Polytetrafluoroethylene (PTFE) fiber reinforced polyetheretherketone (PEEK) composites

doi:10.1016/j.wear.2010.12.003

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Volume 270, Issues 11–12, 5 May 2011, Pages 737-741

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Abstract

This work uses high tenacity expanded polytetrafluoroethylene (ePTFE) filaments as both a fiber reinforcement and a reservoir for solid lubricants. The goal is to reduce the wear of the composites by regulating the PTFE transfer. Expanded PTFE films are a porous network of PTFE nodes and fibrils, while highly oriented ePTFE filaments are aligned crystalline fibers that are regarded as high-tenacity fibers that can be woven into threads or yarns. Reported yield strength of these filaments can exceed 500 MPa. The best performing composites were those that had filaments of ePTFE aligned normal to the counterface. The wear rates obtained from the inclusion of expanded PTFE filaments were better than conventional powder filled PTFE–PEEK composites reaching values as low as K = 7 × 10⁻⁸ mm³/N m and showed stable friction coefficients below μ = 0.125 for over 2 million cycles.

Research highlights

► This paper explores the effectiveness of the inclusion of expanded (ePTFE) filaments in PEEK. ► Inclusion of PTFE in PEEK dramatically reduces friction and the wear rate of the virgin materials. ► ePTFE filaments perpendicular to the sliding surface reduced the wear rate by an order of magnitude. ► The low wear of the ePTFE/PEEK samples were achieved by developing thin transfer films that utilize the highly oriented ePTFE as a lubricant reservoir. The transfer films were brown in color, thin, and contained carbon, oxygen, and fluorine at the surface. ► ePTFE fibers of 5 mm have sufficient surface area in contact to reinforce the matrix and resist pull out.

Introduction

Polymeric composites are frequently used in applications where traditional fluid lubrication cannot be used [1], [2], [3]. Solid lubrication is advantageous due to cleanliness, simplicity and the available range of operating temperatures but achieving a combination of low friction and wear rates remains a challenge. Composite materials offer designers the ability to tune properties and achieve steady, low wear rates while maintaining a low friction coefficient. Fiber reinforced polymers have shown improved wear rates as well as excellent structural properties [4], [5], [6]. The traditional design approach has been to use fibers for strengthening and filler particles for lubrication. Recently there has been some thought with carbon nanotubes to try and have both strengthening and lubrication from the same fibers [7], [8], [9].

Polyetheretherketone (PEEK) is a popular matrix material for tribological composites due to its strength and wear resistance. It is an injection moldable polymer with a high operating temperature and chemical resistance. McCook et al. [10] surveyed fillers in PEEK in various environments. Others have examined ceramic nanofillers and polytetrafluoroethylene (PTFE) in PEEK [11], [12], [13], [14], [15], [16], [17], [18], [19]. The inclusion of PTFE in PEEK yielded a reduced friction coefficient μ; Burris et al. found unfilled μ = 0.36 was improved to μ = 0.11 at 50 wt.% PTFE. Wear rates and friction coefficients of various PEEK composites are shown in Fig. 1.

PTFE has many advantageous properties, such as low friction, low outgassing, high temperature capability and high chemical inertness, which make it an ideal filler material for a wide variety of applications. Expanded PTFE is a fibrillated form of PTFE that has a porous network of PTFE fibrils connected to dense nodes of PTFE. Expanded PTFE films can have a variety of densities and shapes, with porosities ranging from 5 to 90%. Expanded PTFE provides increased strength to weight ratio and creep resistance compared to fully dense PTFE. The mesh-like materials have been used previously in tribology as a coating material with an epoxy matrix [20]. Interestingly, highly oriented PTFE filaments that are extracted from high tenacity expanded PTFE threads have been entirely overlooked as a filler material. These filaments have high crystallinity and strength while maintaining the desirable properties of dense PTFE. While previous work with expanded PTFE has been limited to coating applications; the goal of this work is to design a bulk composite using expanded PTFE filaments in a wear resistant matrix (PEEK). The hypothesis is to regulate the wear by preventing PTFE transfer and subsequently forcing the transfer films to be thin.

Section snippets

Materials

Polyetheretherketone (Victrex 450 PF PEEK) powder of average particle size 10 μm was used as the matrix material in this study. The PEEK was filled with high tenacity expanded polytetrafluoroethylene (ePTFE) thread (PlastomerTech Solar Thread) composed of three filaments of expanded PTFE with characteristic diameters of approximately 180 μm. Prior to processing, the threads were manually separated into individual filaments and trimmed to length (approximately 50 mm for most composites and 10 mm for

Experimental methods

The tribological properties of the samples were evaluated using a linear reciprocating tribometer described in Schmitz et al. [21]. The apparatus is contained in a soft-walled clean room at temperature of 20 °C and exposed to lab air of ∼25% humidity. The samples were mounted directly to a 6-channel load cell and run against a rectangular lapped (R_a = 150 nm) 304 stainless steel counterface mounted to the linear reciprocating stage. A new counterface was used for each test and cleaned with methanol

Results and discussion

Friction coefficients for unfilled PEEK was relatively high and noisy with average values of μ = 0.37. The addition of PTFE reduced the friction coefficient for all loading conditions. These data are plotted against filler loading in Fig. 2a. The addition of 8 vol.% aligned ePTFE filaments reduced this friction coefficient to an average of μ = 0.13 and 10 vol.% aligned filaments yielded a friction coefficient of μ = 0.11. The friction coefficient of the 10 vol.% aligned ePTFE filaments versus sliding

Conclusions

This paper explores the effectiveness of the inclusion of PTFE filaments in PEEK. The wear rates obtained from the ePTFE filled composites were better than conventional powder filled PTFE–PEEK composites. Further, ePTFE–PEEK composites saw little change to the sliding surface. The friction coefficients of the aligned expanded PTFE–PEEK composites were lower than the other composites tested, and were steady throughout testing.

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The inclusion of PTFE in PEEK dramatically reduces both the friction

Acknowledgements

The authors gratefully acknowledge Dr. Jerry Bourne and Jason Bares for their assistance with the SEM work on this project and Eric Lambers with the University of Florida Major Analytical and Instrumentation Center for the XPS work. This material is based upon an AFOSR-MURI grant FA9550-04-1-0367. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Air Force Office of Scientific Research.

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Polytetrafluoroethylene (PTFE) fiber reinforced polyetheretherketone (PEEK) composites

Abstract

Research highlights

Introduction

Section snippets

Materials

Experimental methods

Results and discussion

Conclusions

Acknowledgements

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Tribology International

Tribology International

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