Paralinear Oxidation of Silicon Nitride in a Water‐Vapor/Oxygen Environment

Abstract

Three Si 3 N 4 materials were exposed to dry oxygen flowing at 0.44 cm/s at temperatures between 1200° and 1400°C. Weight change was measured using a continuously recording microbalance. Parabolic kinetics were observed. When the same materials were exposed to a 50% H 2 O–50% O 2 gas mixture flowing at 4.4 cm/s, all three types exhibited paralinear kinetics. The material was oxidized by water vapor to form solid SiO 2 . The protective SiO 2 was in turn volatilized by water vapor to form primarily gaseous Si(OH) 4 . Nonlinear least‐squares analysis and a paralinear kinetic model were used to determine parabolic and linear rate constants from the kinetic data. Volatilization of the protective SiO 2 scale could result in accelerated consumption of Si 3 N 4 . Recession rates under conditions more representative of actual combustors were compared with the furnace data.