Role of surface treatment of carbon fibers on mechanical properties of carbon fiber-reinforced composites

The fiber-matrix interface has been recognized as one of the most crucial parameters affecting properties of continuous fiber reinforced composites due to large surface area of fiber reinforcement in contact with matrix. Many important phenomena leading to limitations of properties of composites may take place at the interface, depending on its structure and local stress field. Present study was carried out in order to obtain better understanding of the role of controllability of the properties of composites via fiber surface treatment and interface modification. Two surface modification methods have been developed and evaluated. These consisted of a chemical vapour deposition technique, used to build up a uniform carbonaceous layer of around 80 nm of pyrolytic carbon on the fiber surface and controlled growth of multiwall carbon nanotubes on fiber surface. The fiber surface modifications were achieved by exposing the fibers to suitable hydrocarbon environment at elevated temperatures. The surface modified fibers were used to fabricate composite specimens with either coal tar pitch or epoxy matrix. The surface modifications are expected to alter damage and microfracture mechanisms occurring at the interface in response to local stress field during loading. Shear properties being amongst the most sensitive to interface behaviour, short beam shear test and flexural tests were conducted on the composite samples made out of carbon fibers with and without surface treatments. A significant increase in the value of these parameters was observed in both the cases of surface modification. Composite specimens made out of PyC coated fibers showed an increase of 300% in flexural strength and 360% in shear strength while the composite specimens made out of CNT grown carbon fibers showed 74% increase in flexural strength and 18% increase in shear strength. Metallographic and fractographic examination showed that pyrolytic carbon layer reduced interface debonding between carbon matrix and carbon fibers. Presence of nanotubes on fiber surface appeared to effectively anchor the carbon fibers into the matrix and reduce fiber pullout during crack propagation and rupture.