Influence of Chromium Alloying on the Cyclic Oxidation Behavior of Nanocrystalline (Mo_xCr_1−x)₅Si₃ Films at 800 °C in Air

In order to investigate the influence of the chromium content on the oxidation performance of the chromium-alloyed Mo 5 Si 3, five kinds of sputter-deposited nanocrystalline (Mo x Cr 1− x ) 5 Si 3 ( x = 1, 0.78, 0.75, 0.64, 0.57) films have been prepared on Ti−6A1−4 V alloy substrates by double-cathode glow discharge. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy have been used to characterize the nanocrystalline (Mo x Cr 1− x ) 5 Si 3 ( x = 1, 0.78, 0.75, 0.64, 0.57) films. The as-deposited films were free of crack having metallurgical bonding with a substrate and consisted of an outer (Mo x Cr 1− x ) 5 Si 3 layer and an inner diffusion layer. The inner diffusion layer could be divided into a β-phase layer with high Mo content (>10 wt %) and an α′′/α′ layer with low Mo content (<10 wt %). During the cyclic oxidation test at 800 °C in air, for the monolithic nanocrystalline Mo 5 Si 3 film, local spallation of the oxide scale took place by internal stresses after 5 min of oxidation, and severe spallation of the oxide scale sharpened the mass loss with an increase in the oxidation time. However, in the case of nanocrystalline chromium-alloyed Mo 5 Si 3 films, spallation resistance to thermal cycling increased significantly with an increase in the chromium addition, and particularly for the (Mo 0.57 Cr 0.43 ) 5 Si 3 film, no sign of spallation or microcracking occurred even after 240 h at 800 °C because of the formation of a fine-grain protective scale.