Hybrid carbon nanotube–carbon fiber composites with improved in-plane mechanical properties

In this study carbon nanotubes (CNTs) were grown on carbon fibers to enhance the in-plane and out-of-plane properties of fiber reinforced polymer composites (FRPs). A relatively low temperature synthesis technique was utilized to directly grow CNTs over the carbon fibers. Several composites based on carbon fibers with different surface treatments (e.g. growing CNTs with different lengths and distribution patterns and coating the fibers with a thermal barrier coating (TBC) layer) were fabricated and characterized via on- and off-axis tensile tests. The on-axis tensile strength and ductility of the hybrid FRPs were improved by 11% and 35%, respectively, due to the presence of the TBC and the surface grown CNTs. This configuration also exhibited 16% improvement on the off-axis stiffness. Results suggest that certain CNT growth patterns and lengths are more pertinent than the other surface treatments to achieve superior mechanical properties.     

Growth of carbon nanotubes on carbon fibre substrates to produce hybrid/phenolic composites with improved mechanical properties

Carbon nanotubes were grown by chemical vapor deposition (CVD) on different carbon fibre substrates namely, unidirectional (UD) carbon fibre tows, bi-directional (2D) carbon fibre cloth and three dimensional (3D) carbon fibre felt. These substrates were used as the reinforcement in phenolic resin matrix to develop hybrid CF–CNT composites. The growth morphology and other characteristics of the as grown tubes were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermal gravimetry (TGA) which confirmed a copious growth of multiwalled carbon nanotubes (MWNTs) on these substrates. The mechanical properties of the hybrid composites was found to increase with the increasing amount of deposited carbon nanotubes. The flexural strength (FS) improved by 20% for UD, 75% for 2D and 66% for 3D hybrid composites as compared to that prepared by neat reinforcements (without CNT growth) under identical conditions. Flexural modulus (FM) of these composites also improved by 28%, 54% and 46%, respectively.     

Comparison of chemical vapor deposition and chemical grafting for improving the mechanical properties of carbon fiber/epoxy composites with multi-wall carbon nanotubes

By engineering the fiber/matrix interface, the properties of the composite can be changed significantly. In this work, we increased the effective surface area of the fiber/matrix interface, to facilitate additional stress transfer between fibers and matrix, by grafting carbon nanotubes on to carbon fibers (in the form of carbon fabric) by two different methods: (1) chemical vapor deposition (CVD) method and (2) a purely chemical method. With the CVD process, carbon nanotubes (CNT) were directly grown on carbon fiber substrate using chemical vapors. For the chemical method, CNT with carboxyl groups were grafted on functionalized carbon fiber via a chemical reaction. The morphology of CNT/carbon fibers was examined by scanning electron microscope (SEM) which revealed uniform coverage of carbon fibers with CNT in both of CVD method and chemical grafting method. CNT-grafted woven carbon fibers were used to make carbon/epoxy composites, and their mechanical properties were measured using three-point bending and tension tests which showed that those with CNT-grafted carbon fiber reinforcements using the CVD process has 11 % higher tensile strength compared to those containing carbon fibers modified with the chemical method. Also, composites with CNT-grafted carbon fibers with chemical method showed 20 % higher tensile strength compared to composites with unmodified carbon fibers. The results of tensile test revealed that both CVD and chemical grafting could significantly improve the mechanical properties of the carbon fiber composites.     

Application of continuously-monitored single fiber fragmentation tests to carbon nanotube/carbon microfiber hybrid composites

To assess the effect of carbon nanotube (CNT) grafting on interfacial stress transfer in fiber composites, CNTs were grown upon individual carbon T-300 fibers by chemical vapor deposition. Continuously-monitored single fiber composite (SFC) fragmentation tests were performed on both pristine and CNT-decorated fibers embedded in epoxy. The critical fragment length, fiber tensile strength at critical length, and interfacial shear strength were evaluated. Despite the fiber strength degradation resulting from the harsh CNT growth conditions, the CNT-modified fibers lead to a twofold increase in interfacial shear strength which correlates with the nearly threefold increase in apparent fiber diameter resulting from CNT grafting. These observations corroborate recently published studies with other CNT-grafted fibers. An analysis of the relative contributions to the interfacial strength of the fiber diameter and strength due to surface treatment is presented. It is concluded that the common view whereby an experimentally observed shorter average fragment length leads to a stronger interfacial adhesion is not necessarily correct, if the treatment has changed the fiber tensile strength or its diameter.     

Interfacial shear strength of a glass fiber/epoxy bonding in composites modified with carbon nanotubes

In recent years, carbon nanotubes (CNTs) grown on fibers have attracted a lot of interest as an additional reinforcing component in conventional fiber-reinforced composites to improve the properties of the fiber/matrix interface. Due to harsh growth conditions, the CNT-grafted fibers often exhibit degraded tensile properties. In the current study we explore an alternative approach to deliver CNTs to the fiber surface by dispersing CNTs in the fiber sizing formulation. This route takes advantage of the developed techniques for CNT dispersion in resins and introduces no damage to the fibers. We focus on unidirectional glass fiber/epoxy macro-composites where CNTs are introduced in three ways: (1) in the fiber sizing, (2) in the matrix and (3) in the fiber sizing and matrix simultaneously. Interfacial shear strength (IFSS) is investigated using single-fiber push-out microindentation. The results of the test reveal an increase of IFSS in all three cases. The maximum gain (over 90%) is achieved in the composite where CNTs are introduced solely in the fiber sizing.     

Nanoscale structure and local mechanical properties of fiber-reinforced composites containing MWCNT-grafted hybrid glass fibers

Carbon nanotubes (CNTs) were grown from the surface of glass fibers by chemical vapor deposition, and these hybrid fibers were individually dispersed in an epoxy matrix to investigate the local composite structure and properties near the fiber surface. High-resolution transmission electron microscopy revealed the influence of infiltration and curing of a liquid epoxy precursor on the morphology of the CNT “forest” region, or region of high CNT density near the fiber surface. Subsequent image analysis highlighted the importance of spatially dependent volume fractions of CNTs in the matrix as a function of distance from the fiber surface, and nanoindentation was used to probe local mechanical properties in the CNT forest region, showing strong correlations between local stiffness and volume fraction. This work represents the first in situ measurements of local mechanical properties of the nano-structured matrix region in hybrid fiber-reinforced composites, providing a means of quantifying the reinforcement provided by the grafted nanofillers.     

Interfacial load transfer in carbon nanotube/ceramic microfiber hybrid polymer composites

Growing carbon nanotubes (CNTs) on the surface of fibers has the potential to modify fiber–matrix interfacial adhesion, enhance the composite delamination resistance, and possibly improve its toughness and any matrix-dominated elastic property as well. In the present work aligned CNTs were grown upon ceramic fibers (silica and alumina) by chemical vapor deposition (CVD) at temperatures of 650 °C and 750 °C. Continuously-monitored single fiber composite (SFC) fragmentation tests were performed on pristine as well as on CNT-grown fibers embedded in epoxy. The critical fragment length, fiber tensile strength at critical length, and interfacial shear strength were evaluated. Significant increases (up to 50%) are observed in the fiber tensile strength and in the interfacial adhesion (which was sometimes doubled) with all fiber types upon which CNTs are CVD-grown at 750 °C. We discuss the likely sources of these improvements as well as their implications.     

Effect of fibre coating and geometry on the tensile properties of hybrid carbon nanotube coated carbon fibre reinforced composite

Hierarchically structured hybrid composites are ideal engineered materials to carry loads and stresses due to their high in-plane specific mechanical properties. Growing carbon nanotubes (CNTs) on the surface of high performance carbon fibres (CFs) provides a means to tailor the mechanical properties of the fibre–resin interface of a composite. The growth of CNT on CF was conducted via floating catalyst chemical vapor deposition (CVD). The mechanical properties of the resultant fibres, carbon nanotube (CNT) density and alignment morphology were shown to depend on the CNT growth temperature, growth time, carrier gas flow rate, catalyst amount, and atmospheric conditions within the CVD chamber. Carbon nanotube coated carbon fibre reinforced polypropylene (CNT-CF/PP) composites were fabricated and characterized. A combination of Halpin–Tsai equations, Voigt–Reuss model, rule of mixture and Krenchel approach were used in hierarchy to predict the mechanical properties of randomly oriented short fibre reinforced composite. A fractographic analysis was carried out in which the fibre orientation distribution has been analyzed on the composite fracture surfaces with Scanning Electron Microscope (SEM) and image processing software. Finally, the discrepancies between the predicted and experimental values are explained.     

Controlled growth of CuO nanowires on woven carbon fibers and effects on the mechanical properties of woven carbon fiber/polyester composites

The effect of CuO nanowires on the improvement of the mechanical properties of woven carbon fiber (WCF)-based polyester resin composite was studied. The composite was manufactured by the vacuum-assisted resin transfer molding (VARTM) process. CuO nanowires were grown on woven carbon fiber sheets in subsequent steps of seeding followed by growth. Scanning electron microscopy (SEM) showed the growth of CuO nanowires on the surface of the carbon fibers; this growth increased with the number of seeding cycles and the length of the growth time. The concentration of the growth solution did not have a significant effect. The maximum amount of growth occurred for 8 seeding cycles with a 60 mM growth solution and a growth time of 8 h. An analysis of the percent weight change, along with X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, supported the above findings. The crystalline peak height of the CuO nanowires increased with the nanowire growth. The new absorption peaks arising in the FTIR spectra also indicated growth of CuO nanowires on the WCF. The mechanical properties in terms of tensile strength, modulus, and impact resistance improved significantly after the growth of nanowires on the carbon fibers: the modulus and strength improved by up to 33.1% and 42.8%, while the impact energy absorption increased by 136.8% relative to bare WCF.     

Enhancement of mechanical performance of epoxy/carbon fiber laminate composites using single-walled carbon nanotubes

Carbon nanotubes (CNT) in their various forms have great potential for use in the development of multifunctional multiscale laminated composites due to their unique geometry and properties. Recent advancements in the development of CNT hierarchical composites have mostly focused on multi-walled carbon nanotubes (MWCNT). In this work, single-walled carbon nanotubes (SWCNT) were used to develop nano-modified carbon fiber/epoxy laminates. A functionalization technique based on reduced SWCNT was employed to improve dispersion and epoxy resin-nanotube interaction. A commercial prepregging unit was then used to impregnate unidirectional carbon fiber tape with a modified epoxy system containing 0.1 wt% functionalized SWCNT. Impact and compression-after-impact (CAI) tests, Mode I interlaminar fracture toughness and Mode II interlaminar fracture toughness tests were performed on laminates with and without SWCNT. It was found that incorporation of 0.1 wt% of SWCNT resulted in a 5% reduction of the area of impact damage, a 3.5% increase in CAI strength, a 13% increase in Mode I fracture toughness, and 28% increase in Mode II interlaminar fracture toughness. A comparison between the results of this work and literature results on MWCNT-modified laminated composites suggests that SWCNT, at similar loadings, are more effective in enhancing the mechanical performance of traditional laminated composites.     

Effect of ZnO nanowire morphology on the interfacial strength of nanowire coated carbon fibers

The ability to tailor interfacial shear strength for a particular fiber and resin system is critical to the development of composite materials that perform optimally in specific applications. One approach to tailor the interface is to introduce a secondary phase between the fiber and matrix, which can act to functionally grade the material properties and enhance load transfer across the interface. This approach has been applied in the past using nanowires, nanotubes, and whiskers and was demonstrated to significantly enhance interface performance. Unfortunately, these processes lack control over the interphase morphology to allow design of the interface for optimal properties. Recently, ZnO nanowires grown on the surface of carbon fibers have demonstrated more than a 110% increase in interfacial strength [1]. Unlike other treatments, this interfacial reinforcement allows precise morphology control. Here, we develop the parameters for the growth of nanowires with varying lengths and diameters and study the influence of the nanowire’s morphology on the interfacial shear strength. ZnO nanowire arrays are grown on carbon fibers, with nanowire diameters ranging from 50 to 200 nm and lengths up to 4 μm. The interfacial shear strength with varying nanowire dimensions is shown to increase by up to 228%, ranging from 45.72 to 154.64 MPa. Unlike existing whiskerization approaches, it is shown that the tensile strength of the ZnO nanowire coated fibers remains constant throughout all growth procedures. The development of an interphase offering control over the interface strength and toughness will provide a means to produce multifunctional composites with optimized performance for multiple applications.     

Structural health monitoring of glass fiber reinforced composites using embedded carbon nanotube (CNT) fibers

In the present work, carbon nanotube (CNT) fibers had been embedded to glass fiber reinforced polymers (GFRP) for the structural health monitoring of the composite material. The addition of the conductive CNT fiber to the non-conductive GFRP material aims to enhance its multi-function ability; the test specimen’s response to mechanical load and the insitu CNT fiber’s electrical resistance measurements were correlated for sensing and damage monitoring purposes. It is the first time this fiber is used in composite materials for sensing purposes; CNT fiber is easy to be embedded and does not downgrade the material’s mechanical properties. Various incremental loading–unloading steps had been applied to the manufactured specimens in tension as well as in three-point bending tests. The CNT fiber worked as a sensor in both, tensile and compression loadings. A direct correlation between the mechanical loading and the electrical resistance change had been established for the investigated specimens. For high stress (or strain) level loadings, residual resistance measurements of the CNT fiber were observed after unloading. Accumulating damage to the composite material had been calculated and was correlated to the electrical resistance readings. The established correlation between these parameters changed according to the material’s loading history.     

The effects of carbon nanotubes on mechanical and thermal properties of woven glass fibre reinforced polyamide-6 nanocomposites

This paper presents a preliminary investigation on the effects of incorporating carbon nanotubes (CNT) into polyamide-6 (PA6) on mechanical, thermal properties and fire performance of woven glass reinforced CNT/PA6 nanocomposite laminates. The samples were characterized by tensile and flexural tests, thermal gravimetric analysis (TGA), heat distortion temperature (HDT) measurements, thermal conductivity and cone calorimeter tests. Incorporation of up to 2 wt% CNT in CNT/PA6/GF laminates improved the flexural stress of the laminates up to 36%, the thermal conductivity by approximately 42% and the ignition time and peak HRR time was delayed by approximately 31% and 118%, respectively.     

Pristine and amino functionalized carbon nanotubes reinforced glass fiber epoxy composites

Multi-phase composites have been studied by incorporating carbon nanotubes (CNTs) as a secondary reinforcement in an epoxy matrix which was then reinforced with glass fiber mat. Different types of CNTs e.g. amino functionalized carbon nanotubes (ACNT) and pristine carbon nanotubes (PCNT) were homogeneously dispersed in the epoxy matrix and two-ply laminates were fabricated using vacuum-assisted resin infusion molding technique. The issues related to CNT dispersion and interfacial bonding and its affect on the mechanical properties have been studied. An important finding of this study is that PCNT scores over ACNT in composites prepared under certain conditions. This is a very significant finding since PCNT is available at a much lower cost than ACNT.     

Impact behavior and fractographic study of carbon nanotubes grafted carbon fiber-reinforced epoxy matrix multi-scale hybrid composites

Carbon nanotubes are the most promising reinforcement for high performance composites. Multiwall carbon nanotubes were directly grown onto the carbon fiber surface by catalytic thermal chemical vapor deposition technique. Multi-scale hybrid composites were fabricated using the carbon nanotubes grown fibers with epoxy matrix. Morphology of the grown carbon nanotubes was investigated using field emission scanning electron microscopy and transmission electron microscopy. The fabricated composites were subjected to impact tests which showed 48.7% and 42.2% higher energy absorption in Charpy and Izod impact tests respectively. Fractographic analysis of the impact tested specimens revealed the presence of carbon nanotubes both at the fiber surface and within the matrix which explained the reason for improved energy absorption capability of these composites. Carbon nanotubes presence at various cracks formed during loading provided a direct evidence of micro crack bridging. Thus the enhanced fracture strength of these composites is attributed to stronger fiber–matrix interfacial bonding and simultaneous matrix strengthening due to the grown carbon nanotubes.     

Effects of carbon nanotubes and carbon fiber reinforcements on thermal conductivity and ablation properties of carbon/phenolic composites

The ablation properties and thermal conductivity of carbon nanotube (CNT) and carbon fiber (CF)/phenolic composites were evaluated for different filler types and structures. It was found that the mechanical and thermal properties of phenolic-polymer matrix composites were improved significantly by the addition of carbon materials as reinforcement. The concentrations of CF and CNT reinforcing materials used in this study were 30 vol% and 0.5 wt%, respectively. The thermal conductivity and thermal diffusion of the different composites were observed during ablation testing, using an oxygen–kerosene (1:1) flame torch. The thermal conductivity of CF mat/phenolic composites was higher than that of random CF/phenolic composites. Both CF mat and CNT/phenolic composites exhibited much better thermal conductivity and ablation properties than did neat phenolic resin. The more conductive carbon materials significantly enhanced the heat conduction and dissipation from the flame location, thereby minimizing local thermal damage.     

Vibration damping characteristics of carbon fiber-reinforced composites containing multi-walled carbon nanotubes

Vibration damping characteristic of nanocomposites and carbon fiber reinforced polymer composites (CFRPs) containing multiwall carbon nanotubes (CNTs) have been studied using the free and forced vibration tests. Several vibration parameters are varied to characterize the damping behavior in different amplitudes, natural frequencies and vibration modes. The damping ratio of the hybrid composites is enhanced with the addition of CNTs, which is attributed to sliding at the CNT-matrix interfaces. The damping ratio is dependent on the amplitude as a result of the random orientation of CNTs in the epoxy matrix. The natural frequency shows negligible influence on the damping properties. The forced vibration test indicates that the damping ratios of the CFRP composites increase with increasing CNT content in both the 1st and 2nd vibration modes. The CNT-epoxy nanocomposites also show similar increasing trends of damping ratio with CNT content, indicating the enhanced damping property of CFRPs arising mainly from the improved damping property of the modified matrix. The dynamic mechanical analysis further confirms that the CNTs have a strong influence on the composites damping properties. Both the dynamic loss modulus and loss factor of the nanocomposites and the corresponding CFRPs show consistent increases with the addition of CNTs, an indication of enhanced damping performance.     

Effect of carbon nanotube waviness on the effective thermoelastic properties of a novel continuous fuzzy fiber reinforced composite

This paper deals with the investigation of the effect of carbon nanotube (CNT) waviness on the effective coefficient of thermal expansion (CTE) of a novel continuous fuzzy fiber reinforced composite (FFRC). This novel FFRC is composed of carbon fibers, sinusoidally wavy CNTs and epoxy matrix. The sinusoidally wavy CNTs are radially grown on the circumferential surfaces of the carbon fibers. Analytical micromechanics model based on the method of cells (MOC) approach is derived to investigate the influence of the waviness of CNTs on the effective CTEs of the FFRC. The present study reveals that if the amplitudes of the radially grown sinusoidally wavy CNTs are parallel to the axis of the carbon fiber then the thermoelastic properties of the FFRC are significantly improved over those of the FFRC being composed of straight CNTs.     

Effect of carbon nanotubes on the interfacial shear strength of T650 carbon fiber in an epoxy matrix

The interfacial shear strength of carbon nanotube coated carbon fibers in epoxy was studied using the single-fiber composite fragmentation test. The carbon fibers were coated with carbon nanotubes (CNT) on the fiber surface using thermal chemical vapor deposition (CVD). The CVD process was adjusted to produce two CNT morphologies for the study: radially aligned and randomly oriented. The purpose of the CNT coating was to potentially produce a multifunctional structural composite. Results of the single-fiber fragmentation tests indicate an improvement in interfacial shear strength with the addition of a nanotube coating. This improvement can most likely be attributed to an increase in the interphase yield strength as well as an improvement in interfacial adhesion due to the presence of the nanotubes.     

A chemical method to graft carbon nanotubes onto a carbon fiber

A simple method is developed for grafting carbon nanotubes (CNTs) onto a carbon fiber surface. CNT and carbon fiber undergo an oxidation treatment. Oxidation generates oxygen, like carboxyl, carbonyl or hydroxyl groups, or amine groups on nanotubes and carbon fiber surface. Functionalized CNTs are dispersed in a solvent and deposited on carbon fibers. The bonds between CNT and carbon fiber are operated by esterification, anhydridation or amidization of the chemical surface groups. The resulting materials are characterized by scanning electron microscopy (SEM). CNTs form a 3D network around the carbon fibers. Likewise, CNT bonding between two fibers is observed.