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Acceptable limits of degradation of TBC for high-efficient turbines (HET TBC)

Acceptable limits of degradation of TBC for high-efficient turbines (HET TBC) Department of Materials (ALSTOM) Lab of Crystallography (ETH Z ürich) CTI project Nr. 7820.3 EPRP-IW Project start Nov. 1. 2005 Project duration 24 months. Outline

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Acceptable limits of degradation of TBC for high-efficient turbines (HET TBC)

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  1. Acceptable limits of degradation of TBC for high-efficient turbines (HET TBC) Department of Materials (ALSTOM) Lab of Crystallography (ETH Zürich) CTI project Nr. 7820.3 EPRP-IW Project start Nov. 1. 2005 Project duration 24 months

  2. Outline Evolution of Sulzer Metco coating microstructure during heat treatment 1. YSZ phase diagram 2. Determination of YO1.5 content 3. XRD of coatings D2429 and D2426 4. Time evolution of coating microstructure • Correlations between coatings degradation and microstructure change SEM and XRD study of coating D03053 1. SEM 2. XRD

  3. T, oC ZrO2 YO1.5 YSZ phase diagram At 1100 ºC < T > 1400 ºC, tetragonal YSZwith ~7 wt% YO1.5should decompose into: TetragonalYSZwith ~4 wt% YO1.5 andcubicYSZwith ~14 wt% YO1.5 TetragonalYSZ should remain tetragonal at RT but can decompose into monoclinic and tetragonalYSZ at T ~ 600 ºC

  4. Determination of YO1.5 content YO1.5 content of tetragonal and cubic YSZ phases can be estimated from cell parameters (Ilavsky et al., 2001): Tetragonal YSZ: Cubic YSZ:

  5. Evolution of coating microstructure with heat treatment Coating D2426 was treated at 1100 ºC for times ranging from 1h up to 1600h and measured by XRD. After 1600 h, spallation of coating occurred Coating D2429 was removed from substrate, treated at 1200 ºC, 1300 ºC or 1400 ºC for 1h, 10h and 100h and then measured by XRD Only traces of monoclinic YSZ were found in both coatings D2426 and D2429

  6. as deposited 10 h at 1300 ºC 100 h at 1300 ºC K2 peaks C-YSZ T-YSZ low YO1.5 % T-YSZ 004 220 004 220 004 220 Higher 2 for (220) peak smaller a-axis value Smaller 2 for (004) peak higher c-axis value Higher c/a ratio smaller YO1.5 content Evolution of coating microstructure with heat treatment The growth of a tetragonal phase with a lower YO1.5 content can be easily seen from XRD spectrum at 2 = 72-76 º

  7. Evolution of coating microstructure with heat treatment 1. Adding one tetragonal YSZ phase with modified cell parameters corresponding to ~4 wt% YO1.5 content Adding more than one tetragonal phase with modified cell parameters corresponding to different YO1.5 contents of 2 - 6 wt% Method to estimate level of decomposition of tetragonal YSZ: The second method did not really improved the quality of the refinement, also most of the tetragonal YSZ with low YO1.5 content was described well by ~4 wt% YO1.5 phase

  8. Determination of YO1.5 content This indicates that we observe: 1. As deposited tetragonal YSZ 2. Fully decomposed YSZ No observation of YSZ undergoing decomposition process Thermodynamic computations (free energy functions) and TEM(*) studies have shown that YO1.5 segregation occurs in the domain boundaries short scale process fast process * Katamura et al. 1998

  9. M-YSZ Determination of YO1.5 content At 1300 ºC after 100 h, there is still no indication of formation of monoclinic YSZ

  10. Determination of YO1.5 content At 1100 ºC amount of tetragonal YSZ with ~4 wt% YO1.5 increases fast during first 250 h and then increases slowly At 1300 ºC after 100 h, more than 50% of tetragonal YSZ is decomposed

  11. Determination of YO1.5 content Phases contents are similar after 10h at 1300ºC than after 250 hours at 1100 ºC At 1300ºC the kinetic of decomposition is one order of magnitude faster than at 1100ºC

  12. 10h at 1300ºC C-YSZ T-YSZ low YO1.5 % T-YSZ Determination of YO1.5 content Peaks of cubic YSZ and tetragonal YSZ with a low YO1.5 content are broader than those of as deposited tetragonal YSZ Refinement of peak shape parameters indicates that micro-strains are higher in cubic YSZ and tetragonal YSZ with a low YO1.5 content

  13. Summary of evolution of coating microstructure with heat treatment: • No observation of monoclinic YSZ • Tetragonal YSZ decomposes into cubic YSZ and • tetragonal YSZ with a reduced YO1.5 content • Strains are higher in the cubic YSZ phase and in tetragonal • YSZ phase with a reduced YO1.5 content • Decomposition rate is one order of magnitude faster at • 1300 ºC compared to 1100 ºC

  14. Correlations between coatings degradation and microstructure change • Coatings produced with Sulzer Metco powder (D2429, D2426), • heated at 1100, 1200, 1300, 1400 oC during 10 - 1000 h: • Traces of m-YSZ can be observed • t-YSZ decomposes into c-YSZ and t'-YSZ with lower Y-content • Peak broadening of c-YSZ and t'-YSZ • Coating 03053: • Dependence of m-YSZ content on operation temperature • t-YSZ decomposes into c-YSZ and t'-YSZ with lower Y-content • m-YSZ is already present after deposition and probably not • related to coating ageing (from our results)

  15. Correlations between coatings degradation and microstructure change • HOSP powder: • - Increase of m-YSZ content with temperature of operation • Change of a', c' lattice constants of t'-YSZ with temperature • Change of Y-content with temperature • Not all parameters of powders have been refined yet due to • shortage of diffractometer measurement time (but this • refinement is in the pipeline)

  16. SEM and XRD study of coating D03053 Outer side EDX: homogeneous element distribution and the presence of traces of S and Ca Some cracks could have appeared during service

  17. SEM and XRD study of coating D03053 Inner side Traces of Ni (90%), Si (10%) and Ca as well as C YSZ with very low traces of impurities Traces of Ca and C

  18. SEM and XRD study of coating D03053 The outer side contains cubic, tetragonal and monoclinic YSZ as well as a phase which was identified as CaSO4 T-YSZ: tetragonal YSZC-YSZ: cubic YSZM-YSZ: monoclinic YSZ The inner side contains tetragonal and monoclinic YSZ as well as two other phases (CaCO3 and NiO)

  19. SEM and XRD study of coating D03053 The asymmetry of the peak shape around 2=74o indicates presence of tetragonal YSZ with YO1.5 content at the outer side. This asymmetry was not observed at the inner side From service time ~10000h, decomposition level and results on SM coatings, operating temperature can be estimated ~1000ºC

  20. SEM and XRD study of coating D03053 • Some cracks appeared at the outer side of the coating during the operation - Equal amounts of ~2 wt% monoclinic YSZ are observed on both sides of coating- Decomposition of tetragonal YSZ is observed on the outer side but not on the inner side - XRD shows that temperature at the inner surface of the coating was lower than at the outer surface- Traces of CaSO4 from the gas combustion are observed at the outer side- Traces of NiO from substrate are observed on the inner side - CaCO3 is observed on the inner surface, its origin has still to be clarified (sample holder?)

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