Advanced Performance Materials 4, 305–315 (1997) c  1997 Kluwer Academic Publishers. Manufactured in The Netherlands. A Diffusion Model for Sword in Sheath Failure Mode in Vapour Grown Carbon Fibers A. MADRO ˜ NERO CENIM, Avda Gregorio del Amo 8, 28040 Madrid, Spain M. VERD ´ U CIDA, Arturo Soria 289, 28033 Madrid, Spain L. FROYEN Dept. Metaalkunde, Katolieke Universiteit Leuven, 3001 Heverlee, Belgium M. DOM ´ INGUEZ Instituto del Fr´ ıo, Ciudad Universitaria, 28040 Madrid, Spain Abstract. A peculiar failure mode named “sword in sheath” was reported in previous studies on Vapour Grown Carbon Fibers (VGCFs). It consists in a gliding between the VGCFs catalytic hard core and pyrolitic soft coating. This is a very interesting failure mode, looking to the additional work of fracture that it supposes. After knowing that the main difference between the hard core and the soften cortical carbon is the hydrogen content, some attempts of increase the hydrogen content in the cortical phase are made at the present work via annealing in a pure hydrogen atmosphere. A mathematical model is set up to describe the different phenomena that take place during the annealing. The hydrogen comes to the cortical carbon from the atmosphere and from the inner core. As a consequence of the very significant gradient of hydrogen concentration in the surrounding of the interface core—cortical phase, a shear mechanism is possible under tensile stress. For this reason sword in sheath takes place, mainly in not very thick fibers. Keywords: carbon fibers failure mode, diffusion coefficient of hydrogen in carbon fibers, hydrogen diffusion in vapour grown carbon fibers, hydrogenation of carbon fibers, vapour grown carbon fibers hardening 1. Introduction Vapour grown carbon fibers (VGCF) are short carbon fibers grown from a hydrocarbon gas. They are a very promising reinforcer for composite materials parts due to their low cost, as much as 10 $/kg [1], which covers recent demand for carbon fiber for use in uses different from aerospace and aeronautics [2]. So, the updating of the knowledge about VGCF peculiarities could be very interesting, in order to optimize their behavior under hard working conditions, just as the ones that automotive parts will face in the future. A peculiar aspect of this sort of fibers is that tensile stresses can produce in them a type of failure mode known as sword in sheath [3] due to the fact that their duplex structure is composed of a hard core and a softer cortical layer, with not a very continuous contact between both phases [4].