• Fiber reinforced composites (FRCs)

A material which consists of two or more physically and/or chemically distinct, suitably arranged or distributed phases with an interface separating them and has characteristics that are not depicted by any of the components in isolation.

• Interface is a region between the reinforcing fiber and matrix where the surface functional groups on the fiber can react with the matrix chemically and/or mechanically to determine the composite properties.

• Benefits of FRCs
  1. Ultimate strength of FRCs > 5x Ultimate strength of Aluminum
  2. Density of FRCs < 0.6x Density of Aluminum
  3. High specific strength and specific stiffness (4x-6x)
  4. (Fatigue life)_FRC > 10x (Fatigue life)_Al
  5. Tailorable, high-degree of optimization
  6. Corrosion resistance
  7. Complex shapes possible

• Limitations of FRCs
  1. Fiber-dominated properties are excellent (in the direction of fibers), but matrix-dominated properties are poor (in the transverse direction).
  2. The out-of-plane (interlaminar) properties are important for laminated composites as they control,
     • Transverse tensile and compressive strength
     • Interlaminar shear strength and delamination

• How to further improve the FRCs?

The performance of FRCs can be improved by enhancing their interlaminar strength. This can be achieved by increasing the load transfer from matrix to the fiber. There are many way by which we can increase the interfacial adhesion or interaction at the interface of fiber and matrix, such as,

1. Oxidative fiber surface treatment typically removes weak outer layers of the fiber, adds functional groups and texture the fiber surface, thereby increasing the interfacial area.
2. Non-oxidative fiber surface treatment involves the deposition of materials on the fiber surface, such as whiskers, grafting of polymer chains or pyrolitic carbon. Improvements are attributed to through-thickness reinforcement and increases in interfacial area but degrade due to damage to the carbon fiber surface due to the high-temperatures ~1300–1800 °C and growth conditions.
3. Use of nanoparticles. The most popular approach is to use nanoparticles such as carbon nanotubes (CNTs) in the composite. Due to strong C-C covalent bonds CNTs have very high aspect ratio (length to diameter ratio) so they have very high specific surface area. If used at the interface, CNTs increase the surface roughness as well as the effective area at the interface thereby improving the interfacial strength. CNTs can be used to reinforce the FRCs in two ways:
   • Matrix reinforcement with CNTs. The presence of van der Waals interactions between CNTs results in their non-homogeneous distribution in the matrix thus leading to agglomeration. Subjecting agglomerated particles to external loading results in premature failure of the final product.
   • Grafting of CNTs on the surface of fiber. A better way to effective harness the reinforceing mechanism of CNTs is by growing CNTs directly on the surface of carbon fibers. In addition to eliminating the ill effects of mixing CNTs in the matrix CNT grafting leads to an increased interfacial area, provides mechanical interlocking between the fiber and matrix and also provides interfacial local stiffening in the matrix.

• Preparation of CNT grafted carbon fibers

Having a uniform catalyst coating on the fiber surface is one of the key steps to have a uniform coverage of CNTs on the fiber surface. Due to the curvature associated with the cylindrical fiber surface, dip coating technique is found to be most suitable and has been used in this work. After removing the polymer sizing from the as received carbon fibers, they are dip coated in an acidic bath at 75˚C. The dip coating time is kept constant for five minutes based on the results of an earlier experimental study (Bedi, Padhee et al. 2016). The composition of dip coating bath and other details of dip coating process can be found elsewhere [7]. Dip coating is followed by drying and placing the catalyst coated carbon fibers in the CVD reactor to grow CNTs on their surface using the growth protocol in Fig. 1 below.

After achieving an initial vacuum of ~10^-2 Torr, the quartz tube reactor is purged with argon (Ar) gas at a flow rate of 300 sccm. Since, Ar also acts as a carrier for other gases, its flow is maintained till the completion of growth cycle. Initially, hydrogen (H₂) gas is mixed in the flowing Ar in a ratio of 1:10 for 10 min, reducing the NiP film deposited on the surface of carbon fibers into Ni or NiP nano-particles which act as nucleation sites for CNT growth. Afterwards, the temperature is raised up to 750˚C and acetylene (C₂H₂) is allowed to flow inside the reactor with C₂H₂ to Ar ratio of 1:10 for 15 min. C₂H₂ dissociates and provides free carbon which gets deposited at the nano-catalyst sites resulting in the formation of CNTs. After CNT growth, the reactor is cooled down in the presence of Ar and the prepared samples are subjected to characterization. The scanning electron micrographs of the surface oif carbon fiber after each step are show in Fig. 2 below.

Systematic experiments are conducted to assess the effect of carbon nanotubes (CNTs) on the interfacial properties in carbon fiber (CF) reinforced epoxy composites by directly growing CNTs on the surface of CF.

• At first, wetting behavior of CNT grafted CF (CNTCF) is investigated with epoxy through contact angle measurements and compared to that of unsized CF (HCF). Both fiber fabric and single fiber filaments are tested with epoxy to have detailed insights on the wetting phenomenon at bulk- and micro-level. An improvement in wettability is observed after the incorporation of CNTs on fiber surface.
• Further characterisation involves the estimation of size and properties of interphase by carrying out micro-mechanical tests on epoxy composites reinforced with single fiber filaments (HCF and CNTCF). Interphase size and stiffness are determined through nanoindentation, whereas, single fiber pull-out test is used to evaluate the interfacial shear strength (IFSS) in HCF/epoxy and CNT/epoxy composites. Complying with the outcome of wettability analysis, CNT grafting enhances the size, stiffness and IFSS of CF/epoxy composites. In addition to this, effect of length and density of the synthesized CNTs on the interfacial properties is appraised by varying CNT growth time inside a thermal CVD (chemical vapour deposition) reactor. Moreover, from scanning electron micrographs of pulled-out fibers, the interface failure criterion is observed to vary in the presence and absence of grafted CNTs.

From the structural integrity view point the presence of an interphase is important as it eliminates the unwanted stress concentration at the interface by gradually transforming the properties from highly stiff fiber to comparatively weaker matrix. It is found that interfacial interaction and hence the size and properties of interphase can be transformed with the aid of grafted CNTs. This opens the pathway to design the interphase in hybrid composites as per the requirement of a specific application.

• Further, macro-mechanical testing is performed to assess the effect of CNT grafting on the bulk mechanical properties of multisclae composites. For this purpose short beam bending and tensile testing are carried out on six ply CFRP laminates as discussed in detail in the pages ahead.