Thermal barrier coatings (TBCs) allow gas turbine engines to operate at higher temperatures, improving efficiency. There are two main methods for depositing TBCs: plasma spraying and electron beam physical vapor deposition. Plasma spraying involves melting and accelerating materials using a plasma jet, resulting in a splatted microstructure, while EBPVD uses an electron beam to evaporate materials into a columnar structure. Common TBC materials include yttria-stabilized zirconia due to its low thermal conductivity. TBCs can fail through oxidation, hot corrosion, or deposition of combustion byproducts on the coating surface.

1. Development of Thermal
Barrier Coating by thermal
spraying for high
temperature application
By Nishant Kumar Mehta

2. CONTENT :
I. Introduction
II. Plasma sprayed TBC
III. Electron Beam- Physical Vapor Deposition
IV. Material Selection
V. Failure
VI. References

3. I. INTRODUCTION :
 Thermal barrier coatings (TBCs)
enable the engines to operate at
higher temperatures without
raising the base metal
temperatures using cooling
systems inside the hot section
components and thus, enhance
the operating efficiency of the
engines.
 TBCs perform the important
function of insulating
components, such as gas turbine
and aeroengine parts, operating
at elevated temperature.
 TBCs provides a temperature
reduction of up to 300 K in the
surface of the superalloy
component.
Layered structure of Thermal
Barrier Coating on a turbine
blade

4.  TBCs have made possible the
increase in operating
temperature of gas turbines.
 TBCs are characterised by their
very low thermal conductivity,
the coating bearing a large
temperature gradient when
exposed to heat flow.
 TBCs have also contributed to
reducing metal temperature ,
thus improving the duration
capability of components.
Thermal barrier coating on
an automotive exhaust
system component

5. II. PLASMASPRAYEDTHERMAL BARRIERCOATINGS:
Plasma-sprayed TBCs have been
extensively used in hot sections of gas
turbine engines.
A high frequency arc is ignited between
an anode and a tungsten cathode. The
gas flowing through between the
electrodes (i.e., He, H2 , N2 or
mixtures) is ionized such that a plasma
plume several centimeters in length
develops.
The temperature within the plume can
reach as high as 16000° K. The spray
material is injected as a powder
outside of the gun nozzle into the
plasma plume, where it is melted, and
hurled by the gas onto the substrate
surface.
PLASMA SPRAYING setup

6.  DevelopmentofTBCinPlasmaSpraying:
 The coatings are formed by successive build-up of splats. The microstructure of the
sprayed coatings is characterized by the existence of splats along with other
microstructure features such as interlamellar and globular pores, intrasplat cracks and
intersplat gaps.
 The lifetime, mechanical and thermal properties of the TBCs are closely related to the
microstructure of the ceramic top coatings. Generally, a certain amount of pores or
micro-cracks is favorable in achieving a high level of thermal shock resistance, because
the micro-cracks and pores can serve as fields of stress concentration where the
damage initiates and coalesces, as well as provide strain tolerance to the coating.
 Due to the relatively weaker bonding between lamellae and rougher interface
morphologies, PS TBCs generally have shorter thermal cycling lifetime when compared
to EB-PVD TBCs. This makes APS TBCs suitable only for less exacting applications in
turbine engines, such as combustors, after-burner flame holders, and stator vanes.

7.  ModificationofPSTBCs:
 In order to improve thermal cycling
performance, microstructure modifications of
PS TBCs have been conducted. TBCs with
extremely high porosities have been
produced by spraying of plastic–ceramic mixed
powders.
 The thermal shock resistance is greatly
improved by the introduction of so-called
segmentation cracks, in particular into the
extremely thick thermal barrier coatings
(>1mm thickness) .
 Recently, some innovative plasma-spray
techniques have been employed to produce
TBCs with lower thermal conductivities and
more durable performance. The solution
precursor plasma spray (SPPS) has been
exploited to deposit TBCs, in which an
aqueous chemical precursor feedstock, rather
than YSZ powders, is injected into plasma jet.
 The lack of large scale ‘splat’ boundaries
effectively toughens the TBCs, making SPPS
TBCs highly durable relative to APS TBCs.
SPPS setup

8. III. ELECTRONBEAM-PHYSICALVAPORDEPOSITION:
 EBPVD is a form of physical vapor deposition in which a target
anode is bombarded with an electron beam which causes atoms
from the target to transform into gaseous phase.
 These atoms then precipitate into solid form ,coating everything in
the vacuum chamber(within line of sight) with a thin layer of the
anode material.
 The EB-PVD process allows attaining coatings with unique
properties. The process parameters are adjusted so that the
deposit has a columnar grain structure perpendicular to the
interface. This morphology maximizes the resistance to strains that
arise from differences in thermal expansion coefficients.

9.  Advantages of EBPVD :
 The material utilization
efficiency is high relative to
other methods
 Offers structural and
morphological control of films.
 Due to high deposition rate, this
process has potential industrial
application for wear resistant
and TBC’s in aerospace
industries
 Disadvantages of EBPVD :
 This process cannot be used to
coat the inner surface of
complex geometries
 Filament degradation in the
electron gun results a non-
uniform evaporation rate
EBPVD setup

10. IV. MATERIAL SELECTION :
Nowadays, considerable effort is being invested in seeking new
materials. The choice of TBC materials is based on several
requirements:
Low thermal conductivity;
High temperature capability;
Phase stability during thermal cycling between room temperature
and operation temperature;
Chemical inertness with TGO and metallic bond coat;
Thermal expansion match with metallic substrate;
Good adherence b/w metallic bond coat and ceramic top coat;
Mechanical properties to sustain strength in high-pressure and
high-temperature environments and resistant to erosion;
Low sintering during high-temperature exposure

11. YSZ
Zirconate pyrochlore
oxides
Fluorite oxides
Mullite
Metal-Glass composites
MCrAlY
(M= Co, Ni or Co/Ni)
NiAl coating
Diffusion barrier coatings
 Typesof Coatings:

12. V. FAILURES :
High Temperature oxidation of La2Zr2O7 TBC
(a) surface (b) Cross-section
Hot corrosion due to V2O5+Na2SO4 molten
salts on YSZ+Al2O3 TBC
Deposition of combustion deposits (CMAS)
on YSZ TBC