## 4 Conclusions  
In summary, the activation of the CFs' surface by a CO\({}_{2}\) laser beam was investigated for the first time as a novel and simple route to enhance the out-of-plane electrical conductivity of fabricated CFRP without simultaneously deteriorating their interlaminar mechanical properties. Irradiated CF yarns were tested prior to the fabrication of the laminated structures, both in terms of tensile strength and electrical conductivity, to examine the impact of the laser irradiation to the CFs themselves. Our results showed that even though there was a decrease in the tensile strength of the irradiated CFs, the electrical conductivity was actually enhanced (up to 18.4%), due to a combination of (i) the laser induced graphitic nanostructuring on the CFs' surface and (ii) laser induced thermal transformation of amorphous to graphitic carbon in the bulk of the fiber. Furthermore, the results of Mode-I interlaminar fracture toughness on the fabricated composites, showed that the interlaminar region was not affected by this decrement in the tensile strength of CFs, as the values of toughness were almost the same. Fractography analysis further established that the CF/epoxy bridging effect was not affected by the laser induced graphitic texturing. Moreover, the out-of-plane electrical conductivity on both through thickness and volume directions was enhanced by 18% and 43% respectively. This improvement on the electrical conductivity of the irradiated laminates is associated with improved conductive paths created within and between the CFs upon irradiation. Overall, the results demonstrate that CO\({}_{2}\) irradiation is a promising route for enhancing the electrical conductivity of the laminated structures without sacrificing the Mode-I interlaminar fracture toughness. However, one limitation is the preservation of tensile strength in single CFs, associated with the high temperatures generated in the bulk of the fiber. Figure 14: Out of plane electrical conductivity (\(\sigma\)) of the fabricated composite structures. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multirow{2}{*}{**Mode-I G\({}_{\text{lc}}\) (J m\({}^{-2}\))**} & \multicolumn{2}{c}{**Electrical Conductivity \(\sigma\) (S/cm)**} \\ \cline{3-4}  & & **Through Volume** & **Through Thickness** \\ \hline reference & \(1268\pm 56\) & \(3.4\,10^{-2}\pm 3.5\,10^{-3}\) & \(5.1\,10^{-3}\pm 2.8\,10^{-4}\) \\ \hline PCF\({}_{4.25\text{ W}}\) & \(1254\pm 80\) & \(4.9\,10^{-2}\pm 3.8\,10^{-3}\) & \(6.0\,10^{-3}\pm 3.5\,10^{-4}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mode I interlaminar fracture toughness and electrical conductivity of the fabricated laminates. Our work demonstrates that controlled CO\({}_{2}\) laser irradiation at reduced laser fluences is an effective route to graphitize the surface of CFs and enhance the bulk composite electrical conductivity needed for large-scale, multifunctional structural applications. The laser induced surface modification of CFs is a straight-forward one step scalable method, when compared to conventional CVD or solution-based methods employed to modify CFs; it eliminates complicating time-consuming processing steps; hence, it is economically viable and can be easily commercialized using widely available CO\({}_{2}\) lasers. The following are available online at [[http://www.mdpi.com/2076-3417/10/10/3561/s1](http://www.mdpi.com/2076-3417/10/10/3561/s1)]([http://www.mdpi.com/2076-3417/10/10/3561/s1](http://www.mdpi.com/2076-3417/10/10/3561/s1)). Figure S1 PCF sample irradiated at 2.5 W (25 J/cm\({}^{-2}\)); Figure S2: PCF sample irradiated at 3.75 W (37 J/cm\({}^{-2}\)); Figure S3. PCF sample irradiated at 4 W (39 J/cm\({}^{-2}\)); Figure S4. PCF sample irradiated at 4.25 W (42 J/cm\({}^{-2}\)); Figure S5. PCF sample irradiated at 4.5 W (44 J/cm\({}^{-2}\)); Figure S6. PCF sample irradiated at 4.75 W (47 J/cm\({}^{-2}\)); Figure S7. PCF sample irradiated at 5 W (49 J/cm\({}^{-2}\)); Figure S8. Area of the irradiation for TCF representative samples.