Transmission electron microscopy of intergranular regions of corroded silicon nitride

Two hot-pressed, dense, silicon nitride materials have been exposed to the flue gases of an aluminium smelting furnace heavily contaminated with Al, Ca and K-containing dust and vapours. Transmission electron microscopy (TEM) has been used to examine microstructural changes occurring in the silicon nitride at the interface between corroded and uncorroded material. Extensive diffusion of contaminant metal into the intergranular glass of the silicon nitride occurs, associated with physical and chemical changes in the intergranular regions, and a marked loss of mechanical strength.