On the effects of surface treatments on the mechanical strength of carbon fibres

TiB2, pyrolytic carbon (C-pyr) and C-pyr + TiB2 double layer coatings have been envisaged as potential protective coatings to prevent the deleterious interface reactions between the carbon fibre and the aluminium matrix durin g composite manufacturing. These coatings have been obtained by low pressur e chemical vapour deposition on T800H carbon fibre yarns. In spite of a 20% increase in the coated monofilaments tensile strength observed for very th in C-pyr coatings, a strength decrease as a function of the coating thickne ss is recorded for single C-pyr or TiB2 layers. For the TiB2 coatings, this decrease fits well with Ochiai's model which simulates the notch effect in duced by a cracked brittle layer. Concerning the C-pyr coatings, a lower da maging effect is observed as compared to TiB2, it may be explained by the d ifference between the Young's moduli of both layers. Although the initial f ibre properties are not perfectly maintained in the case of the double laye r, the crack deviation role of the C-pyr coating succeeds in preserving the coated fibres from the TiB2 notch effect, thus leading to a noticeable str ength increase. (C) 1998 Kluwer Academic Publishers.