Designing the interphase in carbon fiber polymer composites using carbon nanotubes

Abstract

Experiments are conducted to quantitatively assess the effect of carbon nanotubes (CNTs) on the interfacial properties in carbon fiber/epoxy composites by directly growing CNTs on the surface of carbon fiber (CF). Wetting behavior of single carbon fiber filaments with epoxy matrix before and after CNT grafting is investigated through contact angle measurements. An improvement in wettability is observed after the incorporation of CNTs on the fiber surface. Effect of wettability on the size and properties of interphase is studied through well designed micro-mechanical tests on single fiber composites processed with unsized and CNT grafted carbon fibers. While nanoindentation testing is performed for quantitative determination of interphase size and stiffness, single fiber micro-debond tests are used to evaluate the interfacial shear strength (IFSS) in unsized carbon fiber/epoxy and CNT grafted carbon fiber/epoxy composites. Consistent with the wettability analysis, CNT grafting enhances the size, stiffness and IFSS of CF/epoxy composites. In addition, it is shown that even though larger CNT growth time on the surface of carbon fiber provides thicker interphase region but tends to degrade the stiffness of interphase and IFSS of CNT grafted carbon fiber/epoxy multiscale composites. Scanning electron microscopy (SEM) analysis of pulled-out and debonded fibers reveal different interfacial failure mechanism in the presence and absence of CNTs on the surface of CF. The present study quantitatively shows the presence of an interphase region surrounding the carbon fiber after CNT grafting on their surface. The existence of interphase is important from the view point of structural integrity of composites as it eliminates the unwanted stress concentration at the fiber/polymer interface by gradual transformation of properties from highly stiff fiber to comparatively weaker matrix. Moreover, the present study provides us a tool to design the interphase in hybrid composites as per the requirement of a specific application.