The influence of surface microstructure on the scratch characteristics of Kevlar fibers
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The influence of surface microstructure on the scratch characteristics of Kevlar fibers Quinn P. McAllister • John W. Gillespie Jr. Mark R. VanLandingham
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Received: 9 July 2012 / Accepted: 5 September 2012 / Published online: 25 September 2012 Ó Springer Science+Business Media, LLC 2012
Abstract In this work, nanoindentation and nanoscratching experiments are combined with atomic force microscopy to investigate the relationships between contact geometry, apparent friction, and deformation modes of two grades of KevlarÒ (Dupont) fiber—Kevlar KM2 and Kevlar 49. Changes in the relative angle between the scratching probe and the fiber surface, often termed as the attack angle, result in changes in deformation mode, which correlate with the changes in the apparent friction. As attack angle increases, the observed deformation modes of the fiber surface change from a smoothing of the surface, often termed as ironing, to fibrillation, in which the fibrils break and coalesce in front of the progressing probe. A mixture of these two modes occurs at intermediate attack angles. When fibrillation occurs, material pile-up forms in front of the progressing probe. This pile-up introduces an additional component to the frictional response that is largely responsible for an increase in apparent friction with an increasing attack angle and/or scratch length. The level of friction associated with fibrillation is measured to be up to approximately three times higher than previously reported for Kevlar yarn–yarn friction. Fibrillation of
Q. P. McAllister J. W. Gillespie Jr. (&) Department of Materials Science and Engineering, Center for Composite Materials, University of Delaware, Newark, DE 19716, USA e-mail: [email protected] Q. P. McAllister e-mail: [email protected] M. R. VanLandingham Materials and Manufacturing Sciences Division, Weapons & Materials Research Directorate, U.S. Army Research Laboratory, ATTN: RDRL-WMM-B, Aberdeen Proving Ground, Aberdeen, MD 21005-5069, USA
Kevlar KM2 occurs at larger attack angles as compared to Kevlar 49, which is believed to be related to a near-surface region of reduced modulus and hardness previously observed in KM2 fibers. A detailed discussion of the measured response is given based on the interactions between the scratching probe and the fibrillar network and the resulting deformation mechanisms.
Introduction Woven fabrics consisting of high performance polymeric fibers, such as poly(p-phenylene terephthalamide) or PPTA (e.g., KevlarÒ), are used to create a variety of impact resistant materials. Fabric impact performance has been shown to be limited by windowing, or spreading apart of principal yarns during projectile penetration [1, 2]. The degree of windowing can be restricted by increasing the inter-fiber friction [1, 3]. Recent developments in textile composite systems have also shown a reduction in fabric windowing through incorporation of nanosized particulates [4–6]. Microscopy evidence from these studies suggests that the nanosized particulates effectively increase the inter-fib
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