INFLUENCE OF MATRIX AND INTERFACE ON THE MECHANICAL-PROPERTIES OF UNIDIRECTIONAL CARBON-CARBON COMPOSITES

The mechanical properties and microstructure of unidirectional carbon/carbon (UD C/C) were investigated. The strength of one type of UD C/C, produced with an intermediate modulus fibre treated to four different levels. of an oxidative surface treatment, was determined after each step of the production cycle (of loose, impregnated and carbonized impregnated fibre bundles). The impregnated bundle had a strength 1.9-4.3 times the strength of the loose bundle, whereas after carbonization the strength of the bundle dropped below the strength of the loose bundle. It is suggested that this is mainly caused by the formation of defects in the fibres due to the shrinkage process during carbonization. These defects are larger if a good fibre/matrix bond strength in the green material exists. The possibility that the low strength of carbon/carbon could be caused by stress-concentration effects was excluded with the aid of TEM investigations. They showed that the carbon matrix mainly consisted-of vitreous carbon, the modulus of which (E = 35 GPa) does not become effective due to micro- and macrocracks.