Diffusion mechanisms in chemical vapor-deposited iridium coated on chemical vapor-deposited rhenium
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W.M. CLIFT, D.R. BOEHME, K.F. McCARTY,
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. I.
INTRODUCTION
HIGH-temperature, radiation-cooled rocket thrusters present a significant challenge to the materials scientist. Materials for these thrusters must provide both mechanical strength and oxidation resistance at temperatures above the melting point of most metals. No single material has a suitable combination of properties, consequently the strategy of putting oxidation-resistant CVD coatings on materials with suitable mechanical properties has evolved. The Aerojet Propulsion Division has developed thrusters using CVD Re (melting point 3180 ~ as the structural material with CVD 11"(melting point 2410 ~ as an oxidation barrier. The Ir coating is essential, since volatilization of Re oxides results in rapid loss of structural material if oxygen reaches the Re. The probable scenario for failure of Ir-coated Re thrusters involves (1) the diffusion of Re through the Ir coating followed by the removal of Re from the surface by oxidation and/or (2) the direct removal of Ir by oxidation. In order to predict the lifetime of these thrusters, accurate diffusion data for these Ir-Re layers are required. Since diffusion mechanisms and rates are critically affected by sample morphology (i.e., grain size and structure), it is important to study diffusion in actual CVD materials. In this article, we report measurements on Re-Ir diffusion couples prepared by chemical vapor deposition. Samples were annealed in vacuum at temperatures slightly below thruster operating temperatures. Diffusion rates and mechanisms were determined using an electron microprobe.
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