Oxidation Behavior of Rare‐Earth Disilicate–Silicon Nitride Ceramics

The oxidation behavior and microstructure of the oxidized surfaces of RE 2 Si 2 O 7 –Si 3 N 4 ceramics were investigated. The high oxidation resistance of these materials at 1400°C is attributed to the minimization of amorphous phases via devitrification to disilicates that are in equilibrium with SiO 2 , the oxidation product of Si 3 N 4 . Crystals of RE 2 Si 2 O 7 grew out of the surface silicate in prefered orientations that were dictated by crystal structure. The morphology of the microstructure of the oxidized surfaces was shown to be partially dependent on the concentration of impurities; the presence of Ca was found to coincide with the growth of Gd 2 Si 2 O 7 and Dy 2 Si 2 O 7 crystals with high aspect ratios.