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

The codeposition of chromium and aluminum in cementation packs contain ing a Cr-5 weight percent (w/o) master alloy and an NaCl/NH4Cl activat or mixture has been studied at temperatures of 1100, 1120, and 1140-de grees-C. The effects of activator mixture composition, deposition time , temperature, specimen arrangement within the pack, and the loss of v olatile species from the pack on the extent of chromium and aluminum c odeposition onto a Rene 80H substrate were determined. It is shown tha t 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 wit h solid-state diffusion being rate determining. For the times studied (0 to 12 h), the surface composition of the beta-NiAl coating matrix r eached a steady-state value once the pack reached the deposition tempe rature. The mass-transfer model commonly used to describe chemical vap or deposition in a pack-cementation process could not be used because local quilibrium conditions were not maintained. Reasons for this are discussed.