Fabrication of CrSi 2 /MoSi 2 /SiC–Mo 2 C gradient composite coating on Mo substrate and the stabilizing effect of Cr on the coating's anti-oxidation properties

A multilayer CrSi2/MoSi2/SiC–Mo2C composite anti-oxidation coating was in-situ prepared on Mo substrate by a three-step pack cementation method. Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) tests indicated that the coating was composed of CrSi2, MoSi2, SiC and Mo2C. Furthermore, back scattered electron (BSE) and electron probe microanalysis (EPMA) analyses demonstrated that the whole composite coating was about 65μm and the outermost layer was a composite which was composed of CrSi2, MoSi2 and SiC, and just beneath it was a Mo2C transitional layer which connected the outer layer and the substrate Mo. Since the distribution of the elements in the composite coating was gradually increased or decreased and thus no distinguished interface arose during the preparation process. Cyclic oxidation experiments indicated that the mass loss was only 5.01wt.% in 120min at 1600°C for CrSi2/MoSi2/SiC–Mo2C coated samples, which was much lower than that of MoSi2/SiC–Mo2C coated ones. Besides the Si diffusion hindrance effects of Mo2C transitional layer, BSE, surface profilometer and EPMA observations proved that Cr in the form of CrSi2 could not only reduce the mismatch between the coating and the substrate through the formation of Cr2O3, but also accelerate the formation of a mixed protecting oxide scale at low temperature. Moreover, the Cr2O3 in the scale might also stabilize the glassy coating at high temperature by restraining the volatilization of SiO2.