Annika Lange

Environmental/thermal barrier coating systems deposited on Nb/Nb5Si3 based alloy

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 M7Si6-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 Gd2Zr2O7 topcoat survived even 400 cycles at this exposure temperature.

Y2SiO5 environmental barrier coatings for niobium silicide based materials

Abstract The oxidation behaviour of Nb silicide based alloys, considered as potential ultra-high temperature material for gas turbine engine applications, can be improved by chromia–silica forming coatings with M7Si6 structure. To stabilise the protective oxide scales against water vapour corrosion in combustion atmospheres, environmental barrier coatings (EBCs) of yttrium silicate were deposited on an FeB modified M7Si6 based bond coat using magnetron sputtering. The 15–20 μm thick ceramic top coats with an approximate chemical composition of 25Y–13Si–62O (at.-%) were dense and amorphous after deposition. They were annealed in vacuum to get a crystalline structure with the predominant phase Y2SiO5. Samples with this EBC system were tested in rapidly flowing water vapour at temperatures between 1100 and 1300°C for up to 16 h. At 1100 and 1200°C, significantly reduced mass gains were determined for samples with yttrium silicate top coat in comparison to those coated only with FeB containing layers. The EBC partially transformed into the Y2Si2O7 phase and exhibited microporosity. At 1300°C, the yttrium silicate layer decomposed forming yttrium oxides.

PVD thermal barrier coating Systems for Mo-Si-B alloys

Abstract Oxidation protective layers with chemical compositions of Mo–70Al, Mo–46Si–24B, Mo–37Si–15B and Mo–47Si–24Al (at.-%) were deposited on Mo–9Si–8B specimens by magnetron sputtering. After pre-oxidation of the coated samples, ceramic topcoats of yttria partially stabilized zirconia (YSZ) and gadolinium zirconate (GZO) were applied using electron-beam physical vapour deposition. Both as-deposited YSZ and GZO topcoats exhibited good adhesion to the pre-oxidised bond coats. The different thermal barrier coating (TBC) systems were exposed to air at 1000 °C for periods between 20 and 100 h. The YSZ topcoat was tightly-adherent to the borosilicate scale grown on the Mo–46Si–24B bond coat after 20 h of exposure. Similar results were obtained for GZO topcoats deposited on Mo–46Si–24B and Mo–37Si–15B bond coats. The TBC system consisting of GZO topcoat and Mo–47Si–24Al bond coat, which formed a mixed scale of silica and mullite-like oxides, survived 100 h at 1000 °C. However, after this exposure time, the bond coats were approaching their lifetime due to the low layer thickness (5–10 μm). Oxidation of the Mo–Si–B substrate at unprotected areas around the suspension hole of the samples caused severe deterioration of the Mo–70Al bond coat and substantial degradation of the outer region of the GZO topcoat due to chemical reactions with MoO3.