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 during composite manufacturing. These coatings have been obtained by low pressure chemical vapour deposition on T800H carbon fibre yarns. In spite of a 20% increase in the coated monofilaments tensile strength observed for very thin C-pyr coatings, a strength decrease as a function of the coating thickness 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 induced by a cracked brittle layer. Concerning the C-pyr coatings, a lower damaging effect is observed as compared to TiB2, it may be explained by the difference between the Young's moduli of both layers. Although the initial fibre properties are not perfectly maintained in the case of the double layer, 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 strength increase. (C) 1998 Kluwer Academic Publishers.