MICROSTRUCTURE OF METAL-MATRIX COMPOSITES REINFORCED BY CERAMIC MICROBALLOONS

Metal-matrix composites reinforced by ceramic hollow microspheres were produced as special porous metals, called metal-matrix syntactic foams (MMSFs). In this paper the microstructure of the ceramic hollow microspheres as reinforcing elements is investigated in connection with the production of MMSFs with the pressure infiltration. SL150- and SL300-type ceramic microspheres from Envirospheres Ltd (Australia) were investigated. They contained various oxide ceramics, mainly Al2O3 and SiO2. The chemical composition and the microstructure of the microspheres had a strong effect on their infiltration characteristics; therefore, in the view of the MMSF production it was very important to know microstructural details about the microspheres. Due to this energy-dispersive X-ray spectroscopy, maps were recorded from the cross-sections of the microspheres' walls. The results showed that the Al2O3 and SiO2 distribution were not equal; the Al2O3 phase was embedded in the surrounding mullite, while the SiO2 phase occurred in the form of needles. Line energy-dispersive X-ray spectroscopy measurements were performed in order to investigate the possible reaction between different aluminium alloy matrices and the ceramic microspheres. The results showed that, due to an uneven distribution of Al2O3, rich particles of the molten aluminium could reduce the SiO2 rich parts of the microspheres and the walls of the hollow microspheres became damaged and degraded. This chemical reaction between the microspheres and the walls could make the infiltration easier, but the resulting mechanical properties will be reduced due to the damaged microsphere walls.