiber reinforcement have attracted considerable research over the past half century because of their predominant properties such as excellent specific modulus, superior strength, good rupture toughness, high thermal shock resistance, and good corrosion resistance1-3. The carbon fiber reinforced polymer or ceramic composite exhibit a wide range of applications in the areas of auto industry, aerospace, manufacturing sport equipment, construction, and other aspects4-6. However, the excellent properties of polymer or ceramic based composites reinforced with CF not only due to the performance of carbon fiber and matrix but also have a great relationship with the interface structure between the fibers and the surrounding matrix7-10. The relative smooth surface and inert property of carbon fiber lead to the weak interaction and low contact area of the interface with the matrix, resulting in the poor interfacial strength11. In summary, the research of interface performance between carbon fiber and polymer


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or ceramic matrix remains much room to be explored because it has a significant impact on determining overall properties of the composites. Different nanomaterials such as graphene oxide, inorganic nanoparticles, and carbon nanotubes were grown on the CF surface to promote the interfacial performance of CF-based composites12-14. The commonly used modification techniques could be summarized as two classes, which are physical and chemical routes respectively. The representative method of the chemical modification is the chemical oxidation and chemical grafting of CNTs, CNFs, GO etc. to the CF surface. However, the method has a fatal limitation that the acid corrosion or other chemical treatment of the CF resulting in the decrease of mechanical strength of CF. Furthermore, the loading of CNTs, CNFs, GO etc. on the surface of CF is limited and uncontrollable resulted in the restriction of improving the interfacial strength15. Alternatively, the physical method generally refers to the chemical vapor infiltration or chemical vapor deposition route, which always directly grow the carbon nanotubes, carbon nanofibers, or graphene onto the surface of CF. It is noteworthy that the CVD technology provide the direct command of landing, microstructure, and distribution density of CNFs or CNTs, as well as supplying enhanced connection with the matrix. Large amount of homogeneous and dense CNTs, CNFs or graphene could be planted around the CF architecture through CVD technology. The CVD growth of CNTs or CNFs causes the significant promotion of interface strength of CF, especially the surface area and wettability. The growth of CNFs by CVD process uncommonly increases the roughness of CF surface, which resulting in the enhanced mechanical connection of CF and polymer matrix16, 17. Recently, carbon nanofibers (CNFs) as reinforcement have been widely used in composite because of its unique characteristics, such as excellent stiffness, relatively low density, superior


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strength, high electrical capacity, and high thermal stability18-20. However, even though the excellent properties of the CNFs, the incorporation in polymer or ceramic matrix still unable to produce the significantly promoted composite21. The carbon nanofibers were proved to have great potential in increasing the interface area of the composite as revealed with the recent study22. The unavoidable trend of agglomeration of carbon nanofibers in the nanocomposites always decreases the area of interface and generates the stress concentrations which can further lead to new defects of the composites performance22. Clearly, a proper loading and uniform distribution of CNFs in polymer or ceramic are crucial parameters to enhance the comprehensive performance. To avoid agglomeration, CNFs covered continuous carbon fiber hierarchical structures have caused lots of research in improving the interface bonding strength23-25. The polymer based composites, reinforced with the multiscale CNFs-CF structures, exhibit the outstanding

improvements

in

the

mechanical

properties

and

additional

thermal

multifunctionality. Many efforts have been tried to study the effect of different reaction conditions on the performance of CNFs covered CF hierarchical architectures in the previous liturature26-28. Furthermore, adjustment of CVD reaction time is considered as an effective and convenient way to optimize the capability of hierarchical architectures. In this work, plasma enhanced chemical vapor deposition (PECVD) technique was applied to plant CNFs on the surface of CF, in which radio frequency assisted to stimulate and produce plasma to ease the CNFs growth and reduce the reaction temperature. Run time of the radio frequency was adjusted to achieve the optimal length and density of the CNFs. The introduction of ultra-high-temperature ceramics (UHTCs) coating onto the CNFs-CF hierarchical substrates to improve their thermal stability has been shown to be a good method for combining the excellent mechanical property of CF and good high-temperature performance of


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ceramics29,

30

. Zirconium carbide (ZrC) is considered as the promising ceramic material to

prevent CF from oxidation and ablation in the application of ultra-high temperature area due to the excellent performance such as high hardness, outstanding chemical stability, and superior melting point etc.31, 32. In recent years, several methods have been used to coat ZrC or other ceramic layer onto the carbon fiber/fabrics, such as polymer infiltration and pyrolysis method, alloyed reactive melt infiltration, CVI combined with RMI, hot-press and sinter processing, and in situ reaction method33-36. Every processing route has its disadvantages, such as high cost, inconvenience in adjusting reactions, poor distribution of the ceramics phase, and bad performance of the composite. Although scientists have acquired some considerable achievements in this field, the preparation and performance of CNFs-CF coated with ZrC ceramic coating composite fabricated by the CVD combined PIP process have not been reported. In this study, large scale of carbon nanofiber arrays were coated around the 3D-CF architecture substrate with radially aligned morphology through the use of plasma enhanced chemical vapor deposition. CNFs-CF/ZrC preceramic nanocomposite were prepared by impregnating the CNFs-CF hierarchical structure into the ZrC precursor polymer solution. The CNFs-CF/ZrC ceramic nanocomposites were got from the pyrolysis of CNFs-CF/ZrC preceramic polymer composites at 1600 °C. The coating of the CNFs on CF surface promoted the uniform and dense distribution of the ZrC ceramic layer effectively, as well as enhancing the interface performance of carbon fiber and ZrC ceramic powerfully. The microstructure and interfacial properties of the composite are discussed in detail. The CNFs-CF/ZrC ceramic nanocomposite exhibited good thermal stability and mechanical property. 2. EXPERIMENTAL SECTION The 3D carbon fiber architecture (woven with T300 CFs) with a bulk density of 0.15 g/cm3


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supplied by Jiangsu Tianniao High Technology Co. Ltd., China was used as substrates to grow CNFs. HNO3, Ni(NO3)2·6H2O, acetone, xylene and other solvents or chemicals were purchased from Sigma-Aldrich Co. LLC. Epoxy (E-44) and hardener (Triethylene tetramine, C6H18N4) used in IFSS test specimens was purchased from Sinopec Group, China. ZrC preceramic polymer (molecular formula, [Zr(CH3COCHCOCH3)2O2C4H8]n) synthesized according to literature37 was supplied by Institute of Process Engineering, Chinese Academy of Sciences, China. The chemicals and solvents were of analytical grade and used without further purification. The carbon fiber architectures (Weaved from T300 raw fibers) were thoroughly washed with acetone at 70 °C for 48 h to remove the effect of any polymer sizing, and dried under vacuum. The dried carbon fiber architectures were followed by initial slight oxidation with an acid treatment (65% HNO3) for 3 h at room temperature (RT). Ni catalyst was coated around the carbon fibers through dipping of fiber architectures into a 0.05 M acetone solution of Ni(NO3)2·6H2O at RT for 10 h. The Ni(NO3)2 coated carbon fiber architectures were loaded into a quartz boat and moved into the PECVD reaction chamber. Methane and hydrogen were used as reaction sources. The Ni(NO3)2 coated on the surface of CF architectures was firstly treated at 600 °C in the hydrogen gas with a flow rate of 20 sccm for 30 min to transfer into the metallic Ni particles. The reaction chamber was heated up to 700 °C with a heating rate of 5 °C/min, CNFs were grown with the assistance of radio frequency (plasma stimulation) by introducing a mixture of methane/hydrogen (CH4/H2) into the reaction chamber with a flow rate of 40/10 sccm. To get proper CNFs loadings on the carbon fiber architectures, the run time of the radio frequency was maintained from 5 to 30 min. The PECVD chamber was cooled down to RT in H2 atmosphere. The CNFs-CF architectures were taken out from the furnace for later use after the fabrication of carbon nanofibers.


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The orange ZrC preceramic polymer powders were dissolved in the xylene solvent in a rotary evaporator at 80 °C for 5 h to get a homogeneous solution with mass fraction of 65%. The as-prepared CNFs-CF architectures and ZrC preceramic polymer solution were successively added to a beaker, which was then placed into a vacuum kettle. In order to get enough ZrC preceramic polymer penetrating into the holes and gaps of the CNFs-CF architectures and guarantee every single CNFs-CF coated with ZrC preceramic polymer, the kettle was allowed to stand for 3 h under vacuum. The solid samples were collected and curing treated at a temperature of 120 °C in vacuum until the weight was not changed. Then, the CNFs-CF reinforced ZrC preceramic polymer products were prepared. CNFs-CF/ZrC ceramic nanocomposites were got from the pyrolysis of CNFs-CF/ZrC preceramic polymer. CNFs-CF/ZrC preceramic polymer composites in the tube furnace was heated to 1000 °C with a heating rate of 10 °C/min in highpurity argon atmosphere. The tube furnace was maintained at 1000 °C for 1h, and then increased to 1600 °C with a rate of 5 °C/min. The ZrC ceramics was completely converted from the ZrC preceramic polymer at 1600 °C for 1h. The CNFs-CF/ZrC ceramic nanocomposites were formed after the tube furnace cooling down to RT. The morphology of the fabricated materials were investigated using a FEI Helios Nanolab 600i scanning electron microscope with the voltage of 20 kV, and the microstructure of the materials were observed using a FEI Tecnai F30 transmission electron microscope with the working voltage of 300 kV. X-ray photoelectron spectroscopy (Model: Escalab 250) were used to detect the chemical bonding and material component. Nitrogen sorption measurements were carried out using a Micromeritics 2020 ASAP instrument at 77 K. The BET method was used to calculate the surface area of CF and CNFs-CF architectures. An interfacial strength examination system were used to conduct the single fiber pull out tests with a displacement rate of 0.05 µm/s.


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The test diagrammatic sketch was displayed in Figure S1. The values of interfacial shear strength between CNFs-CF and the polymer were concluded from the formula below.

IFSS =

Fmax

π dfL e

(1)

Here, df represents diameter of the single CNFs-CF, Fmax represents maximum load in the test, and Le represents length of the fiber part embedded in matrix. A CNFs-CF single fiber was fixed onto a holder with adhesive plaster. E-44 epoxy resin mixture was painted carefully onto the CNFs-CF monofilament with the merger length of 40-60 mm using a sharp pin. The samples were heat treated at 80 °C for 2 h and then 150 °C for 4 h. The thermogravimetric synchronous differential scanning calorimetry analyzer (Netzsch STA 449 F5, Germany) were used to study the thermal decomposition behavior and quantitative calculation of yield of ZrC preceramic polymers with the heating rate of 10 °C/min under argon atmosphere at temperature of RT-1600 °C. The phase compositions of the materials were detected with X-ray diffraction (X’Pert PRO, PANalytical, Holland) in the 2θ range from 10° to 80° with Cu Kα radiation. The Electronic Universal Testing Machine (Instron 5569, USA) was applied to evaluate the compressive strength of the nanocomposites with the displacement rate of 0.2 mm/min at RT. The specimens were polished and the size specification was 10 mm×10 mm×12 mm. Every compressive strength value was calculated from the average of five test results. Thermal stability of the materials was characterized by thermogravimetric analyzer (Netzsch STA 2500, Germany) with a heating rate of 10 °C/min. 3. RESULTS AND DISCUSSION A new method was put forward to fabricate the ZrC ceramic coating with high temperature resistance on the CF surface by precursor infiltration pyrolysis technology, as shown in Figure 1.


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To enhance the interface performance between CF architecture substrate and preceramic polymer, CNFs were grown on the CF architecture surface with a plasma enhanced CVD method. Immersion of the CF architecture in the 0.05 M acetone solution of Ni(NO3)2·6H2O with stirring guarantee the uniform coverage of catalyst on the CF surface. Better growth effect of CNFs were found when the solvent is acetone rather than aqueous or ethanol in preparation of catalyst solution. Tzeng et al. have reported the similar results in their literature38. The surface of carbon fiber is hydrophobic. Therefore, the acetone solution promised the better wetting and dispersion of nickel nitrate on the CF surface. The concentration of catalytic solution is important in determining the effective dispersion of Ni(NO3)2 on the CF surface. To a certain extent, the catalyst size decided the growth effects and diameters of CNFs directly. The higher the concentration of catalyst solution, the more agglomeration of the catalyst particles, resulting in the larger diameters of the CNFs. Meanwhile, the diffusion path is enlarged with the increase of catalyst particles diameters and the results facilitated the formation of amorphous carbon rather than carbon nanofibers. From our previous exploration and research, the proper concentration of catalyst solution is 0.05 M in loading of catalyst on CF surface. The same result was got in J.G. Zhao’s work39.


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Figure 1. Diagrammatic sketch of the fabrication process of CNFs-CF architecture with ZrC ceramic coating. Scanning electron micrographs of raw CF and CNFs-coated carbon fiber (CNFs-CF) with reaction time from 5 to 30 min have been presented in Figure 2. Untreated carbon fiber has a comparatively smooth surface and no grooves and protrusions could be observed, as shown in Figure 2a and e. After dipping into the acetone solution of Ni(NO3)2·6H2O, it is found that the Ni(NO3)2 particles with a narrow diameter range of 15~30 nm, were uniformly deposited on the CF surface (Figure S2, SI). The smallish and little CNFs were prepared on the CF surface when the radiofrequency time is 5 min (Figure 2b and f). When the growth time is reached to 15 min, neatly arrangement of carbon nanofiber arrays with a narrow range of diameter are appeared. The average diameter of the uniform and dense carbon nanofiber arrays is about 80 nm, and the length of CNF arrays is about 1 µm, as is shown in Figure 2c and g. When the reaction time reach to 30 min, excellent and more uniform coating of carbon nanofibers on the CF surface can be observed as shown in Figure 2d and h. In this case, the diameter of the CNFs is from 80 to 120 nm, and the length of them is between 3 and 4 µm. Because of their increased lengths, the CNFs cannot maintain divergent along the fiber radial and many of them are lying on the CF surface. The CNFs prepared in this condition are not straight but with a worm-like structure. The CNFs are entangled with each other and formed a 3-D network skeleton.


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Figure 2. SEM images of CNFs-CF with the CNFs growth time varied from 0 to 30 min, (e)-(h) are the high-magnification images corresponding to (a)-(d) respectively. Nowadays, the ‘adsorption-diffusion-deposition’ mechanism based on the tip growth model is widely accepted by most researchers in explaining the growth of catalytic CNFs, especially the bamboo-like structure CNFs. As the mechanism, the adsorption and decomposition of metal carbide on the catalyst particles surface is the key process in the growth of CNFs. Liquid metal carbide were generated when the carbon atoms being absorbed and penetrated into the catalyst particles. Continuous feed of carbon reactant results in the excess of carbon atoms, and the excessive atoms spreading through the catalyst particles to deposit and produce CNFs39. In order to characterize the structures in detail, TEM and SAED analysis of CNFs synthesized with the radio frequency time of 30 min were also conducted. Figure 3 shows the TEM micrographs and SAED pattern of CNFs grown on the CF. The intertwined CNFs with diameters of about 100 nm were prepared in Figure 3a. The existence of catalyst particles on the top of CNFs shows a tip-growth model in the preparation of CNFs because of weak catalystsupport interactions40. Figure 3b shows the TEM image of single CNF, which exhibit the herringbone bamboo-like structure of the fabricated CNFs. The herringbone bamboo-like


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structure is a representative type of CNFs in CVD fabrication process with methane catalyzed by metal nickel41. High-resolution TEM image of part of single CNF was shown in Figure 3c. It’s clearly shown that graphite layers of carbon nanofibers are arranged irregularly and irrelevantly. In addition, the SAED pattern (Figure 3d) of the CNFs reveals the crystalline feature and shows clearly its (002), (100) and (110) diffractions, which correspond to d-spacings of 0.34 nm, 0.21 nm and 0.12 nm, respectively.

Figure 3. (a) TEM image of as-prepared carbon nanofibers, (b) TEM image of single carbon nanofiber, (c) HRTEM image of part of the single CNF and (d) SAED pattern of the CNFs. XPS was used to detect the materials compositions of carbon fiber at different processing steps and the spectrum was shown in Figure 4. The elements on the surface of untreated CF are C and O, as indicated in the XPS spectrum. However, after treated by HNO3, the surface of acidified CF mainly consists of O, C and N element. The content of O atom is obviously higher than that of untreated CF. Visibly increased O1s peak and the appearance of the N1s peak are attributed to the oxidation process by concentrated nitric acid. Ni particles were attached on the


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carbon fibers after dipping CF into the acetone solution of Ni(NO3)2·6H2O, and the Ni2p peak was observed in the XPS curve in Figure 4c. At last, after the deposition of the CNFs, the surface of the CNFs-CF sample is mainly composed of only C element (Figure 4d). The peak location and areas in the XPS spectrum of every sample were studied to identify the special binding energy which quite consistent with the characterization results. The C1 peak enlarged from peak deconvolution was presented in Figure S3. With regard to the untreated CF, C-C, C-OH and OC=O peaks were found corresponding to the binding energy of 284.8 eV, 285.6 eV and 288.5 eV. The presence of C-OH and O-C=O peaks are ascribed to the purification treatment and atmospheric oxidation process of CF. However, after the CNFs growth, the C-OH peak at 285.6 eV and O-C=O peak at 288.5 eV are disappeared. In contrast, the main peak of C-C at 284.8 eV is sharpened and strengthened. So the above results of XPS indicated that the CNFs were grown onto the surface of the CF successfully. The characterization results of XPS are adapted with the previous report24.

Figure 4. XPS spectrum of untreated CF, CF treated by HNO3, CF attached with Ni and CNFsCF prepared with the deposition time of 30 min.


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The interface performance between CF and the matrix are usually studied using a single fiber pull out experiment. The experiments were carried on the untreated CF and different CNTsCF samples to understand the influence of reaction time on the interface performance. Figure 5 shows the interfacial shear strength results of the untreated carbon fiber and different CNFs-CF samples. It is clear that the coating of the CNFs on the CF architecture enhance the interfacial strength of the CF effectively. The IFSS of the untreated CF is 39.81 MPa, and increased to 61.66 MPa for the 5 min growth CNFs-CF sample. As for the CNFs-CF sample prepared with the reaction time of 30 min, the IFSS is further increased to 70.51 MPa. Comparing to the untreated CF, the CNFs-CF could get a 77% promotion in IFSS values, which attributed to the hierarchical structure of CNFs-CF. In the research of CF reinforced polymer composite, a weak interface structure between CF and polymer matrix may preserve the integrity of composite and lead to the fibers pulling out from matrix, whereas a strong bond combination may induce brittle fracture behavior. Therefore, the appropriate interface strength is beneficial to the performance of both CF and matrix. The specific surface area of the untreated CF and CNFs-CF hierarchical architectures were characterized by the BET method. The results of BET surface areas of untreated CF and CNFsCF hierarchical architectures are also shown in Figure 5. The BET surface area shows an obvious increase from 0.25 m2/g for untreated CF to 5.18 m2/g for CNFs-CF prepared with the reaction time of 5 min, and further increase to 12.59 m2/g after the deposition of CNFs onto CF with the reaction time of 30 min, as shown in Figure 5. Back to the previous Figure 2, it could be found that the untreated CF contained a relatively smooth surface without any protuberances and depressions (Figure 2e). After the CNFs deposited onto the surface of the CF, the surface roughness is increased noticeably (Figure 2f to 2h), which lead to the typically increase of the


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BET surface areas of CNFs-CF hierarchical architectures. Thus, the coating of CNFs around the CF architecture is beneficial to promote the IFSS and BET surface areas of CNFs-CF hierarchical architectures, and provide more interface contact points. In addition, the appropriate distribution density and proper length of the CNFs on the CF surface can enhance the mechanical interaction and physical adsorption between the carbon fibers and matrix.

Figure 5. Interfacial shear strength and BET surface area of untreated CF and CNFs-CF with CNFs growth time from 5 to 30 min. ZrC preceramic polymers were synthesized on the surface of the three-dimensional CNFsCF architecture substrate, for which zircomiun tetrachloride (ZrCl4), acetylacetone (Hacac, CH3COCH2COCH3), methanol (CH3OH) and 1,4-butanediol (C4H8(OH)2) were used as monomers. Chemical reaction equations shown in Figure 6 present the synthetic route for the precursor used in this study. The CNFs-CF/ZrC ceramic nanocomposites were synthesized from the pyrolysis of CNFs-CF/ZrC polymer composites at 1600 °C for 1 h. As an effective organic ligand, acetylacetone could linked with Zr ion to form a metal complex, which possessed a long shelf life of over 3 months. The oxygen atom in the acetylacetone and butanediol complexed with Zr ion and eventually built up Zr-O-Zr linear macromolecules (Figure 6).


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Figure 6. Synthesis reaction equations of ZrC preceramic polymer. There are two stages in the polymer-to-ceramic transformation process of ZrC preceramic polymer. The first stage is the crosslinking of the monomer molecules at a relatively low temperature resulting in the formation of curing organic-inorganic skeleton. The second stage is the ceramicization process through the pyrolysis with temperature up to 1600 °C resulting in the fabrication of ZrC ceramic. It is noteworthy that, the ZrC ceramic was synthesized from the carbothermal reduction of ZrO2, and oxygen should be segregated strictly throughout the pyrolysis process. Reduction of the weight loss of monomeric molecules and weight loss of pyrolysis in the ceramicization process contributed to the promotion of the ZrC output. The coverage of the ZrC ceramic on the CNFs-CF multiscale structures improved the high temperature stability of CF effectively. The preparation process of ZrC precursor polymer and CNFs-CF/ZrC ceramic nanocomposite was illustrated in Figure 7.


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Figure 7. Schematic illustration of synthesis process for the ZrC precursor polymer and CNFsCF/ZrC ceramic nanocomposite. In order to detailed study the pyrolysis behavior and production calculation of ZrC preceramic polymer, and promotion the polymer transformation, thermal gravimetric (TG) and synchronous differential scanning calorimetry (DSC) analysis within a range of RT-1600 °C was applied to trace the mass change behaviors. The TG, DTG and DSC curves of ZrC preceramic polymer were shown in Figure 8. The ZrC preceramic polymer displays four obvious weight-loss steps with corresponding endothermic or exothermic effect. The weight loss of the first step before 200 °C is 12.69%, corresponding to the endothermic peak on the DSC curve at about 123.1 °C, resulting from the vaporization of residual solvent and the desorption of small molecule material. The second and the third step weight loss range from ~200 °C to 1100 °C in the TG curves are 11.32% and 26.48% respectively, and have no a clear boundary. This corresponds to the exothermic peaks at 312.8 °C and 521.5 °C on the DSC curve, which can be attributed to the dissociation of organic groups, the rupture of macromolecule chain and the organic-to-inorganic conversion. The fourth step weight loss from ~1200 °C to 1600 °C is about


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10.79 %, corresponding to the endothermic peak on the DSC curve at about 1385.1 °C, assigning to the carbothermal reduction. The above results revealed that the ceramic yield after pyrolysis at 1600 °C is about 38.72%.

Figure 8. TG, DTG and DSC curves of ZrC preceramic polymer. XRD were employed to understand crystallization behavior during pyrolytic conversion of ZrC preceramic polymer and the patterns were shown in Figure 9. The diffraction pattern shows that the pyrolysis sample calcined at 1300 °C mainly consists of large amount of t-ZrO2 and small amount of m-ZrO2. When the pyrolysis temperature reach to 1400 °C, the diffraction peaks of t-ZrO2 become sharper and stronger, and the peak of m-ZrO2 is disappeared. The ZrC becomes the predominant phase and firstly detected when the pyrolysis temperature being elevated to 1500 °C, indicating that the majority of ZrO2 have been transformed into corresponding ZrC via carbothermal reduction. But small amount of t-ZrO2 is still existed in this case. The final highly crystal and pure ZrC ceramic powders were obtained at the pyrolysis temperature of 1600 °C, which indicates a complete transformation of ZrO2 to ZrC. Based on this XRD phase identification, all the CNFs-CF/ZrC ceramic nanocomposite were prepared with the heat treating temperature at 1600 °C.


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Figure 9. XRD patterns of reaction products of ZrC preceramic material with the heat treating temperature at 1300 °C, 1400 °C, 1500 °C and 1600 °C. To investigate the effect of CNFs on CF surface to CF enhanced ZrC ceramic nanocomposite, the untreated CF and as-prepared CNFs-CF were infiltrated in the ZrC precursor polymer solution, and then pyrolyzed at 1600 °C to produce CF/ZrC and CNFs-CF/ZrC ceramic nanocomposite respectively. SEM was conducted to analyze the morphology and structure of the CF/ZrC and CNFs-CF/ZrC ceramic nanocomposites. Figure 10 presents the SEM images of untreated CF, CF/ZrC composite, single fiber from CF/ZrC composite, CNFs-CF, CNFs-CF/ZrC nanocomposite, and single fiber from CNFs-CF/ZrC nanocomposite. The surface topography of the untreated CF was almost smooth (Figure 10a), and all the fibers were covered with a layer of uniform and dense CNFs (Figure 10d) after the deposition of CNFs in the PECVD equipment. Figure 10b and e show the morphology of the CF/ZrC and CNFs-CF/ZrC ceramic nanocomposite respectively which were performed six cycles of PIP process of ZrC polymer precursor. The ZrC ceramic is produced in the pores or gaps between the fibers and on the surface of fibers. For the untreated CF/ZrC composite, great amount of CF were still smooth and exposed without ZrC ceramic coating (Figure 10b). However, for the CNFs-CF/ZrC


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nanocomposite, all the fibers were coated with a homogeneous layer of ZrC ceramic coating (Figure 10e). The surface of the fiber from CNFs-CF/ZrC nanocomposite shows the presence of CNFs embedded in the ZrC ceramic matrix (Figure 10f). The ceramic layer on the CNFs-CF surface is uniform and with a thickness of ~8 µm. Obviously, the CF and ZrC ceramic coating are bridged with the CNFs, which is conducive to the enhancement of mechanical connection between CF and the ZrC ceramic coating. Due to mechanical connection between CNFs-CF and ZrC ceramic coating, the interface combination and slip resistance are effectively promoted, which is agree with the previous IFSS analysis.

Figure 10. SEM images of a) untreated carbon fiber, b) carbon fiber reinforced ZrC composites, c) single fiber from carbon fiber reinforced ZrC composites, d) CNFs-CF, e) CNFs-CF reinforced ZrC nanocomposites, and f) single fiber from CNFs-CF reinforced ZrC nanocomposites. Both the CF/ZrC and CNFs-CF/ZrC ceramic nanocomposites are porous and the density of the nanocomposites are investigated and the results are shown in Table 1. The ρ(CF/ZrC) and


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ρ(CNFs-CF/ZrC) when the PIP cycle times is 0 represent the bulk density of pure 3-D CF and CNFs-CF hierarchical structures respectively. The examined results of CF and CNFs-CF structures demonstrated that the growth of CNFs around the CF surface increase the bulk density of 3-D CF architecture from 0.15 g/cm3 to 0.22 g/cm3. The density of both CF/ZrC and CNFsCF/ZrC nanocomposites are increasing with the increase of precursor infiltration pyrolysis cycles. The density of CNFs-CF/ZrC nanocomposites is slightly larger than that of carbon fiber reinforced ZrC composites with the same precursor infiltration pyrolysis cycle due to the growth of CNFs. When the precursor infiltration pyrolysis cycle times is 6, the density of CF/ZrC and CNFs-CF/ZrC nanocomposites are 1.75 g/cm3 and 1.82 g/cm3 correspondingly. Overall, the CF/ZrC and CNFs-CF/ZrC nanocomposites in our work are lightweight because of their porous structure and relatively low density of ZrC ceramic comparing with the other ultra-high temperature ceramics. Table 1. The density of the CF/ZrC and CNFs-CF/ZrC nanocomposites changed with the precursor infiltration pyrolysis cycles. PIP cycle times

0

1

2

3

4

5

6

ρ(CF/ZrC)/ g/cm3

0.15

0.61

0.82

1.12

1.29

1.57

1.75

ρ(CNFs-CF/ZrC)/ g/cm3

0.22

0.74

0.86

1.14

1.39

1.67

1.82

Due to the lightweight and porous features of the CNFs-CF/ZrC ceramic nanocomposite, compressive properties test were studied to represent their mechanical properties. Figure 11 shows the results of compressive strength test of CF/ZrC and CNFs-CF/ZrC ceramic nanocomposite. It is observed that the compressive strength obviously enhanced on account of the coverage of carbon nanofibers. The similar results, in which the growth of carbon nanofibers


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or nanotubes around the CF surface result in the improvements of the IFSS and compressive strength have been reported in previous literature42,

43

. With the increase of the precursor

infiltration pyrolysis cycles, the compressive strength of both CF/ZrC and CNFs-CF/ZrC are increasing gradually. The compressive strength of the CNFs-CF/ZrC is at least twice higher than that of CF/ZrC with the same precursor infiltration pyrolysis cycle. When the precursor infiltration pyrolysis cycles increased to 6, the coating of the CNFs increased the compressive strength from 9.4 MPa (CF/ZrC) to 26 MPa (CNFs-CF/ZrC). The improvement of the compressive strength might be ascribed to the enhanced contact area between CF and ZrC ceramic, and the interlocking geometry of the CNFs and ZrC ceramic matrix. The uniform and dense coating of ZrC on the CNFs-CF architecture also beneficial to the compressive strength improvements for CNFs-CF/ZrC compared with the smooth CF without ZrC coating for CF/ZrC composite. Figure S4 presents the representative compressive stress-strain curves of CF/ZrC and CNFs-CF/ZrC respectively. It is shown that the coating of CNFs around the CF architecture can also improve the deformation ability of CNFs-CF/ZrC, and lead to a significant increase of the failure stress.

Figure 11. Compressive properties of CF/ZrC and CNFs-CF/ZrC ceramic composite.


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The TG was used to study the thermal stability of CNFs-CF, CNFs-CF/ZrC precursor, and CNFs-CF/ZrC ceramic nanocomposite, all the samples were under air atmosphere. Simultaneously, the CF precursor architecture substrate was used as contrast sample. TG results of CF precursor, CNFs-CF, CNFs-CF/ZrC preceramic and CNFs-CF/ZrC ceramic can be seen in Figure 12. CF precursor has a slight weight loss during the initial heating up to 400 °C, which the residual mass is ca. 99%. Then, a significant weight loss appeared between 600 °C to 800 °C, and CF precursor is oxidized completely at about 850 °C. As the temperature increases, the CF oxidized to produce CO and CO2 is the main reason for the remarkable weight loss. The CNFsCF has a slowly decrease of weight from 28 °C to 600 °C, which attributed to the oxidation of the CNFs around the CF surface. Further, the sample also shows a dramatic drop from 600 °C to 800 °C and the residual weight reach to 0% resulted from the complete oxidation of CNFs-CF. As is shown in Figure 12, the residual weight of the CNFs-CF/ZrC preceramic and CNFsCF/ZrC ceramic is 38% and 98% respectively at 1580 °C in air. According to the result, the excellent antioxidant capacity of ZrC ceramic has been revealed. ZrC preceramic polymer need to convert to crystalline ZrC ceramic, so the CNFs-CF/ZrC preceramic has more weight loss compared with the CNFs-CF/ZrC ceramic. Two steps of weight loss of CNFs-CF/ZrC preceramic is attributed to the organic-to-inorganic conversion and carbothermal reduction respectively. After the pyrolysis process, the CNFs-CF/ZrC ceramic shows an excellent thermal stability in the air atmosphere. That reveals the effective protection of the CF substrate from the high-temperature oxidizing environment by the ZrC ceramic layer. The ZrC ceramic could convert to the zirconia at higher temperature, which could seal the pores on the surface of CF to prevent CF oxidation weight loss in high temperature.


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Figure 12. TG analysis of CF precursor, CNFs-CF, CNFs-CF/ZrC preceramic and CNFsCF/ZrC ceramic. 4. CONCLUSIONS In summary, CNFs-CF/ZrC composite was successfully prepared by PECVD and PIP methods. The CNFs-CF architecture substrate was fabricated with catalysts assistant growth CNFs on the CF surface to strengthen the interface binding force between CFs and ZrC preceramic. The interface reinforcement mechanism of CNFs-CF/ZrC composite is that the construction can provide a strong mechanical lock. The growth of CNFs on the CF surface is completely controllable by optimizing PECVD reaction conditions. Pyrolysis of CNFs-CF/ZrC preceramic under inert conditions would lead to the crystallized ZrC ceramic of generating and coating on the surface of CNFs-CF substrate. The presence of CNFs on the CF architecture surface guaranteed the uniform and dense coating of the ZrC ceramic layer effectively. CNFsCF/ZrC composite exhibits an outstanding high temperature stability. The combination between CNFs-CF hierarchical substrate with ceramic matrix could provide a novel way in the preparation of CF reinforced ceramic composite with excellent interfacial properties. ASSOCIATED CONTENT


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Supporting Information. Schematic diagram of the IFSS test between CNFs-CF and polymer material. Different magnification of SEM images of CF coated with Ni(NO3)2 particles: a) 5000×, b) 20000×, c) 80000×. XPS spectra and fitting curve of C1s peak of a) untreated carbon fiber and b) CNFs-CF. Typical compressive stress vs. compressive strain curves: a) CF/ZrC, b) CNFs-CF/ZrC. (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This research was financially supported by the National Natural Science Foundation of China (Project Nos. 51272056, 11121061, 91216301 and 51602076), the National Fund for Distinguished Young Scholars (Project No. 51525201), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No. 11421091) and the China Postdoctoral Science Foundation Funded Project (Project No. 2016M601426). REFERENCES [1] Li, M.; Gu, Y. Z.; Liu, Y. N.; Li, Y. X.; Zhang, Z. G. Interfacial Improvement of Carbon Fiber/Epoxy Composites Using a Simple Process for Depositing Commercially Functionalized Carbon Nanotubes on the


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Figure Captions Figure 1. Diagrammatic sketch of the fabrication process of CNFs-CF architecture with ZrC ceramic coating.


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Figure 2. SEM images of CNFs-CF with the CNFs growth time varied from 0 to 30 min, (e)-(h) are the high-magnification images corresponding to (a)-(d) respectively. Figure 3. (a) TEM image of as-prepared carbon nanofibers, (b) TEM image of single carbon nanofiber, (c) HRTEM image of part of the single CNF and (d) SAED pattern of the CNFs. Figure 4. XPS spectrum of untreated CF, CF treated by HNO3, CF attached with Ni and CNFsCF prepared with the deposition time of 30 min. Figure 5. Interfacial shear strength and BET surface area of untreated CF and CNFs-CF with CNFs growth time from 5 to 30 min. Figure 6. Synthesis reaction equations of ZrC preceramic polymer. Figure 7. Schematic illustration of synthesis process for the ZrC precursor polymer and CNFsCF/ZrC ceramic nanocomposite. Figure 8. TG, DTG and DSC curves of ZrC preceramic polymer. Figure 9. XRD patterns of reaction products of ZrC preceramic material with the heat treating temperature at 1300 °C, 1400 °C, 1500 °C and 1600 °C. Figure 10. SEM images of a) untreated carbon fiber, b) carbon fiber reinforced ZrC composites, c) single fiber from carbon fiber reinforced ZrC composites, d) CNFs-CF, e) CNFs-CF reinforced ZrC nanocomposites, and f) single fiber from CNFs-CF reinforced ZrC nanocomposites. Figure 11. Compressive properties of CF/ZrC and CNFs-CF/ZrC ceramic composite. Figure 12. TG analysis of CF precursor, CNFs-CF, CNFs-CF/ZrC preceramic and CNFsCF/ZrC ceramic. Table Captions


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Table 1. The density of the CF/ZrC and CNFs-CF/ZrC nanocomposites changed with the precursor infiltration pyrolysis cycles.


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102x76mm (300 x 300 DPI)


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Carbon Nanofiber Arrays Grown on Three ... - ACS Publications

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