THE EFFECT OF REACTION AND INFILTRATION ON THE DEGRADATION OF REFRACTORY CERAMICS BY MOLTEN METALS

The degradation of refractory ceramics, used to contain and thermally insulate molten aluminum, in many aluminum processing operations, decreases the efficiency of aluminum production. The decreased efficiency results from the erosion of the refractory ceramic by molten aluminum, which increases thermal losses, and, eventually leads to the stoppage of production when the refractory is replaced.There are two primary modes of refractory degradation; chemical reaction and infiltration. Chemical reaction leads to a steady decrease in the amount of refractory material. To study this phenomenon, tests were performed in which ceramic substrates were immersed in molten metals and the subsequent microstructural evolution was analyzed. Using thermodynamic, kinetic, and mechanical considerations, the critical factors which govern the rate of reaction were identified. From these investigations, a model was developed to determine the equilibrium interfacial composition, and the rate of oxide dissolution. Also, the importance of the Pilling-Bedworth ratio, the ratio of reaction product to reactant molar volume, in the development of refractory ceramics which are resistant to chemical attack was demonstrated. Molten metal infiltration into the cracks and pores in refractory ceramic materials is the second mode of degradation. The infiltration of cracks with metal can lead to further cracking upon thermal cycling, and, if widespread infiltration occurs, the rate of degradation can be rapid. Two experimental methodologies were used to investigate the tendency for molten metals to infiltrate cracks. First, the classic sessile drop method was used to investigate several model metal-oxide systems, to determine the factors which affect the wetting and infiltration behavior of molten metals. Using these results, several parameters which can affect infiltration behavior were identified. Secondly, a new experimental method was developed, the dynamic capillary displacement method, to directly study the infiltration behavior of molten aluminum in refractory ceramics. The contact angles, calculated from the infiltration behavior of aluminum into alumina capillaries, are in good agreement with the contact angles reported in the literature, determined by other methodologies. Combining the investigation of the chemical reactions with the infiltration investigation, a framework was developed for the mechanisms by which molten metals degrade ceramics.