1.
Fatigue life of thermal barrier coatings (TBCs) – physically based modeling
Sten Johansson, Håkan Brodin, Robert Eriksson, Sören Sjöström, Lars Östergren, Xin-Hai Li
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2. Introduction

3. The components of a TBC system
BC µm, TC µm
top coat (TC), zirconia + 6–8% yttria
bond coat (BC), MCrAlY M = Ni or Ni + Co
Substrate, Ni-base (Haynes 230, Hastelloy X)
light optic micrograph of TBC

4. plasma spraying, image from Sulzer Metco
Manufacturing of TBC
Raw material (powder) is fed into a plasma flame.
The coatings are built up by droplets, forming a splat-on-splat structure on impact.
Plasma spraying can be done in: air (APS) or vacuum (VPS).
plasma spraying, image from Sulzer Metco

5. oxidation / corrosion / fatigue
Problems with TBC if combustion temperature is increased, or when new fuel types are used
oxidation / corrosion / fatigue
Fatigue
needs to develop life models for thick TBC
needs to include effect of corrosion in life models
More severe oxidation
Sulphur in fuel might form sulphides
Salts might cause rapid oxidation
damaged oxide in TBC
damaged top coat in TBC

6. Problem description Reliability of TBCs must improve!
stresses the need for more robust life models that takes into account: oxidation, corrosion, BC/TC interface morphology, coating thickness, different types of thermal cycling
How to accomplish this?
Physically based fracture mechanical models

7. Experimental work: Surface roughness

8. interface morphology It influences:
grayscale image
It influences:
adhesion
stress distribution in BC/TC interface
How to model the surface roughness?
binary image
surface profile
filtered profile
model

9. correlation between Ra and TCF life length
Results
Correlation between Ra and life length.
Interface morphology influences fatigue life
correlation between Ra and TCF life length
from Fracture mechanical modelling of a plasma sprayed TBC system, Brodin et al.

10. experimental work: Adhesion of heat treated thermal barrier coatings

11. Isothermal and cyclic heat treatments
isothermal oxidation 1100°C
continuous high temperature exposure
thermal cycling fatigue 1100°C
1 h in furnace => 10 min cooling to ~100°C
burner rig test ~1100°C
75 s heating => 75 s cooling

12. Microstructural degradation
Al depletion of the bond coat is one of the mechanisms that limits life of TBCs
β-depletion in heat treated bond coat, a) BRT 300 cyc. b) BRT 1150 cyc. c) isoth. ox. 1 h d) isoth. ox. 23 h

13. Microstructural degradation
formation of bulky oxide cluster in TCF-subjected specimen
Bulky cluster of oxides formed in the interface may act as crack initiation sites.

14. Fracture surfaces from adhesion test
By gluing the TBC coated specimens to two rods, the adhesion of the TBC can be tested using a tensile machine 3
18.4 MPa 1
19.7 MPa 2
20.9 MPa
Fracture surfaces from adhesion test

15. Changes in adhesion with heat treatment type and treatment length
changes in adhesion with high temperature exposure
Cross-sectioned fracture surfaces from adhesion test, 300 cycles TCF
Burner rig testing gives lower adhesion, but slower decrease in adhesion per cycle. Isothermal oxidation is beneficial.

16. Fractography, fracture in top coat
Failure occurs between splats in the top coat
fractured TC, as-sprayed specimen

17. fracture in top coat Through-splat cracks are rare.
Mostly occur in pre-existing cracks
splat-on-splat nucleation
fractured TC, as-sprayed specimen
fractured TC, as-sprayed specimen

18. High temperature exposure: grain coarsening
a) as-sprayed, b) isothermal oxidation ~300 h, c) burner rig test 1150 cycles, d) thermal cycling fatigue 300 cycles

19. High temperature exposure: sintering
cross-sections showing sintering
a) as-sprayed
b) isothermally oxidised 300 h
fracture surfaces showing in-splat sintering
a) as-sprayed
b) isothermal oxidation ~300 h
c) burner rig, 1150 cycles
d) thermal cycling fatigue, 300 cycles

20. Fracture in BC/TC interface
Increase in amount of interface fracture due to fatigue damage.
Furnace cycled specimens have cracked oxide layer and cut-through oxide clusters.
black fracture: a) as-sprayed, b) 300 cycles of thermal cycling fatigue, c) 300 cycles of burner rig test,
d) 1150 cycles of burner rig test

21. Fracture in BC/TC interface: revealed oxide layer
Al2O3 BC
Al2O3 BC
Cr-rich / spinel
Al2O3 TC
TCF 300 cycles
(Al,Cr)2O3 / (Ni,Co)(Al,Cr)2O4 BC

22. Fracture in BC/TC interface: revealed oxide layer
Al2O3
NiO BC
Cr-rich / spinel TC
TCF 300 cycles
Cut-through oxide clusters
Usually cracks during thermal cycling
NiO
Cr-rich/ spinel

23. Modelling work: influence of chamfer angle on edge cracking

24. Basic failure mechanisms
Spallation from
flat surfaces
Spallation from convex
surfaces
3. Spallation at sharp
edges
Substrate

25. Edge effects: effect of end chamferingTC BC
substrate
TGO x z
TBC chamfer angle
Substrate end angle  (2)
equivalent ’fatigue evaluation’ stress
Surprising conclusion:
Practically no improvement until  (1) is made < 60 º

26. Ongoing experimental work: Oxidation study and corrosion testing

27. Corrosion rig Corrosion cycle:
spraying with Na2SO4+NaCl
1 h in furnace, 650–950 °C
cooling in air, reaching a minimum temperature of ~100 °C
Currently running corrosion on BC coated specimen, SL30 NiCoCrAlY+Si