Enhancing oxidation resistance of Mo metal substrate by sputtering an MoSi₂(N) interlayer as diffusion barrier of MoSi₂(Si) surface coating

Owing to the diffusion of Si elements towards the metal substrate and the “pesting” behavior caused by volatilization of MoO₃, magnetron-sputtered MoSi₂ coating is highly susceptible to failure and unable to protect the metal substrate from oxidation. In this work, therefore, an MoSi₂(N) interlayer was sputtered on Mo substrates as the diffusion barrier, followed by deposition of Si-doped MoSi₂ (i.e., MoSi₂(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 MoSi₂(N) interlayer was amorphous and kept the most stable thermal stability at Ar/N₂ 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 MoSi₂(Si) layer were repaired by the growth of SiO₂ layer via the preferential oxidation of Si. The mechanism for the enhanced oxidation resistance was explained through a model.