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Sol-Gel Thermal Barrier Coating (TBC) for use on Titanium
Substrates with Complex Geometries – Feasibility Study
by
Brittany R. Travis
A Master’s Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the degree of
MASTER OF MECHANICAL ENGINEERING
Approved:
_________________________________________
Sudhangshu Bose, Master’s Project Adviser
Rensselaer Polytechnic Institute
Hartford, Connecticut
December, 2012
© Copyright 2012
by
Brittany R. Travis
All Rights Reserved
ii
TABLE OF CONTENTS
TABLE OF CONTENTS ................................................................................................. iii
LIST OF TABLES ............................................................................................................ iv
LIST OF FIGURES ........................................................................................................... v
ACKNOWLEDGMENT .................................................................................................. vi
DEDICATION ................................................................................................................. vii
ABSTRACT ................................................................................................................... viii
1. Introduction.................................................................................................................. 1
1.1 Background ........................................................................................................ 1
1.2 Component ......................................................................................................... 2
2. Substrate Materials ...................................................................................................... 3
2.1 Steel .................................................................................................................... 3
2.2 Aluminum .......................................................................................................... 3
2.3 Nickel ................................................................................................................. 5
2.4 Titanium ............................................................................................................. 6
3. Coating Materials....................................................................................................... 10
3.1 Metallic Coatings ............................................................................................. 11
3.2 Ceramic Coatings ............................................................................................. 12
3.3 Thermal Barrier Coating Systems .................................................................... 12
4. Coating Processes ...................................................................................................... 14
4.1 Line-of-Sight Processes ................................................................................... 14
4.1.1 Thermal Spray Processes ..................................................................... 15
4.1.2 Cold Spray............................................................................................ 17
4.1.3 Physical Vapor Deposition................................................................... 18
4.2 Non Line-of-Sight Processes............................................................................ 20
4.2.1 Diffusion .............................................................................................. 20
4.2.2 Sol-Gel ................................................................................................. 22
5. Methods Used to Apply Sol-Gel on Substrates ......................................................... 31
5.1 Titanium with applied Sol-Gel TBC ................................................................ 33
6. Conclusion ................................................................................................................. 34
7. Definitions ................................................................................................................. 35
References........................................................................................................................ 37
iii
LIST OF TABLES
Table 1: Aluminum Alloy Series (from Kaufman, J. G. "Chapter 4: Aluminum Alloys."
Handbook of Materials Selection. Ed. M. Kutz. John Wiley & Sons, 2002. 89-134.
Knovel. Web.) .................................................................................................................... 4
Table 2: Comparison of Common High Temperature Nickel Superalloys (from
Bassford, T. H., and Jim Hosier. "Chapter 7: Nickel and Its Alloys." Handbook of
Materials Selection. Ed. Myer Kutz. John Wiley & Sons, 2002. 235-58. Knovel. Web.) 5
Table 3: Typical TBC Thicknesses for Conventional Coating Processes (from Bose,
Sudhangshu. High Temperature Coatings. Elsevier, 2007. Print.) .................................. 12
Table 4: Densities of Ceramic & Metallic Coatings (from Bose, Sudhangshu. High
Temperature Coatings. Elsevier, 2007. Print.) ................................................................ 13
Table 5: Different Types of Gels (from Carter, C. B., and M. G. Norton. Ceramic
Materials: Science and Engineering. New York: Springer Science+Business Media,
LLC, 2007. Print. Table 22.5 pg. 406)............................................................................. 28
iv
LIST OF FIGURES
Figure 1: Strength versus Temperature of Nickel Based Alloys (Donachie, Matthew J.,
and Stephen J. Donachie. "Chapter 10: Selection of Superalloys for Design." Handbook
of Materials Selection. Ed. Myer Kutz. John Wiley & Sons, 2002. 312. Knovel. Web.) . 6
Figure 2: Stress versus Cycles to Failure of Different Forms of Manufactured Titanium
(From Donachie, M. J. (2002). Handbook of Materials Selection: Chapter 6 Selection of
Titanium Alloys for Design. M. Kutz (Ed.). John Wiley & Sons., p. 226) ....................... 7
Figure 3: Titanium Alloys Crystal Structure versus Temperature (from Donachie,
Matthew J., Jr. TITANIUM: A Technical Guide. Materials Park: ASM International,
2000. Print. Fig 3.1 pg. 13) ................................................................................................ 8
Figure 4: Component with Non Line-of-Sight Access ................................................... 15
Figure 5: Hierarchy of Line-of-Sight Coating Processes ............................................... 15
Figure 6: Plasma Spray Deposition (Crawmer, Daryl E. "Introduction to Coatings
Equipment, and Theory." Handbook of Thermal Spray Technology. Ed. J. R. Davis.
ASM International, 2004. Fig. 1. Pg. 43. Knovel. Web.) ................................................ 16
Figure 7: EB-PVD Columnar Grains (Wahl, G., W. Nemetz, M. Giannozzi, S.
Rushworth, D. Baxter, N. Archer, F. Cernuschi, and N. Boyle. "Chemical Vapour
Deposition of TBC: An Alternate Process for Gas Turbine Components." ASME
TURBOEXPO. Munich, Germany: ASME, 2000. Google Scholar. Web. Figure 9) ...... 19
Figure 8: Hierarchy of Non Line-of-Sight Coating Processes ....................................... 20
Figure 9: Sol-Gel Process ............................................................................................... 23
Figure 10: Comparison of Acid Catalyzed vs. Base Catalyzed Gels (a) acid-catalyzed,
(b) base-catalyzed, particulate silica gels aged under conditions of high (c) or low (d)
solubility. (from Brinker, C. J., and George W. Scherer. SOL-GEL SCIENCE: The
Physics and Chemistry of Sol-Gel Processing. San Diego: Academic, 1990. Print. Figure
6 on Pg. 363) .................................................................................................................... 26
v
ACKNOWLEDGMENT
I’d like to thank Professor Bose for his assistance with this paper and patience in
teaching me. Thanks and appreciation go out to all of my friends who helped me through
this. I’d also like to thank all of my co-workers who had to endure me talking about SolGel for months on end.
vi
DEDICATION
This paper is for my father, Arthur Ray Travis, Jr., whose life was taken so abruptly
April 2, 2010. He did the best he could with what he had no matter what and showed me
that you never give up even when life is terrible.
Dad,
You understood me like no other. I also understood you like no other. We wouldn’t have
to speak or look at each other to know what the other thought or felt. I miss our
conversations and your craziness, but most of all I miss your bear hugs!
I will always be your brown eyed ‘squirrel’ and I promise to one day put your ashes to
rest as you requested.
Love,
Brittany
vii
ABSTRACT
Sol-Gel is a non-line of sight coating process which makes it inviting to industries that
utilize coatings on complex geometries that cannot be accessed by traditional line-ofsight coating processes. This paper describes the Sol-Gel [1][2] process for Thermal
Barrier Coating (TBC) application on complex geometries of Titanium alloys, for use in
aerospace applications. The TBC protects the Titanium alloy from heat damage in use.
The paper is structured to give background of various aerospace materials with respect to
the prior work that has been done on Sol-Gel. Then the reasons for utilizing coatings,
typical coating compositions and coating processes are explored, highlighting the SolGel method. Lastly, the viability of using this Sol-Gel applied TBC on Titanium
substrates will be discussed.
viii
1. Introduction
1.1 Background
The continuous evolution of materials technology is important in many different
industries, such as, but not limited to, space exploration vehicles, gas turbine engines,
and steam turbines. Within the gas turbine industry, to improve thermodynamic
efficiency, modern gas turbines are being pushed to operate at higher and higher
temperatures. In order to achieve this, current materials are being pushed to their
thermal limits and to facilitate this, either new, higher-temperature capable materials or
methods to protect existing materials from the environment must be established. One
common method used today to protect substrates in high temperature environments are
by Thermal Barrier Coatings (TBCs). TBCs have low thermal conductivity 1 W/mK to
10 W/mK or higher for metals. Thus, under proper conditions, TBC thermally insulates
the underlying metallic components. TBCs normally consist of a metallic bond coat and
a ceramic top coat, but sometimes have an intermediate layer. The metallic layer is
characteristically NiCoCrAlY [4] providing oxidation protection and an adherent surface
while the ceramic layer is Yittrium-Stabilized Zirconium (YSZ) [4] for thermal
insulation and if an intermediate layer is used it is usually metallic like the bond coat.
TBCs are typically applied using a plasma spray (PS) method or electron beam physical
vapor deposition (EB-PVD). However, these two methods for applying TBCs are lineof-sight dependent and do not work well with complex geometries.
Sol-Gel is a non line-of-sight process that has recently become more effective within the
ceramics industry. The process was initiated in the mid 19th century. There has been
research and testing performed on the application of Sol-Gel TBC mainly on Steel and
Nickel alloys. There has also been research, testing, and use of Sol-Gel anti-corrosion
coatings, containing Zirconium, on Aluminum and Titanium alloys, but there is currently
no published research or testing of Sol-Gel TBC on Titanium alloys. This paper is based
on existing research and knowledge of both the Sol-Gel process and of Titanium alloy
composition. It attempts to best-fit the chemistry of the coating process with the
substrate.
1
1.2 Component
The component being studied in this paper could be any component within a gas turbine
engine, such as but not limited to vanes, blades, cases and shrouds that are utilized in
temperatures up to approximately 1000°F (537.8°C). The component is made up of
complex geometry in which line-of-sight coating processes will not work effectively. A
generic component requiring non line-of sight application of TBC is shown in Figure 4.
2
2. Substrate Materials
The main alloys that are utilized in the aerospace industry are those based on Aluminum,
Steel, Nickel and Titanium. The comparison of the materials strength to temperature
capability as well as density illustrates why the use of Titanium alloys are becoming
more prevalent.
2.1 Steel
Initially Steels were widely utilized in gas turbine engines. They are still being used but
to a lesser extent simply because of their weight impact. The density of steel ranges, but
for the usefulness of this discussion 304 Stainless Steel has a density of 0.286 lb/in3 [56].
Steels are classified into five different groups: ferritic, martensitic, martensitic age
hardening, duplex austenitic-ferritic and austenitic [5]. Steels are widely affected by
several different types of corrosion, such as stress-corrosion cracking, crevice corrosion
and pitting, as well as intergranular and galvanic corrosion [5]. It is important to note
that not all steels are as susceptible to corrosion as others and their susceptibility
depends on the metals that they are alloyed with. Their environmental resistance as well
as their temperature capability limits their use in gas turbine engines. High strength
steels maximum use temperature is 1200°F (650°C) [4] with a maximum yield strength
at temperature of 86ksi (600MPa)[4].
There has been some work done applying Zironia or Alumina Sol-Gel coatings on 304
stainless steel utilizing different methods of application. These can be seen in references
[6], [7], [8], [9] and [10]. This particular steel is one of the most widely developed
austenitic stainless steels and is relatively inexpensive. It also has relatively low strength
at high temperature; hence it is a good specimen to demonstrate that Sol-Gel coating
application can successfully be performed at low temperatures.
2.2 Aluminum
Aluminum alloys offer similar strength to mild steels while providing significant
advantages in weight. The density of Aluminum alloys varies, but for this discussion
3
Aluminum 7075 has a density of 0.101 lb/in3 [56]. However, Aluminum has a low
melting point of 1000°F [11] and can be used only up to 500°F-600°F (260°C-316°C),
dependent on the chosen alloy. Aluminum is also susceptible to corrosion and erosion
problems. The initial digit in the Aluminum alloy series designation gives a great deal of
information as in the strength, formability and corrosion resistance of the alloys, since
each designation correlates with the percentage of alloying materials. The Aluminum
alloy series are shown in Table 1. Aluminum alloys are widely used because of their
good strength-to-weight ratio as well as its desirable cost, but are limited by the
relatively low operating temperatures and lack of fatigue resistance.
Table 1: Aluminum Alloy Series (from Kaufman, J. G. "Chapter 4: Aluminum
Alloys." Handbook of Materials Selection. Ed. M. Kutz. John Wiley & Sons, 2002.
89-134. Knovel. Web.)
Al Alloy
Main Alloying
Ultimate Tensile
Series
Element
Strength (ksi)
2XXX
Copper
62
3XXX
Manganese
41
4XXX
Silicon
55
5XXX
Magnesium
51
6XXX
Magnesium Silicide
58
7XXX
Zinc
88
There have only been a limited number of published studies performed on coating
Aluminum by the Sol-Gel route. In particular reference [9] applied piezoelectric
polycrystalline ceramic (PZT) [12] Sol-Gel with Zirconia and Titania powder additives
to Aluminum foil successfully. The one outstanding application of Sol-Gel to Aluminum
alloys is made by Socomore and is known as SOCOGEL [13] utilized by Boeing as a
corrosion resistant coating applied to aircraft prior to painting. The method by which this
Sol-Gel is applied to Aluminum is non-toxic and does not require a bond coat, but does
require the metal to have a roughened surface as seen in reference [14]. More about this
method will be discussed in the later part of the report.
4
2.3 Nickel
Nickel alloys are able to withstand high temperature environments that other alloys
cannot. They also have excellent resistance to corrosion and high strength capability.
When Nickel is alloyed with certain other elements such as Iron, Chromium, and
Molybdenum, the alloy takes on unique properties and the strength and temperature
capability varies widely. A comparison of common Nickel base alloys, their yield
strength and maximum useful temperatures can be seen in Table 2.
Table 2: Comparison of Common High Temperature Nickel Superalloys (from
Bassford, T. H., and Jim Hosier. "Chapter 7: Nickel and Its Alloys." Handbook of
Materials Selection. Ed. Myer Kutz. John Wiley & Sons, 2002. 235-58. Knovel.
Web.)
Nickel Base Alloy
Inconel 718
Waspaloy
Hastelloy X
.2%YS (ksi)
168
115
52
Temperature °F
1300
1400
2200
Figure 1 illustrates the ultimate strength of various Nickel based alloys versus
temperature. It can be seen that Nickel based alloys have high strength to temperature
ratio. The major disadvantage of Nickel base alloys is their high density, which will
compound the weight of any system. The densities vary with each Nickel alloy, but for
this discussion INCO 718 has a density of 0.297 lb/in3 [56].
5
Figure 1: Strength versus Temperature of Nickel Based Alloys (Donachie,
Matthew J., and Stephen J. Donachie. "Chapter 10: Selection of Superalloys for
Design." Handbook of Materials Selection. Ed. Myer Kutz. John Wiley & Sons,
2002. 312. Knovel. Web.)
The most widely studied Sol-Gel TBC applied coatings on gas turbine engine
components have been on Nickel based alloys referenced in but not limited to [17], [18],
[19], [20], [21], [22] and [23]. It has also been seen that Sol-Gel processing has been
used on the already deposited base and top coats on Nickel based alloys to protect the
coating from degradation as in [24] and [25].
2.4 Titanium
Titanium alloys address some of the inherent disadvantages of both Aluminum and
Nickel. Titanium has a temperature capability that lies between that of Aluminum and
Nickel base alloys while simultaneously having relatively low density, approximately
0.163 lb/in^3 [3]. Titanium, if formed and protected properly with the use of TBC, can
be utilized at reasonably high temperatures. It retains good strength properties at those
temperatures. The fatigue strength of the alloy is affected by the form that it was
manufactured in. As seen in Figure 2, wrought and annealed Titanium is the strongest
and Cast plus HIP is relatively close behind.
6
Figure 2: Stress versus Cycles to Failure of Different Forms of Manufactured
Titanium (From Donachie, M. J. (2002). Handbook of Materials Selection: Chapter
6 Selection of Titanium Alloys for Design. M. Kutz (Ed.). John Wiley & Sons., p.
226)
Within the aerospace industry, casting Titanium is the new norm, because of the
advantages casting lends. The biggest advantage of casting is that it can produce
complex geometries that when formed by other means are either costly or impossible to
machine conventionally. Additionally the ability to use Stereolithography (SLA) for the
production of casting patterns can make casting a shorter lead-time process, especially
during the development phase. One disadvantage to the casting process is that it can
leave excess material behind and the producible tolerances increase with the increase in
size of the substrate. Casting Titanium has only become a viable production method
since the late 1960’s [27].
Titanium is a very reactive metal in its liquid state making it challenging to cast since it
can react with refractories or the atmospheric gases during the molding process. Over the
years the process of casting Titanium has evolved to combine a heat treatment called Hot
Isostatically Pressing (HIP) that leads to better mechanical properties. The basics of the
process are that the metal is heated up to 1650°F (899°C) [3] and compressed in a nonreactive medium to close porosity that was created during the pouring process of molten
metal. Titanium’s melting point is about 3000°F (1649°C) [3], but is typically not used
7
in operating temperatures higher than 1000°F - 1100°F (538°C-593°C) [3]. During
manufacturing, multiple heat treatments over 1000°F (593°C) may take place for repairs
on the part after HIPing, but are restricted to between 1100°F-1200°F (593°C - 649°C)
[3] to ensure the integrity of the mechanical properties last throughout the life of the
part.
The most common Titanium alloy within the aerospace industry today is Ti-6Al-4V.
While Ti-6AL-2Sn-4Zr-2Mo-Si is becoming more prevalent. Both alloys are considered
alpha-beta crystal structure as seen in Figure 3. Here Alpha represents a hexagonal close
packed (hcp) crystal structure and beta represents a body-centered cubic (bcc) crystal
structure. This crystal structure allows for better strength and creep resistance.
Figure 3: Titanium Alloys Crystal Structure versus Temperature (from Donachie,
Matthew J., Jr. TITANIUM: A Technical Guide. Materials Park: ASM
International, 2000. Print. Fig 3.1 pg. 13)
8
Titanium’s corrosion susceptibility is unique compared to that of Aluminum, Nickel and
Steel because when exposed to oxygen a Titania scale forms which does not protect
against further oxidation. Although Titanium has good corrosion resistance to marine
and industrial environments, it is also known to have stress-corrosion cracking (SCC)
issues when subjected to a corrosive environment with tensile stresses that attack a weak
area of the alloy. Another variation of stress-corrosion cracking that Titanium sees is hot
salt stress-corrosion cracking (HSSCC) caused by exposure to halide salts as well as
Sulfides, Fluorides, and dry Chlorides, for a length of time. In addition to SCC and
HSSCC Titanium is particularly sensitive to cracking on exposure to methyl alcohols.
The times and temperatures that cause SCC and HSSCC vary, which is why it is
imperative to perform stress corrosion testing when any processing is performed using a
new substance that includes halide salts or alcohols.
There has been only one published paper found on the application of Sol-Gel coatings on
Titanium, reference [14]. Although, the materials used are applicable to thermal barrier
coatings, the coatings within reference [14] is not utilized for thermal barrier application,
but were formulated specifically for corrosion/erosion resistance. This paper leads the
way to the successful use of Sol-Gel thermal barrier coatings on Titanium substrates.
9
3. Coating Materials
Coatings are applied to protect the substrate from the chemical and thermal environment.
The key to successful coatings is the combination of materials, formation and deposition.
There are several different chemical compositions that are used as coatings. They are
chosen based on the intended use for the coating. The two different categories of
coatings that will be discussed are metallic and ceramic. Metallic coatings can be used
independently or with ceramic coatings as a system. Metallic and ceramic coatings are
typically used for oxidation or corrosion protection as well as for wear resistance, but
ceramic coatings differ in that they are widely used as thermal barriers. These coatings
have several different methods of application, which are expanded on in subsequent
sections.
Oxidation
Corrosion is aggressive oxidation that occurs from the formation of oxide or the scales
on the surface of a metal or alloy when exposed to oxygen, or gases that contain oxygen,
or the corrosives for a period of time. The oxide scale, if intact on the substrate, will be
protective, but once it spalls off, the underlying metal will be exposed and the corrosion
process will continue at an increased rate. Internal oxidation can also occur, which can
lead to detrimental consequences since it reduces the load carrying capability of the
substrate. Additive elements that provide protection against oxidation are ‘passivating’
elements such as Aluminum, Chromium, Titanium, or Nickel with slight additions of
Silicon and Manganese. Oxygen reactive elements, known as REs, improve the
adherence of good oxides such as Aluminum oxide and Chromium oxide because they
are more stable than the oxide scales typically formed on the metal or alloy. REs consist
of but are not limited to Yttrium, Hafnium and Zirconium. Aluminum oxide is the most
chemically stable scale that is oxidation resistant; hence it is frequently used for
corrosion resistant coatings.
10
Hot Corrosion
Hot corrosion, unlike oxidation at low temperatures, occurs with the presence of oxygen
and molten salts that cover the substrate at elevated temperatures between 1300°F1700°F (700°C-925°C) [4]. There are two types of hot corrosion, ‘Type I’ and ‘Type II’
in which Type I occurs above the melting point of the corrosive salt and Type II
typically occurs at the lower of the temperature range and has been seen below 1300°F
(700°C) particularly on Titanium substrates. Two of the most prevalent elements that
protect against hot corrosion are Chromium and Platinum.
3.1 Metallic Coatings
Metallic oxidation resistant coatings in the aerospace industry need to have the following
properties to work effectively: thermodynamic stability, good surface scale adherence,
no undesired phase changes, good substrate adherence, matched properties to the
substrate and should withstand cycling impacts throughout the life of the substrate. The
most widely used metallic coatings that illustrate these properties are Aluminides,
Platinum Aluminide, Chromium containing Aluminide with Yttrium and Zirconium. The
most commonly used metallic coating for the bond coat of TBCs is known as MCrAlX.
The overlay coating MCrAlX is characterized in which M represents either Nickel(Ni),
Cobalt(Co) or Iron (Fe) and X stands for Zirconium (Zr), Hafnium (Hf), Silicon (Si), or
Yittrium (Y) [4, p.96]. The most common form of the metallic bond coat is NiCoCrAlY.
It is important to note that these coatings are assumed to have no load bearing capability
and are just used for surface protection. In reality they have some load bearing
capability that one would not want to credit for because if the coating gets damaged it
lends the substrate to degradation.
11
3.2 Ceramic Coatings
Carbides, Nitrides, Silicides and Glasses are used as ceramic coatings for wear as well as
thermal barriers. This paper will focus on thermal barrier ceramics, such as YSZ, utilized
in the aerospace industry. The ceramics used as TBCs must have a high melting point,
low thermal conductivity, coefficient of thermal expansion close to that of the substrate
material, stable phase, high oxidation and corrosion resistance as well as a high strain
tolerance.
3.3 Thermal Barrier Coating Systems
TBCs typically consist of two or three layers in which each layer is made of a different
chemical composition of varying properties. Initially a metallic bond coat is applied to
the substrate and then a ceramic top coat applied over the metallic bond coat, but in
some instances another metallic coat is applied in between the bond coat and the top coat
as an intermediate metallic layer. There is a unique thermally grown oxide (TGO) that is
generated during the application of the ceramic coat on top of the bond coat that resides
at the bond coat-top coat interface. The TGO is normally Aluminum oxide and is the
link that bonds the metallic coat to the ceramic coat. The thickness of the coatings range
depending on the temperature they are being utilized at and on the coating process.
Typical thicknesses for each of these layers are shown in Table 3.
Table 3: Typical TBC Thicknesses for Conventional Coating Processes (from Bose,
Sudhangshu. High Temperature Coatings. Elsevier, 2007. Print.)
Layer
Bond Coat
TGO
Top Coat
System
Min (in)
0.00200
0.00002
0.00492
0.00694
Min (µm)
50.0
0.5
125.0
175.5
12
Max (in)
0.00500
0.00039
0.03937
0.04476
Max (µm)
125.0
10.0
1000.0
1135.0
Densities of typical TBC coatings applied with the line-of-sight processes are shown in
Table 4.
Table 4: Densities of Ceramic & Metallic Coatings (from Bose, Sudhangshu. High
Temperature Coatings. Elsevier, 2007. Print.)
Ceramic
Top
Coats
Metallic
Bond
Coats
Dense YSZ
APS 7YSZ
EB-PVD 7YSZ
Min NiCoCrAlY
Max NiCoCrAlY
APS NiCoCrAlY
NiAl
(Ni,Pt) Al
Density (g/cm3)
6.10
5.00
5.10
7.00
7.50
6.50
5.90
7.00
Density (lb/in3)
0.220
0.181
0.184
0.253
0.271
0.235
0.213
0.253
Bond Coat
The bond coat is used for three reasons, to provide oxidation resistance, its ability to
adhere to both the substrate as well as the ceramic top coat and to handle thermal
expansion between the substrate and the ceramic top coat. The bond coat is normally
applied by one of two methods, Electron Beam Physical Vapor Deposition (EB-PVD) or
a form of Plasma Spray (PS). These methods will be explained in more detail later.
Top Coat
The ceramic top coat of Yittrium Stabilized Zirconium (YSZ) meets the required criteria
for TBC, and is widely used today. By itself, Zirconium oxide cannot do the job even
though its melting point is 4874°F (2690°C) [4]. Zirconium oxide goes through a phase
transformation during thermal cycling, which causes shear strains eventually leading to
failure of the coating; hence it must be combined with a stabilizing agent such as
Yttrium oxide. The percentage of Yttrium oxide used is crucial to create a stable phase
and has been determined through testing to be 7 wt %. [4, p.161].
13
4. Coating Processes
There are several different coating processes available that are used to apply thermal
barrier systems as well as corrosion prevention coatings. The coating process is
extremely important to the coating as a system because it creates final coating
microstructure that lasts throughout the life of the substrate. One important requirement
of depositing coatings is how they adhere to the substrate as well as how the coating
adheres to itself and other coatings. When faced with the question of “what makes it
stick?”[29], one can say a combination of the following (dependent upon the process):
mechanical, chemical, diffusion or dispersion adhesion. The details of adherence will be
discussed within each process.
The coating process chosen is subject to many variables, including but not limited to the
substrate material, the geometry of the substrate, the availability of coating suppliers and
the processing cost. The coating processes can be broken up into two sub-categories,
line-of-sight and non line-of-sight processes.
4.1 Line-of-Sight Processes
Processes known as ‘Line-of Sight’ are those that need to have linear and direct access
by the coating beam to the portion of the substrate that is to be coated. The best coating
microstructure will generally be found when there is a 90° angle between the applicant
tool and the substrate surface, as shown in Figure 4. If there is not direct line-of-sight the
microstructure will suffer, or coating may not even be deposited in certain areas. As with
all line of sight processes, the coating will have acceptable microstructure when applied
within an angle range, but this must be determined empirically for each specific part and
application method.
14
Figure 4: Component with Non Line-of-Sight Access
There are three categories that make up line-of-sight coating processes and two of those
three processes can be broken down even further as shown in Figure 5.
Figure 5: Hierarchy of Line-of-Sight Coating Processes
4.1.1
Thermal Spray Processes
Thermal spray was developed around 1912 by Dr. M. U. Schoop and associates [28].
Thermal Spray liquefies the desired coating materials by the use of thermal energy and
then using kinetic energy propels the molten coating particles onto the substrate. There
are three main categories of thermal spray processes, Plasma Arc Spray, Electric Arc
15
Spray and Flame Spray. Each has seen variations developed over the years. Each
category will be described below.
Plasma Spray
In this application a source of energy is used to excite a gas to the point where the
energy separates the electrons from the gas ions creating plasma. When the energy is
removed from the gas, the separated electrons and ions attract and combine releasing
energy in the form of kinetic energy producing heat and light [30]. The plasma is
normally created within a plasma gun made up of tubing carrying the plasma gas (either
argon or hydrogen), an electrode, and a nozzle, a feed mechanism for powder, and
cooling water tubing. The process is considered Air Plasma Spray (APS) when it is
performed in air, which can produce a coating with high oxide content. With Low
Pressure Plasma Spray (LPPS) also known as Vacuum Plasma Spray (VPS) [30] the
plasma gun is enclosed in a vacuum or in an inert gas chamber, which reduces but does
not eliminate oxides within the coating. This process is known for its ‘splats’ in the
coating structure, which is what the molten metal droplets look like when deposited onto
the substrate, see Figure 6. First the initial splats adhere to the substrate through
mechanical adhesion where the splats get stuck in the crevices or pores of the roughened
substrate. The splats also chemically adhere to one another through chemical bonding.
Figure 6: Plasma Spray Deposition (Crawmer, Daryl E. "Introduction to Coatings
Equipment, and Theory." Handbook of Thermal Spray Technology. Ed. J. R.
Davis. ASM International, 2004. Fig. 1. Pg. 43. Knovel. Web.)
16
Electric Arc Spray
The electric arc spray process was also developed by Dr. M. U. Schoop in 1910, but was
not significantly utilized until the mid 20th century. The process consists of two wires, a
wire feeder, compressed gas and an electrical source. The wires have the desired
composition that create an arc in a controlled manner by means of the wire feeders,
while the compressed air/gas mixture forces the melted material to spray outward onto
the substrate. The deposition and adhesion onto the substrate is similar to that of plasma
spray.
Flame Spray
There are two different variations of the flame spray process High-Velocity Oxygen Fuel
(HVOF) and Detonation Gun. The basic construction of any flame spray process is a
combustible gas mixture, a nozzle and a powder feed. The most common process used
today is HVOF, which evolved from the original flame spray detonation gun. Unlike
plasma and arc spray the powders are put into a mixture of combustible gases that are
restricted to a tube and then expelled outward toward the substrate. An HVOF gun has a
2-dimensional converging-diverging nozzle [4] that creates diamond shaped shockwaves that are formed from the supersonic velocities in which the flame expels the
particles. The detonation gun flame velocity creates supersonic shockwaves due to its
chamber shape and the combustion that propels the powder. These two processes again
produce ‘splats’ just like plasma spray and adhere to the substrate and themselves
through metallic and chemical bonding.
4.1.2
Cold Spray
Cold spray is formed with kinetic energy which differs from the previous processes
discussed above since they are formed using thermal energy. Powder is fed to a mixture
of gases which then travel through a converging-diverging nozzle so the mixture
becomes compressed and then expands to velocities above supersonic. The velocity is so
high that the particles upon impacting the substrate have widespread plastic deformation
and adhere by metallic bonding.
17
4.1.3
Physical Vapor Deposition
The last section of line-of-sight coating is physical vapor deposition which can be
broken up into three different processes, Electron Beam Physical Vapor Deposition (EBPVD), Sputtering and Ion Plating. The coating produced by these processes adhere to the
substrates through a combination of all four adherence mechanisms, mechanical,
chemical, diffusion and dispersion.
EB-PVD
EB-PVD is the most popular of the three simply because this is one of the few methods,
as well as Plasma Spray, that can melt Zirconium. This method utilizes an electron beam
gun to melt the desired coating material by directing high energy at the raw material in a
vacuum chamber. Once the material melts into a pool, additional heat causes the pool to
boil, and generates vapor. That vapor then fills the chamber and deposits itself on the
substrate. The substrate that is being coated is positioned directly above the melted pool
to direct the vaporized molecules to hit the substrate prior to colliding with one another.
Molecules take a longer amount of time before colliding with one another when they
have a large mean free path. This large mean free path is determined by a simple
equation derived from the kinetic theory of gases and the ideal gas laws. The substrate is
located on a mechanical device that has the ability to rotate and angle the substrate as
needed for the required line of sight.
This process in particular has an inherent disadvantage that limits it’s applicability to
titanium alloys. The process requires heating of the substrate to a desired temperature,
approximately1832°F (1000°C), for the length of the coating process and is necessary
for surface diffusion. This temperature is well beyond the normal processing temperature
limits for Titanium, and for this reason, this process is much more suited to coating
Nickel alloy parts. In order to coat Titanium alloys, the preheat temperature must be
lowered to protect the desired crystal structure of the substrate. Lowering the preheat
temperature can limit the growth of the desired coating composition in which coupons
should be tested prior to application on actual hardware. The microstructure that EBPVD produces is known as a columnar grain as seen in Figure 7 in which there are
18
clearly defined separation lines which lead to foreign object infiltration to the TGO
layer and eventual failure of the coating.
Figure 7: EB-PVD Columnar Grains (Wahl, G., W. Nemetz, M. Giannozzi, S.
Rushworth, D. Baxter, N. Archer, F. Cernuschi, and N. Boyle. "Chemical Vapour
Deposition of TBC: An Alternate Process for Gas Turbine Components." ASME
TURBOEXPO. Munich, Germany: ASME, 2000. Google Scholar. Web. Figure 9)
Sputtering
Sputtering, also known as Ion Beam processes are a unique line of sight processes
because they do not rely on temperature but momentum to adhere to the substrate. Again
this process is performed in a vacuum chamber and gas is expelled from a positively
charged nozzle towards the substrate, which becomes a cathode and the gas ionizes to
form plasma where the plasma is propelled toward the substrate. The substrate is also
held by a mechanical device that can rotate or angle as needed to ensure full coverage.
There are four types of sputtering, which will not be described within this paper in which
each are a slight variation of the process described above. The four processes are known
as Radio Frequency Sputtering, Magnetron Sputtering, Planar Diode Sputtering, and
Triode Sputtering.
19
Ion Plating
The basis of ion plating is the barrage of ions implemented with plasma physical vapor
deposition. The type of ion plating is dependent on how the ions are created within the
environment. There are three main groups in which ions can be created and utilized:
thermal evaporation, electron beam evaporation or arc vapor. Again this is done in a
vacuum environment and the substrate can be held fixed or mechanically turned or
angled for coverage.
4.2 Non Line-of-Sight Processes
There are very few non line-of-sight coating processes that are widely used in the
aerospace industry as shown in Figure 8. Diffusion coatings are dependent on the
substrate alloy to create the coating, but Sol-Gel coatings are a new alternative.
Figure 8: Hierarchy of Non Line-of-Sight Coating Processes
4.2.1
Diffusion
Diffusion processes can be described based on the chemical activity levels of diffusing
species. There are two levels of activity for diffusion coatings, “low-activity” and “highactivity”. Low activity is when the processing temperature is high and the diffusion is
out of the substrate, while high activity process is at lower temperatures and diffuses into
the substrate. Activators such as halide salts (chlorides and fluorides) are utilized
because they react readily with the metals and transport vapors easily essentially forming
20
the coating. The coatings thicknesses are limited because the thicknesses are controlled
by diffusion. To create a TBC of reasonable thickness would require processing times so
lengthy, that they would be unrealistic. The adherence of diffusion coatings is done by
diffusion adherence where the coating and substrate material is soluble within each other
joining the particles together.
Chemical Vapor Deposition (CVD)
Chemical vapor deposition is dependent on the substrate alloy since the chemical
reaction of the vapors with the substrate is what creates the coating. Initially a halide
vapor is created and carried in designated plumbing to the surfaces of the substrate that
require coating. The halide vapors are mixed with hydrogen in order to create the
appropriate chemical reaction, and carried in a medium such as argon. There are two
types of chemical vapor deposition Thermal CVD and Plasma CVD. The two processes
differ mainly by the temperature limitations of the substrate. Thermal CVD temperatures
vary from 1470°F to 2010°F (799°C-1099°C) whereas plasma CVD varies from 570°F
to 975°F (299°C-524°C). A heat treatment is then carried out on the parts after the
coating is applied. It is conceivable that Titanium could be coated with plasma CVD,
except the halide vapor and hydrogen mixture has the potential to damage the substrate
via stress corrosion depending upon how long the halide remains on the substrate.
Above the Pack
Within the above the pack process, a tray is filled with powder of the pack mix and
placed in a furnace where the component is positioned above the tray. When the part and
powder are at the desired temperature, gas is flowed in all directions to carry out the
diffusion process. The pack mix contains halides and the chemical composition required
to deposit the coating through the low activity process. This process applies very
uniform coating and allows for coating to be applied into holes or on intricate shapes.
Although this process can apply uniform coatings on complex geometry, it has the
disadvantage of the presence of halides, which some metals, like titanium, cannot be in
contact with for certain periods of time.
21
Pack/Slurry
In the pack or slurry process, an activator is applied to the substrate in the areas where
coating is desired, and then the substrate is either put in the dry coating composition or
slurry. Once the material is applied to the substrate, it is placed in a vacuum or in a
neutral medium and the coating fully adheres to the substrate with applied temperature
and pressure. This process easily handles complex geometry, except the coating process
lends itself again to damaging halides, temperatures are above 1380°F (750°C) [4] and
create limited coating thickness.
4.2.2
Sol-Gel
A sol is defined, Brinker [1], as “a colloidal suspension of solid particles in a liquid.” A
colloid is a mixture of particles that remain suspended because the attractive forces
between particles in the mixture and that due to the suspension of fluid are greater than
that of gravitational forces pulling down on the particles as sediment. A gel is “a porous
3-dimensionally interconnected solid network that expands in a stable fashion
throughout a liquid medium and is only limited by the size of the container.”[2] The sol
and gel are two separate entities, but when sols are mixed with another medium they can
form gels. Therefore a Sol-Gel is a colloidal solution that gels, is dried into different
structures and then sintered. There are two ways to process Sol-Gels. These are
dependent on the coating one is trying to achieve. There is colloidal dispersion or
polymerization. It is important to note that colloids are used to make polymers. To deter
confusion, colloidal dispersion is sometimes referred to as particulates. This paper will
focus on the polymerization route which is widely used to create ceramic coatings. The
basic method of the sol-gel process by polymerization is outlined in Figure 9.
22
Figure 9: Sol-Gel Process
Creating sols is relatively inexpensive simply because the equipment needed isn’t
complex. The equipment consists of a flask containing the mixed solutions, a mechanical
stirrer to ensure the mixture is uniform, and a reflux condenser. The mixture must be
held at a relatively constant temperature.
4.2.2.1 Precursor
The process begins with a precursor which could be a metal alkoxide, metal salts or
organometallics. A metal alkoxide is denoted as M(OR)n in which M is typically a metal
cation, but can be a non-metal, and n is the number of alcohol groups ROH[2]. R denotes
the alkyl chain [33] and OR is an alkoxy or ligand formed by removing a proton from
the hydroxyl on an alcohol [1]. In the simplest O is oxygen and H is hydrogen. Many
studies have used tetramethoxysilane, Si(OCH3)4, (TMOS) and tetraethoxysilane,
Si(OC2H5)4, (TEOS) as metal alkoxides. These precursors are easily hydrolyzed. Metal
23
salts such as Aluminum Chloride AlCl3, are denoted as MnXm, where X is an anionic
group [2]. The subscript n or m represents the number of atoms present in a molecule of
the compound [55]. Organometallics are linkages with direct metal-carbon bonds rather
than metal-oxygen-carbon bonds as in metal alkoxides [1]. When multiple precursors are
used they must have similar hydrolysis rates to ensure there will not be inhomogeneities
in the resulting gel. Metal alkoxides have low solubility and are sensitive to moisture. As
such, metal salts are often added to the mixture, typically in an alcohol because they
catalyze faster than with water, but if alcohol cannot be used, water can be substituted to
hydrolyze the mixture. The precursor is then mixed into a solvent such as water or an
organic liquid, but water is always present.
4.2.2.2 Solution – Hydrolysis & Condensation
The solution derived from precursors and solvents undergoes hydrolysis, in which a
hydroxyl ion leaves water and attaches to a metal atom. Hydrolysis can be performed
with an acid-catalyst or a base catalyst. Acid catalyzed conditions have a pH < 2.5 and
base catalyzed conditions have a pH > 2.5[33]. These pH values are based on the point
of zero charge (PZC) at which the surface is electrically neutral [33]. Hydrolysis can be
seen as the following reaction:
Hydrolysis begins with forcing alcohol or water out and then a process of condensation
purges the alcohol or water. As the chemicals react further, condensation continues to
occur to form polymers. The condensation reactions that take place can have one of two
forms in which a water or alcohol molecule is released:
or
24
4.2.2.3 Gel Point to Gelation
Once the solution has gone through hydrolyses and condensation, the gel forms through
polymerization. The formation of the gel is dependent on the conditions of the
hydrolysis and condensation reactions that created the sol. The parameters of those
reactions that influence the formation of the gel are the temperature, precursor
concentration, reaction medium and whether or not the catalyst was a base or acid. In
order to move the sol to the gel-point viscosity, either capillary flow or coquette flow is
used. The gel-point is the point at which the solution abruptly forms into a gel with high
viscosity. There are several theories that try to define the transition from a sol to a gel
structure, but no one method has been determined to correctly describe what happens in
real life. It can be seen that acid catalyzed gels form entwined linkages, while base
catalyzed gels form branches and clusters as shown in Figure 6.
25
Figure 10: Comparison of Acid Catalyzed vs. Base Catalyzed Gels (a) acidcatalyzed, (b) base-catalyzed, particulate silica gels aged under conditions of high
(c) or low (d) solubility. (from Brinker, C. J., and George W. Scherer. SOL-GEL
SCIENCE: The Physics and Chemistry of Sol-Gel Processing. San Diego:
Academic, 1990. Print. Figure 6 on Pg. 363)
These formations are considered “a weak skeleton of amorphous material containing an
interconnected network of small liquid-filled pores” [33]. An easy to visualize example
of these concepts is Jell-O. Jell-O is formed with water and is an acid catalyzed reaction
that uses collagen as its acid. Jell-O dry powder is the precursor. Jell-O uses a reversible
gelation reaction that can be seen when the gelatin is refrigerated for 4 hours and then
removed, left in typical room temperature of 70°F (21°C) for the next 4 hours. One
would see the gel turns back into liquid from the breakage of the weak bonds. An
irreversible Jell-O at room temperature, known as ‘Finger Jell-O’, has additional acid
and is only mixed with boiling water. The coatings that are being discussed utilize
different precursors that are processed with the intent to become irreversible.
26
4.2.2.4 Aging
Aging is sometimes categorized within gelation because the chemical reactions that turn
a sol into a gel continues long after the gel-point. There are four processes that make up
the aging process: polymerization, syneresis, coarsening and phase transformation.
“Polymerization is the increase in connectivity of the network produced by condensation
reactions” according to Brinker [1]. The chemical reactions can last an extremely long
time, months at room temperature [1], unless higher temperatures are used. The bonds
that are created from polymerization strengthen the system making it more rigid. Again
hydrolysis may take place leading to syneresis which is the “shrinkage of a gel network
resulting in expulsion of liquid from the pores”[1]. The reaction of syneresis can be seen
here:
and
Next the gel is coarsened or ripened which is the “process of dissolution and
reprecipitation driven by differences in solubility between surfaces of different radii of
curvature”[1]. Small particles have large solubility while larger particles have small
solubility. The dissolution and reprecipitation makes the larger particles transform into
minimized size particles and smaller particles are densified in decreasing the specific
surface area of the gel. Lastly there is phase transformation such as separation of a solid
into a liquid or a liquid into different liquid phases. An example of aging can be seen in
the processing of dairy products such as when one opens a container of yogurt, there is
liquid residing on the top of the gel-like substance.
4.2.2.5 Drying
In order to transform the weak skeleton bonds of the initial gel into a stronger network
the mixture of alcohol and water must be removed. This is done by drying. Drying
chemically consists of two periods, the ‘constant rate period’ (CRP) and the ‘falling rate
period’ in which there is a ‘first falling rate period’ (FPR1) and a ‘second falling rate
27
period’ (FPR2). The transformation from the constant rate period to the falling rate
period is called the critical point. During CRP evaporation takes place and forces the
liquid to the surface of the gel by either capillary forces or osmosis which causes
shrinkage until the gel network becomes so strong that the shrinkage stops. This abrupt
stoppage of shrinkage is the critical point where fracturing can occur if one does not
modify the chemistry appropriately. Evaporation continues into the FPR1 caused by the
vapor pressure on the exterior being lower than the interior of the gel. The liquid, having
a continuous path to the exterior, flows outward during this process instead of diffusing
outward. Eventually the flow of the liquid slows as the gel thickens and the liquid
diffuses into vapor, this process is FPR2 and continues until the gel is completely dry.
The different methods of dying gels can be classified as such in the table 5.
Table 5: Different Types of Gels (from Carter, C. B., and M. G. Norton. Ceramic
Materials: Science and Engineering. New York: Springer Science+Business Media,
LLC, 2007. Print. Table 22.5 pg. 406)
Type
Process of Drying
Autoclave supercritical/hypercritical
Aerogel
evacuation
Xerogel natural evacuation
Sonogel Ultrasound treatment prior to autoclave
Cryogel Frozen
Injection of liquid Tetrachlorosilane (SiCl4)
Vapogel
followed by natural evacuation
Kistler was the first to study supercritical drying of silica gels in 1932[2]. The basic
process consists of heating the gel in an autoclave to temperatures and pressures which
can vary above the critical point of the solvent. Then a dry gas medium is cycled through
the autoclave so that the supercritical solvent can eventually be removed. This process
creates gels that have high specific surface area that could exceed 1000 m2/g [1] which is
good for insulation and acoustics, but has a high level of porosity (>80%) [36]. The
drying process to create a Xerogel is similar to an Aerogel except the evaporation of the
solvent takes place in ambient temperatures and pressures. Xerogel surface areas range
between 500-900 m2/g [1]. As for the Sonogel process, the initial mixture is exposed to
ultrasonic waves for a period of time prior to heat being applied for drying. Sonogel is
used with nanoparticles that are known for unstable dispersions because the method
28
breaks apart aggregates that form and are difficult to separate. Cryogels are solutions
that are freeze dried to reduce porosity. One additional method of drying is Vapogel in
which a liquid Tetrachlorosilane (SiCl4) is injected into acidified water as a catalyst and
then the gel is dried by the Xerogel process. Sol-Gel thermal barrier coatings are
typically processed as Aerogels or Xerogels.
During the drying process the major concerns are fast shrinkage and cracking. One can
use preventative methods such as utilizing supercritical/hypercritical drying,
strengthening the gel with the aging process, decreasing the liquid/vapor interfacial
energy and/or increasing the pore size of the gel which can be accomplished through
modifying the initial chemistry of the solution [33]. Depending upon what the Sol-Gel is
being used for the process can be modified accordingly to produce powders, fibers,
coatings, monoliths, or ordered pores.
4.2.2.6 Sintering
Although Sintering is typically defined as “the process of transforming a powder into a
solid body using heat” [33], Brinker notes that it “is a process of densification driven by
interfacial energy.” This process joins particles together below their melting point, which
can be complex. Sintering is key to producing dense ceramics but can produce porous
ceramics with additional processing. Sintering originated with powder metallurgy.
There are three different methods of sintering: solid-state sintering, also known as
diffusion sintering, liquid-phase sintering and hot pressing. Diffusion sintering begins
with an unfired ceramic, that when introduced into high temperature environments,
particles combine and porosity is reduced. The diffusion of particles can be performed
by several different methods such as surface diffusion, volume diffusion, grain boundary
diffusion, plastic flow or evaporation-condensation. The inter-connection zone of
particles is called a grain boundary. Single crystals have grain boundaries between two
spheres, while polycrystalline has many grain boundaries and glasses do not have any.
When Yttrium Stabilized Zirconia (YSZ) is heated to temperatures between 752°F to
1112°F (400°C-600°C) [9] the material changes crystal structure to tetragonal. If the
coating is needed for high service temperature use then it must be pressurized during
29
heat treat or heated above 1112°F (600°C). When heated above 1112°F (600°C) the
monoclinic phase becomes present. This phase change can potentially lead to cracking,
but metal cations can be added by alloying to sustain the integrity of the coating by
suppressing the phase transformation. Liquid phase sintering occurs when ceramic
powders are in contact with a liquid and the liquid solidifies during cooling. Hot pressing
involves the application of pressure to reshape the particles and fill voids between them.
Sintering is the final step in producing a Sol-Gel and is critical to coating life that is
based on porosity levels.
30
5. Methods Used to Apply Sol-Gel on Substrates
There are currently four ways of applying sol-gel coatings on substrates: dipping,
spinning, spraying and painting. The key to applying Sol-Gel coatings to substrates is to
ensure good adhesion. Prior to applying coating one needs to prepare the surface similar
to that of conventional coatings by cleaning and roughening the surface by either grit
blasting or etching. The Sol-Gel coating adheres to the substrate through chemical
covalent bonding and mechanical adhesion, but dispersion and diffusion adhesion
happen within the chemical reactions that make the coating. The chemical adherence can
be seen by the chemical reaction (where Mn represents the substrate and Mx represents
the coating):
Dipping
Sol-Gel dip coating forms when the substrate is immersed in the liquid solution and then
removed at a designated speed. During this process the initial layer of coating bonds to
the surface of the substrate while a boundary layer divides the liquid and the outer layer
flows back into the pool of solution. The initial layer goes from precursor to gelation
almost instantly as the component is being withdrawn from the pool. The layer thickness
is dependent upon the speed the substrate is withdrawn from the solution [38]. It should
be understood that the thicker the coating the longer it will take to age and dry due to
increasing porosity levels. Viazzi et al.[17] did a study with Hastelloy X applied with a
plasma sprayed NiCrAlY bond coat that was dipped into YSZ Sol-Gel dried at room
temperature then went through two different high temperature heat treatments above
1742°F (950°C). The dip coating performed produced layers on the average of 0.0002in
to 0.0004in (5µm-10µm). It was learnt that achieving a thickness of approximately
.010in (250µm) or greater is difficult. Kurihara [7] also did a study of Sol-Gel dip
coatings on 304 stainless steel using Alumina as the stabilizer instead of Yttria which
has better thermal-mechanical matching. A bond coat was not applied to the component
prior to Sol-Gel application and the thickness of the coating showed to be thicker than
that of any other study at 0.034in (876µm).
31
Spinning
Spin coating is characteristically used on flat substrates since the physics of application
limit the coating uniformity. The process is such that the solution is put in one location,
typically the center of the substrate and then the substrate is spun at high speeds letting
the centrifugal force created spread the coating on the surfaces. This process is mainly
used for thin coatings. The higher the rotational speed the thinner the coating becomes.
Markowitz [6] applied Datec Sol-Gel coating using the spinning method. A dropper was
used to apply the coating in the center of the substrate that was hooked into a spinner.
The substrate was then spun up to 4000rpm for 20 sec. Once each layer of approximate
thickness 0.0003in (7µm) was applied, it was then put through heat treat and the process
repeated until the coating was 0.0014in (35µm) thick.
Spraying
Spray coating with Sol-Gel is much simpler than plasma spray since one can use an air
gun at room temperature to deposit layers of the liquid solution. Since Sol-Gel is a nonline of sight chemical process, the microstructure is not dependent on the spray angle.
Spray and spin coating processes are sometimes combined to ensure the substrate is fully
covered with uniform coating. Markowitz [6] also utilized spray coating in her studies
and specified that the pattern of spray determined the thickness as well as the uniformity
of the coating. This is also true with plasma spray, but plasma spray must have line of
sight, a perpendicular spray angle, and uses a much more complex, and physically larger
gun.
Painting
Zhang et al. [8] came up with thermal pressure and filtration of Sol-Gel painting for
thermal barrier applications. Prior to the application of Sol-Gel, the substrate was coated
with MCrAlY bond coat. The Sol-Gel was then prepared using ammonia, nitric acid and
polyvinyl alcohol as catalysts during different steps of the process. Once the Sol-Gel was
mixed it was applied to the substrate and allowed to dry at room temperature for 60
minutes. The substrate was then covered with filter paper as a separator, and placed in
powder medium within a furnace and then pressure and heat were applied to densify the
gel. It was shown if pressure was not used, the temperature required to cure the gel was
32
to be increased to approximately 1562°F (850°C) [8]. The temperature required for heat
treatment with pressure is approximately 932°F (500°C) [8]. Zhang et al. [8] were able
to produce dense crack free coatings with thicknesses of 0.001in - 0.020in (25µm500µm). It is apparent that if this process was used on a Titanium substrate, the
preferred method would be with pressure, since Titanium’s mechanical properties
degrade after approximately 1000°F (538°C) as described previously in the substrate
section.
5.1 Titanium with applied Sol-Gel TBC
As described before the Sol-Gel process is applicable to Titanium. In particular
Blohowiak et al. [14] proved this by applying a solution of organozirconium compound
with an organosilane sprayed or dipped on grit blasted or etched clean Titanium samples,
without a bond coat, followed by curing at 160°F-250°F (71°C - 121°C) for 15-45
minutes. This particular application of Sol-Gel on Titanium is not utilized in high
temperatures. In order to produce an YSZ TBC via Sol-Gel application on a Titanium
substrate one should test various chemistries as well as processing parameters. The bond
coat may, or may not be necessary to ensure the Sol-Gel adheres to meet the required life
of the substrate if exposed to high temperature thermal cycling. Catalysts that are
normally used in the solutions, such as halide salts or methyl alcohols should not be used
on Titanium. If one uses these catalysts, stress-corrosion cracking tests need to be
performed at high and low temperatures with the coating applied. This will assure that
stress corrosion cracking would not be an issue. During aging, drying and sintering the
maximum temperature should not exceed the maximum use temperature of the Titanium
alloy in order to maintain its mechanical properties.
33
6. Conclusion
Sol-gel based TBC coatings can become a significant factor in improving the weight of
aerospace systems, by promoting the use of Titanium alloys in higher temperature
environments. The ability to TBC coat areas that do not have line-of-sight access is a
fundamental basis in protecting these structures to allow this. In order to manufacture a
functional Titanium component with Sol-Gel applied TBC, one must ensure the use of
appropriate precursors, additives to reduce cracking and the correct heat treatment
regimen. The precursors should not include methyl alcohols and to reduce cracking the
water-alcohol solution ratios should be determined empirically. Lastly, the heat teat
temperatures should not exceed the maximum useful temperature of the Titanium. The
disadvantages of the Sol-Gel process are difficulties in controlling shrinkage during
processing, residual chemicals and being able to apply sufficiently thick coatings. The
advantages of the Sol-Gel process are that it is cost effective, better for the environment
than conventional coating processes and easy to manufacture. The density of Sol-Gel
TBC’s should be determined through testing, since it can be highly dependent on the
mixture of solutions and the final stages of processing. Most importantly Sol-Gel TBC
can be applied to areas that are inaccessible by traditional TBC coating application
processes.
34
7. Definitions
Aggregate - formed by the collection of units or particles into a body, mass, or
amount. [53]
Alkoxides - A compound formed from an alcohol by the replacement of the
hydrogen of the hydroxyl group with a metal. An example is sodium methoxide,
CH3ONa, from methyl alcohol, CH3OH. [54]
Chemical Adhesion – Can occur with ionic bonding, covalent bonding or hydrogen
bonding.
Colloid - A suspension in which the dispersed phase is so small (~1-1000nm) that
gravitational forces are negligible and interactions are dominated by short-range
forces, such as van der Waals attraction and surface charges. [1]
Condensation - a chemical reaction involving union between molecules often with
elimination of a simple molecule (as water) to form a new more complex compound
of often greater molecular weight. [53]
Crystal Structure – The size and shape of the smallest arrangement of atoms, ions
or molecules in a three-dimensional array known as a lattice. [4]
Diffusion Adhesion – The linkage of polymeric chains through diffusion.
Dispersion Adhesion – Utilizes van der Waals forces to join two materials.
Gel - A porous 3-D interconnected solid network that expands in a stable fashion
throughout a liquid medium and is only limited by the size of the container. A gel
forms when the homogenous dispersion present in the initial sol rigidifies. [2]
Hydrolysis - Chemical decomposition in which a compound is split into other
compounds by reacting with water. [54]
Ligand - a group, ion, or molecule coordinated to a central atom or molecule in a
complex. [53]
Mechanical Adhesion – Where a substrates roughened surface geometrically
constrains the liquid applied and as the liquid solidifies it interlocks with the surface
irregularities. [58]
Monolith - if the smallest dimension of the gel is greater than a few millimeters, the
object is generally this. [1]
35
Polymeric - Having the same elements combined in the same proportion but
different molecular weights. [54]
Precursor - A chemical that is transformed into another compound, as in the course
of a chemical reaction, and therefore precedes that compound in a synthetic pathway
[54]
Sol - A colloidal suspension of particles in a liquid [1]
36
References
1. Brinker, C. J., and George W. Scherer. SOL-GEL SCIENCE: The Physics and
Chemistry of Sol-Gel Processing. San Diego: Academic, 1990. Print.
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