Environmental/thermal barrier coating systems deposited on Nb/Nb5Si3 based alloy. (January 2015) - Braun, R.
Abstract : Coupons of a Nb-silicide-based alloy with the nominal composition of 47Nb–25Ti–8Hf–2Cr–2Al–16Si (at-%) were coated with Co-, Ni- and FeB-modified M7 Si6 -based layers using pack-cementation. Subsequently, on one side of the coated samples, environmental/thermal barrier coatings (E/TBCs) of yttria partially stabilised zirconia (YSZ) and gadolinium zirconate were deposited by electron beam physical vapour deposition. The lifetimes of the different E/TBC systems were determined performing thermal cycling tests at 1100 and 1200°C in air. At 1100°C, lifetimes approaching 1000 cycles (corresponding to 1000 h of high temperature exposure) were measured. However, severe oxidation occurred at the edges and rear side of the samples (areas without ceramic topcoat) due to local degradation of the oxidation protective coatings, resulting in substantial material recession with prolonged testing. When thermally cycled at 1200°C, the E/TBC systems exhibited lifetimes exceeding 200 cycles. The sample with FeB containing bond coat and Gd2 Zr2 O7 topcoat survived even 400 cycles at this exposure temperature.