Thermal Barrier Coatings (TBCs) are coatings applied to gas turbine engine components to increase their high-temperature capability. TBCs typically have four layers: a superalloy substrate, bond coat, thermally grown oxide layer, and yttria-stabilized zirconia ceramic topcoat. TBCs allow gas turbine blades to operate at temperatures up to 1500°C, significantly increasing engine efficiency. However, TBCs can fail over time due to thermal expansion mismatches and oxidation of the bond coat, reducing the operating life of coated components.

1. Thermal Barrier Coatings for Gas-Turbine Engine Applications
Report
by
Mainak Saha
Roll No: MM18D400
Department of Metallurgical and Materials Engineering
Indian Institute of Technology(IIT) Madras
Chennai- 600036, Tamil Nadu
September,2018

2. Acknowledgement
With deep regards and profound respect, I would like to express my sincere and hearty gratitude
towards my guide Prof. ing KG Pradeep for providing me the opportunity to carry out the
project in this esteemed institute. My special thanks to him for bringing much awareness in me
about research and allowing me to express myself in research. Besides, I am also strongly
indebted to him for his valuable suggestions, both academically and non-academically which
led to self-motivation and helped me complete the seminar work successfully.
Besides, I am also thankful to all research scholars for their constant help during the project
work.
I also thank my parents all the elders for their support and encouragement to pursue higher
studies.

3. Introduction
In the present world, the need for high efficiency turbine engines leads to an increasing demand
for materials capable to withstand high mechanical loads at elevated temperatures. Since 1930s,
Ni-based superalloys have been the most suitable materials for high-temperature applications
due to their excellent high temperature strength which is provided by the two phase γ / γ/ -
microstructure. Containing a A1 matrix (Ni-rich, disordered fcc, γ - phase) and L12 ordered
intermetallic compound (ordered fcc, consisting of 2 simple cubic sublattices, γ/ -phase), the
microstructure also shows an anomalous flow stress behavior. Ni-based superalloys(possessing
excellent oxidation resistance), also do offer a unique set of properties making them the
material of choice for turbine disks and blades. Conventional Co-based superalloys mostly find
application in mechanically low loaded parts, as because they lack the possibility of γ/-
strengthening at temperatures above 900◦
C. However, Sato et al. discovered a ternary
intermetallic compound Co3(Al,W) with a L12 structure, which showed the possibility of γ / γ/
-strengthening in Co-based alloys. The lattice misfit of new compound at elevated temperatures
is significantly lower than of the L12 Co3Ti phase, which is not suitable for γ / γ/ -strengthening.
Suzuki et al. reported the occurrence of a flow stress anomaly analogous to Ni-based alloys for
Co–9Al–9W and showed that Ta to increase the flow stress at the peak temperature. Shinagawa
et al. showed that boron addition effectively leads to grain boundary strengthening, thus
improving the ductility of polycrystalline Co–9Al–9W alloy.
At present, Thermal Barrier Coatings(TBCs), comprising of low thermal conductivity ceramics
are presently coated on gas turbine blades Ni-based superalloys, thereby increasing the high
temperature withstanding capacity of these alloys. As a result, we find some of the best engines
operating at 1500◦
C, at present. These coatings about 100-300 µm, thick, are used to reduce the
surface temperatures to about 100-400⁰C on the surface of the superalloy blade. These coatings
do also find applications in diesel engines, where they lead to extensive fuel economy. These
coatings, primarily, comprise of 4 layers: 2 metallic and 2 ceramic layers, namely:
superalloy(substrate), Bond coat, topcoat and cooling air film(towards the surface). The
properties that turbine blades are expected to possess are: excellent hot corrosion resistance,
oxidation resistance, thermal fatigue resistance and most importantly, excellent creep
resistance. However, it has also been reported that TBCs may fail due to a number of reasons
among which the two most important reasons are: Thermal expansion mismatch stresses
between superalloy and the TBC and oxidation of metal. TBC is the only system where a large
number of diversified phenomenon occurs: diffusion, oxidation, phase transformation, elastic
deformation, plastic deformation, creep deformation, thermal expansion, thermal conduction,
radiation, fracture, fatigue, and sintering.
Main anatomy of TBCs
For the TBCs, Ni or Co- based superalloys act as substrate on which these coatings are
deposited. Besides, it is also known that at high temperatures, diffusion creep gets highly
predominant and if there are interfaces such as grain boundaries present in material, creep life
at a given temperature, is highly expected to decrease, as because such interfaces provide easy

4. pathways for diffusion of atoms. So, for excellent creep resistance, it is always advisable to use
single crystals, with no interfaces like grain boundaries, instead of polycrystalline materials.
Thus, at present, through special techniques of investment casting, single crystal superalloys,
are widely manufactured. Besides, a superalloy(Ni-based or Co-based), contains alloying
elements, as many as 5-12, which impart a number of interesting properties including good
creep resistance, good oxidation behavior, excellent hot corrosion resistance, but at high
temperatures of about 900-1200⁰C, there is a high possibility of interdiffusion of elements(
particularly low melting point elements like Al), to diffuse between superalloys and these
coatings. Besides, these elements are also sometimes found in the TGO(Thermally grown
oxide) layer, present between the bondcoat and topcoat and even sometimes in topcoat. This
might lead to spallation of TBCs. The TGO is required to possess excellent adhesion with the
bondcoat.
However, the main problem with bondcoat/Thermally grown oxide(TGO) interface is the
segregation of S, there and this drastically reduces the adhesion of the TGO layer. For this
reason, the S-gettering elements like Y, Zr e.t.c. are added in small amounts. Besides, elements
like Si, Hf e.t.c. enhancing the adhesive behaviour of TGO with the bondcoats, are added in
small amounts whereas elements like Ti, Ta e.t.c. are added within acceptable limits. The bond
coats are typically oxidation resistant metallic layers, ~75-150 µm and are made of NiCrAlY
or NiCoCrAlY and are deposited using Electron beam PVD or Plasma spray method. Other
types of Bond coats, in use, are aluminides of Pt or Ni, deposited using CVD or Diffusion
aluminizing.
TGO comes into existence when the bondcoat undergoes oxidation at temperatures as high as
~700⁰C. The TGO(1-10µm) is present between the bondcoat and the ceramic topcoat. It is
generally required to ensure that the TGO is formed as alpha-Al2O3 such that the oxide growth
is slow and uniform, as because such oxide layer has very low ionic diffusivity and prevents
further oxidation of bond by setting up a diffusion barrier and thus, preventing further
oxidation(to be more specific, the TGO is expected to restrict the inward diffusion of oxygen
through it).
Ceramic top coat provides thermal insulation and is typically made of YSZ(Y2O3-stabilized
ZrO2). YSZ possesses a no of interesting properties including thermal conductivity, as low as
2.2 W/mK at 1000⁰C, for a fully dense material and a high concentration of point
defects(oxygen vacancies and substitutional solute atoms), which scatter phonons(lattice
waves). YSZ also has a high thermal expansion coefficient(~11x10^(-6)/C), which helps
alleviate thermal stresses (due to thermal expansion mismatch coefficient) between topcoat and
underlying metal( ~ 14x10^(-6)/C). Besides, YSZ is resistant to both ambient and hot
corrosion, possesses low density(~6.4 mg/cubic m) and has high hardness(~14 GPa). Cracks
and porosities are deliberately engineered into ceramic topcoat, in order to make it highly
compliant(young’s modulus~50 GPa). Although ZrO2 can be stabilized by a host of different
oxides (MgO, CeO2, Sc2O3, In2O3, CaO), Y2O3(~7-8 wt.%) -stabilized ZrO2 (YSZ), to be found
to stabilise tetragonal phase, most suitable for TBC applications. This amount of ZrO2 is
required to ensure that ZrO2 does not undergo Martensitic phase transformation. Although there
are various methods for depositing ceramic coatings on metal substrates, the two most

5. important methods used for TBC top-coat deposition are (i) air-plasma-spray (APS) deposition
and (ii) electronbeam physical-vapor deposition (EB-PVD).
Air Plasma sprayed(APS) TBCs(~300 µm)
This gives rise to “splat” grain morphology(1 to 5 mm thick, 200 to 400 mm diameter) possess
inter-splat boundaries and cracks parallel to metal/ceramic interface. They(these TBCs) are
characterised by very low thermal conductivity and highly undulating nature of metal/ceramic
interface. However, due to the undulating nature of interface and cracks parallel to the interface,
their thermal cycling lives are much shorter than EBPVD coated TBCs. Thus, they are used in
less exciting applications(involving low temperature applications) such as blade and vane
applications of combustors.
In service conditions, it has been reported that stresses are tensile at bond-coat/TGO undulation
crests and compressive at troughs. Thus, as the TGO grows, the tensile stresses lead to cracking
at bondcoat/TGO interface. If there is a significant thermal-expansion coefficient mismatch
between top-coat and the bondcoat/ superalloy, top-coat is put in overall compression at room
temperature. However, these stresses are much lower than the residual stresses in the TGO,
primarily because the porous and cracked top-coat is much more compliant with respect to
TGO, and it has a relatively lower thermal expansion–coefficient mismatch with the bond-coat.
Once again, because of the highly undulating nature of the metal/ceramic interface, out-of-
Fig 1. Cross sectional view of EBPVD coated TBC in a turbine blade,
taken using SEM. Besides, the temperature profile across different layers
of TBC has also been shown. The turbine blade contains internal hollow
channels for air cooling.

6. plane stresses result in the vicinity of the TGO/top-coat interface: tension at the crests and
compression at the troughs. The tension causes fracture along the TGO/top-coat interface at
the crests. It is observed that due to highly undulating nature of metal/ceramic interface, there
are again tensile stresses induced at undulation crests of TGO/top-coat interface and
compressive stresses at undulation troughs. This leads to cracking at highly brittle top-coat in
the near vicinity of TGO/top-coat undulation crest.
Fig 2. Cross-sectional SEM of an air-plasma sprayed (APS) TBC that has
been subjected to 120 thermal cycles

7. Fig 3. (A) Figure showing 4 different cracking mechanisms in APS coated
TBCs and (B) Cross section of failed APS coated TBCs(240 cycles).

8. Electron Beam PVD(EBPVD) coated TBCs
These are characterized by; (i) Presence of a thin region of polycrystalline YSZ with equiaxed
grains (size 0.5 to 1 mm) at or near the metal/ceramic interface; (ii) columnar YSZ grains (2 to
10 mm diameter) growing out of equiaxed-grain region to the top-coat surface; (iii) nanometer-
scale porosity within the columnar grains; and (iv) channels separating columnar grains, normal
to the metal/ceramic interface, giving rise to ‘strain tolerance’ by accommodating thermal
expansion mismatch stresses. The cracks and porosities lead to lowering of thermal
conductivity, here as well, but to a lesser extent than APS coated TBCs as because channels,
separating columnar grains, are found to be parallel to direction of heat flow. However, they
are costly relative to APS coated TBCs but find major application in blades and vanes of aircraft
engines.
Due to presence of channels separating columnar grains, the top-coat in EBPVD TBCs is more
“strain tolerant” than that in APS TBCs, and thus, various cracking events in this system occur
at the bond-coat/TGO or the TGO/ top-coat interfaces. There are 3 major failure mechanisms
in such systems: (i) progressive TGO roughening caused by bond-coat cyclic creep (ii)
accelerated growth of embedded oxides due to localized TGO cracking and (iii) bond-coat
cavity formation. The crests in the case of EB-PVD are “ridges” present on the bond-coat
surface before top-coat deposition.

9. Fig 4.(A) Diagram showing two of the three different cracking mechanisms in EB-PVD TBC.
Cross-sectional view through SEM showing (b) and (C)different mechanisms of failure at
different cycles, and (D) large-scale buckling (1830 cycles) where bond-coat surface
imperfections were eliminated before top-coat deposition.

10. Damage Accumulation and Failure
The formation of alpha alumina TGO results in deficiency of Al in bondcoat which leads to
formation of Y3Al5O12, and Y2O3 which compromises with the structural integrity of the TGO
and accelerates localized oxidation by providing fast oxygen- diffusion paths. During thermal
cycles, leading to cyclic creep, there is roughening(also called ratcheting) of
bondcoat/TGO/topcoat interfaces. During cyclic creep, there is lengthening of TGO, either due
ed primarily due to segregation of undesirable elments such as to cracking in TGO or due to
in-plane growth during cyclic oxidation. Such phenomenon is not observed under isothermal
exposures.
There are a variety of ways by which crack initiation and propagation may occur in TBCs,
however these are TBC specific. However, the ultimate failure of the TBCs has been reported
to occur due to occur due to coalescence of cracks.
In both APS as well as EBPVD coated TBCs, interfacial fracture is caused due to degradation
of interfacial toughness, such as ‘S’. Sintering in the topcoat at operating temperatures, results
in the partial healing of the cracks, a reduction in porosity, and also accelerates TBC failure by
making the top-coat less “strain tolerant”. In addition, sintering increases the thermal
conductivity of the topcoat, which may lead to deleterious consequences.