Lifetime of environmental/thermal barrier coatings deposited on an Nb/Nb5Si3- based alloy with FeB-Modified M7Si6-based bond coat

To enhance the performance of aircraft engines, high temperature materials are required being capable to operate at temperatures significantly higher than the temperature limit of about 1150°C approached for Ni-based superalloys currently employed. Nb/Nb5Si3-based composites are promising candidates for turbine engine applications at temperatures up to 1300°C, exhibiting balanced mechanical properties and reduced density compared to Ni-based superalloys [1]. To use these composites in gas turbine combustion atmosphere, environmental/thermal barrier coatings (E/TBCs) are required to protect them against heat, degradation in flowing water vapour and chemical attack of calcium-magnesium-alumino-silicate (CMAS)

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

International audienceEnvironmental/thermal barrier coatings (E/TBCs) were applied on an Nb/Nb5Si3-based alloy. The substrate material with the nominal composition Nb-23Ti-4Hf-3Cr-6Al-20Si (at.%) was manufactured by plasma melting. Rectangular platelets of this alloy were coated with chromia-silica forming M7Si6-based (M = Ti, Nb, Cr, X with X = Ni, Co, Fe) bond coats produced by pack cementation. The Co- and Fe-modified M7Si6-based layers also contained boron. On these grit blasted bond coats, ceramic topcoats of 7 wt.% yttria partially stabilised zirconia (YSZ) and gadolinium zirconate (GZO) were deposited using electron-beam physical vapour deposition. The E/TBC systems were thermally cycled at 1100 °C and 1200 °C in laboratory air (1 h at high temperature, 10 min at ambient temperature). Cross-sectional examinations of the samples after cyclic testing were carried out using scanning electron microscopy to study the protection capability of the E/TBC systems and failure mechanisms.The lifetimes of the E/TBC systems exceeded the maximum exposure period of 1000 cycles at 1100 °C. When cyclically tested at 1200 °C, the samples with Ni-modified and boron containing Co-modified M7Si6-based bond coats failed within 700 cycles. Failure was caused by spallation of thick oxides scales thermally grown on the substrate after degradation of the bond coats. The E/TBC systems with boron containing Fe-modified M7Si6-based bond coats exhibited lifetimes approaching 1000 cycles at 1200 °C. For both YSZ and GZO topcoats, chemical reactions with the TGOs were observed

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

International audienceEnvironmental/thermal barrier coatings (E/TBCs) were applied on an Nb/Nb5Si3-based alloy. The substrate material with the nominal composition Nb-23Ti-4Hf-3Cr-6Al-20Si (at.%) was manufactured by plasma melting. Rectangular platelets of this alloy were coated with chromia-silica forming M7Si6-based (M = Ti, Nb, Cr, X with X = Ni, Co, Fe) bond coats produced by pack cementation. The Co- and Fe-modified M7Si6-based layers also contained boron. On these grit blasted bond coats, ceramic topcoats of 7 wt.% yttria partially stabilised zirconia (YSZ) and gadolinium zirconate (GZO) were deposited using electron-beam physical vapour deposition. The E/TBC systems were thermally cycled at 1100 °C and 1200 °C in laboratory air (1 h at high temperature, 10 min at ambient temperature). Cross-sectional examinations of the samples after cyclic testing were carried out using scanning electron microscopy to study the protection capability of the E/TBC systems and failure mechanisms.The lifetimes of the E/TBC systems exceeded the maximum exposure period of 1000 cycles at 1100 °C. When cyclically tested at 1200 °C, the samples with Ni-modified and boron containing Co-modified M7Si6-based bond coats failed within 700 cycles. Failure was caused by spallation of thick oxides scales thermally grown on the substrate after degradation of the bond coats. The E/TBC systems with boron containing Fe-modified M7Si6-based bond coats exhibited lifetimes approaching 1000 cycles at 1200 °C. For both YSZ and GZO topcoats, chemical reactions with the TGOs were observed

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

International audienceEnvironmental/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 1h 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.Des revêtements de barrière thermique résistants à l’environnement (E/TBC) de zircone partiellement stabilisée par l’yttrine (YSZ), de zirconate de gadolinium (GZO) et d'une combinaison de Y2SiO5 et de GZO ont été appliqués sur un alliage Nb/Nb5Si3 de composition nominale Nb-25Ti- 8Hf-2Cr-2Al-16Si (at.%). La couche de liaison utilisée est à base de M7Si6 modifié par FeB produite par cémentation. Les couches en céramique ont été fabriquées par dépôt physique en phase vapeur sous faisceau d'électrons (YSZ et GZO) et par pulvérisation cathodique magnétron (Y2SiO5). Les différents systèmes E/TBC ont été testés dans des conditions d'oxydation cycliques dans l'air à 1100°C et 1200°C. Des durées de vie supérieures à 1000 cycles de 1 h (temps de séjour) à 1100°C ont été constatées. Les dépôts YSZ, GZO et Y2SiO5 + GZO ne se sont pas écaillés au cours des tests, montrant une bonne adhérence sur la couche d'oxyde sous-jacente constituée d'oxydes mixtes de Fe, Ti, Cr et Nb noyés dans la silice / borosilicate. Pour les systèmes YSZ et GZO qui ont été cyclés à 1200°C, des durées de vie allant jusqu'à 800 cycles ont été mesurées ; celle du système Y2SiO5 + GZO a dépassé 1000 cycles

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

International audienceCoupons 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 packcementation. 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 degrees C in air. At 1100 degrees 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 degrees 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

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°C and 1300°C for up to 16 h. At 1100°C and 1200°C, significantly reduced mass gains were determined for samples with yttrium silicate top coat in comparison to those only coated with FeB-containing layers. The EBC partially transformed into the Y2Si2O7 phase and exhibited micro-porosity. At 1300°C, the yttrium silicate layer decomposed forming yttrium oxides