Oxidation resistance of Mo(Si₁-ₓAlₓ)₂ nanocrystalline films and characterization of their oxide scales by electrochemical impedance spectroscopy

Abstract

To explore the influence of Al alloying on the oxidation resistance of MoSi2, four Mo(Si₁-ₓAlₓ)₂ nanocrystalline films, with differing Al contents, were fabricated on Ti–6A1–4V substrates by a doublecathode glow discharge technique and their cyclic oxidation behavior was characterized at 500 °C in air. The oxidation kinetics of the four Mo(Si₁-ₓAlₓ)₂ films was found to obey a subparabolic behavior with respect to the overall exposure, and their oxidation resistance was improved by Al additions. On the other hand, the electrochemical behavior of the oxide scales developed on the four Mo(Si₁-ₓAlₓ)2 nanocrystalline films in a 3.5 wt% NaCl solution was studied using electrochemical-impedance spectroscopy (EIS). The impedance data showed that with increasing oxidation time, the oxide scales transformed from a homogeneous and dense structure to a duplex structure consisting of a porous outer layer and a denser inner layer. The resistance of the oxide scales increased with increasing Al addition, implying an enhanced protective ability of the oxide scales by Al alloying in media containing chlorine ions. The findings represent a step forward in improving the surface integrity of alloy components used in the hot zones of jet engines.