"The forensic study of single fibre pull-out specimens using ToF-SIMS"

Arran R Wood

UniS Materials Institute and School of Engineering, University of Surrey, Guildford, Surrey, GU2 7XH, UK

a.wood@surrey.ac.uk

Time-of-flight secondary-ion mass spectrometry (ToF-SIMS) is a highly surface and chemical specific technique that has been demonstrated to be a powerful tool in characterising fibre reinforced composite (FRC) fracture surfaces1. Its use in the fractographic studies of FRC materials enables the precise locus of failure to be determined as interfacial, between fibre and matrix; cohesive, within the matrix; or even within the fibre-matrix interphase. Very thin polymeric layers can be detected increasing the chemical knowledge of a failure to complement optical and mechanical observations.

The single fibre pull-out (SFPO) test has been used to investigate at a fundamental level the interfacial interaction between a glass fibre and a polyester matrix system2. However, the mechanical data alone cannot explain the mechanisms of failure.

The present work employs ToF-SIMS imaging for the forensic examination of fibre surfaces following a SFPO test. Regions of interest have been selected for retrospective analysis. Multivariate techniques have been shown to vastly improve interpretation of SIMS images by statistically correlating multiple peaks, characteristic of particular compounds, to image regions3. One such technique, principle component analysis (PCA), has been utilised to assist the identification of regions of chemical similarity, enabling better image segmentation and removal of topographic effects4.

Results will be presented proposing a failure model based upon these complementary analytical techniques. Negative ToF-SIMS images have revealed a distinct difference in the surface chemistry at the fibre tip compared to the bulk of the “pulled out” region approximately 250µm in length. Further exploration of this area using higher spatial resolution in the positive mode reveals exposed glass stripped of the polymeric size.

This investigation has utilised ToF-SIMS imaging and retrospective analyses to complement mechanical data to better explain the failure mechanism of an SFPO specimen. Regions of stripped size revealing bare glass have been identified at the fibre tip.

Future work will concentrate on the development of an automated fracture stage for in-situ SFPO testing within the preparation chamber of the ToF-SIMS instrument. This will remove the risk of contamination, principally from test machine lubrication present in the atmosphere of failure, and fracture debris from previously tested specimens, and will provide higher secondary ion yields for enhanced spectra interpretation5-7. The automated stage will permit quantifiable mechanical data to be collected while also generating contaminant free failure surfaces.

[1] P. Denison et al, Scanning secondary ion mass spectroscopic studies of the micromechanical and chemical structure in the region of the interface in carbon fibre-epoxy composites, J. Mater. Sci., 1988, Vol. 23, p2153-2156

[2] A.R. Wood et al, Investigation of the interfacial properties between glass fibre and nano-particle modified polymer matrices, Proceedings 29th Annual Meeting of The Adhesion Society, Inc., 2006, p229-23

[3] B. Tyler, in: J. C. Vickerman, D. Briggs (Eds.), ToF-SIMS: Surface Analysis by Mass Spectrometry, Surface Spectra Limited, 2001, p475

[4] B. Tyler, Interpretation of ToF-SIMS images: multivariate and univariate approaches to image de-noising, image segmentation and compound identification, Applied Surface Science, 2003, 203-204, p825-831

[5] P. E. Vickers, PhD Thesis, University of Surrey, 1998

[6] A. C. Prickett et al, ToF-SIMS studies of carbon-fibre composite fracture surfaces and the development of controlled Mode in-situ fracture, Surf. Interface Anal., 2001, Vol. 31, p11-17 7A. C. Prickett, PhD Thesis, University of Surrey, 2001