Double-layer iridium–aluminum intermetallic coating on iridium/rhenium coated graphite prepared by pack cementation

A double-layer iridium–aluminum (Ir–Al) intermetallic coating was prepared on iridium/rhenium (Ir/Re) coated graphite by pack cementation at 750 °C for 1 h. The microstructure and phase of the Ir–Al intermetallic coating were studied by scanning electron microscope and X-ray diffraction, and the mechanical properties of the Ir–Al intermetallic layers were determined using the nanoindentation method. The oxidation resistances of the aluminized Ir coating and pure Ir coating samples were investigated comparatively using the induction heating method. The results showed that the as-formed Ir–Al intermetallic coating exhibited a double-layer structure with an IrAl inner layer and an IrAl2.7(3) outer layer. The sharp interfaces of IrAl2.7(3)/IrAl and IrAl/Ir in the aluminized Ir coating showed excellent adherence without any defects. The hardness and elastic modulus of IrAl layer were 26.9 and 455.2 GPa, respectively, higher than those of IrAl2.7(3) layer, which were 19.6 and 318.4 GPa, respectively. The mass loss rates of the aluminized Ir coating and Ir coating during the oxidation test were 0.004 and 0.811 mg cm− 2 min− 1, respectively, showing that the aluminized Ir coating has superior oxidation resistance compared with the pure Ir coating. The as-oxidized Ir–Al intermetallic coating exhibited a double-layer structure with a continuous Al2O3 outer layer and an inner layer of Al2O3 containing island-like Ir domains.