Graphite flake-carbon composites. II: Fracture behaviour, toughness, notch insensitivity and Weibull modulus

This paper describes the development of tough, high reliability sintered carbon materials by the incorporation of graphite flakes into a sintered mesophase-based carbon matrix, a kind of carbon-carbon composite. The bending strengths are lower than those of the matrix alone because of the presence of misaligned flakes at the surface. However, multiple fracture and high work of fracture can be obtained depending upon the size, volume fraction and orientation of the graphite flakes. The fracture toughness is increased by factors of 2-3, due to energy dissipating mechanisms such as crack deflection, crack bridging and pull-out, all caused by the presence of the flakes in the composite. The reliability of the composites in terms of Weibull modulus is also increased by a factor of 2-3, because the defect population is effectively controlled by the flakes, which are mostly un-bonded to the matrix. A qualitative model is proposed to account for the correlation between failure probability, strength and microstructure. (c) 2007 Elsevier Ltd. All rights reserved.