CODEPOSITED CHROMIUM-ALUMINIDE COATINGS .2. KINETICS AND MORPHOLOGY OF COATING GROWTH

The codeposition of chromium and aluminum in cementation packs containing a Cr-5 weight percent (w/o) master alloy and an NaCl/NH4Cl activator mixture has been studied at temperatures of 1100, 1120, and 1140-degrees-C. The effects of activator mixture composition, deposition time, temperature, specimen arrangement within the pack, and the loss of volatile species from the pack on the extent of chromium and aluminum codeposition onto a Rene 80H substrate were determined. It is shown that a proper choice of deposition time and temperature provides control of the composition of the coating's outer beta-NiAl + alpha-Cr layer. Coatings with outer layer chromium contents ranging from approximately 6 to 40 w/o were obtained. The growth kinetics of these outward-grown coatings were measured and found to follow the parabolic rate law with solid-state diffusion being rate determining. For the times studied (0 to 12 h), the surface composition of the beta-NiAl coating matrix reached a steady-state value once the pack reached the deposition temperature. The mass-transfer model commonly used to describe chemical vapor deposition in a pack-cementation process could not be used because local quilibrium conditions were not maintained. Reasons for this are discussed.