DIFFUSION MECHANISMS IN CHEMICAL VAPOR-DEPOSITED IRIDIUM COATED ON CHEMICAL VAPOR-DEPOSITED RHENIUM

Materials used for radiation-cooled rocket thrusters must be capable of surviving under extreme conditions of high temperatures and oxidizing environments. While combustion efficiency is maximized at high temperature, many refractory metals are unsuitable for thruster service due to rapid materials loss caused by volatile oxides. The Aerojet Propulsion Division has developed thruster chambers using chemical vapor-deposited (CVD) Re coated with CVD Ir on the inside surface which is exposed to hot combustion gases. Iridium serves as an oxidation barrier protecting the Re which maintains structural integrity at high temperatures. In order to predict and extend the performance limits of these Ir-coated Re thrusters, we are studying the diffusion kinetics of CVD materials at temperature. Thruster end ring sections were examined using electron microprobe analysis both before and after exposure to high-temperature vacuum environments. The resulting elemental maps for Re, Ir, and Mo in the near-surface region allow identification of diffusion mechanisms operating at these temperatures. Line scans for Ir and Re were fit using a diffusion model to extract relevant diffusion constants. The fastest diffusion process is seen to be grain-boundary diffusion, with Re diffusing down grain boundaries in the Ir overlayer. The measured dependence of the diffusion rate on temperature will allow prediction of operating lifetimes for these thrusters.