Highly improved mechanical and tribological properties in nanofiber composites

21 July 2010
Bin Li, Weston Wood, Loren Baker, Wei-Hong Zhong, Gang Sui, and Carla Leer
A facile melt-processing method enables uniform dispersion of carbon nanofibers in polyetherimide without any chemical or physical treatment.

High-performance thermoplastic polyetherimide (PEI) has great promise for aerospace materials and electronics subjected to extreme conditions because of its high strength and elastic modulus, excellent thermal stability, and superior electrical and dielectric properties. However, PEI poses two crucial challenges to practical applications: brittleness and poor wear resistance, which significantly affect the processing, performance, and service life of polymer materials.

Toughening of PEI has frequently been reported by blending it with other polymers or adding nanoscale particles to it. However, studies on modification of the wear properties of PEI are still limited to microfillers. The groups of J. Bijwe1 and A. P. Harsha2 have carried out comprehensive research on the tribological (friction and wear) properties of glass fibers, carbon fibers, and their fabric-reinforced PEI composites. In tribological applications, all of these composites required substantial amounts of microfiller—up to ~80vol% (volume percent) in some studies—which can result in the loss of critical properties of PEI, including low mass density and toughness.

We have been able to alleviate these problems by adding very small amounts of carbon nanofibers (CNFs) to the PEI matrix. CNFs not only have very high strength and modulus along the axial direction but also structural flexibility due to van der Waals forces (weak molecular attractions) between the stacked graphene layers: see Figure 1 (inset). CNFs have an attractive set of characteristics for industry because, compared with carbon nanotubes, they are relatively low cost and have high purity. The graphitic structure of CNFs also has potential for improving the tribological properties of polymer materials, for example, by acting as a lubricant, reinforcing transfer films, and breaking up CNFs.3


Scanning electron microscope image of the fracture surface of a polyetherimide/carbon nanofiber (PEI/CNF) composite, and (inset) a transmission electron microscope image of a CNF applied in this study. wt%: Weight percent.

We fabricated PEI/CNF composites using a facile melt-processing method on a twin-screw extruder at elevated temperature (370°C) in the melting zone. Long graphitic CNFs (Pyrograf III®) were used in their pristine form. Figure 1 shows extremely uniform dispersion of CNFs in the PEI matrix. We also applied additional techniques, including DC conductivity measurement of thin composite films, to verify the results.4 Our method of uniform dispersion of CNFs realized by direct-melt mixing processing meets the demands of chemical reduction programs that are increasingly being promoted in the industrial sector.

Figure 2 shows the results of three-point-bend flexural testing (American Society for Testing and Materials D790). The PEI/1.0wt% (weight percent) CNF (pristine) composite fractured in a ductile manner differs from pure PEI and exhibits dramatic improvement in mechanical properties. There is an ~54% increase in flexural strength and an ~550% increase in fracture toughness compared with pure PEI. We conclude that this remarkable improvement is mainly attributable to the effective CNF network formed by spatial entanglement of uniformly dispersed CNFs in the PEI matrix.4 At 3.0wt% CNF loading, excessively weak interfacial regions decrease the flexural properties, although effective CNF networks still form in the PEI.


Flexural stress-strain curves of PEI and PEI/CNF composites. Mpa: Megapascals.

We conducted a sliding wear test4 to evaluate material loss resistance of pure PEI and its composites. A low wear rate corresponds to good wear resistance. The addition of only 0.5wt% CNF effectively reduced the wear rate by ~56%. Furthermore, an approximate relationship between fracture toughness and wear rate was evident (see Figure 3). Obviously, materials with better toughness show a high wear rate because they are much more easily pulled out or scratched during sliding.


Steady-state wear rates of PEI/CNF composites.

With as little as 0.5wt% and 1.0wt% CNF loading, the wear resistance and fracture toughness of PEI/CNF composites show significant improvement over previous materials of this class, which could satisfy the long-term service requirements of PEI composites. However, the optimal balance between wear resistance and mechanical properties is still being explored. We are currently working on the surface modification of CNFs to improve interfacial bonding.


Authors

Bin Li
School of Mechanical and Materials Engineering Washington State University

Bin Li is a PhD candidate in mechanical engineering. He is working on fabrication and characterization of multifunctional PEI/CNF composites.

Weston Wood
School of Mechanical and Materials Engineering Washington State University

Wei-Hong Zhong is a professor. Since 2006 she has also been a consultant for Boeing Engineering in the field of nanotechnology. She has had over 160 publications on nanocomposites, bionanomaterials, electronic materials, and nanomanufacturing technology, including 112 peer-reviewed papers, 51 conference papers, four book chapters, and five patents.

Loren Baker
School of Mechanical and Materials Engineering Washington State University

Wei-Hong Zhong
School of Mechanical and Materials Engineering Washington State University

Gang Sui
Beijing University of Chemical Technology

Carla Leer
Applied Science, Inc.


References

  1. M. Sharma, I. M. Rao and J. Bijwe, Influence of orientation of long fibers in carbon fiber-polyetherimide composites on mechanical and tribological properties, Wear 267 (5–8), pp. 839-845, 2009.

  2. S. Arjula, A. P. Harsha and M. K. Ghosh, Erosive wear of unidirectional carbon fiber reinforced polyetherimide composite, Mater. Lett. 62 (17–18), pp. 3246-3249, 2008.

  3. G. Sui, W. H. Zhong, X. Ren, X. Q. Wang and X. P. Yang, Structure, mechanical properties, and friction behavior of UHMWPE/HDPE/carbon nanofibers, Mater. Chem. Phys. 115 (1), pp. 404-412, 2009.

  4. B. Li, W. Wood, L. Baker, G. Sui, C. Leer and W. H. Zhong, Effectual dispersion of carbon nanofibers in polyetherimide composites and their mechanical and tribological properties, Polym. Eng. Sci., published .

DOI:  10.2417/spepro.003053