Carbon-fiber-reinforced polymer composites are attractive materials for applications requiring lightweight, strong components, such as aviation and automotive. However, their performance has been partially limited by the bonds between the polymer matrix and the embedded carbon fibers. Now, researchers at Oak Ridge National Laboratory (ORNL; Oak Ridge, Tenn.; www.ornl.gov) have devised a way to enhance the mechanical properties of this class of composite materials by engineering the adhesion of the fiber-matrix interfacial bond.
The ORNL approach combines both mechanical and chemical bonding to yield a 50% improvement in tensile strength and a nearly two-fold increase in toughness through use of carefully tailored nanofibers. ORNL project leader Sumit Gupta said, “we found that a hybrid technique using carbon nanofibers to create chemical and mechanical bonding yields excellent results.” Specifically, the ORNL researchers designed a “physicochemical scaffold that incorporates microscopically architected chemically reactive nanofibers.” These reactive nanofibers act as a multiscale bridge between the carbon fibers and the matrix, the researchers wrote.
A carbon fiber is prepared for mounting in a device. Credit: Carlos Jones/ORNL, U.S. Dept. of Energy
The key to the improvements is an innovative technique known as electrospinning, in which a carbon-fiber precursor (polyacrylonitrile) is extruded into fibers in the presence of a strong electric field to produce strands about 200-nm wide. The strands land on a spinning metal drum overwrapped with carbon-fiber fabric.
By varying the strength of the electric field, the speed of the drum and other factors, the researchers can create fibers that chemically bond to the matrix and mechanically bond to other carbon fibers, essentially creating “bridges” between the two dissimilar materials.
“By meticulously controlling the nanofiber architecture through variable surface area, functional-group availability and polymer-chain alignment effects, the extent of covalent bonding between nanofibers and the matrix is manipulated,” the researchers wrote, “ultimately resulting in improved carbon fiber-matrix adhesion.”
The ORNL concept was validated using polyacrylonitrile nanofibers within an acrylonitrile butadiene styrene (ABS) matrix in a discontinuous carbon-fiber-reinforced composite system. The resulting composites demonstrated ~56% and ~175% improvements in tensile strength and toughness, respectively, compared to composites without nanofiber.
The versatility and efficacy of the approach have the potential to address longstanding interphase challenges in the composites industry. The research team will seek industrial partners to license the approach.