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

The influence of the microstructure of the interfaces on the mechanical behaviour of unidirectional T800 carbon fibres/Al - 4.5 Mg matrix composites is investigated to optimize the composite performances. Three composites were selected. In two of them, a fibre coating was introduced 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 (approximate to 300 nm large) is formed by fibre-matrix reaction at the T800/Al interface. The composite is weak (450 MPa) and brittle. The C-p coating (approximate to 100 nm) exhibits a turbostratic structure similar to the fibre one. However, it does not react with the matrix. The composite is stronger (1400 MPa) and tough. The bilayered coating is made of a layer (approximate to 100 nm) of turbostratic carbon next to the 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 intermediate mechanical behaviour (740 MPa and very Little pull out). As a result, it appears that brittle interphases (coating or reaction layer) are detrimental to the mechanical behaviour of the composites. Their influence depends on their composition but also of their granulometry. The C-p coating has a beneficent effect on the mechanical behaviour because it prevents the formation of a brittle interphase and deceases the failure resistance at the interface.