Iridium coatings with various grain structures prepared by electrodeposition from molten salts: Growth mechanism and high temperature oxidation resistance

Iridium (Ir) coatings with various grain structure have been prepared on the rhenium (Re) substrates by electrodeposition from melting chlorides. Effects of current parameters on microstructure of the Ir coatings were investigated and the corresponding growth mechanism was discussed. Two different steps, the nucleation and grain growth step, were contained in the Ir electrodeposition and grain structure of the deposited Ir coating was mainly determined by the nucleation step. If nucleation of Ir grain occurred only at the initial stage of Ir electrodeposition, then the Ir coatings were mainly characterized as columnar grain structure. In contrast, while nucleation of Ir grain could last from beginning to end of Ir electrodeposition, the Ir coatings were consisted of equiaxed grains. Increasing in the applied current density could promote the nucleation of Ir grains. Due to using higher current density, equiaxed grain Ir coatings have been successfully prepared on the Re substrates by electrodeposition. Cyclic oxidation of Ir coatings on the Re substrates showed that oxidation failure of the Ir coatings was co-contributed by uniform oxidation of Ir grains and preferential oxidation of Ir grain boundaries. Life time of the 100 μm-thick equiaxed grain Ir coatings on Re substrates at 2000 °C in stagnant air was about 410 min, much longer than that of the columnar grain Ir coatings with the same thickness. Improvement in oxidation resistance of the Ir coating was attributed to the sluggish of Re diffusion due to the flexuous boundaries in the equiaxed grain Ir coatings.