Ultra High Temperature Ceramics for
Thermal Barrier Coatings on Nickel Alloy Turbine Blades
Shaban Rexha & Paul Weidler
Submitted in partial fulfillment of course requirements for
MatE 115, Fall 2015
17 November 2015
TABLE OF CONTENTS
CHAPTER 1: INTRODUCTION ...................................................................................... 1
1 ULTRA HIGH TEMPERATURE CERAMICS ........................................................................ 1
1.1 PROPERTIES ................................................................................................................ 1
1.2 APPLICATIONS ............................................................................................................ 2
CHAPTER 2: PROCESSING METHODS AND COMPOSITIONAL ANALYSIS ........ 3
2.1 DEPOSITION METHODS ............................................................................................... 3
2.2 COMPOSITION ............................................................................................................. 3
2.3
CHEMICAL AND STRUCTURAL ANALYSIS OF TBC’S .................................................. 4
CHAPTER 3: THERMOMECHANICAL PROPERTIES ................................................. 8
3.1 THERMAL INSULATOR PROPERTIES ............................................................................. 8
3.2 ADHESION AND MECHANICAL STABILITY ................................................................... 9
CHAPTER 4: MICROSTRUCTURAL ANALYSIS AND FRACTURE BEHAVIOR .. 10
4.1 MICROSTRUCTURE ANALYSIS ................................................................................... 10
4.2 FAILURE MODES AND EFFECTS OF PROCESSING ON FRACTURE MECHANICS ............. 12
CHAPTER 5: CONCLUSION ......................................................................................... 14
REFERENCES ................................................................................................................. 16
CHAPTER 1: INTRODUCTION
1
Ultra High Temperature Ceramics
1.1
Properties
Ultra high temperature ceramics (UHTCs) are a classification of ceramic materials that
are able to withstand temperatures in excess of 2000°C without experiencing deformation,
fracture, or corrosion. The property of these ceramic materials to maintain structural stability and
mechanical strength in extremely high temperature environments gives them a unique potential
to be used in various applications where the effects of exposure to high temperatures must be
addressed and mitigated. Ultra high temperature ceramic materials are typically developed using
compounds consisting of diborides, carbides, and nitrides, as well as hafnium, zirconium,
tantalum, and silicon. These elements are used in the development of UHTCs due to their unique
crystalline structures and compositions, as well as the high bonding strength of the covalent
bonds that they typically form. The processing of these ceramics often involves methods of
densification via high temperature and high pressure hot pressing, or special methods of
sintering. Overall, the integration of selected compounds during the unique synthesizing process
of UHTCs allows these ceramics to have high mechanical strength, toughness, and melting
temperatures, as well as maintain high oxidative and corrosive resistance, and high thermal
conductivity in extremely high temperature environments.
1.2
Applications
Ultra high temperature ceramic materials are needed in order to advance and support
ongoing research and development in aerospace and hypersonic vehicle applications. UHTCs
incorporating hafnium and zirconium compounds can be used to coat the exterior surface of
commercial airliners and space shuttles in order to allow for better efficiency, performance, and
structural stability when operating in the presence of extremely high temperature gradients. The
need for the unique mechanical and thermal properties of UHTCs in aerospace applications is
also applied to hypersonic applications as well. These ceramics are used in transportation
applications involving hypersonic speeds where thermal conductivity, oxidation, and structural
stability in extremely high temperature scenarios are design concerns.
2
CHAPTER 2: PROCESSING METHODS AND COMPOSITIONAL ANALYSIS
2.1
Deposition Methods
The usual method for depositing a zirconia TBC is air plasma thermal deposition [1, 2, 5,
7, 8, 9, 11]. This method employs a jet of air to spray particles of ceramic through an electrode
couple at high voltage, resulting in a plasma that turns the ceramic particles into molten droplets.
These solidify into “splats” on impact. The complete TBC is built up of these splats, as layers of
solid form atop each other. The cooling rate and size of the splats, as well as the temperature of
the substrate, are important for the bulk properties and microstructure [2, 5]. The addition of a
polymer filler [7] in the stream of molten particles provides spaces between the ceramic grains,
resulting in a highly porous coating, which can allow for cooling gas to flow through the coating.
A variation of plasma spray is suspension plasma spraying [2], where the ceramic particles are
suspended in a liquid before injection into the plasma.
A bonding layer is often used to improve the matching of thermal expansion, which
reduces cracking at the interface and improves toughness and lifetime [4, 5, 6, 9, 11]. This can
be a YSZ layer deposited under different conditions than the bulk YSZ layer [5, 11], a layer of
YSZ mixed with other metal oxides [9], or a metal oxide layer grown from the substrate using
chemical deposition methods [4, 6]. Magnetron plasma chemical deposition has been used to
chemically deposit an adhesion layer before YSZ is applied [4], and a YSZ/Al2O3 layer was
prepared by electrophoretic deposition of YSZ– Al2O3 in composite suspension [6].
2.2
Composition
The composition of the coating can be manipulated by carefully controlling the particle
3
size and ratio of ceramic metal oxides [1, 4, 6, 8, 10]. For example, scandia, gadolinia, and
yttria, when injected into the air plasma with zirconia particles, will form splats that contain a
stoichiometric mixture of oxides optimized for lattice structure stability over thermal cycling [1].
This is an improvement over the more common Yttria Stabilized Zirconia [4], which displays a
transition between the tetragonal to the monoclinic phase on cooling. This phase change is
undesirable because the internal stresses caused by the change in volume of the two crystal types
can lead to cracking and failure.
Bond coat composition is chosen to improve the matching of characteristics between the
substrate and the TBC. In the case of a Zirconia-based ceramic on a nickel superalloy, NiCrAlY
alloy bond coat [8] with 2 different proportions of Scandia and Yttria in zirconia TBC, showed a
tradeoff between thermal cycling lifetime and thermal expansion mismatch. On aluminum
substrates, YSZ on YSZ/Al2O3 layers improved oxidation resistance at the metal surface
interface [6].
2.3
Chemical and Structural Analysis of TBC’s
The most common method used to characterize the chemical composition of ceramic
coatings, bond layers and substrate is SEM/EDS [2, 8, 9, 10, 11]. EDS detectors on SEM
systems allow researchers to use EDS while obtaining structural information with the SEM. A
section of coated metal can contain several layers. The composition of those layers must be
controlled as closely as possible. SEM/EDS enables determination of elemental composition to a
4
few atomic percent with good quantitative resolution.
TEM and TEM Diffraction are used to determine physical morphology and crystal structure [3,
10]. High Resolution TEM allows imaging of the crystal structure on the atomic level, as in
Figure 1. Figure 2 shows the electron diffraction pattern of ZrO2 in a deposited coating.
Fig. 5. HRTEM images of the three different interfaces in the ternary eutectic
in the as processed condition. (a) Al2 O3 –YAG. (b) Al2 O3 –YSZ. (c) YAG–YSZ.
The main atomic planes are marked in the figures.
Fig. 6. HRTEM images of the three different interfa
after testing. (a) Al2 O3 –YAG. (b) Al2 O3 –YSZ. (c) YA
respond to the superplastically deformed region. Th
marked in the figures.
Figure 1: HRTEM image of phase boundary in YAG/YSZ eutectic [3]. The crystal planes are marked.
Figure 2: TEM Diffraction pattern of ZrO2 [10].
5
XPS enables identification of both elemental and molecular bonding of a material [11]. The
elemental lines are shifted by binding energy of the oxygen bonds, which allows for
identification of the bond type.
Dynamic SIMS is used to characterize multiple layers of material [10]. The ion beam removes a
layer of material at a time as it is repeatedly rastered over a rectangular area as in Figure 3(b).
Figure 3: (a)FIB-EI of cross section microstructure of ZSLO oxidized at 1600◦ C for1h showing∼250umthick
oxidized layers, (b) crater formed at the interface of layers 1 and 2 and (c) positive mass spectra taken on
rectangular crater of (b) showing the presence of B, Si, Zr, La and their respective oxides. [10]
Electron Backscatter Diffraction Analysis shows the orientation of the grains in the deposited
6
coating [2]. The individual splats quickly solidify into crystals, at random orientation. Figure 3
shows random orientation of crystal planes in an experimental mixture of YSZ.
Figure 4: Electron Backscatter Diffraction image of a YCSZ coating [2]
7
CHAPTER 3: THERMOMECHANICAL PROPERTIES
3.1
Thermal Insulator Properties
In regards to high pressure turbine blades in high temperature jet engine applications,
heat resistant zirconia ceramic coatings can be used to protect turbine blades from wear and
degradation due to high temperature exposure. Zirconia coatings for high pressure turbine blades
consist of heat resistant connecting layers bound to an external zirconia ceramic layer. These
zirconia layers can consist of columnar, layer-porous, or fragmented structural layouts, as these
structures are able to provide effective thermal insulation and resistance for high pressure turbine
blades in highly fluctuating temperature environments [5]. A major concern in high pressure and
high temperature turbine blade applications is the rate of heat transfer to the metal alloy turbine
blade during high temperature exposure. Porous zirconia ceramic coatings are used to
significantly reduce the thermal transfer of heat energy from the high temperature operating
environment to the alloy turbine blade. The low conductivity of the ceramic coating reduces the
surface temperature of the metal alloy blade-ceramic coating system and reduces heat transfer to
the metal alloy turbine blade components, thus allowing the blade to maintain a stable
temperature while operating in high temperature conditions [8]. Processing the zirconia coating
to have high porosity allows for greater stability of the coating itself which increases the
durability and heat resistance of the coating and in turn extends the operating lifetime of the
coating during extended service periods in high temperature environments.
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3.2
Adhesion and Mechanical Stability
Processing zirconia ceramic coatings to yield porous or non-porous structural phases is
significant in the resulting levels of adhesion strength and overall mechanical stability. Zirconia
thermal barrier coatings are traditionally produced in the non-porous structural phase, while
spray deposition processing methods can be utilized to develop zirconia coatings with high
porosity [8]. Porous zirconia coatings have been found to have about one-half of the thermal
conductivity value of non-porous forms, thus the porous structural layout is generally more
effective at maintaining thermal insulation during periods of extensive high temperature
exposure [8]. In regards to adhesion strength, the porous structural phase displays low adhesion
strength at about 20% of the strength maintained by the non-porous phase, indicating that the
non-porous phase has greater mechanical stability [8]. Overall, the porous structural phase
displays more desirable thermal properties in terms of high thermal resistance and low thermal
conductivity in high temperature operating conditions when compared to the non-porous phase,
yet displays lower adhesion strength compared to the non-porous phase. Thus, the non-porous
phase maintains greater mechanical stability during long term high temperature exposure yet
may be less thermally effective, while the porous phase remains more susceptible to degradation
due to adhesive failure.
9
CHAPTER 4: MICROSTRUCTURAL ANALYSIS AND FRACTURE BEHAVIOR
4.1
Microstructure Analysis
Suspension plasma spraying is a processing technique that can be used to produce a two-
zone microstructure for zirconia ceramic coatings. This two-zone interface consists of a dense
zone packed with large lamellas, and an unmelted particle zone with fine grains in an irregular
distribution. Recent developments in suspension plasma spraying methods that utilize very fine
particles has resulted in the processing of a columnar microstructure for zirconia based ceramics
[3]. This columnar microstructural arrangement is effective in thermal barrier coating
applications as it improves the thermo-mechanical properties of zirconia ceramics. Producing
this microstructure via the appropriate plasma spraying methods allows for a reduction in
residual stresses, higher durability, thermal shock resistance, and overall ceramic lifetime
extension [3]. Oxidation processing can also enhance the thermal insulative properties of
zirconia based ceramic coatings and increase their effectiveness in ultra high temperature
applications and working environments. Sintering and oxidation processing methods at
temperatures in excess of 1600°C can used to produce high refractory crystalline oxide
intermediate microstructures and surface layers on zirconia ceramics with high thermal insulative
and resistive properties [10]. Integrating these surface oxide layers into the microstructural
surface strengthens weakly linked interface points that are susceptible to wear and fatigue and,
protects against microstructural degradation that can occur in high temperature environments [4].
Implementing directionally solidified eutectic oxides into the surface microstructure provides
high structural stability and desirable thermo-mechanical properties for zirconia coatings in ultra
high temperature applications [4]. Additionally, varying sintering processes can provide zirconia
10
coatings with properties of high emissivity and low surface catalycity. This reduces the
temperature gradients and thermal stresses introduced into the microstructure during high
temperature service conditions and promotes oxidative recombination leading to oxidation of the
microstructural surface [11]. The result of this microstructural oxidation includes increased
strength and thermal resistance of the overall ceramic coating.
Various thermomechanical properties of ceramics such as strength, fracture toughness,
thermal conductivity, and oxidative resistance can be controlled by altering the grain size, grain
shape, and oxide formation within the microstructure via specialized processing methods [9]. Hot
pressing at temperatures in excess of 2000°C for extended time periods is a common processing
method for zirconia ceramic coatings. However, by implementing spark plasma sintering
methods, the processing time and temperature for the ceramic coating can be reduced and also
yield a ceramic microstructure with reduced grain sizes proving the overall coating with
increased strength, hardness, and fracture toughness [9]. Grain shapes can be controlled and
altered by introducing particle coatings into the microstructure during heat treatment and high
temperature processing. This method of grain shape alteration also causes and increase in grain
boundary density in the ceramic microstructure, thus providing more barriers to fracture via
crack propagation and increasing the fracture toughness and fracture strength of the ceramic
coating [9]. Surface oxidation and oxidative resistance can also be controlled via implementing
the appropriate microstructural processing and development methods. Oxidative properties can
be altered by integrating additional elemental species and elemental phases into the zirconia
ceramic coating and zirconia microstructure, as this increases the diffusion rate of oxygen upon
the surface of the ceramic coating and thus increases the oxidation rate [9]. Slightly reducing the
11
zirconia concentration will also supplement this process, while usually little additional elemental
species or increasing zirconia content can reduce the rate of surface oxygen diffusion and oxide
formation and the overall oxidative properties.
4.2
Failure modes and Effects of Processing on Fracture Mechanics
Zirconia based thermal barrier coatings typically experience failure by way of crack
initiation and propagation mechanisms. The processing methods used to interface the zirconia
interlayers with the bond coating and substrate phase often determine the density of cracks and
voids in the final coating. Zirconia based coatings are commonly developed using deposition
methods of atmospheric plasma spraying and electron beam physical vapor deposition. These
methods give zirconia ceramic coatings properties of high strain tolerance and low thermal
conductivity, yet result in ineffective mechanical performance by providing the coating with low
fracture toughness. The main path of fracture typically occurs via crack propagation at the
zirconia bond coat interface [5]. During exposure to high thermal conditions, zirconia grains
parallel to the zirconia bond coat interface can begin to wear which leaves behind a thin residual
zirconia coat layer across the interface surface. This thin layer near the interface is highly
susceptible to crack propagation, as thermal expansion mismatching creates stresses that initiate
crack formation and propagation throughout the overall zirconia coating [5].
In order to prevent this mode of fracture and crack propagation, processing methods
involving the increase in strain tolerance and decrease in surface roughness must be used.
Solution precursor plasma spraying techniques can be used to promote the formation of inter-
12
lamellar bonds and increase the inter-lamellar bonding ratio in the zirconia interface [5]. This
allows the fracture toughness of the coating to increase, yet comes at the cost of reducing heat
insulation properties. However, this can be addressed by promoting inter-lamellar bond
formations during interlayer deposition at high surface temperatures (over 923°C) and
developing additional barriers to crack propagation pathways. This method has shown to
significantly increase the strength of the inter-lamellar bonds at the zirconia bond coat interface,
thus improving the fracture toughness of zirconia coatings and extending the thermal cyclic
lifetime of these coatings in high thermal applications [5].
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CHAPTER 5: CONCLUSION
The engineering challenges are to produce a ceramic coating that is an excellent thermal
insulator, chemically resistant to combustion products at working temperatures, that has good
adhesion, and is able to withstand strain from thermal expansion cycling of the underlying metal.
We have examined these properties, and the techniques to achieve them in zirconia coatings
applied to jet engine turbine blades. Thermal plasma spray deposition is commonly used to
deposit both lamellar undercoat and porous or columnar top layers, with many variations of
spray parameters and substrate conditions that determine the structure of the TBC. This effects
the mechanical properties such as porosity, toughness, resistance to corrosion, and thermal
insulative ability. Yttria Stabilized Zirconia can be combined with other metal oxide ceramics to
adjust the properties of the resulting coating, adding phase stability or chemical resistance.
Bonding layers can reduce failure from cracking at the substrate/coating interface by matching
the thermal expansion properties. This reduces the primary mode of crack failure and spalling.
The ability to implement different processing methods to alter the specific composition
and microstructure in order yield the desired thermal, mechanical, adhesive, and fatigue
properties also makes these coatings adaptable for different thermal applications with specific
design and operational constraints. Overall, ultra high temperature ceramics are a complex and
unique class of materials that are a significant research topic in materials engineering and a
crucial material in the success of high thermal engineering applications.
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15
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