Add time:07/26/2019         Source:sciencedirect.com

Multilayered silicide coatings are developed to protect Nb alloys from high temperature oxidation. Their efficiency is improved using a second halide-activated pack-cementation step consisting in a boronizing treatment. This work focuses on the location and quantification of boron in the multi-layered silicide coating in order to study the role of boron. The observations are performed on the cross sections of coated samples, in both as-coated and oxidised (Air–1200 °C–100 h) states. Characterisation of boron is performed in a FEG-SEM using a coupled WDX-EDX system. It was observed that EDX cannot be used to locate B in a matrix containing Nb, but WDX has sufficient spectral resolution to do it. The synthesis of calibration specimens (TiB2, NbB2 and (Nb0.5 Ti0.5)B2) was required for boron quantitative analysis. The analytical conditions are reported and justified. The compositions determined for borides formed in the coating were relevant with the phase diagrams and the thermodynamic data available for this system. If present in the oxidation products, boron should be in very low amounts because it was not observed during the investigation. Nevertheless, it can be suggested that for high temperature application, the formation of highly stable borides in the coating subsurface is helpful because, in case of crack formation, borides promptly form a boro-silicate glass with a low melting temperature, which is responsible for the self-healing of the oxide scale.

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