Interphases and mechanical properties in carbon fibres/Al matrix composites

Abstract
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 (≈ 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 (≈ 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 (≈ 100 nm) of turbostratic carbon next to the fibre and of a layer (50-200 nm) of small (20 nm) β-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 decreases the failure resistance at the interface.