MICROSTRUCTURE ANALYSIS OF INTERFACE ALUMINA
THERMAL BARRIER COATING FOR HIGH TEMPERATURE
APLICATION
Hariyati Purwaningsih1, Sulistijono2, Lukman Noerochim2, Cartha Kharisma3
1
Materials and Metallurgical Engineering Department
Industrial Faculty of Technology
Institute of Technology of Sepuluh Nopember (ITS)
Campuss ITS Keputih Sukolilo Surabaya 60111
Phone/Fax : +62 31 5997026; email: hariyati@mat-eng.its.ac.id
2
Materials and Metallurgical Engineering Department
Industrial Faculty of Technology
Institute of Technology of Sepuluh Nopember (ITS)
Campuss ITS Keputih Sukolilo Surabaya 60111
Phone/Fax : +62 31 5997026
3
Graduate Student of Materials and Metallurgical Engineering Department
ABSTRACT
This research summarized the microstructure analysis of alumina coating which
deposited to substrate Al-Si alloy by use of PFS (Powder Flame Spraying) technique, and
thermal cyclic where the out put specimen from flame spraying is brought to the boil
inside the furnace at 600oC with several cycles at 20, 50, and 100, and each cycle is held
a weighing. XRD, SEM (Scanning Electron Microscopes) and EDS (Energy Dispersive
Spectrometry) were applied to characterized phases and microstructures. In order to find
out the hardness which it formed at each layer and interface is using the micro hardness
tester. This research has ascertainable, that the bond which is formed at interface has a
high hardness where as thermal cyclic had an effect on TGO figuration and varying
phase. The TGO figuration, as effect of thermal cyclic, caused a reduction of the bonding.
It formed micro porosity which to make a failure at TBC system
1. Introduction
Diesel engine was the machine
that working by fuel hypodermic of air
which have compressed so that have
own the high temperature and pressure
application from 300°C until 500°C,
with the pressure equal to 2492 KPA. On
its operation oftentimes meet the
constraints caused by the existence of
some component from diesel engine
experience of the damage of effect on
high temperature operation, so that its
performance become less be optimal
To increase resilience to high
temperature required a veneering process
use the system of Thermal Barrier
Coating (TBC system). In many research
had mentioned that thermal barrier
coating application at diesel engine aim
to increase performance of machine,
make-up of machine age, and efficiency
of fuel usage (Beardsley, 1997), as
research done by Ramaswamy (2000)
that is use 8YPSZ and Mullite
(3Al2O3.2SiO2) as thermal barrier
coating material of diesel engine piston,
what have been known able to give the
protection to oxidation and degradation.
Some problems arise with the
thermal barrier coating method, that is
abrading of ceramic coat (Top Coat) and
its failure because of thermal stress in its
bearing by forming TGO (Thermal
Growth Oxide) at interface between top
coat and bond coat. Thermal Growth
Oxide have different characteristic with
top coat and bond coat. This coat is
heterogeneous and imperfect, so that
caused crack on the surface of top coat,
interface top coat-bond coat and also at
TGO itself (Kristanto, 2008). So, it is
required a furthermore research to
analysis of microstructure and phase
formed at interface between top coat and
bond coat.
2. Method
The design and experimental
procedures can be explained as follows:
Substrate is Al-Si super alloy and Al2O3
powder for coating ceramic material.
The properties of Al2O3 ceramic
powders, among others; M = 101.94 g /
mol; Specific Surface Area = 120-190
m2 / g; Stamping density = 950-1100 g /
l. And also use Ni-Al powder for bond
coat material. The process of thermal
cycle include heating the specimen at
600oC for 1 hour and cooling down at
room temperature for 15 minutes, this
cycle uses the following standard cycle
testing in high temperature corrosion.
One cycle in this study equal to 1 hour
holding time for heating up and hold 15
minutes for cooling down. The duration
of the heating is done until peeling of top
coat happened at specimen surface, this
is indicates that the failure crack in TBC
systems. Checks carried out each cycle,
by considering specimens with analytical
balance. Then SEM and XRD
characterization was applied to obtain
the phase and microstructure changed.
3. Result and Discussion
3.1 Macrostructure Analysis
In this study using specimens 4,
where 1 specimen without treatment and
the other 3 specimens applied to thermal
cycles, respectively - each is 25, 50, and
100 cycles. Cyclic thermal treatment
performed at a temperature of 600°C,
issued each made 1 hour and weighed to
determine weight changes that occur.
Observations began on specimens
without treatment. As in Figure 3.1.a is
the initial condition of the coating
specimen with flame spraying process
that has not been given heat treatment.
Macrostructure analysis was
observed a corrugated surface and not
smooth and the color of specimen is grey
with little black spots on its surface. This
condition is caused prior to coating with
the flame spraying process, sand blasting
conducted in order to obtain the
substrate surface area that allows
occurred bond between the substratebond coat and bond coat-top coat. Where
a coating material powder particles are
sprayed at high temperatures with a high
speed also, as a result uneven surface of
the specimen. In the specimen is a small
area of the specimen that is not coated,
due to block by the holder during the
coating process, as in Figure 3.1.
After
25
cyclic
thermal
treatments, the substrate had melt,
starting from not coated area. The
growing cycle, the conditions are also
increasingly rough surface, and the color
changed to blackish specimens, as
shown in Figure 3.1.a. Melting causes
cracks at substrate side, it is also due to
differences in coefficient thermal
expansion of each layer, the crack length
with increasing the heating cycle, until
100 cycles, as shown in Figure 3.1.d,
due to fire and fall off the substrate, with
consideration that the thermal cyclic was
stopped.
(a)
(c)
(b)
(d)
Figure 3.1 Macrostructure analysis for (a). as sprayed, (b) 25 cycles, c. 50
cycles, (d). 100 cycles
Weigth (gr)
Weigth (gr)
Cycle (hour)
Cycle (hour)
(a)
(b)
Weigth (gr)
Cycle (hour)
(c)
Figure 3.2 Result of microbalance analysis to obtained mass changed after (a) 25 cycles,
(b) 50 cycles and (c) 100 cycles
In this study also conducted
weighing at each cycle, to be known
weight changes of specimens due to
cyclic thermal, to facilitate the analysis
of weight change data plotted in a graph,
as shown in Figure 3.2, and given the
trend line to see the pattern of the graph.
Based on the graph in Figure 3.2 above,
can be identified due to cyclic thermal
causes severe changes in the fluctuations
in the specimen, all the graphs have
common in the first cycle until some
point cycle increase in weight, due to the
occurrence of oxide, since oxygen react
with elements in the layer. But in the
next cycle, decrease in weight of the
specimen, it is because at around cycle at
20 cycles occurs cracks in the specimen,
and also specimens fall burning, until the
specimen weight decrease significantly,
until the cycle to 100.
3.2 X-ray Diffraction Analysis
Figure 3.3 is the result of x-ray
diffraction analysis prior to cyclic
thermal. Where the results of tests on
specimens prior to cyclic thermal formed
γ-Al2O3 phase with cubic crystal system,
and another phase is formed NiO with
cubic crystal system. Results obtained in
the XRD test, which reflects that the
phase formed at the interface top coat
and bond coat is the result of oxidation
of elements contained in the top coat and
bond coat. This is consistent with
research conducted Ogawa (2003), that
at the interface between top coat and
bond coat TBC systems will be formed
TGO (Thermal Oxide Growth), where
the TGO formed because of oxidation of
porosity with elements that are contained
in the top coat and bond coat, which
starts from the porosity at the interface.
Furthermore the oxygen reacts with the
element to forming the top coat
oxidation, because based on the
Ellingham diagram element Al is the
easiest to oxidize, the oxidation reactions
that form a protective layer of Al2O3.
After thermal cyclic, so the cycle
into 25 phases are formed almost the
same as before the thermal cycle, the
phase γ-Al2O3 and NiO except that
there is little change, ie there was a shift
position slightly left 2θ, where the
highest peak of cycle 25 is occupied γAl2O3 phase with a cubic crystal system
in the 2θ = 44.470°, while referring to
the XRD results on specimens prior to
treatment with the highest peak position
of 2θ = 44.48167. But overall there was
no change crystal phase and the system,
due to the small shift differences. While
NiO phase after 25 cycles on the graph
(in Figure 3.3.b) occupies the other
highest peaks at 2θ = 43.273°, which
when compared with the XRD results on
specimens before treatment, NiO phase
top the view position 2θ = 38.62, it can
be known that there was a shift 2θ value,
which is significantly more right, these
shifts can result in a change of crystal
system. It was appropriate when search
using software analysis, it is known that
the crystal system changes from cubic to
rhombohedral.
In the cycle to 50 based on the
results of XRD analysis (Figure 3.3.c),
the phase also formed γ-Al2O3 and NiO,
but there is a change compared to the
highest peak position in 25 cycles, where
the XRD results at the 50th cycle is the
highest peak position is the position of
2θ = 44.486°, which shifted more right
than cycle to 25, and prior treatment.
Due to the change is not too significant,
the crystals formed system is not much
different end result was γ-Al2O3 phase
with a cubic crystal system. NiO phase
while the intensity increased as shown in
Figure 3.3.c there are several other peaks
that are occupied by such NiO phase in
the position of 2θ = 2θ = 43.29 and
37.24 are NiO phase with rhombohedral
crystal system, this position is not much
different from the 25 cycle , so also no
change of crystal. system.
A
A : Alumina-cubic
N : NiO-cubic
R : NiORombhohedral
A
N
A
N
N
(a)
(b)
R
(c)
(d)
Figure 3.3 X-ray Diffraction Analysis for (a) as sprayed, (b) after 25 cycles (c) after 50
cycles (d) after 100 cycles
After 100 Cyclic thermal
treatment, there is not a significant
change, where the dominant phase is
Al2O3 with cubic crystal system, which
can be seen on the XRD results at Figure
3.3.c, γ-Al2O3 phase with a crystal
system cubic occupies the highest peak
in 2θ = 44.885°. When compared with
the XRD results before treatment, and at
25 and 50 cycles there was a slight
change in the position of the highest
peak, which shifts more right. While
NiO phase obtained at the position of 2θ
= 43.29°, with a rhombohedral crystal
system, there was no change 2θ position.
However, the XRD results for 100
cycles in a new phase of SiO with the
cubic crystal system in the position of 2θ
= 57.43. Based on analysis results, it is
because the thermal treatment occurred
melted substrate, which started in not
coating area, until the 100 cycle, melted
of the substrate reach all parts of the
specimen. Thus expected that element
of the substrate diffusion by porosity at
the interface, so that elements of the
substrate react with oxygen to form SiO
at interface. To clarify the phases and
microstructure formed on the specimen,
it can be observed through SEM/EDS
(Scanning Electron microscopic/Energy
Dispersion Spectrometry) analysis
3.3 Microstructure Analysis using
SEM/EDX
In
this
study
conducted
observations of the interface between top
coat and bond coat by using SEM / EDS
(Scanning Electron microscopic/Energy
Dispersion Spectrometry), to determine
phase and microstructure formed on the
specimen before and after thermal cyclic
done.
Porosity
TC
BC
TGO
Figure 3.4 Microscope Electron analysis to as sprayed specimen. It shown porosity and
also TGO area
Figure 3.4 is the result of
microscope electron before thermal
treatment cycle, it shown porosity at top
coat. This is because there are grains of
alumina is not completely melt during
the coating using a flame spraying,
because it is done in areas with no
vacuum so some gases were trapped
during the spraying process. According
to research conducted by Chen (2004),
whose explained that the interface area
between the top coat and bond coat
porosity will occur as a result of the
spraying process. The existence of
porosity caused the separation or
irregularity of the surface layer of
ceramic with a bond coat, as well as
between the bond coats to the substrate.
Surface irregularity of flame spray
interface, resulting in the emergence of
impurity and porosity which resulted in
the formation of surface defects, such as
particle adhesion inclusion and Sand
Blasting results, especially due to the
melt ceramic particles are fired with a
certain speed and grinding on the bond
coat surface, as shown above, a layer
covering the top coat such as a bond coat
layer, so that the boundary between the
top coat and bond coat is not too clearly
visible, it shown in a different color, and
porosity line blackish color. Shown in
Figure 3.4, the specimens are not
subjected to thermal treatment already
started happening at the interface top
coat and bond coat, but oxidation occurs
is still thin and not very visible, because
the intake of oxidation during spraying.
Oxidation which occurs not destructive,
because it formed a thin layer of
protective nature.
After 25 thermal treatment it was
found that in areas along the interface
between top coat and bond coat will be
formed TGO (Thermal Oxide Growth)
and alumina thin layer of intermixed
zone area (area that contains a blend of
elements contained in the top coat and
bond coat ) are shown with a slightly
blackish color on the interface. As
research conducted by Quadakkers, et al
(2005) explained that during the heat
treatment (thermal cycle) on the
specimens using the TB, resulting in the
emergence of TGO (Thermal Oxide
Growth) in the interface between top
coat and bond coat. This region is the
weakest area and the beginning of the
formation of cracks. The existence of
porosity in the ceramic layer as a result
of oxidation, as shown in Figure 3.5.a
porosity is formed along the cracks.
Porosity in the specimens
increased with increasing time of
thermal cyclic treatment, as well as in
the cycle to 50 levels of porosity in the
top coat and bond coat on the area, as
shown in Figure 3.6. Porosity occurs at
the interface regions top coat and bond
coat, so it looks like fissures that cut
through the interface areas, where there
is TGO evenly spread, where the growth
of TGO give effect to the failure and
peeling of Thermal Barrier Coating.
Where the growth of TGO is depends on
time,
temperature
and
chemical
composition that is in the top coat and
bond coat. Many studies have stated that
the oxidation of the bond coat is the
most important thing in a decrease in
performance of the system at high
temperatures. As in this study, the crack
occurred due melt substrate, where the
melting of the substrate provides a stress
in the layers of the others, it can be seen
in Figure 3.1, cracks occur on the side of
the specimen, which continued to spread
in all parts of the specimen.
After the 100th cycle, cyclic
thermal treatment is stopped. That's
because state specimens are not possible.
Melted Substrate began in the not
coating area and continued to other parts
of the specimen, as in the previous
explanation on macrostructure analysis.
At the 100th cycle and TGO formed
more and more, in general, this layer
exists between top coat and bond coat
because the area contains a lot of
porosity and provides a way for the
oxide to oxidize the elements contained
in the bond coat, but is not possibility
TGO formed inside the bond coat. As
research conducted by Chen (2004),
found TGO trapped inside the bond coat
due partly to avoid the bay area bond
coat surface, so the oxygen is oxidized
and reacts with the elements contained in
the bond coat. According to research
conducted Sulistijono (1998), Al content
in the specimen which reached 8.27%
should be able to protect the metal
underneath the aluminum oxide formed
during the thermal cyclic. Oxide can be
formed Al2O3 as TGO layer (Thermal
Oxide Grown) provides protection by
reacting with oxygen through the TGO
layer (Thermal Oxide Grown). Because
react with Al, so ideally there is no
oxygen enter into the bond coat layer
(according to the Elingham diagram).
But in fact, there are other oxides, to find
out what oxides are formed, its existence
can be detected using EDS (Energy
Dispersion Spectometry).
Table 3.1 is summary result of
SEM / EDS, it can be seen the elements
that form the interface between top coat
and bond coat, are O, Al, Ni. The
composition of the elements are not
equal to the before and after thermal
cycle.
Porosity
TGO
Resin at porosity
Crack
(a)
(b)
Figure 3.5 SEM analysis on specimen after (a) 25 thermal cycles and (b) 50
cycles. It shown porosity, crack and TGO area
Table 3.1 Comparison of element quantity of specimen
ELEMENTS
AS SPRAYED
(% Mass)
25 Cycle
(% Mass)
50 Cycle
(% Mass)
100 Cycle
(% Mass)
C
O
Al
Ni
Si
34,15
26,27
39,57
As sprayed
(% Mass)
38,43
35,10
26,47
25 Cycle
(% Mass)
21,47
0,11
78,42
50 Cycle
(% Mass)
3,97
32,93
21,92
38,64
2,55
100 Cycle
(% Mass)
49,64
50,36
-
66,31
33,69
-
0,21
99,79
-
41,41
49,16
5,46
Phases
Al2O3
NiO
SiO2
Table 3.1 shows that almost all
the elements that formed at the interface
changes after cyclic thermal, these
changes vary from each cycle, elements
and compounds. Before cyclic thermal,
obtained for the elements O was 34.15%
because of the oxidation process during
the spraying process. In cycle 25 there is
an increase due to the occurrence of
oxidation processes during the cyclic
thermal oxidation which occurs both in
the furnace and the moment when issued
free air. Increased % of mass is in
accordance with the weighing done on
visual observation, where the heavy
curve in each of his cycle until the Cycle
25 has increased. However, severe
decline in the next cycle, until the 100th
cycle, this would fit in Table 3.1 it can
observe the cycle to 50 and 100% mass
of O has decreased, it is because of
cracks in the specimen due to substrate
melting.
Almost the same Al element to
the condition O elements, which
increased in % mass after cycle 25,
before the cyclic thermal 26.27% to
35.10%, due to the formation of bonds
with oxygen to form Al2O3, formed a
thin layer of protection. Of course, these
conditions are also accompanied by an
increase in mass% Al2O3 formed
compounds in the cycle up to 25, more
time on the cyclic thermal oxidation rate
is also greater. This causes the content of
Al and eventually thinned out (smialek,
2006). The statement according to Table
3.1, where the number % of the mass of
Al and Al2O3 on the cycle decreased to
50 up to 100 cycles.
Whereas for Ni element in the
cycle 25 has decreased, it could be due
to the time of the shooting on the area
covered with a thin layer of test is more
dominant Al2O3. With increasing time,
the next cycle occurs increased % mass,
reaching 78.42% at 99.79 on Ni and
NiO. This can be caused because at the
time of other elements to form bonds,
not react to form Ni oxide or oxide
formation can be said NiO late. As a
result of the bond coat Nickel Ni from
the substrate accommodating before
forming the nickel oxide resulted in the
mass fraction of Ni grows. According to
research conducted Chen, et al (2004)
explains that after the heat treatment of
specimens will be formed oxide - a new
oxides such as (Cr, Al) 2O3, Ni (Cr, Al)
2O4, which is heterogeneous NiO / non
uniform and cause layer has a low
adhesivity.
. In the cycle to 100, based on
test results of SEM / EDS acquired
elements Si and SiO compounds, which
are not found at previous cycle, this is
because the condition of the specimen
that has undergone melting in the
substrate, where it reaches the overall
melting of the specimen in this cycle,
and based on SEM observation test
(Figure 3.5) where the substrate melts
filled at porosity and top coat bond coat.
From SEM analysis at Figure 3.6
note that after the thermal spraying
occurred many bonding at the interface
between top coat and bond coat such as:
mechanical interlock, chemical bonds
(oxidation reactions), and diffusion.
Mechanical interlock between the
surface of the top coat and bond coat
caused some of them. First, at the time
of the shooting of ceramic particles, not
all particles in the melt state and the
shooting occurred at high speed so that a
portion of the bond coat surface that
does not deform uniformly. Second, the
condition of the bond coat surface is not
flat so it had a high roughness, so that
the particles are fired top coat and follow
the contour of the bond coat. Between
ceramic particles and the other one has a
different cooling rate so that the bond
will be formed interlock mechanical
layer. According to research conducted
by Chen, et al (2004), mechanical
interlock can improve adhesivity
between top coat and bond coat, because
the area of overlap between top coat and
bond coat expanded.
Chemical
bonding
Mechanical
Interlock
Top Coat
BC
Figure 3.6 Microscope electron analyses at interface layer between top coat and
bond coat.
Chemical bonds or result of
oxidation reaction occurs at the interface
area between the bonds coats based
super alloy with oxygen is trapped in
porosity. The oxidation reaction formed
a variety of metal oxides at different
forms. At the top coat and bond coat
Al2O3 and partially formed within the
TGO formed many kinds of oxides as
described previously. Diffusion process
occurs between the surfaces of the top
coat with a bond coat surface with atoms
reactivity.
4. Conclusion
From this research it can conclude that:
1. Thermal growth oxide layer had
form at the interface between top
coat ceramic and bond coat NiAl that formed alumina (Al2O3)
phase
2. Thermal
cycle
at
high
temperature operation caused
diffusion of element on substrate
and bond coat to form oxide at
interface layer
3. At interface layer found chemical
bonding resulted from oxide
reaction
and
mechanical
interlocked bonding
5. Reference
Chen, W. R., Wue X, Marple B. R.,
Patnaik, P. C., 2004. “ Oxidation and
crack nucleation/growth in an airplasma-sprayed thermal barrier
coating with NiCrAlY bond coat”.
Institute for Aerospace Research,
National Research Council of Canada,
1200 Montreal Road, Bidg M-13,
Otawa, Ontanio, Canada, K1A0RG
Kristanto. 2008. “Studi Fasa dan
Mikrostruktur pada Interface Top
coat 8YSZ dan Bond coat
NiCoCrAlY Akibat Thermal
Fatique”. Surabaya: Jurusan Teknik
Metalurgi dan Material- Institut
Teknologi Sepuluh Nopember
Surabaya
Ogawa . K, Gotoh .N, 2003. “ The
influence of thermal barrier top
coating on the initiation and growth
of thermally cycled thermal barrier
coatings”. Sweden : Lund University.
Ramaswamy,P.,2000,“Thermo
mechanical fatique characterization
of zirconia (8%Y2O3-ZrO2) and
mullite thermal barier coating on
diesel engine components: effect of
coatings on engine performance”,
Proquest Science Journals pg 729
Smialek, J. L., jan 2006. “Moustureinduced delayed spallation and
interfacial hydrogen embrittlement
of alumina scales, JOM. Pp 29-35.
Sulistijono, 1998, “ Pelapisan keramik
pada sudu turbin gas untuk
meningkatkan ketahanan korosi pada
temperature tinggi “. Surabaya :
Jurusan Teknik Mesin-Institut
Teknologi Sepuluh Nopember.