INTERPHASES AND MECHANICAL-PROPERTIES IN CARBON-FIBERS AL MATRIX COMPOSITES

The influence of the microstructure of the interfaces on the mechanica l behaviour of unidirectional T800 carbon fibres/Al - 4.5 Mg matrix co mposites is investigated to optimize the composite performances. Three composites were selected. In two of them, a fibre coating was introdu ced by LPCVD. The coating was composed of pyrolytic carbon (C-p) or of a C-p/SiC bilayer. As already known, a brittle Al4C3 interphase (appr oximate to 300 nm large) is formed by fibre-matrix reaction at the T80 0/Al interface. The composite is weak (450 MPa) and brittle. The C-p c oating (approximate to 100 nm) exhibits a turbostratic structure simil ar to the fibre one. However, it does not react with the matrix. The c omposite is stronger (1400 MPa) and tough. The bilayered coating is ma de of a layer (approximate to 100 nm) of turbostratic carbon next to t he fibre and of a layer (50-200 nm) of small (20 nm) beta-SiC grains. Some reaction occurs between the SiC coating and the matrix resulting in magnesium silicide (Mg2Si) and Al4C3. The composite exhibits an int ermediate mechanical behaviour (740 MPa and very Little pull out). As a result, it appears that brittle interphases (coating or reaction lay er) are detrimental to the mechanical behaviour of the composites. The ir influence depends on their composition but also of their granulomet ry. The C-p coating has a beneficent effect on the mechanical behaviou r because it prevents the formation of a brittle interphase and deceas es the failure resistance at the interface.