A model for inert strength reduction in carbon-coated optical fibers

Abstract

The inert strength of carbon-coated optical fibers (hermetic fibers) has been observed to be less than that of standard polymer coated optical fibers. A scaled version of the of the carbon-coated optical fiber was developed in the laboratory and used in an experimental investigation of the mechanical properties of this system. The microstructure and phase similarity between the scaled system and the optical fiber was established using Raman spectroscopy. The mechanical properties, residual stress in the film and the fracture toughness of the scaled system were determined using nano-indentation. A fracture mechanics model was developed to explain the mechanism of this strength reduction. The model, based on the cracking of thin films in residual tension, will be used to predict growth of flaws from the carbon film and penetrating into the substrate. The model can be applied to all brittle coatings where delamination of the coating is not observed. Conditions under which cracks in the carbon film propagate into the substrate were investigated using a recently developed superposition scheme. Possible methods of crack arrest will be discussed.