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.