ELSEVIER zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA S1359435X(96)00005-X Composites Part A 27A (1996) 677-684 Copyright 0 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 1359-835X/96/$15.00 Atomic force microscopy with simultaneous a.c. conductivity contrast for the analysis of carbon fibre surfaces zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA H. Sturm” and E. Schulz Federal Institute for Materials Research and Testing (BAM), Unter den Eichen 87, 12205 Berlin, Germany A scanning force microscope (SFM) has been extended by an a.c. circuit, which measures the current between a metallized tip and the excited sample. Using the lock-in principle, the resulting current amplitude and phase shift information can be visualized by the image processing unit of the SFM. Like in scanning tunnelling microscopy, it is possible to achieve atomic resolution on highly oriented pyrolytic graphite (HOPG). Experiments were performed with four different samples: a carbon fibre embedded in poly(phenylene sulfide) matrix and pulled out, carbon fibres with two different plasma treatment times and an untreated reference fibre. A grey-scale threshold analysing procedure is discussed to evaluate the amount of bare and covered fibre surface. (Keywords: zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA carbon fibre; surface analysis; conductivity microscopy; scam&g force microscopy) INTRODUCTION In scanning force microscopy @FM)‘, the availability of localized measurements of electrical properties is a new investigation tool. With conductivity microscopy in the scanning force microscope @FM), a wide range of applications is opened up in materials research. The improved and extended SFM overcomes the limited applications of scanning tunnelling microscopy (STM) in the field of insulating materials and for samples with a heterogeneous electrical surface conductivity. Examples are insulating protective coatings on strip conductors, polymeric or oligomeric coupling agents on carbon fibres and conductive organic layers with local variations in electrical conductivity. The advantage of an analysis of the conductivity contrast on carbon fibres leads to the new possibility of a comparison of fibre surfaces before and after sizing. Additionally, the characterization of delaminated fibres from prepregs or samples from a single-fibre pull-out or indentation test can be performed. The investigation of carbon fibre-polymer adhesion can be done in three experimental steps. 1) Using conductivity microscopy in the SFM, coupling agents (which are usually electrically insulating) can be easily distinguished from the conductive carbon * To whom correspondence should be addressed 2) 3) fibre. Thus the quality and heterogeneity of the sizing or a surface activation by pyrolysis can be character- ized quantitatively before a prepreg is formed or a single fibre is embedded in the polymer matrix. Single fibres or prepregs can be tested mechanically on the nanoscale and these results can be correlated with those of experiments in step (1) for evaluating the efficiency of the surface treatment. After the pull-out of a carbon fibre from its polymeric matrix, the surface can be characterized in terms of the polymeric residues to describe the local fracture mechanism. With the described experimental modes, microscopic and macroscopic results can be compared with one another. In our case, scanning force and conductivity microscopy have become an essential tool for a better understanding of single-fibre pull-out or single-fibre indentation. The work presented here concentrates on experimental steps (1) and (3). In its present stage of development, an analysis of chemical composition with the SFM (e.g. via local measurements of the dielectric constant) is not possible and therefore time-of-flight secondary ion mass spectrometry (t.o.f.-s.i.m.s.)2 or related techniques may be used as a powerful tool to complete the SFM results. The aim of this work is the investigation of the interface and/or interphase between carbon fibres and polymers in composites. In this paper, first results are presented using the conductivity contrast analysis in the SFM. This enables detection of the surface covering on 677