Aluminizing by pack-cementation to protect CoSb₃ from oxidation

The CoSb₃ skutterudite compounds are thermoelectric materials used to produce electricity from heat. Unfortunately, these materials quickly oxidize in air at elevated temperature. To solve this problem, this work explores the possibility to produce a protective aluminide surface layer to limit the interactions with oxygen. To synthesize the layer, this research work experiments the pack cementation process. A good understanding of the process requires the knowledge of the thermodynamic phase equilibria of the involved elements. However, this ternary system has never been considered in literature. Therefore, the first part of the research concerns the study of the isothermal section at 600 °C of the Al–Co–Sb ternary system to improve the knowledge of the phases that can be formed during the pack cementation process. Next, the aluminide surface layer is synthesized, then characterized by coupling scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis. The results reveal that the obtained coating is made of two crystalline phases, AlSb is the major one and Al₉Co₂ is the minor one. At last, the oxidation experiments point out the protective properties of the surface layer against oxygen penetration, keeping the substrate safe for 1000 h under a flow of synthetic air at 527 °C. Consequently, the aluminized CoSb₃ is a promising thermoelectric material useable in oxidative environments.