Transmission electron microscopy investigation of the oxidation of hot isostatically pressed silicon nitride with and without sintering aids

Analytical transmission electron microscopy of thin-foil cross sections has been used to examine the oxidation behavior of hot isostatically pressed silicon nitride (Si3N4) materials. The transmission electron microscopy (TEM) cross sections are prepared by a special technique that provides electron transparency through the entire oxide, interfacial, subscalar, and matrix regions simultaneously. The materials are oxidized in an alumina furnace at 1250°C for 100 h. TEM investigation indicates that oxidation of Si3N4 occurs in an oxidation reaction zone that is comprised of the scale, oxide/matrix interface, and subscalar regions; therefore, the silica (SiO2)/Si3N4 interfacial surface area that is available for oxidation is very large. The oxidative attack on the Si3N4 grains is not uniform or sequential, and oxygen diffuses into the matrix before the surface grains are consumed. Gas bubbles, probably nitrogen gas, accumulate at all levels of the scale, and no evidence is found for the existence of an “oxynitride” layer. Disintegration of the secondary phase, Y2Si2O7, in the subscalar region is observed to occur, indicating that secondary, oxidation-related phenomena are occurring.