SILICON NITRIDE AT VERY HIGH TEMPERATURES: BUBBLE AND PIT FORMATION

摘要

From oxidation studies of sintered ceramics and pure Si_3N_4 and phase equilibrium studies it is concluded that the bubble formation in scales and the bloating of powder compacts have a common cause. This common cause is thought to be the formation of ,,native oxides", which are metastable amorphous solids of the type SiN_xO_y. As the eutectic temperature of the system is approached they recrystallise or decompose. The latter case leads to the bubbling/bloating. In pure Si_3N_4 this is in the vicinity of 1600 deg C at atmospheric pressure and defines the upper HT-limit of use. Pit formation seems to require bubble formation but not vice versa. Pits probably form at surface defects. They interact with microcracks forming during mechanical loading in oxidising atmospheres at high temperatures.