Effects of thermal exposure on Ir-based alloys with and without Pt coating

Isothermal oxidation tests were performed on Ir, Ir–3Hf, Ir/Pt, and Ir–3Hf/Pt in static air at 1100 °C for 200 h. The microstructural evolution and changes in the composition of the alloys were characterisation by performing scanning electron microscopy (SEM) and field-emission electron probe microanalysis (FE-EPMA). The results indicated that the specimens of all the Ir-based alloys exhibited linear mass loss due to the formation/evaporation of gaseous Ir oxide species (mainly IrO3). The oxidation behaviour, including mass change kinetics, evaporation of gaseous IrO3, and the role of Hf and Pt are discussed.

Research highlights
► An interesting surface morphology with faceted structure is formed on pure Ir after oxidation.
► Above 1100 °C, Ir forms evaporative oxide species and exhibits linear mass loss.
► Hf addition is not effective to protect Ir–3Hf alloy due to the currently porous feature of HfO2.
► Pt coating alleviates the mass loss by reducing the exposed surface area or serving as a metal quilt.