Abstract
Composites investigated in this article owned the highest fracture resistance after pyrolysis at 650 °C. Above 700 °C the mechanical properties of the composites deteriorated as the matrix transformation and fibre crystallisation progressed. In this work, it was found that the crystallisation processes in fibres were affected by diffusion of pyrolysis gases through the fibre–matrix interface. Additional annealing in oxidative atmosphere distinctly changed the crystallisation processes, resulting in a slightly improved fracture toughness of the composites. Microstructural changes in the composite were investigated using SEM, TEM, and Mössbauer spectroscopy. Crystals of elemental iron (Fe0) were found in the fibres. Their formation can be explained by the reduction of ferrous (Fe2+) ions initially present in the fibres by pyrolysis gases (hydrogen, methane) released from the pyrolysing matrix. The BCC lattice of the Fe0 particles was clearly assigned by comparison of measured diffraction patterns with reference data as well as by the Mössbauer spectroscopy.
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