Mechanical & Tribological Properties of Carbon Fiber Composite

Carbon fiber composite is composed of extremely strong carbon fibers bound by polymers, such as resin, epoxy or others. Its high strength-to-weight ratio and rigidity make carbon fiber composite an ideal material for a variety of industries, such as aerospace, automotive and civil engineering, sports goods and other consumer and technical applications. For example, its usage has substantially increased in the most advanced aircraft: the Airbus A350 XWB is built of 53% carbon fiber composite, including wing and fuselage components, and the Boeing 787 Dreamliner of 50%. It is also used extensively in high-end automobile racing and supercars.

Unlike isotropic materials such as steel and aluminum, carbon fiber composite often has directional strength properties due to the texture layouts of the carbon fibers. Therefore, the local mechanical and tribological properties of the carbon fiber composite vary depending on the direction of the carbon fibers and the proportion of the carbon fibers relative to the polymer matrix at the test location. A reliable and accurate evaluation protocol for the local mechanical and tribological properties becomes vital for developing and comparing carbon fiber composites in quality control and research and development.

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A high-strength lightweight carbon fiber sheet is used for the comprehensive mechanical and tribological evaluation.

The local hardness and Young’s modulus were measured using the Micro Mode of the NANOVEA Mechanical Tester. The test conditions are shown in Table 1.

The scratch resistance of the carbon fiber sheet was tested using the Micro Scratch Mode with a diamond stylus (200 µm radius). Images of the whole scratch (panorama) were automatically generated, and the different areas of the scratch were correlated with the friction coefficient by the system software. The test parameters are listed in Table 2.

The wear behavior of the carbon fiber sheet was evaluated using the linear reciprocating setup of the NANOVEA Tribometer. The test conditions are listed in Table 3.

The hardness, H, and Young’s modulus, E, of the high-strength carbon fiber sheet were tested at different locations using a Knoop indenter in order to investigate the effect of directional carbon fiber on the strength enhancement, as shown in Fig. 2.

The load-displacement curves are shown in Fig. 3 and the test results are listed in Table 4. It can be observed that the carbon fiber composite exhibits different H and E values at different locations and carbon fiber orientations. The sharper Vickers tip created a deeper penetration at the same load compared to the Knoop tip. As shown in Fig. 4, the Knoop indenter across the fiber texture left a smaller indentation mark on the sample, primarily attributed to the strength and rigidity reinforcement of the carbon fiber underneath the indent. The resulting H and E values are 0.68 and 12.2 GPa, respectively. In comparison, the Knoop tip with the sharper edge along the carbon fiber texture penetrated deeper into the gap between fibers, leading to substantially decreased H and E values of 0.22 and 6.5 GPa, respectively. The carbon fibers are patterned and combined by an epoxy or resin matrix. The indentation on the matrix at the border of the pattern shows a large mark. The Vickers tip has a symmetric shape and thus less impact from the direction of the carbon fiber pattern, resulting in measured H and E values of 0.46 GPa and 7.5 GPa.

In this indentation study we show that the NANOVEA Mechanical Tester equipped with a Knoop tip is an ideal tool for analyzing the local mechanical properties of composite materials with directional strength reinforcement.

The scratch resistance of the carbon fiber composite on the texture and the matrix is compared using scratch tests at a constant load of 10 N, as shown in Fig. 5. The penetration depth varies along the scratch track: the scratch along the carbon fiber texture shows a width of ~0.18 mm, while that across the carbon fiber texture barely shows a sign of a shallow scratch. The scratch at the border of the pattern exhibits a deep groove, due to the relatively soft matrix. This demonstrates that the carbon fibers significantly enhance the scratch resistance of the test composite sample.

Such observation under the optical microscope is in agreement with the evolution of the coefficient of friction (COF) recorded in situ during the scratch tests. The closed-loop control of the load with a separate precision load cell ensures the consistency of the applied constant normal load. The COF exhibits a relatively constant value of ~0.15 when the stylus travels on the carbon fiber texture, and peak values above 0.25 as it digs into the groove at the border of the pattern. The scratch on the matrix along the pattern border shows a larger track and higher COF.

The wear tracks were compared using the microscope and 3D profilometer, as shown in Fig. 6 and Fig. 7, respectively. The NANOVEA 3D profilometer allows visual examination of the 3D wear tracks and direct accurate determination of the wear volume and depth, as summarized in Table 5. The wear test on the carbon fiber texture created a smaller wear track compared to the one on the matrix. Such a difference in wear resistance is attributed to the strength enhancement of the carbon fiber texture, which gives rise to improved mechanical properties, including high H and E and scratch resistance. The enhanced mechanical properties from the carbon fibers cut the wear rate by half compared to the polymer matrix.

In summary, carbon fibers play a critical role in the enhancement of mechanical and tribological properties in the composite material. It is important to be aware that the values obtained in the indentation and wear tests are highly dependent on the location and fiber pattern orientation of the composite material.

In this study, we show that the NANOVEA Mechanical Tester, Tribometer and 3D Profilometer complement each other by measuring different aspects of essential material properties. They can provide a complete mechanical and tribological evaluation of composite materials and allow users to get more insight into the physico-mechanical behaviors of materials under different applications.

The Nano, Micro or Macro modules of the NANOVEA Mechanical Tester all include ISO and ASTM compliant indentation, scratch and wear tester modes, providing the widest and most user friendly range of testing available in a single system. The NANOVEA Tribometer offers precise and repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes, with optional high temperature wear, lubrication and tribo-corrosion modules available in one pre-integrated system.