Thermal Barier coating (TBC) - Introduction & failure mechanism

1. Thermal barrier coating (TBC)
• Introduction
• Structure of TBC
• Processing
• Failure mechanism
• Approaches in the field

2. Introduction
• The combination of mechanical stresses and high
temperatures is highly destructive to metallic parts, and
could lead to various type of failure such as, thermal
fatigue, hot corrosion, creep, erosion.
• High motivation to improve performance of engineered
parts at elevated temperatures:
• Life cycle
• Efficiency
• Reliability
• Gas turbine blades, combustor chamber, cutting tool.

3. Tensile strength of some superalloys as a function of temperature [7].

4. Introduction - TBC
• Thermal Barrier Coatings are highly advanced material systems applied to metallic surfaces,
operating at elevated temperatures.
• Multi-layer structure with thermal isolation ceramic top-coat, that provide good resistance to
erosion, wear and corrosion.
• Substrate – NickelCobalt super alloys
• Bond coat – usually NiCoCrAlY, promote adhesion of ceramic
layer, prevent oxidation of substrate
• Top-coat – ceramic layer (YSZ – Ytria-stabilized Zirconia), high
hardness, low thermal conductivity.
Matching of thermal expansion is crucial!
Multi-layer structure of TBC [1]

7. APS – Air Plasma Spraying EB-PVD – Electron Beam PVD
• Columnar macro-structure of ceramic top-
coat
• Flat interface surface
• Expensive process
• “Strain tolerant”, higher durability
• Lower thermal conductivity
• Drop-like macro-structure.
• Wavy interface between layer.
• Cheaper process
[2]

8. TGO – Thermal Grown Oxide
• Oxide layer forms during use of the
coated parts in high temperatures.
• The growing of TGO occurs due to
diffusion of metallic (Al, Cr, Y) atoms
from the bond coat and oxygen from
YSZ layer, at elevated temperatures.

9. TGO – Thermal Grown Oxide
• This layer develops extremely large residual
compressions stress, as the system cool to
ambient, due to it’s thermal expansion mismatch.
• The form of 𝛼- Al2O3 is preference to other oxides
because of it’s low oxygen diffusivity, superior
adherence and slow, uniform and defect free
growth.
• The strain energy in the TGO scales linearly with
the TGO thickness and quadratically with the TGO
stress.[1]

10. Failure Mechanism - Spallation
• Ultimately the durability is governed by spalling of the external
insulating oxide, as deduced from components removed from
engines.
• Spallation: It is a process by which the TBC peels off of the
substrate; and naturally, after the coating has spalled, the
continuous thermal protection layer no longer exists.

12. Top-coat/TGO delamination [6]
This mechanism is accompanied by rumpling (or
ratcheting) of the TGO, manifest as undulations
that, locally, penetrate into the bond coat.

13. Rumpling/ratcheting
• As the TGO grows, it also concurrently
develops a compressive stress. Because it’s
attached to the bond-coat, the only way in
which it can decrease its elastic strain energy is
by undulating.
• This undulation requires the bond-coat to
deform to accommodate the undulation. This
accommodation is by plastic deformation of
both the TGO and bond-coat during thermal
cycling.

14. Oxide Imperfection

16. Al depletion layer [2]
• When the Al concentration falls bellow a
critical value, aluminum oxide is no longer
the thermodynamic preferred phase and
other oxides form. (notably spinels ,Cr2O3
and Y2O3)
• These other oxides do not form such a
protective scale, and consequently the alloy
oxidizes faster.
• In addition, the formation of these oxides is
associated with an increase in volume that
can be disruptive and possibly have lower
fracture energies
𝛽𝑁iAl → 𝛾′
𝑁𝑖3 𝐴𝑙

18. Nickel diffusion through TGO
formation of spinel NiAl2O4 [5]

19. FOD – Foreign Object damage [3]
Erosion FOD

20. 1st approach – heat treated TBC [9]
• This approach include pre-treatment of the Bond-coat (before YSZ deposition), in
vacuum at 1000ºC for 2 and 24 hours.
Oxidation Resistance
As - deposited
2hr
24hr

22. Cross-sectional SEM
morphologies and element
distributions for the bond coat
after the thermal cyclic tests for
24hr pre-treated sample

23. 2nd approach - Development of gradient thermal barrier coatings.[10]
• In this paper, a new type of gradient thermal barrier coating was produced by the co-
deposition of Al-Al2O3-YSZ or Al2O3-YSZ and YSZ onto NiCoCrAlY bond coat by means
of EB-PVD.
• The electron beam current for evaporating the gradient layer increased gradually from
0.1A to 1.0A As the vapor pressures of the components of the mixtures are different. (PAl >
PAl2O3 > P ZrO2)
Substrate – Ni alloy (GH140)
Bond coat - NiCoCrAlY
Gradient Layer
Al-Al2O3-YSZ or Al2O3-YSZ
Top coat- YSZ

24. Al2O3-YSZ Al-Al2O3-YSZ

25. Microstructure and composition distribution
• it was concluded from SEM-EDS and XRD
patterns that the fundamental microstructure
across the thickness of the two gradient coatings
is as follow:
NiCoCrAlY / Ni3Al / Ni3Al + Al2O3 / Al2O3 /
Al2O3 + YSZ / YSZ
XRD patterns of gradient region

26. Oxidation and hot-corrosive behavior

27. Thermal Cycling test of coating
• It was found that spallation did not take place
in the Al-Al2O3-YSZ gradient coatings even
after 500hr !!
• It can be explained that the thermal stresses
arising from the different thermal expansions
were greatly relaxed in this gradient coating.

28. References
• [1] Nitin P. Padture, Maurice Gell, Eric H. Jordan, Thermal Barrier Coatings for Gas-Turbine Engine
Applications, (April 12, 2002), Science 296 (5566), 280-284.
• [2] D.R. Clarke and C.G. Levi, MATERIALS DESIGN FOR THE NEXT GENERATION THERMAL BARRIER
COATINGS, Annu. Rev. Mater. Res. 2003. 33:383–417.
• [3] J. R. Nicholls, R. G. Wellman & M. J. Deakin (2003) Erosion of thermal barrier coatings,
Materials at High Temperatures, 20:2, 207-218
• [4] Mechanisms controlling the durability of thermal barrier coatings.
• [5] M. J. Stiger, N. M. Yanar, M. G. Topping, F. S. Pettit, and G. H. Meier, Thermal Barrier
Coatings for the 21st Century
• [6] A.G. Evans , D.R. Clarke, C.G. Levi, The Influence of Oxides on the Performance of
Advanced Gas Turbines, Journal of the European Ceramic Society, 2008.

29. References
• [7] Abdullah Cahit Karaoglanli, Kazuhiro Ogawa, Ahmet Türk and Ismail Ozdemir,
Thermal Shock and Cycling Behavior of Thermal Barrier Coatings (TBCs) Used in
Gas Turbines.
• [8] D. R. MUMM, A. G. EVANS, ON THE ROLE OF IMPERFECTIONS IN THE FAILURE
OF A THERMAL BARRIER COATING MADE BY ELECTRON BEAM DEPOSITION,
Princeton Materials Institute, 2000.
• [9] Xiaofang BiU, Huibin Xu, Shengkai Gong, Investigation of the failure
mechanism of thermal barrier coatings prepared by electron beam physical vapor
deposition.
• [10] Huibin XuU, Hongbo Guo, Fushun Liu, Shengkai Gong, Development of
gradient thermal barrier coatings and their hot-fatigue behavior.

30. Appendix A – Composition of bond coat
• Composition and role of additions
• The M of MCrAlY stands for either Ni or Co, or a combination of both (when applied to steels, it can also be Fe), depending on the type of
superalloy. Co-based appear to have superior resistance to corrosion.
• Cr provides hot-corrosion resistance, but the amount that can be added is limited by the effect it is expected to have on the substrate, and the
formation of Cr-rich phases in the coating.
• Al content is typically around 10-12 wt%. Since oxidation life is essentially controlled by the availability of Al, it would be tempting to increases the
alulminium content. However, this results in significant reduction of ductility (for example, Sivakumar, 1989).
• MCrAlY also typically contain 1 wt% yttrium (Y), which enhances adherence of the oxide layer. It was initially thought that Yttrium helped the
formation of oxide pegs which helped anchor the oxide layer to the coating.
However, it has been shown that there is little if any correlation (Smeggil, 1987), and it is now believed that the main role of Y is to combine with
sulfur and prevent its segregation to the oxide layer, which is otherwise detrimental to its adhesion.
Additions of hafnium (Hf) play a similar role
• The effect of other additions has also been investigated (Nicoll, 1982). It was found that silicon (Si) significantly improved cyclic oxydation
resistance, however it also decreases the melting point of the coating. 5 wt% are enough to lower the melting temperature to about 1140 oC.
There is also evidence that it affects phase stability. For cyclic oxidation at 1000 oC, 2.5 wt% was found to be the optimum content. Further
additions were detrimental.
• Additions of rhenium (Re) have been shown to improve isothermal or cyclic oxidation resistance, and thermal cycle fatigue (Czech et al., 1994).
• Additions of tantalum (Ta) can also increase the oxidation resistance.