Carbon Nanotube Hierarchical Composites for Interlaminar Strengthening

The exploitation of conventional, continuous fibre-reinforced plastics in engineering structures has been steadily diversifying from sports equipment and high performance racing cars, to helicopters and most recently commercial aeroplanes. The main benefits of composite materials, such as their excellent specific strength and stiffness properties, must be viewed with respect to in-plane fibre-direction applications. However, if a composite plate is subjected to significant out-of-plane stresses subsurface delaminations may develop between layers due to the weak through-thickness cohesive strength of the composite (2). Previously, techniques such as Z – pinning, stitching and 3D – braiding have been investigated to improve through-thickness properties but these tend to reduce the in-plane performance of the laminate by damaging primary fibres and inducing fibre waviness (1).

Carbon Nanotube interfacial strengthening

Throughout the last decade the huge interest in Carbon Nanotubes (CNT) has been fuel by their extraordinary intrinsic mechanical, electrical and thermal properties, which make them ideal candidates for multifunctional structures (3). To overcome the weakness of interlaminarstrength considerable research has been conducted to develop hierarchical composite structures by using nanoscale CNT reinforcement alongside microscale carbon and glass fibers. Examples in nature such as cell walls and animal shells show that excellent mechanical properties can be obtained from spreading reinforcement over a number of length scales, even if the original constituents are fairly weak (4). This paper reviews the progress in developing such hierarchical composites to improve delamination resistance and through-thickness properties by intra- and interlaminar reinforcement of multiwall carbon nanotubes (MWCNT).

In an attempt to improve the through-thickness properties the introduction of CNTs should,

● Ideally be attached radial to the primary fibres and extend into the surrounding matrix to stiffen the fibre/matrix interface, improve the primary fibre surface area and facilitate mechanical interlocking, all of which improves stress transfer.

● Result in a uniform distribution of CNTs.

● Not reduce the in-plane laminate properties.

● Not introduce other secondary or additional modes of failure by damaging the primary fibres.

● Allow a scalable, straightforward processing technique that can be easily incorporated with conventional manufacturing processes such as VARTM or pre-preg.

In the literature there are currently two popular methods to achieve this,

1. Dispersing CNTs in a polymer matrix followed by infusion of pre-forms with the CNT-reinforced resin,

2. A direct attachment of CNTs onto the external surface of the primary fibres subsequently infused with a pristine resin.

In the following sections the details of the two manufacturing approaches (shown schematically in Figure 1) are outlined and the implications of each approach on through-thickness performance such as interlaminar shear strength, and Mode I and Mode II critical fracture energy discussed.

Fig. 1. Schematic diagram of conventional CFRP and hierarchical CFRP with CNTs in matrix and grown on fibres (4).

CNT-reinforced Matrix

The simplest method to manufacture hierarchical nanocomposites is by mechanically or ultrasonically shear-mixing CNTs into low-viscosity thermosetting resins, and then infusing or impregnating the primary fibre stack using conventional techniques such as VARTM (5; 6; 7). To date the most uniform dispersion of MWCNTs throughout the matrix have been achieved by shear mixing using a three-roll mill (8; 9). On the other hand this approach is limited to short CNTs < 1 mm at low volume fractions of 1 – 2%, which greatly limits the reinforcement potential. Higher volume fractions are to date not possible since the viscosity of the matrix increases rapidly with CNT content leading to incomplete infusion (10) or CNT agglomeration/depletion in different areas of the fabric (11).

Flexural tests of hierarchical composites with glass and carbon primary fibres show that the in-plane stiffness and strength are not impaired by the MWCNTs (5; 8). Qiu et al. (5) actually showed an improvement in tensile strength and stiffness of a glass-fibre composite of 15.9% and 27.2% respectively, while Veedu et al. (12) showed improvements of 142% and 5% for carbon composites. Most importantly, as tabulated in Table 1 short beam shear (SBS) and compression shear tests (CST) have shown increases in the matrix-dominated interlaminar shear strength (ILSS) between 8% and 33%. Scanning electron microscopy (SEM) images show that the MWCNTs in the resin lead to better fibre-to-matrix adhesion as well as pullout and rupture of the MWCNTs before final matrix failure, which consumes additional fracture energy (Figure 2).

Fig. 2. SEM images showing much more matrix stuck to the fracture surface of CNT reinforced matrix suggesting better matrix/fibre adhesion (7)

The SEM images also indicate that the alignment of the CNTs is heavily influenced by the direction of the resin flow during infusion and local orientation of the primary fibres (4). As resin infusion generally occurs in the through-thickness direction the VARTM approach can give some control in aligning the CNTs in the preferred direction for improving transverse properties, although a certain degree of random alignment remains. Furthermore, one study has shown (5) that functionalised MWCNTs resulted in slightly higher SBS shear modulus and strength (~3%) compared to a pristine un-functionalised MWCNTs. Using SEM imagery the authors showed that this stemmed from a superior interfacial bonding between the CNTs and the matrix.

Delamination resistance is generally investigated using Mode – I double cantilever beam (DCB) tests and Mode – II end-notched flexure (ENF) tests. Table 1 summarises the significant improvements of up to 98% and 75% for Mode I and Mode II fracture toughness respectively compared to non-hierarchical composites. The characterisation of the fracture surfaces using SEM imagery has shown that the additional pullout and bridging of the CNT is responsible for the toughening. Similarly, Garcia et al. (13) have developed an efficient technique of growing CNT mats on growth substrates and then “transfer printing” the CNT mats in between tacky pre-preg plies using a roller. Since this process better controls the CNT alignment in the through-thickness direction much higher improvements of fracture toughness of 152% in Mode I and 214% in Mode II were observed. However, the process of “transfer printing” CNT films at every ply interface is a very time consuming endeavour and may therefore not be as applicable to scalable industrial integration as the VARTM process.

Table 1.    Improvements in ILSS and delamination resistance of CNT-reinforced composites.

Fibre	Matrix	Nanofiller	Nano- Reinforced Region	Test Method	Improve-ment	Ref. And Year
woven glass	VARTM epoxy	1 wt% of pristine and functionalised MWCNT	entire matrix	SBS (ILSS)	7.9%	(5), 2007
woven glass	epoxy	0.5-2 wt% MWCNTs	entire matrix	Compression Shear Test (ILSS)	9.7% (0.5%) 20.5 (1%) 33% (2%)	(14), 2008
carbon	epoxy	5 wt% cup stacked CNTs	entire matrix	DCB (Mode I) – ENF (Mode II)	98% – 30%	(15), 2007
carbon	epoxy	1 wt% MWCNTs	entire matrix	DCB (Mode I) – ENF (Mode II)	60% – 75%	(16), 2009
UD carbon	pre-pregepoxy	~1% CNT forests	layer between pre-pregplies	DCB (Mode I) – ENF (Mode II)	152% – 214%	(13), 2008

The fabrication of hierarchical composites by impregnating microscale primary fibres with nanoscale-modified resins is limited to maintaining low matrix viscosities. Furthermore, resin flow during impregnation tends to align CNTs parallel to the primary fibre direction, the least desirable orientation for improving through-thickness properties. In this respect growing or “grafting” CNTs directly onto the surfaces of primary fibres followed by infusion with a pristine, low-viscosity matrix allows higher volume fractions and is ideal for orientating fibres radial to the primary fibres. Furthermore, this approach overcomes the problems of CNT agglomeration or self-assembly into bundles as observed when CNT are freely dispersed in a matrix. Three techniques for attaching CNTs onto fibres were found to be most popular in the literature: CNT-modified Fibres

1. Direct growth of CNTs onto fibres via Chemical or Thermal Vapour Deposition (CVD and TVD) (12)

2. Electrophoretic deposition (EPD) (6)

3. Coating of primary fibres with CNT-modified sizing agents (7)

The first example of synthesising CNTs onto carbon fibres via CVD was conducted in 1991 by Downs and Baker (18). In this approach the primary glass or carbon fibres are initially oxidised with nitric acid and the iron catalysts then deposited onto the fibres using incipient wetness techniques such as sputtering, thermal evaporation or electrodeposition (4). The ultimate result is the growth of highly aligned and dense CNT forests onto fibre cloths (Figure 3) that are then stacked and impregnated by infusion techniques such as VARTM (12). Experiments have shown that the CNT forests are efficiently wet-out by liquid resins and polymer melts as a result of capillary forces (6; 19).

Fig. 3. SEM images showing CNT forests (b) grown in woven pristine fibre cloth (a) (12)

Recently, Injection CVD (ICVD) techniques have been favoured to then grow the CNTs on the primary fibres via a pyrolysis of solutions containing a catalyst precursor and a hydrocarbon source (20). The ICVD technique has resulted in better degree of orientation and growth of longer CNTs compared to classical CVD approach.

The most crucial parameters in grafting CNTs onto glass or carbon fibres are,

● Choosing a good catalyst for strong anchoring interaction between CNTs and fibres to maximise stress transfer and reduce damage during manufacturing processes, While

● At the same time prevent oxidation damage to the primary fibres by to aggressive a catalyst.

Fig. 4. Electrophoresis (6)

Oxidation and gasification are especially problematic for carbon fibres since the active catalysts deposited onto the fibres etch into the surface and thus may reduce their strength by up to 55% (4). As a solution Bekyarova et al. (6) selectively deposited multi- and single-walled CNTs onto woven carbon fabric using electrophoresis. In this approach MWCNTs are first produced as is using a classical CVD process and then dispersed in an aqueous media between two negative electrodes to charge the CNTs (Figure 4). The dry carbon fabric was then immersed in the CNT doped media and sandwiched between two steel plates connected to a positive charge. Driven by the electric potential, the CNTs are thus deposited onto the carbon cloth and the CNT-carbon fibre performs then infused with epoxy using VARTM. A very simple approach has been presented by Zhu et al. (7) who sprayed nanotubes directly onto woven fibers prior to VARTM processing. The drawback of this technique compared with direct growth methods is relatively little control over the CNT orientation (4).

The pioneering work of Downs and Baker (21) reported a 4.75x increase in interfacial shear strength (IFSS) of a nanofibre-grafted carbon composite, although such incredible improvements have not been repeated thus far. Table 2 summarises interlaminar and delamination resistance enhancements taken from different sources in the literature and based on multiple primary fibre, CNT and matrix combinations. Veedu et al. (12) showed improvements of 348% and 54% for GIC and GIICrespectively for MWCNT enhanced SiC woven fabrics using a classical CVD technique; Bekyarova et al. have found improvements of 27% in ILSS for CNT enhanced carbon fabrics using electrophoresis deposition; while Zhu et al. demonstrated improvements of 45% in ILSS of MWNT doped glass fiber reinforced vinyl ester composites using a simple spray up with only 0.015 wt% of CNTs. In all three studies SEM imagery showed that the improvements arise from the increased surface area of the primary fibres and excellent wettability, which facilitates a strong bond between fibres and matrix by mechanical interlocking.

Based on these results the general consensus is that the damage tolerance of a structure can readily be improved by CNT grafting (4). However, there is also a large variability in the results arising from the different manufacturing processes, material combinations and CNT loadings applied that conceal the exact effectiveness of the method. There is agreement that the degree of enhancement is greatly dependent on the orientation and length of the grafted CNTs and further experimental research is required to ascertain the optimal morphology and manufacturing technique to achieve this (4).

Table 2.  Improvements in interlaminar strength and delamination resistance for nano-grafted composites.

Fibre	Matrix	Nanofiller	Manufacturing Technique	Test Method	ILSSImprov.	Ref. And Year
woven glass	vinyl ester	0.015% SWCNTs and MWCNTs	Spray-up between plies	SBS	20-45%	(7), 2007
carbon	epoxy	0.25 wt% MWCNTS	Electro-phoresis	SBS	27%	(6), 2006
SiC	epoxy	2 wt% MWCNTs	CVD	DCB (Mode I) – ENF (Mode II)	348% – 54%	(12), 2006

Perspectives

The research so far has focused on demonstrating the great potential of CNTs to improve the through-thickness of properties of conventional FRPs. In the future research should focus on,

● Developing scalable manufacturing processes that may find application in real, large-scale industrial applications.

● Finding new approaches that solve agglomeration and high viscosity issues to allow higher loadings of CNTs.

● Functionalisation of CNTs to improve CNT dispersion and stress transfer with the host matrix.

● Reducing or preventing the reduction in strength of primary fibres induced by grafting fibres onto external surface.

● Ascertaining the optimal CNT orientation and aspect ratio to optimise the through-thickness performance.