HOT HYDROGEN EXPOSURE DEGRADATION OF THE STRENGTH OF MULLITE

Near-stoichiometric mullite (3Al(2)O(3) . 2SiO(2)) that contained smal l amounts of calcium and magnesium was exposed to pure dry hydrogen ga s at elevated temperatures, Exposure temperatures were 1050 degrees an d 1250 degrees C, and exposure times were up to 500 h, Preferential at tack of the aluminosilicate glass that was present it the grain bounda ries of the mullite occurred after 125 h at 1250 degrees C. Hydrogen s crubbing off the silica from the glassy grain boundaries and the mulli te grains yielded a porous alumina-rich surface. The room-temperature strength increased after short exposure times at 1250 degrees C (up to 125 h) and then decreased by 53% after exposure for 500 h. At 1050 de grees C, all exposure times (25-500 h) decreased the strength. The roo m-temperature strength of mullite decreased 22% after 500 h in hydroge n at 1050 degrees C, We also observed a rapid 25% strength loss after short exposure times at 1050 degrees C, which was attributed to the ca lcium/hydrogen-assisted crystallization of the glassy grain-boundary p hase.