Carbon fiber reinforced polymer (CFRP) composites are widely used to produce structural components. However, their low interlaminar strength makes them susceptible to delamination, limiting structural applications. Aiming to solve this problem, this work proposes adding carbon nanotubes buckypaper (BP) into CFR thermoplastic composites as an interlayer to enhance the interlaminar strength through the BP bridging effect. Despite this objective, the carbon nanotube BP changed the delamination behavior in mode-I, creating an easy pathway for crack growth (smooth fracture surface) and reducing the interlaminar strength. An opposite behavior was observed for mode-II, in which BP acted as an obstacle for crack growth through the shear direction due BP bridging effect, which slightly improved interlaminar strength, resulting in a rougher surface. The experiments demonstrated through the energy involved in crack growth, the roughness of the fracture surface, and the amount of fracture mechanisms when BP was incorporated that in mode-I the delamination strength decreased, while it increased under the shear mode. This evidences that the BP bridging effect is influenced by the loading mode. Finally, this work highlights the need to study individual modes I and II in composites with buckypaper as an interlayer, since it influences the interlaminar toughness differently.

The use of carbon nanotube buckypapers (BP) has boosted their use in polymeric composites. However, their low permeability makes processing difficult and limits their use. This work's essential contribution is to define the best parameters to process the buckypapers through vacuum filtration technique and the use of electrospun poly (ether imide) (PEI) mats as a substrate during the BP processing to favor matrix/nanoparticle adhesion and improving the handling. Three different sonication power (50, 75, and 100 W) were used in the buckypaper processing to evaluate its influence. From the results obtained, the chosen power was 100 W due to large pore dimensions (100-200 nm) and a more significant structural order observed through L002 (crystallite size) of 2.80. The addition of BP/PEI mats favored an increase in PEI laminates' thermal stability by increasing its initial decomposition temperature by 10 °C and an increase of 9 °C in the glass transition temperature. For poly (aryl ether ketone) (PAEK) laminates, the addition of BP/PEI mats did not impact thermal stability and melting temperature but promoted a 6 °C reduction in crystallization temperature. On the other hand, the BP without PEI mats promoted a reduction of the studied properties, which can be associated with a lousy impregnation of the matrix in the buckypaper resulting in a weak interface. DMA analyses show that BP/PEI in both laminates (PEI and PAEK) favored the interaction between the constituents and present a larger region of interphase compared to BP without PEI/mats thus optimizing the load transfer throughout the interface.

This work concerns the fatigue behavior at three different temperature conditions (−40, 20, and 80°C) and the addition of multiwalled carbon nanotube (MWCNT) into a carbon-fiber reinforced poly(ether-imide) composite. The incorporation of MWCNT into the composite increased the tensile strength and Young's modulus by up 5 and 2%, respectively. At low temperature, the incorporation of the nanoparticles improved the fatigue strength of the laminates by 15%. The shear strength results obtained by interlaminar shear strength and compression shear test tests have shown an increase of about 16 and 58%, respectively, by the introduction of nanotubes into the laminates. Fractographic observations revealed that the surface of carbon nanotube laminate (PEI/MWCNT/CF) presented a ductile behavior, and differences in the fracture aspects of the material compared to the traditional PEI/CF laminate have been observed.