Diffusion model for sword in sheath failure mode in vapour grown carbon fibers

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.