Characterization of a multiphase coating formed by a vapor pack cementation process to protect Nb-base alloys against oxidation

In order to improve the oxidation resistance of Nb-Nb5Si3 composites, a multilayer coating with a top coating consisting of mixed phases of oxides and chromium borides was made. The substrates were firstly coated with a pure Cr layer by cathodic triode sputtering and then borosiliconized according to a two-step pack cementation process. The as-obtained coatings were characterized using scanning electron microscopy (SEM), energy and wavelength dispersive X-ray spectroscopies (EDS, WDS), X-ray diffraction (XRD), infrared (IRRAS) and Raman spectroscopies. The results showed that the multiphase layer is composed of mixed phases of oxides (silica and borosilicate) and chromium borides (outer layer) and chromium silicide (inner layer). The coating formation mechanism involves a eutectic liquid (43 mol%SiO2-57 mol%CrO) created by the reaction between Cr and SiO gas. Oxidation of the chromium borides leads to the formation of boron oxide, which enhances the fluidity of borosilicate phases with a healing effect, the efficiency of which depends on the mass coating.