Evaluation of Electrical, Mechanical and Wear Behavior of Laminated Epoxy/Carbon Fiber Composite with Different Fillers
Abstract
Epoxy resin reinforced with carbon fibers are widely used in applications where requirements of lightness and high mechanical strength are needed. Composites containing fillers with different properties can improve important characteristics of components. In aerospace applications, the use of low removal rate due to the impact of suspense particles must be satisfied in order to reduce maintenance costs of components. Since carbon fibers have low electrical resistivity and are in the range of semiconductors, these composites can be used as radar absorbing materials (RAM). The addition of conductive fillers may increase this intrinsic property of the composite, and also improve the mechanical and wear properties of the laminate. Results showed that the addition of micronized graphite (MG), multi-walled carbon nanotubes (MWCNTs) and short steel fibers (SSF) reduce the weight loss on abrasion test, but reduce values of flexural module (except for the addition of 5% of MG). In terms of electrical properties, fillers showed tendency to increase the electrical conductivity mainly for reduce fraction of fillers added, showing that higher contents can produce a non-homogeneous structure since that the mechanical and wear properties were affect in the same order.
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Introduction
Manufacturing techniques and development of composites have attracted increasing interest mainly due to its applications in the aerospace industry, such as military or civil aviation. Composites based on epoxy resin reinforced with directional carbon fiber had presented excellent corrosion and mechanical properties (mainly the high specific strength), allow greater freedom in order to obtain structural parts and are still promising to reduce maintenance and operation costs of aircraft. According to Librantz (2006) [1] the substitution of aluminum for polymer composites can result in a reduction of up to 30% on the final weight and 25% in acquisition costs of structural parts.
Use of fillers can improve mechanical strength and abrasive wear. A frequent question when reinforcing or adding fillers in composite materials is on the improvement of other interesting properties such as mechanical and wear abrasion strength, essential for building aircraft. Very pronounced effect on aircraft crashes due to dust particles, sand, ice that generate significant cost in terms of maintaining the areas exposed to these events. Although the carbon fibers possess sliding characteristics, their structure is different from carbon black or graphite. In this case, the incorporation of these materials on composite laminate can improve the wear characteristics around the failure mechanisms, like a fiber thinning, fiber-matrix debonding and fiber pull-out and low cycle fatigue [2]. The author also refer that the carbon fiber provides less wear resistance and higher coefficient of friction than glass fiber. According to Takeichi (2008)[3] the reinforcement with carbon black decreases linearly the friction coefficient of PTFE composites. Nanometric particles (around 25 to 38nm) were used in this work. The 10% mass of carbon black was the best concentration for the highest wear resistance.
Conclusion
Considering the effect of adding fillers for improving mechanical and electrical properties, the percentage of 5%wt of MG composite produced the best combined properties. In all three cases, the addition of high concentrations of filler caused a decrease in electrical conductivity and flexural modulus values, indicating a poor dispersion in the resin matrix and likely increasing the distance between the layers of carbon fiber, expected like most responsible for conduction. The percentage of MWCNTs evaluated at this work did not present a significant change in the electrical conductivity
Morphological analyzes showed that the increasing of filler fraction made the adhesion more difficult, resulting in problems of delamination. The fillers added cannot be seen by electron scanning microscopic, attributing a successfully mixture method, therefore the bundles of fillers were not verified on the cross section of cryogenic fractured specimens.
The wear strength for weight loss had similar results for the addition of MG and SSF, with a mean value of 0.005 g. With the addition of MWCNTs, occurred a larger loss of mass of the order of 0.030 g. The difference in the chemical structure between the graphite and MWCNTs may explain the changes on properties analyzed. The MG material has sliding characteristics and MWCNTs, with their crystalline structure, does not have functionally slip planes.