Oxidation behavior of Cu-Cr environmental barrier coatings on Cu-8Cr-4Nb

Oxidation behavior of some Cu-Cr alloys (with 8.5-25.6 wt.% Cr, in both stand-alone coupons and spray coatings on Cu-8Cr-4Nb) was studied by TGA and cyclic oxidation. Judging by the more stringent cyclic oxidation results, protection of the substrate Cu-8Cr-4Nb was effective for the whole 10-h duration of testing up to 750 °C only for coatings with 21% or higher Cr. Similar protection was exhibited by Cu-17Cr only up to 650 °C, and only for <3 h at 750 °C. In all cases the Cr2O3 subscale nucleated discretely (and always under a non-adherent scale of Cu oxides), and passive protection did not start until the nuclei joined into a continuous barrier and parabolic oxidation kinetics set in. The only protection achieved before that stage of Cr2O3 coalescence was via sacrificial oxidation of the coating.