Microstructures and strength of magnesia-graphite refractory composites

The relationships between fabrication variables, microstructure and selected properties of carbon bonded magnesia-graphite refractory composite materials have been investigated. A novel optical microscope method of characterizing the morphology of flake graphites was developed and used to determine distributions of length and thickness and average aspect ratios for the four graphite samples used in the study. The compaction behaviour of magnesia alone and in combination with the flake graphites has been studied in some detail and the microstructures of the products elucidated. It is shown that the amount of magnesia of small particle size plays a significant role in determining the graphite-graphite contact area in the structure. An irreversible volume expansion is observed on firing composites, the magnitude of which can be related to the microstructure and the graphite content. A phenolic resin binder restricts this expansion. It is shown that the carbon binder does not bond to the graphite phase and only weakly, if at all, to the magnesia. Consequently the strengths and moduli are low and show only a small variation with graphite type. The effect of adding graphite to carbon-bonded magnesia is to lower the strength slightly, but increasing the graphite content from 20-30% causes a small increase in strength. Increasing the amount of carbon bond from pitch has little effect on strength at levels of 5-15% whereas over the range 5-13% the resin binder has a more pronounced effect. The most significant factor affecting the strength and modulus of fired composites is the amount of silicon or aluminium, added as oxidation inhibitors, which react to form carbide and nitride phases. Finally, a brief study of slag penetration shows that this can be reduced by decreasing the amount of oxide fines in the composite because of the changes in microstructure that, result.