Lifetime of environmental/thermal barrier coatings deposited on a niobium silicide composite with boron containing M₇Si₆‐based bond coat

Abstract

Environmental/thermal barrier coatings (E/TBCs) of yttria partially stabilised zirconia (YSZ), gadolinium zirconate (GZO) and a combination of Y2SiO5 and GZO were applied on a Nb/Nb5Si3‐based alloy with the nominal composition of Nb‐25Ti‐8Hf‐2Cr‐2Al‐16Si (at%). A B‐containing M7Si6‐based (M = Ti, Nb, Fe, Cr) layer produced by pack cementation was used as bond coat. The ceramic topcoats were manufactured by electron‐beam physical vapour deposition (YSZ and GZO) and magnetron sputtering (Y2SiO5). The different E/TBC systems were tested under cyclic oxidation conditions in air at 1100 and 1200 °C. Lifetimes exceeding 1000 cycles of 1 h dwell time at 1100 °C were determined. The YSZ, GZO and combined Y2SiO5 + GZO topcoats did not spall off during the maximum exposure time, exhibiting good adherence to the thermally grown oxide scale which consisted of mixed oxides of Fe, Ti, Cr and Nb embedded in silica. For the E/TBC systems with YSZ and GZO which were thermally cycled at 1200 °C, lifetimes up to 850 cycles were measured; that of the system with a combined Y2SiO5 + GZO topcoat exceeded again 1000 cycles.

Environmental protection of Nb/Nb5Si3-based alloys by E/TBC systems

“…Results obtained in the present work are in agreement with our previous studies of similar E/TBC systems applied on MASC specimens [22,23]. For the latter substrate material, lifetimes determined for E/TBCs of YSZ and GZO also exceeded 1000 cycles at 1100°C.…”

“…After high temperature exposure of the coated samples, thorough cross-sectional examinations were carried out to study the compatibility of the ceramic topcoats with the thermally grown oxides. Microstructural analyses of the E/TBC system with a combined Y 2 SiO 5 + GZO topcoat applied on MASC samples indicated interfacial reactions [23]. Therefore, in the present work, microstructural investigations focused on chemical reactions between TGO and the ceramic topcoats.…”

“…Large oxide nodules formed there. After their spallation subsequent break-away oxidation of the substrate resulted in material recession and chipping of the E/TBC at the rims [23]. Nevertheless, the E/TBC system was intact, except edge chipping caused by disintegration of the subjacent substrate material.…”

“…The promising results obtained encouraged further optimisation of the bond coat manufacturing process, in particular that of boron containing Fe-modified M 7 Si 6based coating. With this bond coat, enhanced lifetimes of E/TBC systems with YSZ, GZO and combined Y 2 SiO 5 + GZO topcoats were determined [23]. However, substantial material recession was observed with prolonged furnace cycling testing.…”

PVD thermal barrier coating systems for Mo–Si–B alloys

“…Lifetimes up to 1000 cycles (60/10 minutes at high/ambient temperatures) were obtained for these E/TBC systems from furnace cycling tests [31,32]. For Mo-Si-B alloys, only one study on thermal barrier coatings has been published in the literature [33,34].…”