PSI - Issue 14

Harpreet Singh Bedi et al. / Procedia Structural Integrity 14 (2019) 168–175 Harpreet S. Bedi, Prabhat K. Agnihotri/ Structural Integrity Procedia 00 (2018) 000–000

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Where, d and l e are fiber diameter and embedded length of a polymer micro-droplet on the carbon fiber, respectively (refer Fig. 1b for details). 3. Results and discussion 3.1 CNT grafted carbon fiber The size and texture of the surface of unsized and CNT grafted CFs is analysed using scanning electron microscope (JEOL, JSM-6610LV, Japan). The diameter of CF found to be ~7 µ m (Fig. 3a). Figure 3b-c shows the uniform growth of CNTs on the fiber surface. The large surface area of nanotubes has the capability to modify the surface characteristics of the fiber on which they are grown. Grafted CNTs increase the interfacial area (Li et al., 2013; Li et al., 2012; Qian, Bismarck, et al., 2010) and also the surface free energy of carbon fiber (Li et al., 2016; Peng et al., 2013; Wang et al., 2014). This affects the interfacial adhesion between the fiber and the matrix and hence the average properties of CNT based hybrid composites. This change in interfacial properties, as introduced by grafted nanotubes, is assessed by performing a detailed analysis on wetting behavior of CF with epoxy in the next section.

Figure 3. SEM micrograph of (a) unsized carbon fiber (HCF) and carbon fiber grafted with carbon nanotubes for (b) 15 min (CNTCF-15) and (c) 30 min (CNTCF-30). 3.2 Effect of CNT grafting on wettability of carbon fiber Epoxy micro-droplets formed on unsized (HCF) and CNT grafted (CNTCF) carbon fiber filaments are presented in Fig. 4 along with the average value of contact angle ( θ ). It is to be noted here that CNTCF-30 i.e. fiber with CNTs grown for 30 min is not considered in the wettability analysis. This is due to the fact that prolonged heating of fibers for 30 min inside the CVD reactor makes them so brittle that it becomes difficult to separate single fiber filaments of required length so as to carry out the test efficiently. Therefore, only HCF and CNTCF-15 fibers are considered in this part of the study. Symmetric shape of the polymer micro-droplets (Fig. 4a-b) formed on fiber is a characteristic of good adhesion between the fiber and polymer (McHale and Newton, 2002). Moreover, a decrease in contact angle from 74˚ for HCF/epoxy to 62˚ for CNTCF/epoxy indicates that wettability of CF is improved after growing CNTs on the fiber surface. Therefore, adhesion and hence the interfacial bond strength between the fiber and polymer can be enhanced by synthesizing carbon nanotubes on the surface of carbon fibers. The wetting behavior of epoxy with HCF and CNTCF is further quantified by estimating the stiffness and strength of the interface in CFRP composites as discussed in the coming sections.

Figure 4. Epoxy micro-droplets on: (a) unsized (HCF) and (b) CNT grafted carbon fiber with nanotubes grown for 15 min (CNTCF-15).