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Owing to the diffusion of Si elements towards the metal substrate and the "pesting" behavior caused by volatilization of MoO 3 , magnetron-sputtered MoSi 2 coating is highly susceptible to failure and unable to protect the metal substrate from oxidation. In this work, therefore, an MoSi 2 (N) interlayer was sputtered on Mo substrates as the diffusion barrier, followed by deposition of Si-doped MoSi 2 (i.e., MoSi 2 (Si)) layer to suppress the "pesting" behavior. The as-deposited coatings were annealed subsequently at 1000 °C in an Ar atmosphere to obtain a stable structure. To evaluate the oxidation resistance, the annealed coating was heated in a muffle furnace in air at 700 °C, 1000 °C and 1200 °C for different durations. The coating structure, surface morphology, cross section and chemical composition were examined by X-ray diffraction, field emission scanning electron microscopy, and energy dispersive spectroscopy. The results showed the MoSi 2 (N) interlayer was amorphous and kept the most stable thermal stability at Ar/N 2 flow rate of 20/20 sccm. It effectively blocked the downward diffusion of Si. Free state Si elements only diffused towards the coating surface. "Pesting" pores in the MoSi 2 (Si) layer were repaired by the growth of SiO 2 layer via the preferential oxidation of Si. The mechanism for the enhanced oxidation resistance was explained through a model. • Amorphous MoSi 2 (N) interlayer is an effective diffusion barrier. • MoSi 2 (N) interlayer blocks diffusion of Si elements in MoSi 2 (Si) coating to enhance oxidation resistance of Mo substrate. • "Pesting" pores are repaired by growth of SiO 2 layer via oxidation of Si. [ABSTRACT FROM AUTHOR]

To solve the issue of elemental interdiffusion between protective coating and superalloy substrate during high-temperature service, an AlCoCrNiMo high-entropy alloy (HEA) diffusion barrier was deposited by direct current magnetron sputtering between NiAlHf protective coating and Ni-based single crystal superalloy (René N5). The interdiffusion behavior and high-temperature oxidation resistance of the coating system were investigated via isothermal oxidation at 1373 K. After long-term oxidation at 1373 K, no interdiffusion zone (IDZ), secondary reaction zone (SRZ) and rod/needle-like topologically close-packed (TCP) precipitates were observed in the coating system with HEA diffusion barrier. Besides, the NiAlHf coating exhibited an improved oxidation resistance with the addition of HEA diffusion barrier. The results indicate that the HEA diffusion barrier can suppress the interdiffusion of alloying elements between substrate and coating effectively, which is attributed to the sluggish diffusion effect of HEA and the in-suit formed alumina layers at the substrate/HEA and HEA/NiAlHf interfaces during high-temperature oxidation. Moreover, it was found that the Co element in HEA layer also plays an important role in suppressing the formation of SRZ and TCP precipitates in the superalloy. [Display omitted] • An AlCoCrNiMo high-entropy alloy is adopted as the diffusion barrier. • Diffusion barrier effectively retards the elemental interdiffusion at 1373 K. • NiAlHf coating with diffusion barrier shows an improved oxidation resistance. • Co element plays an important role in suppressing the formation of precipitates. [ABSTRACT FROM AUTHOR]