Assessment and Control of Surface Reactions of Carbon Fibres in Light Weight Metal Matrix Composites

The high potential of continuous carbon fibers for strength reinforcement in aluminium based light weight matrices is seriously impaired by brittle Al sub 4 C sub 3 -crystals that are formed at the interface between the fibers and the liquid matrix during infiltration. To suppress fiber degradation three measures have been identified and evaluated: minimization of infiltration time and temperature in order to thermodynamically attain low carbide formation rates, replacement of the Al-matrix by a Mg-based matrix, with Al-additions to control the amount of interface carbides, diminuition of the surface reactivity of carbon fibers by deposition of phosphorus on the surface. By application of optimized processing parameters (5s and 670 deg C for duration and temperature of infiltration) to a Mg-matrix containing 6-9 wt.% Al ROM strength values of approx1900 MPa were achieved. A low cost phosphorus fiber pre-treatment shifted the onset for fiber oxidation from 530 deg C to > 700 deg C, indicating also a higher resistivity against Al sub 4 C sub 3 -formation.