Hot Hydrogen Exposure Degradation of the Strength of Mullite

Near‐stoichiometric mullite (3Al 2 O 3 2SiO 2 ) that contained small amounts of calcium and magnesium was exposed to pure dry hydrogen gas at elevated temperatures. Exposure temperatures were 1050° and 1250°C, and exposure times were up to 500 h. Preferential attack of the aluminosilicate glass that was present in the grain boundaries of the mullite occurred after 125 h at 1250°C. Hydrogen scrubbing of the silica from the glassy grain boundaries and the mullite grains yielded a porous alumina‐rich surface. The room‐temperature strength increased after short exposure times at 1250°C (up to 125 h) and then decreased by 53% after exposure for 500 h. At 1050°C, all exposure times (25‐500 h) decreased the strength. The room‐temperature strength of mullite decreased 22% after 500 h in hydrogen at 1050°C. We also observed a rapid 25% strength loss after short exposure times at 1050°C, which was attributed to the calcium/hydrogen‐assisted crystallization of the glassy grain‐boundary phase.