Microfriction of various phases in a carbon fiber, polytetrafluoroethylene, graphite and polyetheretherketone composite blend as measured by atomic force microscopy

Abstract

Atomic force microscopy and friction force microscopy have been used to conduct microfriction studies on short carbon fiber reinforced PEEK/PTFE composite blends. The relative micro-scale coefficients of friction of different filler particles (PTFE, CF in normal and anti-plane orientation, and graphite) have been compared with the matrix PEEK. The order of the nanoscopic coefficients of friction was: carbon fiber in normal orientation>carbon fiber in plane orientation>PEEK matrix>graphite flake>carbon fiber in parallel orientation>PTFE particle. Additional microhardness studies resulted in qualitative hardness comparisons of the various phases. Their order of the form: carbon fibers ≫ PEEK and graphite>PTFE also reflects the contribution of the various phases to the wear resistance of the composite blend. The latter can be estimated from the AFM-topography traces of the polished composite surfaces.

Introduction

Progress in the fundamental understanding of tribology is crucial for the design and engineering of mechanical components. Recent developments of new tools[1]provide the opportunity to study the phenomena of adhesion[2], friction[3]and lubrication[4]on the micrometer scale. One of these new methods is scanning force microscopy (SFM)[5]. The development of a modified scanning force microscope[3]gives researchers a novel device to probe the topography and tribological behaviour of micro- and nanostructures.

A few years after the invention of the scanning tunneling microscope (STM)[6], the atomic force microscope (AFM)[5]was developed. However, since its inception, a myriad of new operation modes have been developed which can measure, often simultaneously, various sample properties. Perhaps the most notable extension of AFM capabilities so far was the realization that the lateral force between the tip and the sample could also be measured. It was thus recognized that the atomic-scale origins of friction could be probed with this technique, usually described as friction force microscopy (FFM)[3]. For example, Mate et al.[3]measured the atomic friction of a tungsten tip sliding on a basal plane of a single grain of highly oriented pyrolytic graphite by a modified AFM; Kaneko et al.[7]and Miyamoto et al. [8]investigated the microscopic friction force on magnetic disk media and head sliders by a FFM; Germann et al.[9]measured the atomic scale friction of a diamond tip on diamond surfaces using an ultrahigh vacuum force microscope; and Overney and Meyer[10]studied the micro-scale friction on a four-layer Cd-arachidate LB film. In FFM, normal and lateral forces can be measured simultaneously with atomic-scale resolution. This emerging field, known as nanotribology11, 12, focuses on the microscopic understanding of tribological properties.

Thus far, FFM has been applied to layered compounds[10], organic materials[13], ionic crystals[14]and magnetic recording media[15]. Apart from the possibility of determining tribological properties, it has been demonstrated in manifold ways that FFM is a powerful tool for distinguishing between different materials. In the last few years, FFM has been performed in different environments, such as ambient air[3], controlled atmosphere[16], liquids[17]and in ultrahigh vacuum[14]. Various instrumental designs have been introduced to realize FFM, such as designs based on interferometric[3], capacitive[18]and beam-deflection methods[19]. Parallel to the experimental progress in the field of nanotribology, various theoretical attempts were made. However, on the microfriction properties of short carbon fiber reinforced PEEK/PTFE composite blends no report has been published so far. It was therefore the objective of this paper to study the micro-friction responses of the various phases in such a composite system, technically used for injection molded sliding elements and bearings.