FIELD OF THE INVENTION
The invention relates to a process of depositing thick Thermal Barrier Coating
(TBC) on Gas Turbine components to protect the substrate superalloy from high
temperature oxidation.
BACKGROUND OF THE INVENTION
Gas turbine hot path components like combustion liner, transition piece, etc. are
made of cobalt/nickel based superalloy, which are enabled to sustain higher
temperature than the alloyed steel. Further, alloys are also high temperature
oxidation resistant. Some of the high rating gas turbines operate near to melting
point of superalloys or higher than melting point of the superalloys. In those
cases, it is very essential to coat the surface of the alloy with insulating material
like alumina, zirconia, mullite. In order to avoid the oxidation, the hot gas path
components are protected by zirconia thermal barrier coatings (TBCs). These
coatings are used to extend the life of the metal components by creating a
temperature drop across the coating, permitting the base metal to operate at a
reduced temperature.
TBCs are a two-layer system composes of a zirconia layer ~0.300 mm thick,
which faces the hot combustion gases, and an ~0.100 mm bond coat (typically,
NiCrAlY alloy). The bond coat provides strong coating adherence and enhances
the oxidation resistance of the substrate metals.
The coatings are applied by the plasma spray technique. In this process, an
electric arc ionizes an argon gas to form a plasma. Ceramic powders are injected
into the plasma, heated to a "semiplastic" state, and accelerated toward the
coating surface. When the particles impact the target, a very complex
interlocking microstructure is formed, which is highly porous in nature. Pure
zirconia is not used as the top coat because pure zirconia at room temperature
has monoclinic structure which transforms into tetragonal structure on heating.
While cooling down, it comes back to monoclinic structure and with 3% volume
expansion, which finally results in cracking of the coating. According to prior art,
Yttria stabilized zirconia (YSZ) has been very successful as thermal barrier
coating (TBC) for GT components. YSZ contains 8-10% yttria which stabilize the
structure of zirconia as cubic structure at room temperature. However, YSZ has
low thermal coefficient compared to the base metal, which results in thermal
expansion mismatch, between the base metal and the coating during high
temperature operation. This leads to cracking in the TBC coating. In order to
reduce the mismatch of thermal expansion, an intermediate coat of MCrAlY
(M=Ni/Co) is applied according to prior art which is called bond coat.
In the conventional TBC coating the Bond coat is applied to a thickness of 100 to
120 microns. Before bond coating, the substrate is prepared by grit blasting to
remove the dirt and oxides present in the substrate as well as to achieve high
adhesion of coating by mechanical anchoring. After the grit blasting the bond
coat is applied. Over the bond coat a top coat of YSZ is applied to a thickness of
200 to 225 microns. Both the coatings are applied by known plasma spray
system like Metco 9 MB, Metco 4 MB system. In these systems the argon gas is
used as a primary gas for generating the plasma and Hydrogen gas is added to
increase the enthalpy of the plasma. Powder can be fed internally as well as
externally. Plasma produces high temperature which leads to overheating of the
coating substrate. Overheating leads to coating cracking as well as oxidation of
the coating substrate. To overcome such problem, compressed air is used to cool
the coating substrate.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a process of
depositing thick thermal barrier coating on gas turbine components to protect
the base metal of the components from high temperature oxidization.
Another object of the present invention is to propose a process of depositing
thick thermal barrier coating on gas turbine components to protect the base
metal of the components from high temperature oxidization, which reduces high
temperature oxidation of the substrate alloy.
SUMMARY OF THE INVENTION
Power generation by gas turbine is one of the known technique. Advanced gas
turbines operate at higher temperature (>1200°C). Gas turbine components
dispose along the gas path get oxidized at this high temperature. In order to
protect these components from high heat flux, according to the invention a thick
thermal barrier coating (TTBC) is applied on the substrate. According to the
invention, the top coat is made of 8 wt% Yttria partially stabilized zirconia, which
does not change to monoclinic structure. This material possesses higher thermal
shock resistance than the fully stabilized zirconia and pure zirconia. For high
rating gas turbines, the components are subjected to higher temperatures close
to the melting point of the base metals. In order to protect the base metals from
high temperature oxidation, a process for application of a thick thermal barrier
coating is proposed. The TTBC coating is made of two layers; one is bond coat
which is made of NiCrAlY and other is top coat which is made of yttria stabilized
zirconia. Zirconia being one of the most effective insulating compounds, it is used
as TTBC. This coating is applied by plasma spray process which is the only
known method by which such ceramic powder can be deposited. As the plasma
spray process produces high heat which oxidized the coating substrate, sufficient
forced air (compressed air) cooling of the coating should be followed. The new
coating upon application on a substrate was evaluated by X-ray diffraction (XRD)
for phase integrity, for porosity by image analyzing microscope, for bond
strength by pull-off adhesion tester. Porosity of coating was found in the range
of 6-10%. Bond strength of the coating was measured in the range of 2000 to
3000 psi. The technical parameters used in the inventive coating process is
tabularized in Table - 1 of the disclosure.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows an image of X-ray diffraction (XRD) for phase detection of the
coating according to the invention.
Figure 2 - Shows an optical micrograph of the thick thermal barrier coating
(TTBC) of the invention.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention, a known Metco 9MB system is used as plasma spray
system. Bond coat and top coat of YSZ are applied using an ABB robot for
uniform coating. A plasma gun is mounted on the robot. Robot programming is
done for coating sample flat plate. These plates are made of superalloy, which is
used in gas turbine. These flat plates are grit blasted by using alumina grit for
cleaning the surface. Bond coat and top coat is applied using the parameters as
given in Table-1. Two passes are applied for bond coat whereas thirty passes are
applied for top coat of YSZ. Top coating thickness is built up at an interval of 3
passes followed by cooling. Forced cooling of compressed air is continued until
the job is cooled to room temperature. Plasma spray coating is performed inside
an acoustic chamber because of high sound.
Characterization of coating:
Thermal barrier coating was characterized by x-ray diffraction (XRD) for phase
detection, by image analyzing microscope for evaluating porosity in the coating,
and by adhesion tester for checking the bond strength. XRD pattern of the
coating as shown in Fig.l. Pattern shows the presence of tetragonal phase of
yttria stabilized zirconia (YSZ).
Coating thickness was measured using non-ferrous thickness probe. After bond
coat, the thickness reading was taken as well as after YSZ powder coating.
Coated sample was cut using slow speed cutting machine for evaluating the
microstructure by optical microscope. Before cutting the coating was protected
by applying epoxy. Cut sample was mounted and polished for viewing in
microscope. A typical microstructure is shown in Fig. 2. Coating porosity was
evaluated using image analyzing software. Coating porosity was found in a range
between 6 to 10%.
Thermal barrier coated sample was also evaluated by adhesion tester for finding
the bond strength. Pull off adhesion tester with a standard dolly of 20 mm size
was used for the measurement. Dolly was fixed using epoxy on the coated plate.
Bond strength was found in a range between 2000 to 3000 psi.
We Claim
1. A plasma sprayed ceramic coating process for obtaining thick thermal
barrier coating (TTBC) for protecting gas turbine (GT) components
disposed along the hot gas path from high temperature oxidation of the
base metal of the GT components, the process comprising the steps of
surface preparation of the proposed coating surface of the gas turbine
components by grit blasting using alumina grit powders; applying
intermediate base/ bond coating of NiCrAlY by plasma spraying process to
a thickness range 100 to 150 microns; applying thick ceramic yttria
stabilized zirconia (YSZ) coating by plasma spraying to a thickness range
of 1000 to 1200 µm; wherein the distance of the spray gun from the
coatable component is respectively maintained at 110 - 120 mm and 50-
55 mm for bond coating and YSZ coating , wherein the current maintained
during plasma spraying for the bond coat and YSZ coat is respectively 550
- 550 amps and 550 - 600 amps, and the voltage during the both bond
and YSZ coating is between 52 - 55V, wherein the primary/ secondary
gas flow rate is restricted within 60 - 70 lp_m and 70-80 Ipm and 5-10
Ipm respectively for the bond coat and YSZ coat, wherein the powder
feed rate and carrier gas flow rate is maintained between 4-6 rpm and 3
- 4 rpm, and 10-15 Ipm and 8-12 Ipm respectively for bond coat and YSZ
coat, and wherein the spray gun velocity during both bond coating and
YSZ coating is maintained at 200 - 250 mm/sec.
2. The process as claimed in claim 1, wherein a thick zirconia coating is
applied on a superalloy substrate.
3. The process as claimed in claim 1, wherein the coating having a porosity
in the range of 6 to 10%.
4. The process as claimed in claim 1, wherein the coating having bond
strength in the range of 2000 to 3000 psi.

ABSTRACT

A plasma sprayed ceramic coating process for obtaining thick
thermal barrier coating (TTBC) for protecting gas turbine
(GT) components disposed along the hot gas path from
high temperature oxidation of the base metal of the GT
components"
The invention relates to a plasma sprayed ceramic coating process for
obtaining thick thermal barrier coating (TTBC) for protecting gas turbine (GT)
components (combustion liner, transition piece, etc.) disposed along the hot
gas path from high temperature oxidation of the base metal of the GT
components, the process comprising the steps of surface preparation of the
proposed coating surface of the gas turbine components by grit blasting using
alumina grit powders; applying intermediate base/ bond coating of NiCrAlY by
atmospheric plasma spraying process to a thickness range 100 to 150
microns; applying thick ceramic yttria stabilized zirconia (YSZ) coating by
plasma spraying to a thickness range of 1000 to 1200 µm. This thick thermal
barrier coating was evaluated by X-ray diffraction (XRD) for phase/compound
identification, by optical microscope cum image analyser for porosity
measurement, by pull off adhesion tester for finding bond strength
measurement. Coating porosity was found between a range of 5 to 6% and
the bond strength was found between a range of 2000 to 3000 psi.