Improved fracture toughness of carbon fiber composite functionalized with multi walled carbon nanotubes

Woven carbon fiber (CF) laminae are functionalized in situ with carbon nanotubes (CNTs) to test the hypothesis that growing CNTs on CF (i.e. bundles or tow) would enhance the properties of polymeric carbon composites, specifically epoxy/carbon composites that are used in aerospace applications. The CNT as-grown on the woven CF were shown to substantially improve the fracture toughness of the cured composite on the order of 50%. This was accompanied by no loss in structural stiffness of the final composite structure. In fact, the flexural modulus increased approximately 5%. The significant increase in the fracture toughness as tested under the ASTM D 5528 standard indicates that the damage tolerance of a composite structure would benefit from the CNT material applied in this way. Our approach has allowed for significantly larger samples to be uniformly functionalized with CNTs than is reported elsewhere in the open literature. In addition, this work demonstrated CNT functionalization on flexible substrates that remains flexible after functionalization, whereas most CNT growth substrates are rigid in order to withstand the high (>800°C) growth temperatures often encountered in CNT synthesis. * Georgia Tech Research Institute, Atlanta, GA 30332-0826; Email: jud.ready@gatech.edu; Fax: 404-407-9036 2 1. Literature/Background: Over the past decade and a half there has been extensive literature on the unique properties of carbon nanotubes (CNTs) including the ability to possess a very high aspect ratio while maintaining extraordinary strength and stiffness [1, 2]. Previous research has shown that mechanically, CNTs are 100 times stronger than steel but their density is six times lower [3, 4]. CNTs also have a high thermal conductivity of approximately 6000 W/m•K, implying that only a small amount of CNTs is required to significantly alter the properties of a material [5]. CNTs have displayed thermal stability up to 2800°C under vacuum conditions while maintaining an electrical conductivity several times higher than copper [4]. Because of these unique properties, both multi walled CNTs (MWCNT) and single walled CNTs (SWCNT) have numerous potential applications. One area of interest is CNT applicability in the reinforcement of woven carbon fiber (CF) laminar composite systems. CF is an optimal substrate for CNT reinforcement because it is strong, light weight, and can withstand the extreme temperatures used in growing CNTs [6]. These characteristics make these two materials excellent for lightweight reinforcement for numerous applications ranging from aerospace to performance automotive. Traditionally these systems have excellent in-plane two-dimensional (2D) properties but poor interlaminar properties [7]. This work will investigate if the interlaminar properties could be improved by introduction of a three-dimensional (3D) composite system using CNTs, particularly if the CNTs were grown perpendicular to the fiber to improve bonding between the lamina. Most implementations of reinforced CF composite systems explore CNT as a means of reinforcement by mixing the pre-made CNT into the polymer matrix [8]. This method used a high-intensity ultrasonic liquid processor to mix MWCNT with an epoxy resin before applying it