Protection of N-Type (Ni,Fe)TiSb Half-Heusler Materials against Static and Cyclic Oxidation Using a Si-Doped Cr Coating

In this study, Cr–Si coatings were deposited on N-type (Ni,Fe)TiSb thermoelectric (TE) materials by using a closed-field unbalanced magnetron sputtering PVD technique. Oxidation behavior was evaluated under both isothermal (static) conditions (500 °C for 10 h and 600 °C for 50 h) and thermal cycling regimens (500 and 600 °C for 10 or 50 1 h cycles). Mass gain, surface morphology, cross-sectional microstructure, elemental distribution, and phase composition were examined by using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). Regardless of exposure mode, uncoated samples oxidized severely: a duplex scale formed, consisting of an outer TiO₂layer and a subjacent NiSb-rich zone, accompanied by extensive cracking and delamination. In sharp contrast, the Cr–Si coatings remained thermally stable and highly oxidation-resistant, maintaining the substrate’s integrity during both static and cyclic tests. After exposure, coated samples showed negligible mass gain, no discernible morphological change, and no mechanical damage, confirming that the Cr–Si layer markedly enhances thermal durability and prevents surface degradation.